Fracture failure processes in polymers. I: Mechanical tests and results

Abstract

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.

Strength of epoxy adhesive-bonded stainless-steel joints

“…Moreover, tensile strengths were very close to shear strengths. There is indeed some evidence [17,18] that polymers have similar tensile and shear strengths and actually undergo tensile failure in shear tests. However, it is well known that the suitability of BJT tests for measuring adhesive tensile strengths is highly questioned [3], due to significant radial stresses and edge effects.…”

Mechanical testing of natural-fiber composites

“…12.19, even when the plane joining the notches in the sample is subjected to a pure shear load, in a plane located at an angle of 45ë, the stress components are represented by tensile and compressive stresses. 22 That is, even though shear strains can be generated, true shear failure does not occur. In the case of brittle polymers, tensile failure would occur in a plane located at 45ë (Fig.…”

Studies on effect of pre-crack length variation on Inter-laminar fracture toughness of a Glass Epoxy laminated composite

“…Depending upon the type of load, the energy release rate (G Ic , G IIc and G IIIc ) are evaluated which corresponds to Mode-I, Mode-II and Mode-III load [7]. Many industries shown the failure occurs due to the presence of Mode-I & Mode-II load [8,9,10]. Researchers [9, 10, 11,] estimated Mode-I and Mode-II inter-laminar fracture toughness of laminated composites using DCB & ENF specimens.…”