Dr Evangelos Zympeloudis is the CEO and co-founder of iCOMAT, a company based in the UK that is developing automated manufacturing equipment for tow-steered composites. Fibre-reinforced plastics, such as carbon-fibre or glass-fibre composites, hold great promise for high-performance and lightweight design due to their excellent stiffness and strength properties at low material density. Traditional fibre-reinforced plastics are manufactured using straight uni-directional fibres or with straight fibres woven into a fabric.
Generally speaking, a fibre-reinforced composite derives its strength by aligning the fibres with the direction of the dominant load path. The novelty of tow-steered composites is that strips of composite material, so-called fibre tows, are steered along curvilinear paths such that the fibre direction is not straight, but varies continuously from point to point. This characteristic has benefits in structural design as the reinforcing fibres can now be used to smoothly tailor stiffness and strength throughout the structure. For example, tow-steered composites can be used to curve the reinforcing fibres around windows in an aircraft fuselage in order to improve strength and facilitate net-shape manufacturing.
In this episode of the Aerospace Engineering Podcast, Evangelos and I talk about:
his background as an engineer and entrepreneur
the manufacturing challenge of making defect-free tow-steered composites
the capabilities of iCOMAT’s rapid tow-shearing process
the benefits of tow-steering for manufacturing cost and design
and some of the projects iCOMAT is currently working on
Disclosure: I currently work with iCOMAT on a number of projects and am a consultant to the company.
Sam Bousfield is the founder and CEO of Samson Sky, a company that is developing the first truly useful flying car. Sam is an architect by training, but a passion for aviation led him to work on a supersonic aircraft with Boeing. Out of this experience came the idea of building a flying car called the Switchblade.
Harking from an architectural background, Sam approached the problem of designing a flying car slightly differently. Rather than asking the question of how you could make a car fly, Sam and his team focused on the architectural question of how a vehicle that can both fly and drive should be designed. Answering this question led the Samson team to some unique design choices, such as a three-wheel layout and wings that stow and swing out from underneath the vehicle. One of the other challenges in designing a flying car is striking the right compromise between on-road and off-road performance. For example, a car should preferably create downforce, while a plane should create lift. To achieve this Samson Sky has made some very clever design choices in terms of the layout and shaping of the Switchblade, as well as the positioning of the wings and centre of gravity, and the use of lightweight composite materials. In our conversation, Sam and I talk about:
why it has taken so long for a functional flying car to be built
the main design challenges that need to be overcome
the changes that need to be made to the vehicle when switching between flying and driving
the way that Sam envisions the Switchblade to be used in practice
Mark Crouchen is the managing director of Rockwood Composites, a company in the UK that specialises in manufacturing complex composite components using compression and bladder moulding. These manufacturing processes use fibre mats of carbon fibre, glass fibre, Kevlar, or any other material, which are pre-impregnated with a resin matrix and then placed in a mould, where they are cured at elevated temperature with the addition of external or internal pressure.
The team at Rockwood has been supplying the aerospace, defense, medical and nuclear industries for over 25 years, with customers ranging from Leonardo Helicopters and the McLaren Formula 1 team to Safran and Facebook’s Aquila internet drone. In 2018, Rockwood won the Innovation in Manufacture award at the Composite UK industry event for their innovative use of advanced composite materials on the Tokomak ST40 nuclear fusion reactor. Composite materials have many benefits in terms of their excellent strength and stiffness at low weight. However, there is a common misconception that metal or ceramic components can easily be replaced one-to-one with composite components. The performance of any composite component is closely linked to the quality of the manufacturing process, and designing and manufacturing quality components is an area where Rockwood Composites particularly excel. In this episode of the podcast, Mark and I talk about:
his background in engineering
the types of structures that Rockwood Composites manufacture
why composites manufacturing is a challenge
and the special solution Rockwood found for the Tokomak ST40 fusion reactor
“There’s been a lot of good press from the science community on self-assembly of atoms. Well, I guess what I’m looking for is self-assembly and disassembly of large-scale structures…There is all sorts of exciting things we can do when [engineering] structures re-configure themselves.” — Prof. Paul Weaver
This episode features Prof. Paul Weaver, who holds a Bernal Chair in Composite Structures at the University of Limerick in Ireland, and is the Professor in Lightweight Structures at the University of Bristol in the United Kingdom. Lightweight design plays a crucial role in the aerospace industry, and Paul has worked on some fascinating concepts for more efficient aircraft structures. Paul’s research has influenced analysis procedures and product design at NASA, Airbus, GKN Aerospace, Augusta Westland Helicopters, Vestas (and many more), and in this episode we cover some of his past accomplishments and his vision for the future.
Central to this vision is artificial metamorphosis, which is a term that Paul coined to describe structures that re-configure by dis-assembly and re-assembly to adapt and optimise on the fly. Although Paul thinks that this vision of engineering structures is still 50 years into the future, he is well known for his work on a related technology: topological shape-morphing. The simplest example of a morphing structure is a leading edge slat, which is used on all commercial aircraft today to prevent stall at take off and landing. Paul, on the other hand, envisions morphing structures that are more integral, that is without joints and which do not rely on heavy actuators to function. Apart from artificial metamorphosis, Paul and I discuss
his teenage dreams of becoming a material scientist
his work with Mike Ashby at Cambridge University on material and shape factors
interesting coupling effects in composite materials that can be used for elastic tailoring
his work with Augusta Westland helicopters on novel rotor blades
why NASA contacted him about his research on buckling of rocket shells
Aeroelasticity is the study of the interactions between dynamic, inertial and aerodynamic forces that arise when a body is immersed in airflow. The unique challenge of aeroelasticity is to analyse how vibrations, static deflections and lift and drag forces combine, and to make sure that any interaction of these three forces does not lead to inferior aircraft performance or even failure.
The triangle in the figure below is known as Collar’s triangle and each vertex shows one of the forces mentioned above. When all three forces interact simultaneously we are in the realm of aeroelasticity and common failure modes include wing flutter and buffeting. When inertial and elastic forces combine in the absence of aerodynamic forces we are in the classical domain of structural dynamics and essentially dealing with any sort of mechanical vibration that you would experience on any piece of moving machinery. The interaction of inertial forces and aerodynamic forces gives rise to aerodynamic stability problems. How does an aircraft react to small disturbances – do the oscillations dampen out or do they get worse over time? Finally, the interaction of aerodynamic forces and elastic forces can give rise to a phenomenon known as divergence, which is an effect where twisting of the wing becomes theoretically infinite and can cause wings to twist off.
The Collar Triangle defining aeroelasticity as “the study of the mutual interaction that takes place within the triangle of the inertial, elastic, and aerodynamic forces acting on structural members exposed to an airstream, and the influence of this study on design.”
The two most dramatic aeroelastic effects are flutter and divergence. Flutter is a dynamic instability, often of the wing, caused by positive feedback between the wing’s deflection and the aerodynamic lift and drag forces. The flutter speed is the airspeed at which the wing, or any other part of the structure, starts to undergo simple harmonic motion – much like the simple to and fro motion of a simple pendulum – and this vibration occurs with zero net damping. Zero net damping means that there is no dissipation of energy (think of a pendulum swinging for eternity) and so any further decrease in net damping will result in self-oscillation – the structure is basically forcing itself to vibrate more and more, which at some point, will naturally lead to failure.
As we all know, the lift force acting on a wing will tend to bend it upwards, but what is less well-known is that this lift force can also cause the wing to twist. This is because the centre of pressure, the point where the total sum of the lift pressure field is assumed to act on an airfoil, is not necessarily coincident with the shear centre, the point through which a bending load needs to be applied to get pure bending without any twisting. Imagine holding a ruler in one hand and pushing up on it with your other hand. If you apply the load along the central axis of the ruler, the ruler will only bend, but if you apply the load at one of the two sides you can see the ruler bend and twist ever so slightly. Most of the time, the shear centre of an airfoil is not coincident with the centre of pressure, and so a lift force produces both bending and twisting. A critical phenomenon called divergence can occur when this twisting of a wing increases the angle of attack, which consequently increases the lift force further or creates further mismatch between shear centre and centre of pressure, so that a feedback loop ensues until the wing diverges or essentially shears off. In fact, one of the Wright Brothers’ main rivals in the race to being the first at heavier-than-air flight was Samuel Langley, whose prototype plane crashed into the Potomac river in Washington D.C., and this is now believed to have occurred as a results of torsional divergence. Furthermore, torsional divergence was a large problem with many WWI fighter planes and required a lot of additional stiffening of the wings.
