An investigation into variations in roughness-induced crack closure in high strength aircraft alloys under fatigue loading

Crack closure, driven predominantly by plasticity and roughness, has been identified as a critically important issue affecting fatigue crack growth in high strength alloys. This paper details an investigation into the closure behaviour four materials which exhibit a broad range of crack surface roughness. The work includes quantitative evaluation of the crack profiles using optical interferometry, compliance measurement of crack closure levels under constant amplitude loading, and quantitative fractography interpretation of closure effects during spectrum loading. Roughness was found to account for a major rise in closure under constant amplitude conditions, but was found to be much less significant under spectrum loading.