Strain Measurement in the Adhesive Layer of a Bonded Joint Using High Magnification Moire Interferometry

Abstract

Adhesive bonding is acknowledged as superior to riveting in terms of structural efficiency because of the weight penalty of introducing a mechanical fastener and the improved stress distribution and rigidity of the bonded joint. However, the adoption of adhesively bonded joints in structural applications has been inhibited by the lack of trusted design codes. The complexity of the stress distributions in bonded joints has led to their analysis using the finite element (FE) method (Harris et al. [1]), the predictions of which have to be experimentally verified (Tsai and Morton [2–4]). In the current paper we use high magnification moiré interferometry (HMMI) to measure surface strains within the narrow (200 μm) adhesive layer in the fillet region of the joints, and compare the experimental measurements with FE simulations. The strain distribution in this region of the joints is of key importance because it is there where the crack initiates in the case of failure. We also implemented significant improvements in MI instrumentation over previously reported systems, including automated analysis of the interferograms by phase shifting techniques, and the construction of an interferometer with reduced sensitivity to environmental disturbances.