One of the domains where divergence is particularly pernicious is in forward-swept wings. Simply put, wing sweep delays the onset of shock waves over the wings and therefore reduces the associated rise in aerodynamic drag caused by boundary layer separation. In slightly more detail, as air flows over a curved object, such as an aircraft wing, it accelerates due to centripetal forces and this means that an aircraft travelling slightly slower than Mach 1.0 (the speed of sound) can develop pockets of supersonic flow over areas with high local curvature, typically the wings or the canopy. For thermodynamic reasons, supersonic flows terminate in a shock wave which results in a sudden increase in the density of the air. This effect disturbs the smooth flow over the wing and creates vortices behind the aircraft, which means it is a form of parasitic drag. Sweeping the wing reduces the curvature of the body as seen from the airflow by the cosine of the angle of sweep. For example, a 45 degree sweep reduces the effective curvature by around 70% ([latex]\cos 45^\circ = 0.71[/latex]) compared to the straight-wing case. As a result, this increases the airspeed at which supersonic pockets start to form by about 30%, such that the aircraft can reach speeds much closer to Mach 1 before shocks occur.
Another way to think about the effect of sweep is to imagine the airflow over the wing as shown in the figure below. The effect of sweeping is such as to break the airflow into a component normal to the wing chord (“normal component”), and one along the span of the wing (“spanwise component”). The maximum curvature of the wing occurs along the wing chord, and the normal velocity component for the swept wing ([latex] V \cos \psi [/latex]) is always less than the normal component for a straight wing ([latex]V[/latex]).
The figure above highlights another critical aspect of swept wings: the spanwise component. On a backward-swept wing the spanwise flow is outwards and towards the tip, while on a forward-swept wing it is inwards towards the root (see the figure below). Firstly, with the air flowing inwards towards the fuselage, wingtip vortices and the accompanying drag are reduced. Wingtip vortices form when the higher pressure air underneath the wing is sucked up onto the lower pressure top surface of the wing, thereby reducing the effective lift-generating surface of the wing. On most modern backward-swept airliners, winglets and sharklets prevent this phenomenon from occurring. Forward-swept wings similarly minimise this effect by re-routing some of the flow towards the wing root, and therefore allow for a smaller wing at the same lift performance. The second advantage of forward-swept wings is that shockwaves tend to develop first at the root of the wing, rather than towards the tips, and this helps to reduce tip stall. Aerodynamic control surfaces such as ailerons are typically located near the tips of the wings, because the further outboard, the greater their effect on controlling the rolling action of the plane. Tip stall essentially renders these ailerons useless, and therefore jeopardises the pilot’s control over the aircraft. As a result, the dangerous tip stall condition of a backward-swept design becomes a safer and more controllable root stall on a forward-swept design, providing better manoeuvrability at high angles of attack.
For all their merits, forward-swept wings suffer from one detrimental flaw – divergence. In a forward-swept wing configuration, the aerodynamic lift causes a twisting force that rotates the leading edge upward, causing a higher angle of attack, which in turn increases lift, and twists the wing further. With conventional metallic construction, additional torsional stiffening is typically required which adds weight, and is therefore sub-optimal in terms of aircraft performance.
Enter the Grumman X-29
The Grumman X-29 was an experimental aircraft developed by Grumman in the 1980’s, and flown by NASA and the US Air Force. The X-29 tested a forward-swept wing, canard control surfaces, and computerised fly-by-wire control to counter balance the various aerodynamic instabilities created by its airframe. From my perspective, the most important innovation, however, was the novel use of composite materials to control the aeroelastic divergence of forward-swept wings. At the time, composite materials were popular in the high-performance aircraft community as a means of creating stiff and strong structures at very low weight. In fact, composites were mainly used to save weight. However, the X-29 showcased a second advantage of this new material over classic metallic structures – multi-functionality.
Metals are isotropic materials, meaning that their stiffness is the same in all directions. The relationship between stress and strain along one direction of an aluminium panel is the same as in any other direction. Because composite materials are a union of stiff fibres held together by a resin matrix, we can manufacture panels that are stiffer in one direction than in another. This is because the composite material will be very stiff along the fibre direction but relatively compliant perpendicular to the fibre direction. In most fibre-reinforced composite materials, such as fibreglass and carbon fibre, this variation in stiffness is restricted to the plane of a single sheet of material known as an orthotropic lamina.
Consider one such layer of a continuous fibre-reinforced composite in the figure above. The material axes 1-2 denote the stiffer fibre in the 1-direction and the weaker resin in the 2-direction. If we align the fibres with the global x-axis and apply a load in the x-direction, the layer will stretch along the fibres and compress in the resin direction (or vice versa). However, if the fibres are aligned at an angle to the x-direction (of say 45°), and a load is applied in the x-direction, then the layer will not only stretch in the x-direction and compress in the y-direction but also shear. This is because the layer will stretch less in the fibre direction than in the resin direction. This effect can be precluded if the number of +45° layers is balanced by an equal amount of -45° layers stacked on top of each other to form a laminate, e.g. a [45,-45,-45,45] laminate. However, this [45,-45,-45,45] laminate will exhibit bend-twist coupling because the 45° layers are placed further away from the mid plane than the the -45° layers. The bending stiffness of a layer is a factor of the layer-thickness cubed plus the distance from the axis of bending (here the mid-plane) squared. Thus, even if the +45° and -45° layers have the same thickness, the outer 45° layers contribute more to the bending stiffness of the [45,-45,-45,45] laminate than the -45° layers do. Therefore, stretching-shearing coupling is eliminated in a [45,-45,-45,45] laminate as the number of +45° and -45° layers is the same, but bend-twist coupling will occur because the +45° layers are further from the mid-plane than the -45° layers.
Let’s now apply this effect at a wing level, i.e. a [latex][+\theta,-\theta][/latex] layup is used for the top wing surface and a [latex][-\theta,+\theta][/latex] layup for the bottom wing surface. At the global wing level, the layup is balanced because we have an equal number of [latex]+\theta[/latex] and [latex]-\theta[/latex] layers, but the [latex]+\theta[/latex] layers are further away from the wing mid-plane than the [latex]-\theta[/latex] layers. This means that the bending stiffness is dominated by the [latex]+\theta[/latex] layers, and the wing will twist when it bends.
In the Grumman X-29, this bend-twist coupling was successfully exploited to prevent divergence in the forward-swept wings. As aerodynamic lift forces the wing tips to bend upward, the forward-swept wing wants to twist to higher angles of attack, but the inherent bend-twist coupling of the composite laminates forces the wing to twist in the opposite direction and thereby counters an increase in the angle of attack – divergence is avoided!
Bend-twist coupling in Grumman X-29 wings. Both top and bottom wing skin may have the same number of +theta and -theta fibre angles, but if the +theta angles are further from the wing mid-plane then they will dominate the bending behaviour and cause the leading edge to twist down as the wing bends up.
The Grumman X-29 is an excellent example of an efficient, autonomous and passively activated control system. Rather than adding more material to the wing to make it stiffer (but also heavier) an alternative solution is to use the bend-twist coupling capability of composite laminates. This capability is an example of elastic tailoring, and remains one of the most under-exploited advantages of composite materials. As the big aircraft manufacturers overcome the initial hurdles of using composites on a large scale with the 787 Dreamliner and A350-XWB, expect more and more of these multi-functional capabilities of composites to find their way onto aircraft components.
In previous posts I have discussed the unique characteristics and manufacturing processes of a certain type of composite material, namely continuous fibre-reinforced plastics (FRPs). Just like many other composite materials, FRPs combine two or more materials whose combined properties are superior (in a practical engineering sense) to the properties of the constituent materials on their own. What distinguishes FRPs from other composites such as short-fibre composites, nanocomposites or discrete particle composites are the highly aligned, long bundles of fibres typically glass or carbon that are arranged in a specific direction within some resin system.
The biggest advantage of FRPs compared to metals is not necessarily their greater specific strength and stiffness (i.e. strength/density and stiffness/density) but the increased design freedom to tailor the structural behaviour. Metals and ceramics, being isotropic materials, behave in an intuitive way since the majority of the coupling terms in the stiffness tensor vanish. If you a imagine a three-dimensional cube and pull two opposing faces apart then the other two pairs of opposing faces will move towards each other. This phenomenon of coupling between tension and compression is known as the Poisson’s effect and aptly captured by the Poisson’s ratio.
The Poisson’s effect in action
In bending, a similar phenomenon occurs known as anti-clastic curvature. If you have ever tried bending a thin, beam-like structure made out of a soft material e.g. a rubber eraser, you might have noticed that the beam wants to develop opposite curvature in the transverse direction to the main bending axis. The structure morphs into some form of saddle shape as shown in the figure. The phenomenon occurs because the bending moment applied by the person in the picture causes tension in the top surface and compression in the bottom surface in the direction of applied bending. From the Poisson’s effect we know that this induces compression in the top surface and tension in the bottom surface in the transverse direction. By analogy, this is exactly the reverse of the bending moment applied by the hands and so the panel bends in the opposite sense in the transverse direction.
