Stress Intensity Factor Distributions in Bimaterial Systems - A Three Dimensional Photoelastic Investigation
Stress-freezing photoelastic experiments are conducted using two different sets of photoelastic materials to investigate stress intensity behavior near to and coincident with bimaterial interfaces. Homogeneous, bonded homogeneous, and bonded bimaterial single edge-cracked tension specimens are utilized throughout the investigation for comparative purposes. The first series of tests involves machined cracks obliquely inclined to the direction of far field tensile loading. Mixed-mode stress intensity factors are observed and quantified using a simplified analytical algorithm which makes use of experimentally measured data. In this series of tests, the bimaterial specimens consist of a photoelastic material bonded to the same material containing a moderate quantity of aluminum powder (for elastic stiffening purposes). Moderate yet similar increases in stress intensity factors are observed in bonded homogeneous and bonded bimaterial specimens, suggesting the presence of bondline residual stresses (rather than elastic modulus mismatch) as the primary contributing factor. The second series of tests involves the bonding of mutually translucent photoelastic materials whose elastic module differ by a ratio of approximately four to one. Cracks are placed both near and coincident to the bimaterial interfaces. Mode-mixity and increases in stress intensity are found only in bimaterial specimens whose cracks are placed close to the bondline. Using the materials from the first series of tests it is shown that the increases in these near-bondline experiments are due to thermal mismatch properties (incurred during the stress freezing cycles) rather than mechanical mismatch properties.