Some observations on air and corrosion fatigue fracture surfaces of aluminium - 7.5 zinc - 2.5 magnesium

Examination of the fracture surfaces associated with Stage I (shear mode), and Stage II (tensile mode) fatigue crack growth on air and corrosion fatigue specimens has revealed certain characteristic features of each mode of growth. Stage I fracture surfaces show parallel lamellae lying in the direction of crack growth, while striations, made up of a zone of brittle fracture and a zone of ductile fracture produced during one stress cycle, are observed on Stage II fracture surfaces. Two striation forms are produced by Stage II fatigue crack growth and have been designated Type A and Type B. Type A striations predominated on air fatigue fracture surfaces while Type B, containing an increased brittle component and associated with river markings characteristic of cleavage fracture, predominated on corrosion fatigue fractures. It has been shown that Type B striations occur on a fracture plane that is either (100) or a plane very closely oriented to it.