Intergranular and transgranular cracking of Cu–30Zn brass during fatigue loading

Fatigue tests have been performed on annealed α–brass to examine the dependence of fracture morphology on ∆K. The cracking was predominantly intergranular at low values of ∆K_Q (the stress intensity factor range when the specimen does not comply with plane strain conditions) and changed progressively to transgranular cracking at high ∆K_Q values. Detailed scanning electron microscopy has been performed on the fracture surfaces of the specimens, especially from matching areas on opposite faces. It has been shown that matching extrusion and intrusion pairs as well as one–to–one matching of fine slip lines occurred on the intergranular facets indicating that plastic deformation causes the intergranular cracking. Intergranular cracking persists at low ∆K_Q values even though the crack growth rate is smaller than for transgranular cracking because the latter is difficult to initiate. Transgranular cracks form only at regions of localized strain, e.g. coarse slip bands, or at cold–worked surfaces but such transgranular cracking cannot be maintained at low ∆KQ values.

MST/209