Anisotropy; the variance of different physical properties of a material in different directions, is a recurrent attribute of sheet metals (specifically with regard to their mechanical properties), which have under gone extensive plastic deformation during rolling process. This drawback is mainly associated with the existence of preferred orientation of their grain structures from the rolling process. Up to date, extensive research has been devoted to the characterization and modeling of anisotropic plasticity behavior only. Recently, scholars have started advancing studies focused on tackling their anisotropic fracture behavior with extreme preference being given to the plastic anisotropy when under plane stress conditions. Regardless, much of what has been going on has only concentrated on the in-plane anisotropy of these materials, overlooking their out of-plane fracture behavior. Furthermore, the plane stress; a simplification that is valid only under certain loading conditions, has been the epicenter of most research thereby flouting the more commonly experienced three dimensional loading state that undergoes significant out-of-plane loading such as stretch-bending process with a tight radius, which is of practical importance for studies of this nature.
Recently, Professor Shuhui Li, Dr. Ji He, Bin Gu, Yixi Zhao, and Professor Zhongqin Lin at Shanghai Jiao Tong University in collaboration with Danielle Zeng, and Cedric Xia from Ford Motor Company in USA proposed a study whose main objective was to theorize that the fracture strength of a thin sheet metal in out-of-plane shear is inherently weaker than that under in-plane shear, analogous to the inter-laminar properties of composite laminates. Moreover, they purposed to design novel out-of-plane specimens and set up relevant out-of-plane tests for sheet metals. Lastly, they aimed at establishing a complete three-dimensional description of fracture strength model involving anisotropic fracture for sheet metals. Their work is currently published in the research journal, International Journal of Plasticity.
The research method employed here commenced with the development of a novel experimental technique to accurately measure the out-of-plane shear fracture strength. Next, the researchers undertook simulations of the shear specimen so as to analyze the stress state and strain distribution in the shear zone. From this, the scholars proceeded to develop a new anisotropic ductile fracture model to account for the out-of-plane fracture strength for advanced high strength steel sheets. Finally, the team engaged in further experimentation using intricate machinery to illustrate the effectiveness of the novel model.
The authors observed that based on the DP980 sheets results, the out-of-plane shear fracture strength was lower than the in-plane shear fracture strength, which indicated that an early fracture would happen when the out-of-plane shear stress is dominant. Additionally, the results of the DP980 sheet pointed out that the out-of-plane shear fracture strength was indeed about 15% lower than that measured under the in-plane shear condition.
The Shanghai Jiao Tong University study proposed and advanced a novel double notched specimen for out-of-plane shear and experimental approach. It was seen that the anisotropic fracture strength of thin sheet metal could be measured through this new technique. Based on the novel double-notched specimen, the ratio of the length of shear zone to the thickness of the specimen was seen to be the critical parameter to determine whether failure by shear was prior to the necking of arm or not. Altogether, their report has presented out-of-plane tests that should be further investigated since they have untapped potential to offer more than fracture strength measurements and provide new ways to fathom the thin sheet metal properties from out-of-plane.
Shuhui Li, Ji He, Bin Gu, Danielle Zeng, Z. Cedric Xia, Yixi Zhao, Zhongqin Lin. Anisotropic fracture of advanced high strength steel sheets: Experiment and theory. International Journal of Plasticity, volume 103 (2018) page 95–118Go To International Journal of Plasticity