Fatigue crack propagation in aluminum alloys

Fatigue crack propagation in aluminum alloys was studied in thin plate under plane bending the microscopic observation of the surface of specimen and electron fractography.There were two types of fatigue crack propagation according to the conditions of ageing. One was a continuous growth type; for instance, in Al-1.5%Mg2Si alloy after natural ageing, the fatigue crack was generated at the central hole of the specimen and was developed to grow up until final failure. The other was a discontinuous propagation type; for instance, the fatigue crack of the above alloy, fully age-hardened, predominantly propagated by the linking of many isolated cracks.The results of analysis for the growth of fatigue crack by Master curve method showed that there were two stages of the growth and only the 1st stage depended on ageing conditions. The rate of growth in the 2nd stage would be a function of the product of grass strain amplitude, eG and a square root of half crack length, √l. The transition from the 1st to the 2nd stage also depended on the ageing conditions.The examination of the fatigue fracture by electron fractography showed that characteristic feature of the fracture suface depended on the stage of crack growth and the ageing conditions. The 1st stage was the shear mode of crack growth and the surface was often characterized by parallel lamellae along the direction of the growth of the crack. The 2nd stage was the tensile mode of crack growth and the surface was characterized by the striations normal to the local direction of the crack growth. Each striation represented the position of the front oftranscrystalline crack at each load cycle. There were observed two types of striations; ductile and brittle. However, the latter was observed only in aluminum alloys fully age-hardened.