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) irra diation of a polymer. An ethylene carbon monoxide copolymer (ECO) was chose n to make the specimen because of its rapid degradation under UV light. ECO becomes stiffer, stronger and more brittle with increasing irradiation tim e. By controlling exposure time we produced specimens with continuously var ying mechanical properties. We conducted single edge notch fracture tests o n both homogeneously irradiated ECO and functionally graded EGO. A hybrid n umerical-experimental method was used to evaluate fracture parameters. The stress versus crack length relation, obtained experimentally, was used as t he boundary conditions at each increment of crack growth in a Finite Elemen t Analysis (FEA). From the FEA we calculated the J-integral, energy release rate acid stress intensity factor for 5-, 60- and 106-h uniformly irradiat ed EGO, as well as for several FGMs. The fracture toughness of homogeneousl y irradiated ECO showed a rise following initiation and then remained const ant during crack growth. The magnitude of the fracture toughness decreased with increasing irradiation time. For the case of FCMs 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. (C) 2000 Elsevier Science Ltd. All rights reserved.