FRACTOGRAPHY OF WIRES
Fractography is the interpretation of features observed on fracture surfaces, and can be fairly difficult. This is particularly the case on high strength quenched and tempered steels, or in alloys (such as cast irons and pearlitic steels) where the microstructure affects the crack path. For metallic alloys, significant reference sources exist, often in the form of atlases of fractographs (see, for example, the references below). The cases presented in this case study are, however, straightforward at low magnification. Once the fracture mechanism types have been identified at low magnification, the high magnification fractographs can be used for confirmation and to build up a 'reference library' of fractographs.
In general terms, there are three basic crack growth mechanisms possible in fast fracture - intergranular along grain boundaries, brittle fracture via cleavage along crystallographic planes, and transgranular ductile fracture via microvoid coalescence. Fatigue, by definition, involves ductile fracture, which is usually transgranular (through the grains), although intergranular fatigue is possible under certain special circumstances. Although it may prove difficult to fractographically distinguish between fast fracture, fatigue or stress corrosion cracking under certain circumstances, overall consideration of the facts and circumstances of a particular case usually allow correct interpretations of the evidence. In the present case, fast fracture and fatigue are the likely contenders for identification.
Several internet sites exist to aid in identification of the fractographs shown, for instance, the various mechanisms of fracture (including fatigue) are outlined on a Virginia Tech page on fractography.
The fractography resource associated with this set of interactive failure analysis case studies can be accessed from the fractography resource link.