On the significance of the standardized notched impact strength for materials selection and design

Abstract

AbstractExperimental data on the standard Charpy notched impact strength (CNIS) for a large sampling of particulate‐filled and rubber‐modified polypropylenes were analyzed. To determine the significance of the CNIS for material selection and design, CNIS data were compared with fracture toughness measurements expressed as the critical strain energy release rate G′c, measured under impact loading. A scale factor representing the state of stress at the crack tip was calculated, assuming small scale yielding, Class I linear elastic fracture mechanics (LEFM). Based on the data, three principal groups of materials were identified. In the smallest of the three, CNIS and G′c measurements showed the same functional dependence on the material variables studied and were characterized by a scale factor independent of the composition. In these cases, one can justify comparisons of toughness of different compositions using single values of the CNIS, since the materials are probably being compared in equivalent states of stress. In a second group, CNIS and G′c also show the same functional dependence on material variables, but exhibit large variations in the scale factor with composition. One should not compare the toughness of different materials from this group based on single values of CNIS, since it is likely that the comparisons would not be made relative to equivalent states of stress. In these cases, only measurements of G′c can separate the effect of specimen geometry from those of the intrinsic properties of the material. The majority of materials studied fell into a third group, in which the CNIS and G′c exhibited substantially different functional dependencies on the material properties, and the scale factors depended also on composition. In these latter cases, a comparison of CNIS values for different compositions is not a reliable indication of the relative toughness and is of little value as a parameter for material selection and design.