Cure-dependent thermal-mechanical and mode I fracture behavior of a low-Tg bio-based epoxy

Bio-based epoxy represents a promising alternative to petroleum-based epoxy in response to growing environmental concerns, its thermal and mechanical characterization is essential for engineering applications. In this study, a commercial bio-based epoxy was investigated under two curing conditions: room-temperature curing (CC-RT) and post-heating curing (CC-PC). Thermal and mechanical properties were systematically characterized using differential scanning calorimetry, dynamic thermal mechanical analysis, tensile, compact tension (CT), and double cantilever beam (DCB) tests. Despite achieving a nearly complete degree of cure, the two curing protocols exhibit significantly different mechanical responses. The CC-RT epoxy exhibited substantially greater ductility than the CC-PC epoxy in both tensile and CT tests, primarily because of its low glass transition temperature and reduced storage modulus near room temperature. The fracture resistance of the bulk epoxy in CT specimens is estimated using the J-integral based on both ASTM methods and digital image correlation (DIC). In the presence of plastic deformation, the DIC-based J-integral increases with contour size and gradually approaches the ASTM values. In addition, the epoxy is assessed as a thin adhesive layer under confined conditions in DCB tests. Under these conditions, both cured epoxies exhibit brittle fracture due to the limited development of the fracture process zone, in contrast to the CT configuration. Overall, this study provides a comprehensive understanding of the thermal–mechanical behavior and fracture performance of bio-based epoxy and offers useful guidance for its application in engineering structures.