Mechanisms of strain localization, crack initiation and fracture of polycrystalline copper in the VHCF regime

Development of areas with strain localization is regarded as first step of fatigue damage and failure. At small load amplitudes only very few such areas are generated, but their number increases with number of cycles, until all grains are covered by them. Slip bands become persistent (PSBs) during high enough numbers of cycles. Formation of numerous persistent slip bands together with deep intrusions or small stage I (mode II) shear cracks, however, is not sufficient to form a long and propagating crack and thus specimen failure below a specified strain/stress amplitude. Specimen fracture requires an approximately 100% higher cyclic load than generating PSBs. The threshold value for PSB formation is lower than traditionally reported. Quantitative results on cyclic plastic strain measurements, TEM observations of the dislocation structure in the VHCF regime and determination of the critical crack length by use of a Kitagawa diagram are reported. Its relevance for fracturing and endurance limit is discussed.