Strain distributions for tensile and shear loading around multiple holes in a thermoplastic composite introduced by thermally assisted piercing

Abstract

A multiple thermally assisted piercing process has been developed as a method of making equally spaced holes in thermoplastic composites. The consequences for the mechanical properties of the composite of introducing a limited set of inline holes into cross-ply laminates have been investigated. Open-hole tension and Iosipescu shear testing has been carried out on specimens containing drilled or pierced holes aligned with the direction of loading; microscopy and digital image correlation techniques have also been used to investigate local changes in fiber orientation and strain distributions under load. The strain fields for inline holes in drilled and pierced specimens under tensile loading can be understood in terms of local changes to the modulus as a consequence of the piercing or drilling process; in addition, some features of the strain fields can be predicted with the aid of a shear‒lag model developed for modeling matrix cracking in cross-ply laminates. Although significant differences were found between the strain fields of the drilled and pierced specimens, no consistent improvement in strength was observed for the pierced composites compared to drilled composites for different holes spacings. Under shear loading, the pierced composites were found to have a significantly poorer response compared to drilled composites, which is related to the premature collapse of the holes in shear due to (a) localized fractures in regions of low fiber volume fraction and (b) intact fibers being pulled across the holes causing hole collapse.