A study of damage development in a weft knitted fabric reinforced composite. Part 2: Stress-strain and early cyclic behaviour of composite laminates with realistic fabric layups

The mechanical behaviour of knitted fabric composites, consisting of four or five layers of weft knitted glass fabric in a Derakane vinyl ester matrix, has been investigated under simple tensile loading and in cyclic tests at increasing peak strains for a range of loading angles with respect to the wale direction of the knitted fabric. Stress-strain curves are non-linear from very low strains for all loading angles and they exhibit a rapidly reducing tangent modulus with increasing strain. Acoustic emission results and the results of the cyclic loading, together with comparisons with the results on the model specimens in Part 1, suggest the following sequence of damage development: damage develops initially between yarns at the loop cross-over points in the knitted fabric structure, progresses to the development of matrix cracking damage from these initiation sites at weak planes in the composite due to the fabric architecture, and then to the pulling out of loops across matrix cracks. The pulling out of loops across matrix cracks, together with matrix non-linearity, gives rise to a stress-strain curve with a very low tangent modulus at high strains and to hysteresis loops which do not close on unloading and reloading.