Observation of initiation and early growth of small cracks during high-temperature low-cycle fatigue in 304 stainless steel.

An accurate prediction of fatigue fracture at elevated temperatures depends on the elucidation of the small crack initiation and propagation behavior. Experimental studies on the small cracks, however, have been quite limited. In this study, two types of high-temperature low-cycle fatigue tests, C-P type of a trapezoidal stress waveform with tension hold and P-P type of a high-frequency triangular stress waveform, were conducted by using smooth specimens of 304 stainless steel, and the behavior of initiation and early growth of surface small cracks was observed continuously during the tests with a high-temperature microscope. Furthermore, a detailed study was made by the interrupted tests with the replicas or scanning electron microscopy. The results obtained are summerized as follows. (1) The crack of C-P type fatigue initiates at a grain boundary between two triple points as it opens uniformly and gradually. (2) The crack growth to the adjacent grain boundary in the early stage of C-P type fatigue is caused through the same process as the crack initiation, showing a stepwise and discontinuous manner. (3) The process mentioned at (1) is the process of growth and coalescence of voids on the grain boundary owing to the grain boundary sliding. (4) The crack initiation of P-P type fatigue takes place at the intersection of slip bands and a grain boundary, and most of the cracks propagate along the grain boundaries in the early stage of fatigue. (5) The crack growth rate of P-P type fatigue decreases suddenly when the crack tip comes near to the triple point. (6) The fracture mechanisms of C-P type fatigue and P-P type fatigue are quite alike to that of creep and that of low cycle fatigue at room temperature, respectively.