On fracture of layered materials
Sammanfattning: This thesis is concerned with fracture in layered bimaterials. Fracture of coatings is considered to some extent. The behaviour of cracks growing normal to bimaterial interfaces is studied with the objective to clarify conditions for crack-tip shielding and amplification. Further, crack growth towards an interface along curved paths is investigated. Manufacturing of layered structures frequently results in high residual stresses which can induce fracture. An experimental method for determination of residual stress and thermo-elastic properties is considered. Finally, the effect of non-planar interfaces on stress concentrations and interfacial fracture in layered structures under residual stress is studied. The influence of plasticity on crack growth away from an interface between two materials with difference in elastic properties is investigated by finite element calculations. The growth of a virgin crack terminated normal to the interface is considered. The neartip state is found to be K, characterised when the crack has extended a distance of about the plastic zone size. For shorter crack extension, a decrease in crack opening displacement is observed at crack growth into stiffer material. This indicate stable crack growth, and is opposed to the prediction achieved by an elastic model. Crack growth towards an interface between materials with different fracture process properties (or yield stress) is investigated using a cohesive zone model. Very clear results for conditions under which crack-tip shielding can be expected are obtained by an analytical solution. A crack approaching a material with higher cohesive stress is shielded, and may be arrested before penetrating the interface. An expression for the maximum level of shielding for a crack growing towards a material with smaller critical crack opening displacement is given. The crack tip is exposed to an amplified load when the crack approaches a material with lower cohesive stress. Curved crack paths towards an interface to a much stiffer material are analysed using the boundary element method. Conditions for crack path instability and the fragmentation of coatings are discussed. A crack growing towards a thick substrate is deflected, and a crack approaching a very thin substrate layer is attracted normally to the interface. Numerical results are compared with experiments on polycarbonate-steel specimens. A method to determine residual stress and thermo-elastic properties of thin coatings with high accuracy using curvature measurements on bimaterial plate specimens is developed. Expressions relating curvature and mismatch strain for arbitrary coating thicknesses and for large deflections are compiled and discussed. A robust fitting method based on the mean absolute deviation is used. Experiments are performed on a TiN coating. Edge effects and influence of compressive residual stress on the cracking of a diamond coating are discussed. The stress concentration at a curved residually stressed interface is studied by finite element analysis. Interfaces are generally non-planar at some length scale. Smooth non-planar interfaces in elastic bimaterials under residual stress are analysed by finite element calculations. The effect of different interface profiles, ratio of layer thickness to interface roughness, and elastic mismatch is investigated. Generally, interface stresses of a magnitude comparable with the residual stress level in a layer with plane interface are induced. The initial stages of growth of an interface crack is studied, and the crack is shown to be arrested when reaching a neighbouring inflexion point of the interface profile.
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