Stress Corrosion Cracking in Aircraft Structures
Stress corrosion cracking (SCC) occurs, or can occur, in all major alloy systems used in aircraft structures. The consequences of stress corrosion failures may be serious, even leading to loss of an aircraft. This chapter first surveys the types of struct
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Stress Corrosion Cracking in Aircraft Structures R.J.H. Wanhill and R.T. Byrnes
Abstract Stress corrosion cracking (SCC) occurs, or can occur, in all major alloy systems used in aircraft structures. The consequences of stress corrosion failures may be serious, even leading to loss of an aircraft. This chapter first surveys the types of structures and materials used in aircraft and the environments encountered by them. Case histories from a wide variety of aircraft are used to illustrate the problems caused by SCC in service. Guidelines are given for preventing and alleviating these problems.
Keywords Stress corrosion Alloys Primary structures Mechanical systems Fluid systems Case histories Aluminium alloys Stainless steels High-strength steels Magnesium alloys
19.1
Introduction
Stress corrosion cracking (SCC) in metals and alloys occurs because of the simultaneous presence and interaction of three factors: 1. Metal or alloy susceptibility. 2. An aggressive environment. 3. Sustained (constant) stresses. Many metals and alloys are susceptible to SCC, including the structural alloys used in aircraft and spacecraft [1]. SCC also occurs in a variety of environments: some are ubiquitous, like salt water, and some are exotic, like liquid fuels for spacecraft launchers.
R.J.H. Wanhill (&) NLR, Emmeloord, The Netherlands e-mail: [email protected] R.T. Byrnes Defence Science and Technology Group, Melbourne, Australia © Springer Science+Business Media Singapore 2017 N. Eswara Prasad and R.J.H. Wanhill (eds.), Aerospace Materials and Material Technologies, Indian Institute of Metals Series, DOI 10.1007/978-981-10-2143-5_19
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R.J.H. Wanhill and R.T. Byrnes
The sustained stresses may come from service loads, structural assembly (fit-up stresses), or component manufacture and heat-treatment (residual stresses). Assembly stresses should be minimized by careful design, and residual stresses should be alleviated by stress relief treatments during the manufacturing process. However, residual stresses may still be present, since it may be difficult to eliminate them from thick-section components.
19.1.1 Background to Aircraft SCC In the 1920s and 1930s metals and alloys gradually replaced wood and canvas in aircraft structures, owing to their higher structural efficiencies, more consistent properties and reliable fabricability. The use of metals also avoided the problems of moisture stability and fungus attack that affected wooden structures. However, many structural alloys were later found to be susceptible to environmental degradation, including corrosion, hydrogen embrittlement and SCC. Stress corrosion cracking service failures peaked in the late 1960s, mainly due to the widespread use of the aluminium alloys AA2024-T3, AA7075-T6 and AA7079-T6, all of which are highly susceptible to SCC in the short transverse (ST) direction. Improvements in chemistry and processing (aluminium alloys) and control of strength and corrosion protection schemes (high-strength steels) have helped reduce the n
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