Anticlastic curvature in action (1)
For isotropic materials the fundamental linear constitutive equations between stress and strain eliminate a lot of the possible coupling behaviour. There is no coupling between applied bending moments and twisting. No coupling between stretching/compressing and bending/twisting. And also no coupling between stretching/compressing and shearing. FRPs, being orthotropic materials, i.e. having two orthogonal axes of different material properties, can display all of these effects. Consider a single layer of a continuous fibre-reinforced composite in the figure below. The material axes 1-2 denote the stiffer fibre in the 1-direction and the weaker resin in the 2-direction. If we align the fibres with the global x-axis and apply a load in the x-direction, the layer will stretch/compress along the fibres and compress/stretch in the resin direction in the same way as described previously for isotropic materials. However, if the fibres are aligned at an angle to the x-direction say 45°, and a load is applied in the x-direction then the layer will not only stretch/compress in the x-direction and compress/stretch in the y-direction but also shear. This is because the layer will stretch/compress less in the fibre direction than in the resin direction. This effect can be precluded if the number of +45° layers is balanced by an equal amount of -45° layers stacked on top of each other to form a laminate, e.g. a [45,-45,-45,45] laminate. However, this [45,-45,-45,45] laminate will exhibit bend-twist coupling because the 45° layers are placed further away from the mid plane than the the -45° layers. The bending stiffness of a layer is a factor of the layer thickness cubed and the distance from the axis of bending (here the mid plane) squared. Thus, the outer 45° layers contribute more to the bending stiffness of the laminate than the -45° layers such that the coupling effects do not cancel.
A single fibre reinforced plastic layer with material and global coordinate systems
Using metals, structural designers were constrained to tailoring the shape of a structure to optimise its performance i.e. thickness, length and width, and overall profile/shape. FRPs however add an extra dimension for optimisation by allowing designers to tailor the properties through the thickness and thereby achieve all kinds of interesting effects. For example, forward-swept wings on aircraft have and still are a nightmare due to aeroelastic instabilities like flutter and divergence. Basically, sweeping a wing forward is a neat idea because the airflow over swept wings flows spanwise towards the end furthest to the rear of the plane. Therefore, the tip-stall condition characteristic of backward-swept wings is moved towards the fuselage where it can be controlled more effectively. The drawback is that as the lift force bends the wingtip upwards the angle of attack increases, further increasing the lift and thereby causing more bending, and so on until the wings snap off or fail. Rather than adding more material to the wing to make it stiffer (but also heavier) an alternative solution is to use the bend-twist coupling capability of composite laminates. This was successfully achieved in the iconic Grumman X-29. As the bending loads force the wing tips to bend upward and twist the wing to higher angles of attack, the inherent bend-twist coupling of the composite laminate used forces the wing to twist in the opposite direction and thereby counters an increase in the angle of attack. This is an excellent example of an efficient, autonomous and passively activated control system to prevent divergence failure.
Grumman X-29 with forward-swept wings
In this manner, straight fibre composites allow structural engineers to change the stiffness and strength properties through the thickness in order to tailor the structural behaviour. The concept of variable stiffness composites adds a further dimension to the capability for tailoring. Currently this is achieved by spatially varying the point wise fiber orientations by actively steering individual fibre tows using automatic fibre placement machines. One early application that was considered by researchers was improving the stress concentrations around holes by steering fibres around them.
Automated Fibre Placement machine (2)
This concept can be generalised by aligning fibres with the direction of local primary load paths which could vary across different parts of the structure. Tow steering creates the possibility for designing blended structures by facilitating smooth transitions between areas with different layup requirements. One promising application of variable stiffness composites is in buckling and postbuckling optimisation of flat and curved panels. As a panel is compressed uni-axially the capability of the panel to resist transverse bending loads reduces until a critical level is reached where the panel has lost all capability to sustain any bending loads. At this point known as the buckling load, the fundamental state of compression becomes unstable and the panel buckles outward in a single or multiple waves. It has been found that variable stiffness composites can double the buckling load of flat panels by favourably redistributing the load paths in the fundamental, pre-buckling compression state. Essentially, the middle of the panel where the buckling waves will occur is offloaded, and the edges of the panel are forced to take more load. Thus, the aim is to redirect loads to locally supported regions and remove load from regions remote from supported boundaries. This concept has also been extended to improving aircraft fuselage sections and blade-stiffened panels.
A variable angle tow laminate (3)
This new technology is viewed as a promising candidate for further reducing the mass of future aerospace structures. In fact recently NASA Langley Research Centre announced that they are investing heavily in this capability. The possibility of manufacturing integrated structures with smooth flow of material between components and minimal joints will not only revolutionise stress-based design, but also simplify manufacturing and facilitate entirely new aircraft designs that are currently unfeasible. In trees for example, there is a smooth transition of fibres from the trunk into the branches to strengthen the connecting joint. With the variable stiffness capabilities investigated by NASA we could apply this concept to simplify and even strengthen critical interfaces such as fuselage-wing connections.
The treatment of defects in aircraft structural design has been an important aspect in aircraft structural design during the last 50 years. Various different catastrophic events have led to key insights that now shape the design philosophy for primary aircraft structures. One of these is the distinction between Safe-Life and Fail-Safe structures. Safe-Life components are designed to go through their service life without cracks and defects playing a major role in the stress state of the component. Thus, the required fatigue life to initiate a crack is kept below the anticipated service life. This design approach is mainly used for components for which there are no back-ups in place and where failure would lead to the loss of the aircraft. A typical example of a Safe-Life component is the landing gear and this remains one of the reasons why landing gears are made from high-strength steel for which engineers have a long history of structural data. The second “Fail-Safe” design philosophy assumes that any real manufacturing process will induce defects within the part that, even if microscopic, may vary between different batches and may grow during the service life. Thus the Fail-Safe components are structurally designed to withstand all imposed loads up to a certain certain level of defect, known as the “critical size”, which can usually be detected by eye and act as stress concentrators. In this manner critical components are continuously monitored at specific service intervals to make sure that no crack exceeds the critical defect size, and is subsequently replaced if this happens. Furthermore, crack propagation analyses are employed in order to ascertain how many flights/load cycles it will take to grow a crack to the critical size. Most of these insights stem from the experience engineers have gained during the last 50 years with metal aircraft and in fact there was quite a steep learning curve during the transition years from wood to metal aircraft.
Today we are facing a similar transition from metal to mostly fibre reinforced plastics and other advanced materials whose failure mechanisms are often much more complex than that of metals. First, in metallic structures a crack typically initiates at an imperfection or stress concentration and then propagates under fatigue loading until final failure. The damage morphology in composites however is completely different: a large number of microscopic defects, such as micro-cracks that occur during post-cure shrinkage of the resin are present over a large volume of the material and these may develop into different failure mechanism over time. Second, most metals have a ductile failure mechanism such that overloading can visually be detected by the onset of plastic deformation. Therefore there is often a warning period between a structure being overloaded and failing catastrophically. Fibre reinforced plastics, especially carbon fibre composites on the hand fail by more brittle and therefore sudden mechanisms. Third, while a major driver of component design for metal structures is crack growth, which can be predicted quite accurately today using analytical methods or Finite Element codes, fibre reinforced plastics have a plethora of other failure mechanisms and manufacturing defects that are equally important. Some examples are fibre breakage, matrix cracks, matrix-fibre debonding, delaminations, voidage, misplacement of plies, lack of impregnation and fibre waviness. Interlaminar failures such as delaminations are especially important since they can occur very quickly when a laminate is loaded through the thickness, for example at stringer run-outs, in corner-radii of C-spars or simple impact events such as tool drop in the factory. Since there are typically no reinforcing fibres in the perpendicular direction the structural integrity is only guaranteed by the weak matrix. Due to this inherent weakness different plies may literally be pulled apart at their lamination interfaces. Techniques such as through thickness reinforced such as 3D braiding, Z-Pinning or nano-fibre reinforcement are currently being researched. Under compressive forces these delaminations may form blisters, so called delimitation buckling, which can easily propagate along the lamination interface leading to disintegration of the part.
Fig. 1 Delamination Buckling in Composite Laminate
Finally, different failure mechanisms actually interact making accurate predictions of the failure load including a defect extremely difficult. Furthermore, even experimental data for laboratory sized specimens cannot readily be used for real-sized components since the scaling up of structures has been found to greatly alter the dominant failure mechanism. Finally, failure sites in fibre reinforced plastics are often internal meaning that an engineer will not be able to detect them by simple visual investigation during service intervals. As a result, the increasing use of fibre reinforced plastic construction during the recent years and near future means more sophisticated evaluation techniques are required for guaranteeing safe design and operation of aircraft. Another key question is how these new types of defects can be taken into account reliably in structural design?
