The effect of asymmetric loading on fracture toughness of metallic materials

Most studies related to mixed-mode fracture behavior of metallic materials are performed with materials characterized by linear-elastic small-scale yielding behavior. In case of brittle fracture the mode II toughness is usually close to or higher than mode I fracture toughness suggesting that the mode I toughness is a sufficient estimate of fracture behavior of the material. On the contrary, on ductile materials the results are not as unambiguous. In addition, relatively few studies have been published with materials experiencing ductile fracture and large plastic strains. This is mainly due to the difficulties associated with the experimental testing and numerical analysis under elastic-plastic mixed-mode conditions. The current study is a contribution to this subject and describes the results of a test series conducted with four different materials each experiencing large nonlinear strains and a ductile fracture mechanism. The experiments have been conducted with the asymmetric-four-point-bend arrangement at room-temperature under mixed-mode I-II conditions. The materials used were JRQ (A533B) pressure vessel steel, AISI 304 austenitic stainless steel, F82H ferritic stainless steel and CuAl25 copper alloy. The numerical calculations include 2D-linear and nonlinear models for determining the stress intensity factors and the calibration factors for J-integral representation. Additionally, effects related to plasticity were simulated with a 3D-nonlinear model. The experimental studies include the determination of fracture resistance curves for all the studied materials through the mixed-mode envelope. After testing, the specimens were examined with SEM and optic studies for fractography and crack growth were taken. The main results of the study are the fracture resistance curves, which show a trend of decreasing fracture toughness as the portion of mode II component is increased. When approaching to the limit of mode I the values are compared to counterparts from different mode I tests and a good correspondence between the present results and analysis methods is observed. From the SEM fractographs the micromechanical changes associated with the ductile mixed-mode fracture are interpreted and with the help of EDS-analyses the lowering of the fracture toughness is attributed to the changes in fracture nucleation and growth mechanisms. Ductile crack growth under mixed-mode situations is described in detail. Notices to criteria and parameters describing ductile mixed-mode fracture are given. The results demonstrate that mixed-mode loading conditions can produce situations where estimates based on mode I fracture toughness are un-conservative when the fracture process is ductile with elastic-plastic behavior.