Experimental investigation of the quasi-static fracture of functionally graded materials

In this paper we describe the fracture testing of model Functionally Graded Materials (FGMs). The FGMs were created by selective ultraviolet (UV) irradiation of a polymer. An ethylene carbon monoxide copolymer (ECO) was chosen to make the specimen because of its rapid degradation under UV light. ECO becomes stiffer, stronger and more brittle with increasing irradiation time. By controlling exposure time we produced specimens with continuously varying mechanical properties. We conducted single edge notch fracture tests on both homogeneously irradiated ECO and functionally graded ECO. A hybrid numerical-experimental method was used to evaluate fracture parameters. The stress versus crack length relation, obtained experimentally, was used as the boundary conditions at each increment of crack growth in a Finite Element Analysis (FEA). From the FEA we calculated the J-integral, energy release rate and stress intensity factor for 5-, 60- and 106-h uniformly irradiated ECO, as well as for several FGMs. The fracture toughness of homogeneously irradiated ECO showed a rise following initiation and then remained constant during crack growth. The magnitude of the fracture toughness decreased with increasing irradiation time. For the case of FGMs it was observed that fracture toughness increased throughout crack growth (i.e. did not reach a plateau) when the crack propagated from the stiffer to the more compliant region.