INTERGRANULAR TRANSGRANULAR FATIGUE OF COPPER - INFLUENCE OF ENVIRONMENT ON CRACK PATH AND PROPAGATION RATES/

Rising Delta K fatigue tests were conducted on annealed, electrical gr ade, polycrystalline copper plate (25.4 mm thickness) at 95 Hz in seve ral aqueous solutions and desiccated air at 24 degrees C and an R-rati o of 0.33. Crack propagation and crack path behavior were examined in the threshold and low Delta K region. Testing conditions were primaril y under plane stress. It was concluded that intergranular (IG) crackin g was an environmental effect promoted by oxidizing conditions, partic ularly anodic dissolution processes. Transgranular (TG) cracking domin ated under conditions where copper was immune to oxidation, such as ca thodic polarization. Crack rates under predominant IG cracking conditi ons were at least one to two orders of magnitude higher than those und er predominant TG cracking. Cyclic crack propagation rates for both ty pes of crack path situations obeyed a general relationship of the form da/dN = A(Delta K - Delta K-th)(n), where Delta K-th* was a best fit ting (true) threshold value close to the experimental value. The best fitting Delta K-th values were 2.1 and 3.8 MPa m(1/2) for IG and TG c racking conditions, respectively. Factors governing preferential crack propagation along grain boundaries were discussed in terms of cyclic loading effects on enhanced dissolution (oxidation) processes, includi ng deformation-induced vacancy generation and transport of trace solut e impurities to grain boundaries within the crack tip plastic zone.