Interface History on Strain Field Evolution in Epoxy Resins

To determine if full-strain measurements are sufficient in measuring interfacial phenomena in additively manufactured thermosets, we devised a testing method to examine the effect of interface history (i.e., time between interface formation and original material deposition) on localized mechanical properties. Interfaces were formed in diglycidyl ether of bisphenol A (DGEBA)/diethylenetriamine (DETA) epoxy resins with various DETA concentrations. Times to the gel point and full cure were assessed by rheology and Fourier transform infrared spectroscopy (FTIR) for each composition. From here, dogbone samples were fabricated with an interface formed at either the gel point or at full cure for each composition. Key findings show that tensile strength and Young’s modulus deteriorate globally with the presence of an interface regardless of the history or initiator (DETA) concentration. Samples with an interface demonstrate high strain regions near the interface prior to fracture. Micro X-ray computed tomography revealed high density regions at the interface that increased in number with both cure times and initiator concentrations. FTIR revealed that the interface demonstrated a higher cure completion than the sample interior, resulting in a stiffer epoxy localized at the interface versus the bulk. These findings were confirmed by atomic force microscopy modulus mapping at the interface. Finally, computational modeling of epoxy in uniaxial tension with an increasing number of stiff inclusions demonstrated that inclusion contents correlated with increased, localized stress concentrations. These findings will aid in the understanding of fracture phenomenon in additively manufactured thermosets and point to digital image correlation as a useful tool in epoxy interface detection.