Strength of rough interfaces: A micro-scale approach to steel-epoxy and composite systems

The load-carrying capability of current multi-component materials is highly dependent on the performance of interfaces. This thesis studies the mechanical strength of planar interfaces and focuses on the effects of microscopic roughness. The first part of the thesis concentrates on a stainless steel-epoxy bi-material. The effect of the preparation at an elevated temperature is studied by analysing the residual stress state using an X-ray diffraction method. The bi-material's interface strength is determined on the basis of the energy release rate (Gc) and particular attention is paid to advancing the notched coating adhesion (NCA) test method. A systematic pre-crack preparation technique and a definition for the critical point of the interface collapse are developed in order to obtain a representative Gc and also to distinguish the effects of the substrate morphology. In this work, grain boundary grooves with a depth of 1-2 microns were found to produce the highest Gc values. A stabilizing aging effect was observed to reduce the interface strength. A finite element model was generated based on atomic-force microscopy and mode II dominated crack growth was simulated on a micro-scale. The simulation revealed an M-shaped crack-tip mode-mixity distribution for the interface at grain boundary grooves and a 100-fold toughening effect; however, the toughening effect decreased 35% due to modelled voids at the grain boundaries. The second part of the thesis concentrates on planar interfaces in composite systems. A literature review was conducted to review the various effects of roughness patterns due to peel ply surface treatments. An experimental study focuses on overlaminated interfaces in a glass-fibre-reinforced, unsaturated polyester composite. It was found that a combination of moisture and elevated temperature cause a significant decrease in the interface strength and durability of composites pre-treated using either polymeric peel plies or an impregnated tear ply. The work produced a model of a series of experiments, which can be used to describe the essential factors of strength for stainless steel-epoxy interfaces. The thesis concludes that a rough metal-polymer interface can be successfully analysed on a micro-scale when residual stresses, substrate morphology and flaws are realistically incorporated. The developed simulation scheme can be used to interpret the influence of groove-type micro-roughness on macroscopic performance. Also, an experimental study was conducted to offer a baseline for future simulations of systematically roughened, composite-composite interfaces.