Compared to metallic materials composites have a very unique characteristic in that the material and structure/part are created simultaneously. This means that the amount of imperfections in the part is greatly dependent on the manufacturing process. In composite materials the fail-safe design philosophy of degrading the material properties to that including a “critical defect size” is not only important to reduce the probability of failure as in metallic structures but also because a manufacturing process free from imperfections would be financially prohibitive. Thus, the degree of process and quality control depends greatly on the safety requirements of the industry. For example, the high-volume and competitive automobile sector needs to guarantee passenger safety while keeping manufacturing costs at a minimum. In the aerospace industry however the mass of components is absolutely critical and takes precedence over the manufacturing costs. As a result the automobile industry relies more on out-of-autoclave infusion processes that allow high production volumes such as Resin Transfer Moulding, while the aerospace industry currently relies on the high-temperature, high-pressure curing environments of the autoclave that allow the manufacture of high performance parts with low, controlled level of imperfections.
Non-destructive testing (NDT) methods are often employed to detect defects inside or on the surface of a material. In general they are broken down into surface methods, bulk volume methods and global methods. These methods are typically used at the end of the manufacturing process as a quality control measure or during the life of the part to monitor and assess its fitness for continuing use. Surface methods include visual inspection techniques such as scanning the surface for obvious cracks, porosities, resin rich/starved regions or surface waviness. This is often coupled with endoscopes to examine remote or difficult to access locations. Furthermore a common technique is dye penetrant inspection where a dye is applied to external surfaces and illuminated with an ultraviolet light in order to highlight cracks on the surface that the dye has crept into. This technique was quite popular for aero engine components but is inherently quite time and labour intensive.
1. Section of material with a surface-breaking crack that is not visible to the naked eye. 2. Penetrant is applied to the surface. 3. Excess penetrant is removed. 4. Developer is applied, rendering the crack visible. (1)
Bulk volume methods range from the simple tap test to ultrasonic screening to the most sophisticated X-ray and computer tomography techniques. The choice of the method depends greatly on the type of defect that is to be detected and criticality of cycle time and production costs. Simple surface defects, core crush in sandwich structures may easily be detected using visual techniques, while tap tests can be used very effectively to determine delaminations or large internal voids. In a tap test the component is tapped lightly with a hard object such as a coin or ring which emits a very dull sound if a delimitation lies beneath the testing point. On the other hand the exact location and size of a delimitation, possible contaminations, voids or micro-porosities can only be detected with ultra-sonic or C.T. techniques. In this respect ultra-sonic scanning has developed to be the most widely-used NDT technique in the aerospace industry due to its high detection fidelity, compactness and relative low-cost compared to C.T. techniques. In ultra-sonic scanning ultrasound is projected into a component and by measuring the strength and time delay of the echo it is possible to detect inclusions (air, solid objects etc.) that differ from the host composite material.
Fig. 3. Principle of ultrasonic testing. LEFT: A probe sends a sound wave into a test material. There are two indications, one from the initial pulse of the probe, and the second due to the back wall echo. RIGHT: A defect creates a third indication and simultaneously reduces the amplitude of the back wall indication. (2)
One of the drawbacks of ultrasonic scanning is that some sort of coupling agent (typically water or a gel) is required between the probe and surface of the part to guarantee a high-quality reading. Furthermore, the scanning of large areas can be very time intensive even with the use of multi-probe ultrasonic arrays that can be rolled across a surface or controlled by a robotic arm, such that this technique is typically restricted to critical or highly-stressed components. Finally, CT techniques are currently only widely used in academia where they can give very useful insight into the exact 3D morphology of a cured part and show how and where cracks are initiated and when they propagate. Some pieces of equipment like Synchrotron radiation computed tomography at the University of Southampton can produce extremely detailed 3D plots and videos of parts under load that are very useful to help researchers understand what drives failure in composite materials.
Fig. 4. 3D Synchrotron Image (3)
Finally, in recent years global methods such as structural health monitoring have been a hot research topic. In structural health monitoring sensors such as strain gauges or fibre-bragg grating systems are embedded within the structure and provide real time data on the stress state. In this manner the health of the structure can be monitored in real time and service intervals and replacement parts be installed at the required times. However, these systems can probably not be embedded throughout an entire aircraft and require an incredible amount of storage to cope with the continual data stream.
Understanding the detrimental effects of imperfections and the damage mechanisms is essential in order to take full advantage of the benefits that high performance composites have to offer. In this respect non-destructive testing is a very valuable tool for investigating and mapping the internal condition of a component. One of the challenges facing the aerospace and automobile industries in the future is deciding what detail of non-destructive testing is required to guarantee the structural integrity of the products to a high degree of probability during the entirety of its service life and balancing this against the cost that the specific techniques incur.
In the aerospace industry the benefits of exploiting the excellent specific strength and stiffness properties of composites in terms of lightweight structural design are immediately apparent. Other advantages of fibre reinforced plastics, such as the relative ease to manufacture complex shapes, and their excellent fatigue and corrosion resistance, have made FRP composites increasingly attractive in the renewable energy sector. Considering the predicted growth of production in wind turbines, accounting for nearly 60% of the entire advanced composites market by 2020 [1], a wide variety of scientific material has been published in recent years regarding the optimisation of advanced composites usage in wind turbines. Furthermore, considerable “blue-sky” research is being conducted that aims to take advantage of the multifunctional capabilities of advanced composites in order to extend their integration in less obvious applications such as tidal turbines and solar panels. The objective of this post is to give a general overview of the novel research conducted to facilitate these new technologies, while giving a more detailed insight into the challenges that engineers face in designing the new generation of 100m wind turbine blades.
Overview
In the last 25-30 years the use of wind turbines for electricity generation has grown from a grass-root “green” initiative to a financially sustainable
Fig. 1. Correlation of increasing rotor diameter and power rating throughout the last 30 years [3].
primary energy resource [2]. The increasing maturity of the industry can be traced from the small 100-150 kW turbines constructed throughout the 1980s to the large 2-5 MW projects installed both on- and offshore today. This growth can largely be attributed to innovations in the integration of lightweight fibre-reinforced plastics (FRPs), which have facilitated increasingly larger blade lengths as shown in Figure 1. Fibre-reinforced plastics represent a prime material choice for wind turbine blades in terms of structural efficiency since the high specific stiffness limits tip deflections, reduces gravity-induced loading and decreases rotor inertia. Furthermore, the excellent fatigue resistance of FRPs helps to minimise material degradation and maintenance costs over the 20-year design lifespan [4]. A few of the currently largest wind turbines including their blade materials are summarised in Table 1.
Table 1: Summary of various Megawatt wind turbines with defining characteristics and blade material choices [5] – [9].
Company
Model
Blade
Length (m)
Rotor ø
(m)
Power
(MW)
Blade Materials
Vestas
V120-3MW
54.65
NA
3
glassfiber/carbon spars with glassfiber
airfoil shells
Enercon
E-126
NA
127
7.5
glassfiber/epoxy with steel mesh for
lightning strike
Siemens
SWT-3.6-120
58.5
120
3.6
glassfiber/epoxy composite
Gamesa
G136-4.5 MW
66.5
136
4.5
Organic matrix composite reinforced
with fiber glass or carbon fiber
Suzlon
S88-2.1MW
NA
88
2.1
glassfiber/epoxy composite
However, as governmental subsidies run out the long-term growth of wind technology depends on increasing the energy capture efficiency and therefore turbine sizes. This will require further innovation in lightweight structural design by means of multi-functional and stronger materials, as well as cost-effective manufacturing and installation. A current base model wind turbine section is shown in Figure 2.
Fig. 2. A base model wind turbine section with load-carrying box and attached shells [10].
The Challenge of Designing a 100m Blade
Glassfibre reinforced plastics (GFRPs) were selected in the early wind turbine days because of good material availability and well-documented processing technology. The weight of a turbine blade can statistically been shown to increase with the cubic of the length as shown in Figure 3, resulting in a gravity-induced bending moment that varies with the fourth power of the blade length. To improve on this exponential trend carbon fibre reinforced plastics (CFRPs) are now replacing GFRPs in large turbine blades due to their superior specific stiffness and strength properties. To date a hybrid CFRP-spar/GFRP-skin design is the most widely established solution (Table 1), since this presents the best compromise between improved performance and the higher cost of carbon fibre [11].
Fig. 3. Weight/blade length trend for older GFRP and more recent hybrid GFRP/CFRP blades [11].
