Critical Condition of the Branching Behaviour of Running Brittle Cracks in Steels

Fumiaki Tonsho, Tomoya Kawabata, and Shuji Aihara

The University of Tokyo, Tokyo, Japan

Abstract: In this paper, using the devised under-matched welded joint to change the volume of high stress areas near the crack tip, the mechanism of crack propagation is investigated by experiments and the dynamic finite element method (FEM) analysis in which the rational shape of the crack front line are assumed. For evaluating the crack running behaviour, ESSO tests were performed under -100deg.C and several stress conditions and strain gauges and crack gauges were instrumented in measuring the crack propagation velocity which is used for input data of FEM analysis. The results of experiments show that the crack propagation velocity tends to be a little higher in the material with soft welded joint in which the expansion of high plastic strain region is significantly concentrated in the softer material and consequently the exceedingly-high stress region over 4-5 times σy0 (yield strength at ambient temperature and in static) is widely distributed than in the homogeneous material. Interestingly crack bifurcations were observed in both specimens at low crack velocity position (around 800 m/s), contrary to the findings of the elastic dynamic fracture mechanics. Further, the soft welded joint tends to branch in a shorter distance. This is also thought to be due to the high stress concentration in the vicinity of the crack tip in soft welded joint. The triaxiality situations were computed by FEM analysis and the result shows agreement with the above estimate.

Key words: brittle crack propagation, steel, dynamic fracture, strength mismatch, crack branching