Shear strength of polymers and fibre composites: 1. thermoplastic and thermoset polymers

The shear strengths of eight thermoplastics and three DGEBA-based epoxies in sheet form have been tested by the punch and Iosipescu tests. The testing temperatures ranged from 20 to 120°C, and the glass transition temperatures were measured as well. The shear strengths of the epoxies were also estimated from compressive tests on short cylinders. The Iosipescu test gave very unreliable results for polymers in the rubbery state because large deformations were induced before failure, and this caused high tensile stresses to develop instead of high shear stresses. With the punch test the force often had two maxima before failure, and the discs punched out did not have straight sides, so there was also much doubt about the purity of the shear stresses developed. The two methods were often in sharp disagreement. However, they gave comparable results with epoxies tested at room temperature. Comparison with compressive tests indicated that the ratio of compressive yield strength to shear yield strength varied from 1.5 to 2.4. In view of the uncertainties in the tests, compressive testing may be a good method to obtain an approximate value for the shear strength. The Tresca criterion (i.e. divide the compressive yield strength by two to get the shear yield strength) is probably good enough in view of the uncertainties in the shear tests. It works equally well for the ultimate shear strength. A new and better test is clearly needed for the estimation of the shear strength of polymers in sheet form.