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Abstract
During fracture, a propagating crack will interact with microstructural features, resulting in fracture surface perturbations. These regions, where a crack did not remain planar (caused by weak interfaces or residual strains from second phase particles), comprise what are known as crack deflections. The deflections diminish the stress intensity of the propagating crack, and hence, improve the fracture toughness. This thesis addresses the crack deflection process at the fundamental level, as required to provide an essential basis for toughness optimization. This is attained by modelling the process, followed by experimental verification of two primary features of the crack deflection model.
The crack deflection process has been modelled using a fracture mechanics approach. Two crack-particle interactions have been analyzed in the deflection process: the initial tilting and the maximum twist/tilt of the crack front, to determine the interaction that provides the lower crack driving force, and hence, the greater toughening. The analysis has been performed for the three dominant morphologies, the spere, the rod and the disc. For all morphologies, the maximum twist of the crack front is the toughness determining step.
The crack deflection model provides guidelines for the design of tougher brittle materials. A prime feature of the model is the invariance of fracture toughness with particle size. Particle morphology, however, plays a significant role in the determination of toughness increases; rod-shaped particles are most effective; discs and spheres are less effective, respectively. In addition, the spacing distribution between speres effects the toughening magnitude; spheres that are nearly contacting result in twist angles approaching (pi)/2, and consequently, higher toughness tht equi-spaced spheres. Additions greater than 20 volume per cent are unnecessary; as no further toughening is afforded for the three morphologies.
Experimental evidence concurs with two of the main features of the crack deflection model. A hot-pressed silicon nitride with vary aspect ratio grains demonstrates an increase in toughness with increased aspect ratio. Additionally, a series of lithium-alumino-silicate glass ceramics, having a range of crystallite sizes, verifies that toughness is invariant with particle size. Both systems correlate well with the crack deflection model.
