Caustics analysis of the crack initiation and propagation of graded materials

In this paper, dynamic crack initiation and propagation of functionally graded materials (FGMs) are experimentally studied using the optical method of caustics. First, through a novel numerical iteration method, the caustics governing equations that relate to the caustics principles and the elastic solution of mode I propagating crack in FGMs are obtained in terms of the dynamic stress intensity factor, material constants, graded index, crack velocity and the characteristic size of the caustics. Second, the three-point bending beams with crack located at both the stiff and the compliant sides are experimentally studied combining dynamic caustics and high-speed photography. A series of dynamic caustics patterns surrounding the propagating crack tip are recorded at different time intervals. Some dynamic fracture parameters are extracted including dynamic stress intensity factor and crack velocity. Finally, the influences of gradient variation on dynamic fracture behaviors in FGMS are analyzed and discussed based on both experimental result and finite element modeling.