Although the exploitation of advanced composite materials in the aerospace industry is steadily increasing, high strength metallic materials, particularly aluminium alloys, are still the first choice for large-scale fleets such as the Airbus A320 and the Boeing 737. Since the introduction of stressed-skin “semi-monocoque” aircraft structures in the 1930’s the structural design philosophy has developed considerably, and the history of this development has been greatly influenced by in-service failures.
- 1930 – 1940: Early commercial transport aircraft. Design and structural design focus primarily on static strength with little regard to long- term material degradation by mechanical fatigue i.e. cracking, creep etc.
- 1940 – mid 1950’s: Aluminium alloys with higher static strength are developed to reduce material usage but with little improvements or even reductions in fatigue strength. A number of catastrophic in-service failures leads to the increasing awareness of fatigue failure for safe design.
- mid 1950’s – present: The terms “fail-safe” and “damage tolerant” design are coined, which account for damaged and cracked structures before service. The embedded damage is expected grow during service as a result of cyclic loading. Safety is ensured by pre-service testing to ascertain the extent of damage that will induce ultimate failure, and regular inspection, repair and replacements in-service before the critical damage size is reached.
Four case studies are generally considered to be critical milestones in the development of current structural design for metallic aircraft structures (2-5).
Table 1. Four milestone aircraft failures that influenced future aircraft structural design (1)
|
year
|
aircraft failure
|
lessons learned
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1954
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Two DeHavilland Comet aircraft crash as a result of fuselage explosions
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First indicator and seed for awareness of finite aircraft fatigue life as a critical design factor in modern thin-skinned aircraft shell structures. Development of full-scale fatigue testing.
|
|
1969
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F-111 wing failure as a result of an undetected initial material flaw
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Initial material flaws and defects have to be accounted for prior to service and monitored in-service. Aircraft should be damage tolerant.
|
|
1977
|
Boing 707 tailplane lost as a result of fatigue failure in a spar
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The older the aircraft the more susceptible it is to fatigue cracking. Also crack growth accelerates with increasing size.
|
|
1988
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Boeing 737 loses part of fuselage skin due to multiple fatigue cracks in spars
|
Multiple-site fatigue damage may occur in ageing aircraft. Joints in the structure are especially critical.
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References
(1) R.J.H Wanhill (2002). Milestone Case Histories in Aircraft Structural Design. National Aerospace Laboratory. NLR-TP-2002-521
(2) J. Schijve (1994). Fatigue of aircraft materials and structures. Int. J. Fatigue. Vol. 16 (1) pp. 21-32
(3) T. Swift (1987). Damage tolerance in pressurised fuselages. 11th Plantema Memorial Lecture. New Materials and Fatigue Resistant Aircraft Design (ed. D L Simpson) pp 1 – 7. Engineering Materials Advisory Services Ltd., Warley, UK.
(4) A.J. McEvily (2002). Metal Failures: Mechanisms, Analysis, Prevention. Chapter 1. John Wiley & Sons, Inc. New York, USA
(5) A.F. Blom (2002) Fatigue science and engineering – achievements and challenges. 18th Plantema Memorial Lecture, ICAF’2001: Design for Durability in the Digital Age. Vol I, pp 3-64. Toulouse, France.