Effect of triaxial stress on mixed-mode fracture

Abstract

Triaxial stress has a large influence on fracture behaviour of materials. Present work seeks to investigate effects of triaxial stress on mixed-mode fracture and determines the direction of the crack initiation in dependence on the inclination crack angle with mixed-mode loading. For small scale yielding, based on the three-dimensional (3D) stress field around a crack tip, the distribution of triaxial stress at the inclined crack front in 3D space is obtained. In order to determine the crack initiation angle for a given inclination angle, a criterion is defined by means of maximum triaxial stress: along a contour around a crack tip with equal distance to the crack tip, the crack initiates along the direction where triaxial stress is maximal. According to the criterion the relation between the crack initiation angle and the inclination angle is determined. The maximum triaxial stress around a crack tip increases with increasing the inclination angle and decreases with decreasing Poisson's ratio. Experiments to check the theoretical predictions from the criterion have been conducted. CCT-specimens with the same ligament have been designed for the experiments. The steel FeE 550 was tested at temperature −140°C and with loading speed 50 mm/sec. The experimental results have shown a good agreement with theoretical predictions from the present criterion, especially at small inclination angles.