ORAL REFERENCE: 535 EFFECT OF BRIDGING LIGAMENTS UPON CRACK KINKING IN GRADED INTERFACES Mark Hoffman1, Lyndal Kidson1 and Christoph Deneke2 1School of Materials Science & Engineering, University of New South Wales, NSW 2052, Australia 2Department of Materials Science, Darmstadt University of Technology, 64287 Darmstadt, Germany ABSTRACT A graded interface involves a spatially changing composition gradient at an interface which removes the stress singularity which normally occurs at bimaterial interfaces. A number of analytical models have shown that, when a crack initiates perpendicular to the direction of a composition gradient, the cracks will kink as it propagates. This is a significant issue in the design of functionally graded materials. These existing analytical models show that the nature of kinking is controlled by the size and profile of the graded region and the elastic moduli of the two constituents. In this present work a model ceramic/polymer interface is constructed and the nature of fracture within this interface investigated experimentally. The sample developed ensures mixed-mode loading of the crack while ensuring no material composition gradient in the crack-tip field. Additionally, the choice of polyester and alumina means that there are negligible thermal residual stresses in the sample. It is found that the extensive crack bridging by the polymer phase occurs and that crack kinking is significantly less than that predicted by using existing analytical models which are based upon the elastic moduli of the ceramic and polymer. A numerical model is developed and demonstrates that it is the observed crack bridging which hinders crack kinking. An analytical model is then presented which confirms this hypothesis and also elucidates the influence of interface microstructure upon crack kinking in graded interfaces. KEYWORDS composite, graded, interface, kinking, fracture, crack, bridging INTRODUCTION Functionally graded materials often consist of a gradual spatial change from a ductile to a brittle material, e.g. a metal to ceramic. Often this results in the same type of structure as occurs in a ductile phase reinforced brittle material except, that in this case, there is a spatial change in the volume fraction of reinforcing phase. Because the 'matrix' and 'reinforcing' phases have different failure strains, the ductile phase leads to crack bridging and subsequent crack growth resistance or R-curve behaviour results.
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