Intergranular/transgranular fatigue of copper: Influence of environment on crack path and propagation rates

Rising ΔK fatigue tests were conducted on annealed, electrical grade, polycrystalline copper plate (25.4 mm thickness) at 95 Hz in several aqueous solutions and desiccated air at 24°C and an R-ratio of 0.33. Crack propagation and crack path behavior were examined in the threshold and low ΔK region. Testing conditions were primarily under plane stress. It was concluded that intergranular (IG) cracking was an environmental effect promoted by oxidizing conditions, particularly anodic dissolution processes. Transgranular (TG) cracking dominated under conditions where copper was immune to oxidation, such as cathodic polarization. Crack rates under predominant IG cracking conditions were at least one to two orders of magnitude higher than those under predominant TG cracking. Cyclic crack propagation rates for both types of crack path situations obeyed a general relationship of the form da/dN = A(ΔK-ΔKth*)n, where ΔKth* was a best fitting (true) threshold value close to the experimental value. The best fitting ΔKth* values were 2.1 and 3.8 MPa m1/2 for IG and TG cracking conditions, respectively. Factors governing preferential crack propagation along grain boundaries were discussed in terms of cyclic loading effects on enhanced dissolution (oxidation) processes, including deformation-induced vacancy generation and transport of trace solute impurities to grain boundaries within the crack tip plastic zone.