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Tensile tests and Izod impact tests were carried out on eight thermoplastics and an epoxy. The results are compared with shear strengths determined previously, and it is shown that for seven of the polymers, the nominal tensile strength is equal to the yield strength and is approximately the same as the shear strength measured by the punch test. (The true ultimate tensile strength is not always a useful concept because the more ductile polymers are radically changed during the tensile testing process.) In shear tests the shear strains can be extremely high before eventual failure. Because of this the Iosipescu test appears to be unsuitable for measuring the ultimate shear strengths of polymers. Classical shear sliding of failure appears to be rare, and instead most of the polymers appear to be failing in tension when tested in the punch test. (Shear is equivalent to tension at +45° and compression at −45° to the shear directions.) The polymers with shear Strengths that are significantly less than the tensile strengths are brittle ones, and these differences are probably due to flaw statistics. Thus the idea first advanced to explain unduly high interface shear strengths in centrosymmetric systems, i.e., that most polymers ultimately fail in tension when tested in shear, appears to be validated.

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Tensile fracture and punch shear failure surfaces have been observed in a scanning electron microscope. In most cases tensile cracks were visible in the shear fracture surfaces and crack initiation points could be found in the tensile fracture surfaces. The punch test imposes very large shear strains on the materials (many thousand percent) and this appears to reorient the chains so that, so long as the chain length is great enough (>4000 repeat units for polyethylene), chain scission is observed, and the failure process at the molecular level is tensile rather than shear. The exceptions are the shorter chain polyethylenes and polypropylene, which are probably truly liquid‐like, with little or no chain scission being involved in tension or shear.

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