In-situ SEM measurements of crack tip displacements in composite laminates to determine local G in mode I and II

Abstract

The strain energy release rate, G, in cracked composite test coupons and laminates is usually calculated from global parameters such as applied loads and displacements. The global G does not necessarily reflect the conditions at the crack tip, e.g. when fibres are bridging the crack. This paper presents a method for the experimental determination of the local strain energy release rate based on the crack opening displacement (COD) and crack shear displacement (CSD) profiles at the crack tip, for the case of mode I and mode II loading of composite laminates. It is shown analytically that the singular region, in terms of displacements, is three times larger behind the crack tip, than the singular region in terms of stresses and strains ahead of it, making the technique well suited for the experimental determination of the local G. The local G is determined as follows: load is applied to short Double Cantilever Beam (DCB) and End Loaded Split (ELS) specimens using shims and a special loading fixture, respectively. The loaded specimens are placed inside a scanning electron microscope (SEM) where a series of micrographs, of the region behind the crack tip, are taken. From the micrographs COD and CSD profiles are generated, from which it can be determined if a square root singularity exists. If a square root singularity exists, the local energy release rate can be calculated from the COD and CSD profiles using orthotropic linear elastic fracture mechanics (LEFM). The technique was applied to unidirectional AS4/3501-6 delamination beam specimens. It was found that there is a square root singularity near the crack tip, and that the magnitudes of the measured COD and CSD profiles agree well with analytical LEFM predictions. For the tested DCB specimen, with a laminate thickness of 3.4 mm, it was seen that the singular region, in terms of the displacements, extends to approximately 500 μm behind the crack tip.