Evaluation of the mechanical and damage behaviour of tufted non crimped fabric composites using full field measurements

This paper investigates the effect of tufting on the mechanical properties of non crimped fabric (NCF) composites. In-plane behaviour is examined under tension and compression in the axial [0/90] and shear [±45] directions. Cyclic experiments in the bias and axial directions combined with a digital image correlation method (DICM) allow the investigation of damage distribution through the reduction of apparent stiffness and operations on the strain field. The out of plane mechanical response is studied via delamination tests in mode I and mode II. After studying each loading case individually, small structures of both composites are subjected to multi-loadings. Experimental results show that tufting reduces both the in plane stiffness and the strength in the axial direction (by approximately 10%), while it greatly enhances delamination resistance in the normal and shear directions. On the other hand tufts influence on in plane properties is moderate in the bias direction. But large differences were monitored between compression and tension response in the bias direction for standard and tufted composites. Discs punched tests inducing multi-loading (in and out of plane loading) show greater energy absorption but lower failure load for tufted specimens than for untufted. It was found that cyclic loading experiments monitored with DICM yields damage maps that offer a useful insight into damage development. Large damage differences are recorded between different load cases and lay ups. The results also show that the tufts influence the damage progression in the NCF.