Development of sandwich test coupons with continuous protective layers for accurate determination of the tensile failure strain of unidirectional carbon fibre reinforced composites

Recently introduced unidirectional (UD) carbon fibre reinforced epoxy (CF/EP) tensile test coupons with continuous protective layers were developed further by comparing three coupon designs with different layer integration techniques. Consistent experimental data was generated with high sample number and low scatter. Thermal residual strains were considered in case of two coupon designs where the layers were integrated at elevated temperature. A curve-fitting-based strength evaluation method is proposed for the sandwich coupons since this parameter cannot be evaluated directly. The sandwich type coupons yielded statistically significant increase in their average failure strain compared to that of the baseline tabbed coupons. In contrast, the three sandwich coupon types did not show significant differences. Therefore, the sandwich coupon type made by bonding cured UD composite layers together at room temperature is proposed for further application as they allow for full delamination at CF/EP layer fracture and do not require thermal strain correction during the evaluation.