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.