Trajectories of unstably growing cracks in mixed mode I–II loading of marble beams

Summary

The knowledge of the stress field under which a crack in an engineering material or a rock is propagated and the trajectory it follows during its growth, is very important. In this paper the prediction of the crack growth trajectory and its initial angle θ _c under mixed mode I–II loading is studied experimentally in marble beams. The three point bending test is used, together with the strain energy density theory (SED) of Linear Elastic Fracture Mechanics (LEFM). Experimental results are compared with theoretical predictions. The experimental results showed that: marble behaved as a brittle material; crack propagation was unstable and its trajectory followed, mainly, the grain boundaries; the prediction of the critical angle θ _c and the first part of the trajectory, from the crack tip up to the point G, where a global minimum of the strain energy density is reached, can satisfactorily be made with the SED theory. Beyond point G, the initial stress field is different, and for application of the SED theory, the boundary conditions need redefinition.