Dynamic fracture of compositionally graded materials with cracks along the elastic gradient: experiments and analysis

Crack tip deformation and fracture parameter histories in compositionally graded glass-filled epoxy are evaluated for low velocity impact loading. The situations when the elastic gradient is unidirectional with crack orientation along the gradient are examined. The fracture behavior of graded compositions is studied relative to homogeneous counterparts made of identical constituents. Optical method of CGS and high-speed photography are used to measure crack tip deformations prior to crack initiation and during dynamic crack growth. The apparent stress intensity factors prior to crack initiation are determined using dynamic equivalent of the stationary fields for FGMs while crack tip fields for steadily growing cracks in FGMs are used for post-initiation situations. Results from finite element simulations up to crack initiation are in excellent agreement with the experimental evaluations. Post crack initiation stress intensity factor histories and crack growth resistance behaviors for FGMs with monotonically increasing and decreasing elastic gradient are strikingly different. When the crack growth occurs into material with progressively increasing filler volume fraction, continuously increasing K(ID)(t) is seen while a decreasing trend is observed when the gradient is of the opposite sense. The fracture behaviors are explained by independent fracture tests and fractured surface micrographs. (C) 2001 Elsevier Science Ltd. All rights reserved.