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Measurement of delamination crack tip displacement in G/E laminates using scanning electron microscopy Ferguson, James S.


A method of measuring delamination crack tip displacements in graphite/epoxy specimens in a Scanning Electron Microscope (SEM) is presented. The technique involves the application of a grid to the specimen edge by evaporating a layer of gold through a thin mesh. The specimens are then placed into a jig which allows the application of Mode I or II loads individually or simultaneously. The specimen is placed into the SEM and a series of photographs are taken from slightly ahead of the crack tip to approximately 800 microns behind the crack tip. The photos are subsequently enlarged to 8"x10" size, and the gold grid then provides a reference system for making measurement of the relative displacements that occur under load. This method was used to examine the actual crack tip displacement profiles, which were then compared with the profiles that are predicted by Linear Elastic Fracture Mechanics (LEFM). It was found that LEFM was quite accurately able to predict the shape of the profiles, but that the magnitudes were influenced by a number of physical phenomena that were encountered. The measured displacements in Mode I were nearly always smaller than predicted due to the presence of fibre bridging in the specimens. Mode II displacements appeared to be influenced by variations in the local fibre volume fraction of the specimens which resulted in local modulus changes. Mixed-mode testing revealed an interaction that is not predicted by classical LEFM. The addition of a small Mode I load to a Mode II load resulted in increased shear displacements in nine of ten cases. The addition of a Mode II load to a Mode I load, however, resulted in increased, decreased or unchanged opening displacements. Examination of the displacements at the crack starter insert, prior to any crack growth, indicated that its behaviour is distinctly different than an actual crack. This is most likely due to the presence of a large resin pocket, which forms during manufacture at the end of the insert.

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