Influence of microvascular structured void content on composites subjected to in-plane shear loads

Microvascular fiber reinforced composites can be used as multifunctional structures capable of self-healing, thermal regulation, and communication, among others. The inclusion of a small volume of microchannels generally has a minimal impact on the mechanical properties of the composites. In the current work, the in-plane shear properties of microvascular composites containing embedded stainless steel and wall-less microchannels produced through sacrificial material removal were characterized for the first time. Microvascular unidirectional composites were manufactured with precisely located microchannels both aligned with and transverse to the fiber direction, and their in-plane shear properties were tested. Microchannels aligned with the fiber direction were shown to cause a substantial (−7%) decrease in shear strength but had no effect on shear modulus or failure strain. Distortion of the laminate surface around the channels oriented transverse to the fiber axis was observed, resulting in an increase in void content (1–4%), a 9% loss in shear modulus, and a 27% loss in shear strength. The use of a caul plate made the loss in shear strength statistically insignificant, increased the shear modulus by 12–15%, and decreased the shear strain by −25% relative to the baseline composite without microchannels. Digital image correlation showed that the surface strains for transversely oriented samples were interrupted near the microchannels, but post-test x-ray computed tomography and optical microscopy do not show crack redirection. The newfound understanding of the shear response of microvascular composites provides important insights that will enable future engineering designs of multifunctional aerospace structures.