An experimental investigation of dynamic crack growth past a stiff inclusion

The mechanics of transient crack growth past stiff inclusions embedded in a relatively compliant matrix are studied optically under stress wave dominant loading conditions. An ultra high-speed rotating mirror-type CCD digital camera is used to record gray scales in the crack–inclusion vicinity at rates of up to 300,000 frames per second and 1000 × 1000 pixel resolution in real time. By analyzing the images before and after deformations, crack-tip deformation histories from the time of impact up to complete fracture are mapped and fracture parameters are extracted. The effects of inclusion–matrix adhesion strengths (weak and strong) and eccentricity of the inclusion relative to the crack path in the crack-tip vicinity are examined. A weakly bonded inclusion attracts and traps a dynamically growing mode-I crack momentarily whereas the same is deflected away by the inclusion if it is bonded strongly to the matrix. As a result, significantly higher re-initiation crack velocities are seen in weakly bonded inclusion cases upon re-initiation when compared to the strongly bonded counterparts. The effective stress intensity factor histories extracted from measured full-field displacements show a spike in values corresponding to higher crack velocities. Further, crack-tip mode-mixities correlate well with crack attraction and deflection mechanisms. The measured surface roughness is found to be consistently higher for weakly bonded inclusion specimens compared to the strongly bonded ones.