Towards the characterisation of automotive specific woven composite laminates at intermediate strain rates

Abstract

This thesis examines the sensitivity of an automotive specific T700 12K 2 x 2 twill carbon fibre epoxy composite to intermediate strain rate loads. Literature to date has focused on characterising uni-directional laminates at intermediate strain rates with few individual studies on aerospace 1K and 3K woven fabrics, however, there is a dearth of information on the strain rate effect of large tow woven fabrics at intermediate strain rates. Preliminary ASTM standard quasi-static experiments highlighted the heterogeneity of the surface strains in both tension and in-plane shear through the application of 3D digital image correlation full field measurements. Experimental studies in literature conventionally use small specimens to induce elevated strain rates however, this thesis demonstrates that due to the heterogeneous strain field, small specimens induce erroneous failure mechanisms in a material with such a large reciprocating tow structure. To overcome this, a new specimen sizing methodology was employed based on determining the representative surface element of the specimen through quadrangular windowing of the measured surface strains. Experimental correlation showed no statistical difference in comparison to the ASTM standard results. Since the specimens induced failure loads of ~ 50 kN, a new high capacity slack adaptor was designed and commissioned as composite materials with this failure load had previously not been explored at intermediate strain rates. The experimental response of the representative tensile and shear specimens was investigated at discrete intervals between nominal longitudinal strain rates of 2.2 x 10-4 s-1 and 1.0 x 102 s-1. Surface strain analysis at all strain rates using 3D DIC enabled review of the damage mechanisms occurring over the specimen. Woven tensile specimens were shown to be more sensitive to strain rate than UD laminates with T700 carbon fibres that have previously investigated in literature with large increases in strength and modulus observed. This is thought to be a result of the large resin rich regions created by the interstitial sites of the 400 gsm - 12K fabric and the large elliptical tow boundaries. Ultimate tensile strain was also shown to increase significantly at low strain rates prior to stabilising, this was hypothesised to be the due to the rate effect of damage coalescence whilst the fibres remain insensitive to strain rate. Shear specimens showed statistically significant increases in modulus, yield shear strain, yield shear strength and ultimate shear strength whilst ultimate shear strain was shown to be truncated with increasing strain rate. It was shown that the currently available finite element material cards for modelling strain rate sensitivity within LS-DYNA lack accuracy to model the strain rate effect of longitudinal tensile, shear and quasi-isotropic specimens. The modelled tensile response was shown to be more ductile than the experiment at fracture. This inaccuracy was compounded when attempting to model the tensile strain rate sensitivity due to the inability replicate the stiffness increase with rate. In comparison shear modelling was capable of predicting the bi-linear response with rate, however, it was unable to terminate the element, inducing high inaccurate virtual strain energies. This thesis highlights the critical importance of strain rate modelling of automotive specific woven composite materials for CAE vehicle development through extensive experimental studies, and it recommends that the current material cards and appropriate phenomenological models require further research and development