Fractographic Study of Mode II Fatigue Crack Growth on 7075-T6 and 2017-T4 Aluminum Alloys

Abstract

A fractographic investigation on mode II fatigue crack growth has been made on 7075-T6 and 2017-T4 aluminum alloys. Not like in mode I growth which is known to show a stage I-stage II transition, no transition has been observed in mode II growth but there has been a gradual change in fracture appearance with an increase in ΔK_II, whose growth is characterized by crystallographic growth in the lower ΔK_II region while in the higher ΔK_II region the smooth curved surface which seems to be formed by glide plane decohesion becomes dominant. Another characteristic pattern, which is often observed regardless of the magnitude of ΔK_II, is what looks like piled-up thin lamellas. Stress ratio R and superimposed static K_I have no influence on the appearance of mode II growth. Mode II growth rate depends on the range of the stress for both cases of R=0 and-1. A model which is to explain these characteristics of mode II growth has been proposed.