Experimental and numerical investigation of ductile fracture for AA6061-T6 sheets at room and elevated temperatures

Warm forming is largely employed to enhance the poor formability of aluminum alloys at room temperature. The knowledge of the effect of stress state and temperature on the forming limits is an important aspect for the control of warm forming operations. Therefore, the objective of this work is to investigate the influence of temperature on the ductile fracture of AA6061-T6 aluminum alloy sheet metal under different stress states, ranging from shear to biaxial tension. For this purpose, experiments and numerical simulations of uniaxial tensile tests on dog-bone shaped specimen, notched specimens with different radius, specimens with a central hole and shear specimens are conducted at room and elevated temperatures. The hybrid experimental-numerical approach is used to identify the fracture strain and the corresponding stress state parameters (i.e. stress triaxiality and Lode parameter) to derive the fracture loci at room temperature, 150 degrees C and 200 degrees C. To accurately model the material behavior, the positive strain rate sensitivity in the flow stress response at elevated temperature is considered and attention is paid to constrain the finite element model by using the real experimental boundary conditions. The strain rate dependence of fracture strain is not considered in this work. A strong dependency of the ductile fracture on the temperature and stress state is evidenced. Also, an extension of Lou's ductile fracture criterion which includes the impact of temperature on ductile fracture prediction is proposed. The prediction accuracy of the original and extended criteria is evaluated. It is found that Lou's ductile fracture criterion is well suited to predict accurately ductile fracture at elevated temperatures under isothermal conditions whereas the extended form is very useful to predict ductile fracture initiation under non-isothermal conditions.

Automated summary

Warm forming is an effective method for improving the formability of aluminum alloys at room temperature. This study focuses on investigating the influence of temperature and stress state on the ductile fracture of AA6061-T6 aluminum alloy sheet metal. Experimental tests and numerical simulations are conducted to determine the fracture strain and stress state parameters under different conditions. The results show that temperature and stress state have a strong impact on ductile fracture. A modified ductile fracture criterion that considers temperature effects is proposed and evaluated.