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 |
|
1954 |
Two DeHavilland Comet aircraft crash as a result of fuselage explosions |
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 |
F-111 wing failure as a result of an undetected initial material flaw |
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 |
The older the aircraft the more susceptible it is to fatigue cracking. Also crack growth accelerates with increasing size. |
|
1988 |
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. |