Currently the design of wind turbine blades is based around placing unidirectional fibres along the spar axis to provide bending stiffness, while ±45° layers in the skins and webs are used to resist twisting and shearing [12]. Sandia National Laboratories performed a trade-off study concerning innovations in materials and manufacturing processes to ascertain an improved, cost-effective blade design for the next multi-megawatt turbine generation [11]. The researchers conducted finite-element analyses of a baseline fully E-glass/epoxy blade under extreme gust conditions, which showed that the increasing gravity-induced bending loads called for structural reinforcement at the blade root if the blade length was to be scaled up to 60m. Rather than reinforcing the existing design with more E-Glass, replacing the outer half of the spar cap (50% span) with a stitched CFRP laminate was found to result in 32% and 16% reductions in total blade mass and manufacturing cost respectively. The researchers based their decision of the span-wise extent of replacing GFRP with CFRP on a parametric assessment aimed at finding the best compromise in terms of manufacturing cost and increased structural rigidity.
In the future full-span CFRP spars will lead to further reductions in weight and tip deflection with a direct effect on the rotational inertia, aerodynamic performance and energy capture efficiency of the blade. Furthermore, it is estimated that full GFRP rotor blades of 120m in diameter will require 2.5 tons of resin [3] such that through-thickness dissipation of exothermic heat at the thick root sections during cure will become increasingly problematic. In terms of cost, it is currently unclear if the increased demand in carbon fibre by the aerospace, energy and automotive sectors will drive prices up or lead to economies of scale that will further reduce CFRP costs [11]. In the future carbon nanofibre-GFRP hybrid materials may be potential candidates for use in future turbine blades as they combine high strengthening and stiffening potential of carbon nanofibres with relatively cheaper GFRP [13]. The use of carbon fibre for wind turbine blades is further discussed in [14] – [16].
Manufacturing of Turbine Blades
Wet hand lay-up in open moulds has naturally developed as the traditional manufacturing technique for GFRP wind rotor blades due to its process maturity and cost-effectiveness compared to other techniques [2]. In 2008 a survey of wind turbine operators revealed that 7% of all wind turbines blades have to be replaced as a result of failure induced by manufacturing defects [17]. Furthermore, with the expected doubling of production volume in the next 5 years [1], there has been a natural drive towards faster yet more consistent manufacturing processes that facilitate superior material properties. Toward this end pre-preg technology and vacuum-assisted resin transfer moulding (VARTM) have emerged to be promising replacement techniques [11].
VARTM is currently the industrial norm since combining and curing the resin and fibres in one step significantly lowers manufacturing costs. Nevertheless, two of the largest manufacturers in the world, VESTAS and GAMESA, use pre-preg technology to guarantee more repeatable material properties, higher fibre-volume fractions and reduce the degree of fibre-waviness [17]. The main reductions in cost of the VARTM process can be attributed to the use of thicker ply lamina and the elimination of high-temperature and pressure autoclave curing [11]. However, the use of thicker plies exacerbates the magnitude of ply drops in a tapered blade and increases the likelihood of hidden flaws, which may result in the development of delaminations and a shorter fatigue life compared to pre-preg laminates [19] – [20]. Quite recently VARTM has been proven to lend itself to process automation with a subsequent scope for further reductions in cost, and improvement in the aforementioned mechanical shortcomings. MAG Industrial Automation Systems have developed the Rapid Material Placement System (RPMPS), which is an automated blade moulding facility that is capable of laying-up glass and carbon fibre on moulds, cutting the manufacturing time of a 45m blade by 85% [21]. Grande (2008) outlines the Siemens’ innovative IntegralBlade technology that makes blades in one piece, unlike the typical blade that is made in two shells and glued together [22]. The process is based on vacuum infusion with a closed outer mould and an expanding, flexible inner bladder. The IntegralBlade system reportedly offers several advantages, including shorter cycles and more efficient use of manpower and space. Additionally, there are no tolerance issues between the shells and structural spars. Most importantly the blade is an integral structure with no glued joints that could weaken and potentially expose the structure to cracking, water entry, and lightning strikes.
It is clear that both pre-preg technology and VARTM have merits in terms of their application to large turbine blades but the myriad of design factors and possible volatility of material costs currently prohibits the definition of an optimum solution. To guarantee the financial sustainability of wind power the evolution of current manufacturing technology should be of paramount importance, and automated systems such as RPMPS point in the right direction.
Offshore Wind Turbines
As the power of wind turbines has grown and the blade sizes have increased, there has been an increasing amount of wind turbines installed in
Fig. 5. Floating turbine concepts [26].
offshore locations; this presents a number of problems in supporting the turbine. In shallow waters up to about 30m in depth, the turbine can be supported with a monopole. Beyond this depth, the monopole must have some other supporting members and beyond 50m the turbine needs to be on a floating platform with cabled supports into the seabed [25]. Floating a wind turbine presents unique challenges as the platform must resist the motion of the sea and minimise pitch, roll and yaw whilst still maintaining the weight of the turbine. However, the wind industry has not converged on a standard design and more research is needed to fully overcome the challenge. Floating wind turbines open the possibility for combining wind and tidal power in one construction site and therefore increase the energy captured per installed structure. This hybrid design may be a solution to offsetting the high initial capital costs of renewable energy systems.
Future Developments – Thermoplastics and Morphing
Recently there has been a drive towards using thermoplastic resins in wind turbines in order to take advantage of their higher toughness, faster curing times, unlimited shelf life and the potential for recycling. Although BASF have developed a new acrylonitrile styrene acrylate (ASA) polymer for wind turbine use, the inferior fatigue resistance and high moisture absorption restricts the matrix to being used in small-scale GFRP turbines [27]. However, in the light of the forecasted increase in demand of wind turbines Andersen et al. (2007) make the prediction that by 2040, 380 000 tonnes of FRP will have to be disposed of annually [28]. As around 60% of the scrap created during the incineration of FRP is inorganic ash, and only 30% of FRP waste is currently being recycled, further research into overcoming the structural shortcomings of thermoplastics is essential for a truly eco-friendly use of advanced composite in wind technology. Furthermore, research at TU Delft suggests that the ability to fusion-bond thermoplastics may make it cost-effective to redesign turbine blades with more internal stiffening elements that ultimately facilitate a lighter design solution [29].
Fig. 6. Deflection capabilities of the morphing trailing edge [32].
In the future the anisotropic behaviour of non-symmetric laminates may be exploited by forcing blades to twist under strong gusts; thereby reducing fatigue loading and allowing the design of longer blades [20]. To improve fatigue life Ong et al. (1999) suggested rotating the primary span-wise fibres by off-axis 20°, which lead to the design of the TX-100 prototype developed by Sandia National Laboratories with 45% volume fraction of carbon fibre at 18° off-axis angle in the spar cap, and 13° for the skins [30] – [31]. Although the TX-100 is less stiff than its non-twisting CX-100 counterpart it increased the fatigue life by 150% [20]. Hulskamp et al. (2011) demonstrated another method of reducing fatigue loads using sensors and actuators to control trailing-edge flaps along the span of the blade. A significant load reduction was found with this small-scale experiment, however issues with scale-up and the integration and reliability of the electronics must still be addressed for this technique to have industrial applications [33]. Continuously cambered morphing trailing edge flaps have significant advantages over hinged flaps as they reduce the complexity of the design leading to a lower part count, simpler manufacturing techniques and increased aerodynamic efficiency [34]. Towards this end Daynes & Weaver (2011) have successfully manufactured a prototype of a continuously cambered morphing trailing edge as shown in Figure 6 [32]. The trailing edge produces the same lift characteristics as a traditional hinged flap with 34.4% less flap tip deflection (13.1 degrees to 20 degrees), thereby reducing the required actuator work by 69.2% under maximum aerodynamic pressure loading. The trailing edge flap is manufactured from a NOMEX honeycomb core sandwiched between woven CFRP upper and silicon lower skins, and actuated by a CFRP push-pull linkage as schematically depicted in Figure 9.
Fig. 7. Schematic of the internal mechanism actuating the morphing trailing edge designed by [32]
Corrosion and erosion of FRP blades are substantial problems for offshore wind turbines. Offshore turbines suffer from increased wind, UV and high salinity with wetting-drying cycles that have been found to increase corrosion. Erosion may also occur in a number of environments due to ice formation on the blades and the impact of sand, earth and insects. An exhaustive review of wear in FRP materials is presented in [35]. Surface coatings have been considered in order to reduce the effects of corrosion and erosion. Non-stick coatings may be used to resist insect-impact and tapes have been applied to the leading edge of blades in order to protect this erosion-prone area [36]. Coating GFRP with electroless Ni-P has also been found to increase resistance to NaCl corrosion [37] and superhydrophobic coating has proved very successful at preventing water and UVC damage, although more work is needed to prevent icing [38]. Lightning protection is also an issue, with taller blades and carbon reinforcements making turbines increasingly attractive to lightning strikes. One suggestion is to use two down conductors instead of one, which protect the turbine by connecting it to the ground [39].
