Masao Toyoda

Study on Mechanism of Stable Fibrous Crack Initiation and Growth Under Constant Loading

Constant loading tests for double edge-cracked tensile specimens and smooth round bar tensile specimens of mild steel have been carried out at room temperature. And according to these results and visco-elasticity theory, time-dependent deformation behavior has been revealed. Moreover, monotonic loading with extremely low loading rate of cracked tensile specimens and smooth round bar specimens have been carried out. Comparing with these results and constant loading tests, consideration has been conducted on the mechanism of stable fibrous crack growth behavior under constant loading.Under constant loading, time-dependent strain-that is creep strain-appears; and plastic strain in the vicinity of crack tip increases with time. This time-dependent strain contributes fibrous crack initiation and crack growth under constant loading. The crack growth occurs until time-dependent strain is saturated due to strain-hardening in the vicinity of crack tip, that is, fibrous crack extension under constant loading reaches that obtained at the same load level under monotonic loading with extremely low loading rate. Crack growth is arrested when time-dependent strain is saturated, but unstable fracture occurs when holding load level is higher than maximum load under monotonic loading with extremely low loading rate. Accordingly, the time-dependent strain and strain-hardening behavior plays an important role on the mechanism of stable fibrous crack growth under constant loading.