Light-induced stress relief to improve flaw tolerance in network polymers

We demonstrate the ability to use photoactivated stress relaxation to improve flaw tolerance in network polymers. Unlike most self-healing polymers, which effectively close flaws by locally introducing healing agents (such as uncured resins), here light is used to relax elevated stresses around a flaw before it reaches a critical state, which reduces the threat that the flaw poses to the structural integrity of the material. In this study, we fabricate specimens with well-defined flaws and uniaxially stretch them to failure. By irradiating the specimens with UV light (365 nm) before failure, the nominal strain at failure is increased by 70% and the corresponding nominal stress is increased by 30% compared with nonirradiated specimens. To better understand the phenomena that occur at the multiaxial stress state at the flaw, we model the photomechanics using a recently developed finite element approach that accurately describes the light propagation, photochemistry, radical-induced network evolution, and the mechanical behavior of the material. Model predictions agree well with the experimental results and elucidate the role that photoinduced stress relaxation has on improving flaw tolerance.