A technique for measuring delamination crack tip behavior in composite materials inside a scanning electron microscope

This paper presents a technique for measuring the crack opening displacemen t (COD) and crack shear displacement (CSD) profiles at a delamination crack tip, and using them to evaluate the effective 'local' Mode I and Mode Il s train energy release rates. These values can then be compared to the 'globa l' strain energy release rates calculated in the usual manner from the glob ally applied conditions using Linear Elastic Fracture Mechanics (LEFM). The fixture that has been developed can accommodate full size standard delamin ation specimens, which can be loaded to failure in Mode I, Mode II, and mix ed mode inside a Scanning Electron Microscope (SEM). The globally applied c onditions (loads and displacements) and the local crack tip displacements c an be simultaneously recorded and compared. The specimen preparation, the d ata acquisition and test control system, and the image acquisition and anal ysis system are described in detail. As an example, the results for a Mode I test of a 24-layer unidirectional A S4/3501-6 laminate are presented here. The COD profiles have been measured for 10 load levels covering 3 crack lengths. A square root singularity was observed, for roughly 500 Lm behind the crack tip. Furthermore, there is pe rfect agreement between the local and global strain energy release rates, i ndicating that the globally applied load is transmitted fully to the crack tip. These results, for a brittle material with no fiber bridging, demonstr ate the validity of the technique. In future publications, we, will apply t he technique to study fiber bridging in Mode I loading, crack waviness effe cts under Mode II loading, mixed-mode loading, and other cases of interest.