Study on large deformation characteristics of knitted fabric reinforced thermoplastic composites at forming temperature by digital image-based strain measurement technique

The continuous fiber reinforced thermoplastic composites, such as textile composites, have many attractive properties and functions and they are used widely nowadays in industrial applications. The important point in the design and manufacturing is that the composites as well as the reinforcing textiles undergo large deformation during thermoforming, which is much influential on the overall properties. Hence, the large deformation characteristics must be investigated at thermoforming temperature. For this sake, the digital image-based measurement technique of large strain distribution is developed. It is applicable to arbitrary complex shaped components with the help of 3D-CAD. As the large strain definition, Biotʹs strain is adopted that is calculated by the polar decomposition of the deformation gradient tensor based on continuum mechanics. The developed strain measurement technique is applied to the deep-drawing of aramid rib-knitted fabric reinforced polypropylene composites. The comparison between rib and plain-knitted fabrics reinforced composites is studied and many differences are found. The large deformation characteristics are also investigated from microscopic point of view under uniaxial tension at thermoforming temperature.