FRACTURE FAILURE PROCESSES IN POLYMERS - I - MECHANICAL TESTS AND RESULTS

Tensile tests and Izod impact tests were carried out on eight thermopl astics and an epoxy. The results are compared with shear strengths det ermined previously, and it is shown that for seven of the polymers, th e nominal tensile strength is equal to the yield strength and is appro ximately the same as the shear strength measured by the punch test. (T he true ultimate tensile strength is not always a useful concept becau se the more ductile polymers are radically changed during the tensile testing process.) In shear tests the shear strains can be extremely hi gh before eventual failure. Because of this the Iosipescu test appears to be unsuitable for measuring the ultimate shear strengths of polyme rs. Classical shear sliding off failure appears to be rare, and instea d most of the polymers appear to be failing in tension when tested in the punch test. (Shear is equivalent to tension at +45 degrees and com pression at -45 degrees to the shear directions.) The polymers with sh ear strengths that are significantly less than the tensile strengths a re brittle ones, and these differences are probably due to flaw statis tics. Thus the idea first advanced to explain unduly high interface sh ear strengths in centrosymmetric systems, i.e., that most polymers ult imately fail in tension when tested in shear, appears to be validated.