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

Abstract

A B S T R A C T 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.A = pixel area of a hot spot A1 = fitting parameter A2 = fitting parameter B1 = fitting parameter B2 = fitting parameter C1 = fitting parameter C2 = fitting parameter τ ult = average failure shear stress of the adhesive for all specimens δ centre = nearest neighbour distance based on the centroids of hot spots δ edge = nearest neighbour distance based on the edges of hot spots δ min = smallest resolvable displacement for digital image correlation ε min = minimum detectable strain between two features for digital image correlation ε th = threshold strain used to define a hot spot σ ia x = shear stress at the adhesive and inner adherend interface σ oa x = shear stress at the adhesive and outer adherend interface τ ult = failure shear stress of the adhesivẽ δ þ À centre = median nearest neighbour distance based on the centroids of hot spots δ þ À edge = median nearest neighbour distance based on the edges of hot spots à = median pixel area of a hot spot A coh = percent of epoxy failure ANOVA = analysis of variance A ROI = pixel area of the field of view σ x = average bond line shear stress DICT = digital image correlation and tracking Correspondence: C. L. Muhlstein.

High-cycle fatigue damage accumulation in paper

“…The spatial distribution of deformation and strains were evaluated by an inhouse DIC programs developed in Python 3,16 , which were based on both conventional feature tracking and optical flow (Lucas-Kanade) algorithms 17,18 . Strain maps based on the motion of tracked positions were plotted using a linearly interpolated Voronoi tesselation scheme.…”