In linear elastic fracture mechanics (LEFM), crack tip stress intensity factor (SIF) represents the magnitude of the stress field in the vicinity of the crack tip. Structural components like pressure vessels, aircraft fuselages, piping components, etc., develop cracks at multiple locations during service. Crack interaction occurs when two or more cracks are present in close vicinity, under various loading conditions leading to effects such as stress shielding, stress amplification, the coupling of the normal and shear modes, etc. Crack interaction studies in the literature are limited to only uniaxial loading. In many practical applications, the stress field is usually biaxial and many applications involve thermal loading conditions. This study brings out the effects of interaction for a certain class of crack configurations under biaxial and thermal loading. Crack tip stress field parameters are evaluated experimentally using an over-deterministic non-linear least squares iterative process incorporated in a simple user-friendly software PSIF (Photoelastic evaluation of Stress Intensity Factor), which is upgraded and suitably modified for this study.
The SIFs of asymmetric cracks oriented at different angles in a biaxially loaded tension-tension specimen is evaluated. Normalized SIF values are calculated for different crack orientation angles (α) under varying biaxial ratios (β). The effects of crack interactions viz., amplification and shielding are determined. It is brought out that unlike a single crack under tension-tension biaxial loading, crack interaction can cause amplification under certain loading conditions based on the orientation and location of the crack tips. Empirical relations for estimating SIFs under biaxial loading from the uniaxial results, given the values of α and β, are established and verified with both experimental and numerical results.
A rectangular plate subjected to edge heating/cooling is modeled using finite elements (FE) to do a parametric study involving different crack configurations. The thermal boundary conditions to be set are found to be not trivial. Experimental isochromatic fringe patterns from an uncracked specimen is used to aid the FE modeling by establishing a correct set of boundary conditions. Using this hybrid photoelastic FE analysis, an important phenomenon of micro-lifting of the order of microns, occurring when the plastic specimen is in contact with the metal hot plate is brought out. Ignoring this would have yielded wrong results in FE modelling. The asymmetric crack configuration depicted a strong amplification effect, and for this configuration, results are validated experimentally for a distance to crack length ratio of b/l = 1. The normalized Mode I SIFs are evaluated using FE and photothermoelastic experiments for cracked specimens with a close match in results, thus verifying the FE model. The study has truly brought out the importance of experimental results in correctly modeling the boundary conditions in a seemingly simple problem for numerical modeling and the advantages of using photoelastic fringes over other contours such as isotherms.
Analyzing cracks interacting with the bimaterial interface becomes important as it can lead to crack initiation and debonding. The SIFs mainly depend on the properties of the constituent materials and crack geometry. In this study, the complex crack tip stress field equation for a crack terminating at an arbitrary angle to the bimaterial interface is incorporated into the PSIF software as a separate problem module for the determination of unknown crack tip stress field parameters and SIFs, experimentally. Two types of bimaterial specimen viz., a compact bimaterial Brazilian disc and a bending strip are used to study the crack tip stress field near the crack terminating at the interface. The interfacial SIFs at the crack tips are evaluated as a function of crack geometries. Observations pertaining to the effect of material combinations and crack geometry on the SIFs are brought out.