| In this experimental investigation the fracture behavior of functionally graded materials (FGMs) was studied by means of fracture experiments carried out on model polymer-based FGMs. Model graded materials were manufactured by selective ultraviolet irradiation of ECO [poly(ethylene carbon monoxide)], a photo-sensitive ductile copolymer that becomes more brittle and stiffer under exposure to ultraviolet light. The mechanical response of the irradiated materials was characterized using uniaxial tensile tests. Single edge notched tension graded ECO specimens possessing different combinations of spatial variations of Young's modulus, failure stress and failure strain, were tested under remote pure opening (mode I) and in-plane mixed mode loading conditions. Mixed mode loading was generated by positioning the edge crack at an angle with respect to either the applied load or the specimen property gradient direction, or both. A hybrid full-field digital image correlation technique was used to measure in real-time the displacement field around the crack tip while it propagated through the graded material. The measured displacement field was then used to extract fracture parameters KI, KII and T-stress, and thus construct resistance curves for crack growth in FGMs. It was found that the nonsingular T-stress term in the asymptotic expansion for stresses plays a very important role in accurately measuring the fracture resistance in FGMs. In addition as FGMs, in contrast to homogeneous materials, exhibit spatial variation of failure properties, the influence of these local properties on crack growth resistance was illustrated in detail. From the results of the mixed mode fracture experiments, it was found that the maximum tangential stress and maximum energy release rate criteria, used to predict crack kinking in homogeneous materials, can also be applied to the case of FGMs for initial crack kinking. However, for accurate crack path prediction at a length scale comparable to the intrinsic material gradient, detailed material property information is required. In general, the crack will propagate towards a region that exhibits less fracture toughness and along a path where K II is not necessarily equal to zero, unlike the case of homogeneous materials. |
