Correlating bonded joint deformation with failure using a free surface strain field mining methodology

In this work we present a strain field mining methodology where the intensities and spatial distributions of strains in double lap strap epoxy (EA 9394) bonded joints are measured, analysed and correlated with bond strength. While the global behaviour of the bonded joints was linear elastic at room temperature, discrete regions of elevated (relative to the full field average) shear strains were observed. The size, spatial distribution and other features of these strain ‘hot spots’ were described with a metric called the homogeneity index. This index captures the uniformity of the strain field with respect to regions of positive and negative strain at a specific stress level. The linearity of the progression of the homogeneity index and its magnitude was correlated with a joint's fracture strength and failure mode. The index was a robust predictor of double-lap strap bonded joint performance between 30% and 90% of a specimen's strength and can be used to improve joint design, manufacturing, quality assurance, maintenance and inspection.