Biaxial Experimental Determination of In-Plane Matrix Fracture Envelope of Unidirectional Composite

A test procedure for the determination of the in-plane fracture envelope of unidirectional fibre reinforced polymers (FRP) is presented. In particular, the determined fracture envelope covers combined in-plane shear and transverse (perpendicular to the fibre direction) matrix strength. The proposed test procedure allows the manufacture of specimens for material fracture characterisation in the same way that real composite structures are usually produced for the automotive industry. The biaxial testing is performed using a custom-made dual actuator test machine and keeping the ratio of transverse and shear load constant until fracture. The experimentally obtained transverse-shear strength relation can be well represented by the matrix fracture model by Puck. It is shown that the stress concentrations in the gauge section of the flat biaxial specimens can be avoided by the introduction of a thickness reduction, whereas the stress concentrations within biaxial specimens without such a thickness reduction lead to significantly lower strength.