Torsion tests on pultruded GRP sheet

Details of a series of simple torsion tests on rectangular cross-section strip specimens cut from 6·4 mm thick pultruded GRP sheet are described. Stiffness and strength data are presented for specimens with their torsion axes parallel with and transverse to the pultrusion direction. The differences in the development of failure in the two types of specimen are described. The stiffness data are analysed on the basis of simple torsion theory for orthotropic rectangular cross-section bars to determine both the in-plane and through-thickness shear moduli for the pultruded GRP sheet and it is shown that the average value of the former modulus is somewhat higher than (but nevertheless consistent with) values derived from shear tests.