Conclusions
As the demand for renewable wind energy will continue to increase in the coming years there is a real incentive to build considerably larger wind turbines in order to improve the overall energy capture efficiency. Carbon fibre reinforced plastics will play an essential role in facilitating longer turbine blades but certain reluctance prevails in the industry regarding the higher material costs compared to glass fibre reinforced plastics. For this reason, improvements in component quality produced by out-of-autoclave processes such as VARTM and the development of cost-effective pre-preg materials is of paramount importance. Another promising alternative to reducing blade weight and manufacturing cost is the integration of multifunctional composites, where embedded technologies such as SHM or self-healing will enable the reduction of safety factors and therefore decrease material usage. As the use of advanced composites continues to grow a major research effort will have to focus on developing new resin systems that lend themselves to ecological recycling.
References
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I have just returned from the International Conference for Composite Materials (ICCM) in Montreal, Canada and would like to share a few observations and key points about the developments in the composite world that may not be so easily accessible to a broader audience.
1) The Great Advance – Applications
ICCM is the biggest conference for composite materials and this year united over 1500 delegates from academia and different industrial representatives from the classical sectors aerospace, wind energy and high performance cars to newer sectors such as mass market cars (e.g. BMW i3), biomedical applications and even musical instruments. The motto of the conference “Composite Materials: The Great Advance” aptly captures the current state of technology in the industry. Since the 1960 considerable amount of research has been conducted to elucidate the mechanical and chemical properties of the fibre material, matrix and cured composite under various conditions such that the global behaviour of these materials is now sufficiently characterised. This maturity in technology coupled with the ever decreasing costs and the inherent benefits of high specific stiffness and strength that fibre-reinforced plastics have to offer, has led to the increasing application of composite materials in very different industries that we see today. Thus the “great advance” of composite materials towards wide-spread use in many industrial sectors.
Fig. 1. Composite materials growth broken down by sectors (1)
Fig. 2. Carbon Fibre Market (2)
2) The Great Advance – Novel Technologies
Furthermore, “The Great Advance” also relates to novel composite materials with much greater complexity that blur the lines between what is a material and what is a structure. Of course on a macroscopic scale one could say the steel in a steel bridge is the “material” that has been used to construct the “structure” that is the bridge. Therefore in this classical interpretation steel is just the building block to make the bridge, while the structure itself is the final product that performs a function. However on a microscopic scale we could argue that steel is a structure in itself since it is “constructed” of different sized grains that contain different metallic compounds and is thus an arrangement of small particles i.e. a microstructure. We could of course continue this argument further and further up to the atomic scale at which point we have reached the field of nanotechnology. This field of research has enjoyed much popularity in recent years since by manufacturing our products from the ground-up, i.e. from the nanoscale to the macroscale, we can control the properties of our product at multiple length-scales and therefore tailor the characteristics to be optimal for the desired function in service or even add some sort of multi-functionality to the structure/material. Since the material and structure are built at the same time the dividing line that used to distinguish between these two concepts is blurred. Even for a simple composite laminate comprised of a stack of individual layers this divide is no longer so clear since we can define the properties of each ply in the stacking direction and therefore have control over one more length scale.
Therefore in the future there will be a great advance towards novel and multifunctional materials/structures that perform so much more than carrying structural loads. Currently the design of composite structures is still in some cases dominated by a “black aluminium” approach. That is taking the current designs that have worked so well over the last decades using aluminium and replacing them by an equivalent composite design. The problem with this is that on one hand the composite material may not be suitable to carry loads in the same configuration e.g. loads through the thickness have to be avoided to prevent delaminations. Most importantly however, such a design approach hinders the greatest advantage of this new material system, which is to facilitate entirely new structures in terms of functionality and shape that arise as a results of their inherent properties. Only by completely re-designing structures from the ground-up and taking the intricacies of this new material system into consideration can we arrive a new optimal solutions or conversely ascertain that a metal solution actually works better under some circumstances. In the following I want to share a few exciting technologies that you may see in the near future.
1) Variable stiffness technology
This is my field of research and essentially what we are currently doing is changing the fibre direction over the planform of the plate such that we have curvilinear fibres rather than the straight fibre laminates that we use today. In many aerospace applications we require different laminate stacking sequences in different parts of the structure. Abruptly changing from one stacking sequence to another can lead to stress concentrations and thus structurally weaker areas at the interface. Using the variable fibre concept we can easily spatially blend from one layup to another to reduce these problems. Furthermore, we can arrange the fibre paths to follow the dominant load paths as for example around a window in an aircraft fuselage. Loads in a structure always follow the path of highest stiffness. So by aligning the fibres in the load direction in supported areas of the laminate (for example the vertical edges in Fig. 3 below if the load is applied vertically onto the horizontal edges), a large portion of the stress can be removed from the unsupported centre of the panel, which can greatly improve the elastic stability of the structure. This has great potential for future wing structures since the design of wing skins is greatly governed by local buckling (Fig. 4). It has been shown that the buckling loads can be improved by 70%-100% using variable stiffness technology (5), thus the possibility exists to reduce the weight of wing structures by up to 20% using this technology.
Fig. 3. A variable angle tow laminates (3)
Fig. 4. Buckling analysis of a stiffened wing panel. The stiffeners break the buckling mode shapes into smaller wavelengths that require higher energy to form than a single wave (4)
Another form of various stiffness technology is placing material in areas where it is needed and removing it from areas where it is not required. Nature is an expert in achieving this and many of our current design are based on bio-mimicry. For example, your bones are continuously being re-modelled based on the stresses that are placed on your skeleton. In this way the density of your bones is increased in highly-stresses areas and decreased in areas that are not used so much. In the same way sea-sponge arranges its structure in a way to achieve the most efficient design. Similarly, wood possesses an incredibly complex microstructure that is composed of different structural hierarchies at different length scales. This is similar to a rope where individual fibres are twisted together to make strands, strands are twisted together to make bundles, and bundles twisted together to make the complete rope. This approach of designing at multiple length-scales makes wood very ductile and resilient to cracks. In this manner attempts have been made to reproduce such a hierarchical design by arranging short fibres using standing ultrasonic waves.
Fig. 5. Microstructure of wood. Notice the different structures at different length scales that gives wood its inherent strength (6).
2) Self Healing
Yes, materials can heal themselves. The most popular example is that of self-healing asphalt, which was presented a few years ago at a TED conference. In terms of composites 100% recuperation of mechanical properties have been achieved when the mode of failure has been dominated by matrix cracks. In high performance composites the matrix is currently some sort of thermoset or thermoplastic, which allows vascules of uncured resin to be included in the structure which may break open as a crack propagates. The uncured resin then permeates through the open crack and cures in-situ to repair the full functionality of the part. The dissemination of the healing process can also be achieved using very thin vascules that are arranged throughout the part. In this manner the structure starts to behave very much like a living organisms with the vascules serving as pathways for repair very similar to the veins in an organism. Recently, a great article by the BBC summarised the major achievements in this field.
Fig. 6. Self healing capsules (7)
Fig. 7. Self healing vascules (7)
3) Nanotechnology
Nanotechnology has been extremely popular during the last 20 years due to the fact that theoretical predictions promise incredible benefits for almost all applications in engineering. In terms of advanced composites however, there are still problems of evenly dispersing nanotubes in resins with agglomeration or alternatively producing continuous nano-strands at low costs. In the aerospace industry they show great promise in increasing the electrical conductivity of laminates to improve their resistance against lightning-strike, creating structures for magnetic shielding and providing interlaminar strengthening using nano-forests. One of the cooler things I saw at ICCM was research conducted on nano-muscles, which are essentially nano-fibres that have been twisted into a rope and can achieve very high actuation forces and strokes at very little mass.
4) Structural Batteries / Energy Harvesting
Solar power has incredible potential as an energy source since it is the largest form of energy available for consumption on earth and is limitless. However, solar power is sporadically dependent on the weather conditions, which makes energy conversion rather cost intensive and inefficient. However, solar energy harvesting might find increasing use if actively integrated into load-bearing components as a multi-functional structure. Bonding thin-film solar cells onto lightweight composites would eliminate the material redundancy of stand-alone supporting structures and could easily be integrated into current laminate manufacturing technology. Photovoltaic (PV) cells have been embedded in composite laminates and their performance has not been impeded by the curing process. However, the performance of the PV cells diminishes rapidly under static loading since the loading causes cracks in the cells. Similarly there are ideas to create structural batteries such that the load carrying chassis of a car can be “charged-up” to additionally serve as the battery for an electric powertrain. Of course this would have the great advantage that the heavy batteries used today could be eliminated to some extent. BAE systems are working on technology to embed battery chemistries into the carbon fibre fabric.
