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 displacement (COD) and crack shear displacement (CSD) profiles at a delamination crack tip, and using them to evaluate the effective ‘local’ Mode I and Mode II strain energy release rates. These values can then be compared to the ‘global’ strain energy release rates calculated in the usual manner from the globally applied conditions using Linear Elastic Fracture Mechanics (LEFM). The fixture that has been developed can accommodate full size standard delamination specimens, which can be loaded to failure in Mode I, Mode II, and mixed mode inside a Scanning Electron Microscope (SEM). The globally applied conditions (loads and displacements) and the local crack tip displacements can be simultaneously recorded and compared. The specimen preparation, the data acquisition and test control system, and the image acquisition and analysis system are described in detail. As an example, the results for a Mode I test of a 24-layer unidirectional AS4/3501-6 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 μm behind the crack tip. Furthermore, there is perfect agreement between the local and global strain energy release rates, indicating that the globally applied load is transmitted fully to the crack tip. These results, for a brittle material with no fiber bridging, demonstrate the validity of the technique. In future publications, we will apply the technique to study fiber bridging in Mode I loading, crack waviness effects under Mode II loading, mixed-mode loading, and other cases of interest.