Observations on the Analysis of Mixed Mode Delamination in Composites

A review will be given of the analysis of mixed mode delamination failure in composites and adhesive joints. There will be discussion of the issue of mode II failure and how it occurs only in laminates. In homogenous material mixed mode loading results in local mode I failure by the crack kinking. If the crack is constrained in its path, as in laminates, then an apparent mode II failure occurs consisting of a series of local mode I micro cracks. This has a rough surface and, in mode II (shear) does not open leading to an increased surface area and friction. This apparent mode II fracture, and its mixed mode variants, gives higher toughness values than in mode I.Measuring these mixed mode toughness values presents significant experimental problems but standard protocols have been developed to give reliable values and hence failure loci for mixed mode failure. It is a feature of such data that any mode II components gives significant scatter so there is inherent uncertainty present.The prediction of the mode mix in any configuration also presents significant problems. For elastic failures without damage there is a established solution arising from the stress singularities at the crack tip and effects of this are usually present when there is damage. However prediction of the mode mix depends on the form of the damage and is load level dependent. There are approximate ways of determining this mode mix which will be described but numerical calculations are often neededWith this background in mind the practical value, in terms of refining design methods, by using such analyses will be discussed.