SHEAR-STRENGTH OF POLYMERS AND FIBER COMPOSITES .1. THERMOPLASTIC ANDTHERMOSET POLYMERS

The shear strengths of eight thermoplastics and three DGEBA-based epox ies in sheet form have been tested by the punch and Iosipescu tests. T he testing temperatures ranged from 20 to 120 degrees C, and the glass transition temperatures were measured as well. The shear strengths of the epoxies were also estimated from compressive tests on short cylin ders. The Iosipescu test gave very unreliable results for polymers in the rubbery state because large deformations were induced before failu re, and this caused high tensile stresses to develop instead of high s hear stresses. With the punch test the force often had two maxima befo re failure, and the discs punched out did not have straight sides, so there was also much doubt about the purity of the shear stresses devel oped. The two methods were often in sharp disagreement. However, they gave comparable results with epoxies tested at room temperature. Compa rison with compressive tests indicated that the ratio of compressive y ield 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 m ethod to obtain an approximate value for the shear strength. The Tresc a criterion (i.e. divide the compressive yield strength by two to get the shear yield strength) is probably good enough in view of the uncer tainties in the shear tests. It works equally well for the ultimate sh ear strength. A new and better test is clearly needed for the estimati on of the shear strength of polymers in sheet form.