Influence of mixed mode I/III loading on fracture toughness of mild steel at various strain rates

Abstract

The effect of loading angle &phis; on the fracture toughness of mild steel at various strain rates has been studied. The fracture toughness was found to decrease with increasing loading angle (or increasing mode III component) at strain rates 10^-5 to 10⁰ s^-1 where ductile fracture was observed. Under impact conditions (strain rate 10² s^-1), fracture was by cleavage and the fracture toughness was found to increase with increasing loading angle. The results showed that the mixed mode fracture behaviour of mild steel changed from Class C in the strain rate range 10^-5 to 10⁰ s^-1 to a combination of Class A and B under impact conditions. In the strain rate range 10^-5 to 10^-2 s^-1, the fracture toughness behaviour with increasing strain rate was found to be similar for the three loading angles studied, namely &phis;= 0, &phis;= 30 and &phis;= 45°. At the strain rates 10^-2 to 10² s^-1, fracture toughness at &phis;= 0° decreased sharply, while for loading angles &phis;= 30° and &phis;= 45°, the fracture toughness increased with strain rate. The increase in mixed mode fracture toughness with strain rate in this strain rate regime has been attributed to the inertial effects which are known to reduce the T stress ahead of the crack.