5) Morphing
Finally, morphing or shape-changing structures have been extensively studied since the 1970’s for providing aircraft with the possibility of adapting the shape of their wings to provide the optimal lift for different flight scenarios. Of course this is to some extent already used in aircraft with the aid of leading edge slats and trailing-edge flaps to increase the lift-coefficient for slower flight regimes such as landing and lift-off and in Formula 1 using drag reduction system of the rear wing. However, slats and flaps on an aircraft greatly increase the drag of the profile during deployment and increase the weight of the structure do the heavy actuation mechanism. Therefore the aim is to design an integral system such as the trailing-edge design shown below. Other examples of morphing structures include air intakes for cars, noise-reducing chevrons on jet-engines, or high-temperature composites used for jet-engine turbine blades that change there angle of attack based on the temperature of the airflow around them.
Fig. 8. A morphing trailing edge using a flexible honeycomb (8).
However, in most cases these technologies are very difficult to apply to primary aircraft structures. This is because there is a direct conflict between the high-stiffness, high-strength requirement for carrying loads and the low-stiffness, large-deflections required for shape-changes. Thus, a driver to facilitate these technologies will be the development of materials that change there mechanical properties under different circumstances.
3) The Great Advance – Solving “big” problems for larger scale implementation
Finally, one of themes during the conference was trying to solve some of the major problems faced by the industry hindering further implementation of current composite technology in all industrial sectors. Of course for some industries such as mass consumer automobiles the biggest barrier to entry is cost. The new BMW i3, which will enter the marketplace at the start of 2014, will cost £30,000+ and is therefore quite a big investment for a small city vehicle. Of course some of the cost can be attributed to the cost of the electrical drivetrain and batteries but other manufacturers such as Renault have shown that a lot of these costs can be reduced by employing a scheme based on renting batteries rather than buying them with the vehicle. In case of the i3 a lot of the extra cost is simply down to the fact that BMW are the first to build a mass produced automobile using a large amount of fibre-reinforced plastics in primary structural parts. Not only is cost of raw material much higher than for lightweight metals such as aluminium but the manufacturing processes and supply chain management required for reliable mass production were simply not in-place beforehand. Furthermore, a shift in design methodologies is required since the chemical and mechanical behaviour of composites is so different from the metal environment that the automobile industry is so used to dealing with. As an example, proving the structural integrity for the incredible rigorous crash/impact certification using rather brittle composite materials compared to more ductile metals is a challenge in itself. Thus, the relatively high price-tag of the i3 incorporates some of the research and development costs that BMW have had to face in developing composite technology for their market sector. No doubt the cost of mass market composite cars will reduce drastically in the next decade as the raw material price further reduces and design methodologies and manufacturing processes mature.
Another major issue hindering the implementation of composites especially in the aerospace industry is the difficulty of predicting the failure behaviour of these materials. On problem is the large number of failure modes that may occur: fibre breakage, matrix cracks, delamination, fibre crimping, fibre-matrix debonding, global and local buckling etc. and thus finding accurate failure loads for all these phenomena under different load cases. Since a larger number of these failure mechanisms originate on a local, micro-mechanical scale high-fidelity 3D Finite Element models are often needed to fully understand the mechanisms of failure and predict the load-carrying capability of different structures. Considering the size of any commercial aircraft it is absolutely inconceivable to apply such detailed and computationally expensive analysis tools to every part of an aircraft. Furthermore, the failure mechanisms are not as well defined as for metal materials. That is in classical tensile or compressive tests a specimen may undergo some form of non-linearity that may for a metal specimen be classified as a failure event but for the composite considerable residual strength is available. Conversely the failure behaviour of composites can be very brittle with very little warning compared to the gradual, ductile failure mechanism of most metals used in the aerospace industry. Considering the intricacies of composite failure modes and the fact that the individual failure modes may interact or even change in criticality depending on the size of the component and environment in which it is used, it is no wonder that currently very conservative safety factors are being employed for primary composite aircraft structures that greatly offset the weight-savings that are possible using this new material system. Thus, one of the biggest if not the biggest topic in composite structural design for the next couple of years will be the challenge of developing simple and yet robust failure criteria to be used for composite designers.
(5) Gürdal Z, Tatting B, Wu C. (2008). “Variable stiffness composite panels: Effects of stiffness variation on the in-plane and buckling response”. Composites: Part A, 39(5), pp. 911-922.
(6) Greil P, Lifka T, Kaindl, A. (1998). “Biomorphic Cellular Silicon Carbide Ceramics from Wood: I. Processing and Microstructure”. Journal of European Ceramic Society, 18(14), pp. 1961-1973.
(7) Rincon, P. (2012). “Time to heal: The material that heal themselves.”http://www.bbc.co.uk/news/science-environment-19781862
(8) Daynes S & Weaver P.M. (2011). “A Morphing Wind Turbine Blade Control Surface”. Proceedings of the ASME 2011 Conference on Smart Materials, Adaptive Structures and Intelligent Systems. Phoenix, AZ: ASME.
Aircraft have changed enormously over the last century from the early Wright Flyer flown at Kittyhawk to the supersonic SR-71 Blackbird flown today. Of course the developments in aeronautical engineering can be broken down into separate divisions that have developed at different rates: a) the aerodynamics, b) power plant engineering, c) control, radios and navigation aids, d) airframe engineering (e.g. hydraulic/electrical systems, interior fittings etc.), and finally e) the structural design. For example, power plants have developed in two large steps separated by a series of sudden burst of ingenuity. In order to facilitate the first successful flight the Wright Brothers had to find a light yet powerful engine system. The next stride was the ingenious invention of the jet engine prior and during WWII by Sir Frank Whittle and Hans von Ohain. In between, the power output of piston engines “increased almost 200 times from 12 bhp to over 2000 bhp in just 40 years, with only a ten times increase in mass (3) “. As will be outlined in this article, the design of aerospace structures on the other hand has only made one fundamental stride forward, but this change was sufficient to change the complete design principle of modern aircraft. Today however, the strict environmental legislation and advent of the composite era may induce further leaps in structural design.
Fig. 1. A schematic drawing of the Wright Flyer (1)
Fig. 2. The modern supersonic SR-71 Blackbird (2)
1) Wire Braced Structures
If we look at the early design of aircraft such as the Wright Flyer in Figure 1 there can really be no misunderstanding of the construction style. The entire aircraft, including most notably the wings, forward and rear structures were all constructed from rectangular frames that were prevented from shearing (forming a parallelogram) or collapsing by diagonally stretched wire. There were two major innovative thoughts behind this design philosophy. Firstly, the idea that two parallel wings would facilitate a lighter yet stronger structure than a single wing, and secondly, that these two wings could be supported with two light wires rather than with a single, thicker wooden member. The structural advantage of the biplane construction is that the two wings, vertical struts and wires form a deep light beam, which is more resistant to bending and twisting than a single wing. Much like a composite sandwich beam it can be treated as two stiff outer skins for high bending rigidity connected by a lightweight “core” to provide resistance to shear and torsion.
Fig. 3. Cutaway drawing of the 1917 Sopwith Camel (3)
Fig. 4. Cutaway drawing of the 1935 Hawker Hurricane (3)
The biplane construction with wire bracing was the most notable feature of aircraft construction for much of the following years and paired nicely with lightweight materials such as bamboo and spruce (Figure 3). Wood is a composite of cellulose fibres embedded in a matrix of lignin and the early aeronautical engineers knew to take advantage of its high specific strength and stiffness. Strangely enough, after the era of metals we are now returning back to the composite roots of aircraft, albeit in a more advanced fashion. The biplane era lasted until the 1930s at which point metal was taking over as the prime aerospace material. Initially the design philosophy was not adapted to take full advantage of thin sheet metal manufacturing techniques such that wooden spars and struts were just replaced by thinner metal tubing. Consequently there remained a striking similarity in construction between a 1917 (Figure 3) and a 1931 (Figure 4) fighter. Even though some thin metal sheets were being used these components generally did not carry much load such that the main fuselage structure featured 4 horizontal longerons supported by vertical struts and wire bracing. This so called “Warren Girder” design can also be seen in some of earliest monoplane wing constructions such as the 1935 Hawker Hurricane. Aeronautical engineers were initially “unsure how to combine the new metal construction with a traditional fabric covering (3)” used on earlier aircraft. The onset of WWII meant that some safe and conservative design decisions were made to facilitate monoplane wings and the “Warren Girder” principle was directly copied to the internal framework of monoplane wings (Figure 5). These early designs were far from optimised and perfectly characterise the transition period between wire-frame structures and the semi-monocoque structures we use today.
