Analyzing the J-integral of bonded joints under fatigue loading conditions by determining beam rotations with edge detection

• J-integral from silhouette images of DCB specimens determined by edge detection. • Soft, structural and brittle adhesives analyzed under δ- and SERR-control. • Image evaluation software developed for automated analysis of DCB images. • Comparison with LEFM methods showed no deviations under δ-control for all adhesives. • High deviations under SERR-control for glass/EVA laminates with soft adhesive layer. This study presents an edge detection method for analyzing load point rotations and evaluating the J-integral in fatigue-loaded double cantilever beam (DCB) specimens. Images of backlit DCB specimens were captured at maximum load point displacement during fatigue tests. Load point rotation was assessed using an image processing algorithm. The analysis involved binarization, alignment, and edge detection. To validate the approach, J-integral values from image-based analysis were compared with linear elastic fracture mechanics (LEFM) methods, including compliance calibration and elastic foundation models. Three laminate systems, incorporating soft (ethylene vinyl acetate copolymer, EVA), structural (acrylic, DP8810), and brittle (acrylic, Kisling 2206) adhesives were tested under constant amplitude or constant strain energy release rate (SERR) fatigue loading at temperatures ranging from 23 to 80°C. Glass/EVA laminates with a soft adhesive showed no significant difference between J-integral and LEFM methods under displacement control. However, under SERR control, J-integral values were up to 20 % lower at a constant SERR of 75 J/m 2 and 80 °C. Fractographic analysis revealed predominantly interfacial failure under displacement control and cohesive failure under SERR control. Aluminum/DP8810 laminates bonded with a structural acrylic adhesive exhibited no significant deviations between J-integral and LEFM results under both, displacement- and SERR-control. Failure was cohesive under both loading conditions. Similar behavior was observed for brittle electrical steel/acrylic laminates. No deviations were discernible under displacement control. The edge detection-based approach enabled accurate, efficient J-integral determination without compliance calibration or material parameters, reducing error sources and broadening applicability to cases involving large damage zones and elastic–plastic adhesive behavior.