Fatigue Crack Propagation and Crystallographic Orientation of an Aluminum Alloy

Abstract

The effect of crystallographic orientation on fatigue crack propagation was studied with an aluminum alloy 2017-T4 by means of the etch pit technique. In the range of high crack propagation rate around 0.5 micron per cycle, the fracture surfaces were of typical striation type with moderate orientation dependence, {100} fracture surface -<110> propagation direction being somewhat preferred. As the propagation rate decreased into the range less than 0.05 micron per cycle, fracture surfaces changed to the brittle type with river-like or trough-like markings and with strong orientation dependence, {100}-<110> type fracture being predominant. High resolution fractographs revealed indications of ductile fracture such as striations and slip lines on the apparently brittle fracture. These observations indicate that a similar mechanism of fatigue crack propagation is operating in the whole range of propagation rate studied in spite of the different fracture appearances. A fracture mechanics calculation suggests that the transition of fracture morphology with propagation rate appears when the size of the crack tip plastic zone is comparable to the width of the trough-like markings which is one order smaller than the grain size of the test material.