Fig. 5. The Hawker Hurricane wing construction (3).
2) Semi-Monocoque Structures
The internal cross-bracing was initially acceptable for the early single or double seater aircraft, but would obviously not provide enough room for larger passenger aircrafts. To overcome this, inspiration was taken from the long tradition and expertise in boat building which had already been applied to construct the fuselages of early wooden flying boats. The highest standards of yacht construction at the time featured “bent wooden frames and double or triple skins…with a clear varnished finish…and presented a much more open and usable fuselage interior (3)”. The well-established boat building techniques were thus passed on to aircraft construction to produce newer aircraft with very smooth, aerodynamic profiles.
Fig. 6. Semi monocoque fuselage construction of an early wooden flying boat (4)
The major advantage of this type of construction is that the outer skin of the fuselage and wing no longer just define the shape and aerodynamic profile of the aircraft, but become an active load-carrying member of the structure as well. Thus, the structure becomes “multifunctional” and more efficient, unlike the braced fuselage which would be just as strong without the fabric covering the girders. As a consequence the whole structure is generally at a uniform and lower stress level, reducing stress concentrations and giving better fatigue life. Finally, as the majority of the material is located at the outer surface of the structure the second and polar moments of area, and therefore the bending and torsional rigidities are much increased. On the other hand, the thin-skinned construction means that compression and shear buckling become the most likely forms of failure. In order to increase the critical buckling loads the skins are stiffened by stringers and broken up into smaller sections by spars and ribs.
Fig. 7. Components of a semi monocoque wing (5)
Because the external skin is now a working part of the structure this type of construction became to be known as stressed skin or semi-monocoque, where monocoque means “shell in one piece” and “semi” is an english addition to describe the discrete discontinuities of internal stiffeners. The adoption of the semi-monocoque construction and a change from wood to metal naturally coincided since sheet metal production allowed a variety of thin skins to be easily manufactured quite cheaply, with better surface finish and superior material properties. Furthermore, metal construction was conducive to riveting which would overcome the adhesive problems of early wooden semi-monocoque aircraft such as the deHavilland Mosquito.
Fig. 8. Cutaway Drawing of the recently released A400M aircraft (6).
Figure 8 shows the typical construction of a modern aircraft. There have been numerous different structural arrangements over the past number of years but all generally feature some sort of vertical stiffener (ribs in the wings and rings in the fuselage) and longitudinal stiffener (called stringers). Over the years the main driver has been towards a) a reduction in the number of rivets by reverting to bonded assembly or ideally manufacturing separate components as a single piece and b) understanding the effects and growth of cracks under static and fatigue loading by building structures that can easily be inspected or have multiple redundancies (load paths). The design and manufacturing methods of semi-monocoque aircraft are now so automated that the development of a new aluminium, medium sized airliner “could be regarded as a routine exercise (1)”. However, the continuing legislative pressure to reduce weight and fuel consumption provides enough incentive for further development.
3) Sandwich Structures and Composite Materials
One of the major disadvantages of thin-skinned structures is their lack of rigidity under compressive loading which gives them a tendency to buckle. A sheet of paper nicely illustrates this point, since it is quite strong in tension but will provide no support under compression. One way of improving the rigidity of thin panels is by increasing the bending stiffness with the aid of external stiffeners, which at the same time break the structure up into smaller sections. The critical buckling load is a function of the square of the width of the plate over which the load is applied. Therefore skins can be made 4 times stronger in buckling by just cutting the width in half. As a wing bends upwards the main compressive loads act on the top skin along the length of the wing and therefore a large number of stringers are visible across the width.
Fig. 8. Buckling analysis of a stiffened wing panel. The stiffeners break the buckling mode shapes into smaller wavelengths that require higher energy to form compared to a single wave (7)
Another technique to provide more rigidity is sandwich construction. This generally features a very lightweight core, such as a honeycomb lattice or a foam, sandwiched between two thin yet stiff outer panels. Here the role of the sandwich core is to carry any shear loads and separate the two skins as far as possible. The second moment of area is a function of the cube of the depth and therefore the bending rigidity is greatly increased with this technique. Ideally, in this manner it would be possible to design an entire fuselage without any internal rings or stringers and the Beech Starship is an excellent example of a successful application. However, there are problems of forming honeycomb cores onto doubly curved shells since the material is susceptible to strong anticlastic curvature, forming a saddle shape when bent in one direction. Furthermore, there are problems with condensation and water ingress into the honeycomb cells and the ability to guarantee a good bond surface between the core and the outer skins. There is the possibility to use foam cores instead, but these tend to be heavier with lower mechanical properties. Perhaps the current trend is away from sandwich construction (10).
Fig. 9. A carbon fibre composite/honeycomb sandwich panel (9)
Fig. 10. The Beech Starship whose fuselage was design using sandwich construction with minimal internal bulkheads and ribs (8)
One of the major applications of honeycomb structures has been in combination with composite materials. Stiff carbon composite panels are the ideal candidate for the outer skins and the whole assembly can be co-cured together in an autoclave without having to perform any secondary bonding operations. Furthermore, the incredible specific strength and stiffness of carbon composites makes this combination an ultra lightweight yet resilient structure for aerospace applications. Indeed, we are now at the start of the “black” carbon age in commercial aircraft design. Apart from their excellent specific strength and stiffness properties composites exhibit the ability to tailor optimum mechanical properties by orientating the majority of plies in the direction of the load and allowing for less material waste during manufacture. As a result, the first generation of commercial aircraft that contain large proportions of composite parts, such as the Boeing 787 Dreamliner and Airbus A350 XWB, are planned to enter service throughout the next years.
Fig. 11. Considerable delamination leading to catastrophic failure (11)
Considerable effort has been made to mature composite technology in order to reduce manufacturing costs, guarantee reliably high quality laminates, understand the highly complex failure criteria and built hierarchical, multifunctional or self-healing structures. One of the major shortcomings is that the structural advantages of fibre-reinforced plastics must be viewed with respect to applications where the primary loads are aligned with the fibre direction. However, if a composite plate is subjected to significant out-of-plane stresses subsurface delaminations may develop between layers due to the weak through-thickness cohesive strength of the composite. These intralaminar delaminations are a significant problem as they are difficult to detect by visual inspection and may reduce the compressive strength of the laminate by up to 60%.
4) Novel Designs
With environmental legislation becoming ever so strict it is adamant that new concepts for lightweight and fuel efficient aircraft are found swiftly. Although the pressure on developing advanced composite materials is high it must be remembered that 100 years of innovation were required to reach the stage that large metal semi-monocoque structures could be manufactured in the 1940s and another 30 years to fully understand all failure criteria. Thus we may still require significant research and development before all current issues with composite materials are resolved. Apart from carbon fibre and other composites other researchers have been looking into completely redefining the shape of aircraft. Researchers at MIT have been developing the blended wing concept and NASA are exploring the technology of morphing or shape-changing aircraft, taking inspiration directly from nature.
Fig. 12. Illustration of the MIT Silent Aircraft concept (12).
Fig. 13. NASA morphing wing aircraft (13)
Whatever the final solution might look like the next 5o years in aerospace engineering will be incredibly innovative, ground-breaking and an exciting industry to be part of!
by Prof. J E Gordon
![Fig. 1. Correlation of increasing rotor diameter and power rating throughout the last 30 years [3].](https://rgroh.com/wp-content/uploads/2013/08/Fig1.jpg)
![Fig. 2. A base model wind turbine section with load-carrying box and attached shells [10].](https://rgroh.com/wp-content/uploads/2013/08/Fig2.jpg)
![Fig. 3. Weight/blade length trend for older GFRP and more recent hybrid GFRP/CFRP blades [11].](https://rgroh.com/wp-content/uploads/2013/08/Fig3.jpg)
![Fig. 5. Siemens IntegralBlade Manufacturing Technology [18]](https://rgroh.com/wp-content/uploads/2013/08/Fig5.jpg)
![Fig. 6. Floating turbine concepts [26].](https://rgroh.com/wp-content/uploads/2013/08/Fig6.png)
![Fig. 8. Deflection capabilities of the morphing trailing edge [32].](https://rgroh.com/wp-content/uploads/2013/08/trailingedge.jpg)
![Fig. 9. Schematic of the internal mechanism actuating the morphing trailing edge designed by [32]](https://rgroh.com/wp-content/uploads/2013/08/Fig8.jpg)
