Loading rate effect on ductile fracture

The Charpy impact test is widely used to monitor the quality requirements of industrial processes. It was also adopted by engineers and scientists to monitor material embrittlement resulting from environmental effects like for example irradiation. In this work, we investigated the effect of loading rate, namely quasi-static versus impact loading, on the ductile fracture behavior. Two low alloyed steels used in the reactor pressure vessel industry were selected, namely A533B and 20MnMoNi55. These steels were extensively characterized from the flow, Charpy impact and fracture toughness properties [1]–[2]. Figure 1 shows how the loading rate affects the ductile to brittle transition curve for both materials. As can be seen, the major effect of loading rate is located in the fully ductile regime where quasi-static loading requires significantly less energy to full fracture than dynamic (impact) loading. Two temperatures, namely 25°C and 290°C were selected to investigate these loading rate effects. At both temperatures, tensile and crack resistance measurements were performed at both quasi-static and dynamic loading. For the A533B steel, at 25°C, the fracture is not fully ductile and therefore only tests at 290°C were considered for this material. The results are shown in Table 1 for the various materials and conditions. These result clearly show that empirical correlations [3] relating fracture toughness to Charpy impact energy are not applicable without an in-depth analysis. Moreover, the loading rate effects on the crack resistance cannot be solely attributed to the strain rate sensitivity of the material.