Assessing strain fields in unbalanced unidirectional non-crimp fabrics

Automation of fabric preforming for resin transfer moulded composite parts motivates the need to characterize the response of dry fabric, which is required for the development of simulation models to predict potential draping defects. In this study, the in-plane tensile-shear response of a heavy-tow unidirectional non-crimp fabric (UD-NCF) subjected to bias tensile loading was investigated. Challenges associated with fabric surface texturization and associated strain measurement through digital image correlation was addressed by using a mixture of oil-based paint and mineral spirits to create a suitable speckle pattern. Custom clamps were also designed to prevent the test specimen from damaging or sliding from the grips. Strain maps revealed that the off-axis extension tests induced combined shear, tensile and compressive strains in the fabric test specimens. The fabric deformation response and proposed methods are relevant aspects for characterizing unbalanced UD-NCFs and calibrating corresponding draping simulation models.