Mechanical testing and fracture studies of a nickel-alumina functionally graded material system

Elastic modulus and fracture toughness was determined for a layered nickel-alumina functionally graded material (FGM) system. FGM samples with a linear compositional gradient profile were formed by sequentially stacking different Ni-Al ₂O₃ cermet compositions and bonding together by solid-state sintering. From these bulk samples tensile and compact-tension (C-T) specimen geometries were used to determine the elastic moduli and fracture response within each specific layer of the FGM structure, respectively. Moiré interferometry techniques were used extensively to determine the physical displacement across the entire tensile specimen surface and the surface directly adjacent to the crack tip during testing. Interferometry results were used to verify the accuracy of a series of computer simulations calculating the fracture response through a material interface. While the macroscopic effects of residual stress resulting from the layered gradient structure were anticipated for this study, defects within the microstructure were shown to dominate the mechanical and fracture response in this material system. The level of microdamage was illustrated by much lower elastic modulus values measured for cermet layers possessing an interpenetrating network microstructure. These microstructures had considerable contact between the two constituents, nickel and alumina, providing maximum opportunity for microdamage within these cermet interlayers. Using the measured moduli values the elastic fracture response for a crack propagating from an 80%-Al₂O₃ layer toward a 60%-Al ₂O₃ layer was both measured experimentally and calculated using finite element modeling techniques. As expected, the fracture toughness of the FGM increased as a crack propagated within the more ductile layer. Relatively straight, brittle fracture was seen in the 80%-Al₂O ₃ microstructure while crack bridging, deflection, and bifurcation were all observed within the 60%-Al₂O₃ layer. A gradual rise in fracture toughness as the crack approached the sharp material interface was calculated from computer simulations illustrating the effect on fracture toughness as the material (and thus elastic modulus) is changed in front of the crack tip. Again, microstructural damage dominated the fracture response in the form of significantly reduced elastic moduli while macroscopic residual stresses had only a minimal effect, due in part to the crack propagation direction.