Impact Behavior Of Functionally Graded Multi-layered Sheet Metals

To solve the impact issues of the aerospace vehicles, high-speed trains and automobiles in crash of the crashworthiness structures and the anti-impact performance for body armor, for instance, the functionally graded multi-layered sheet metals used in the automotive as crashworthiness structures, this study aims at investigating the mechanical response and the penetration resistance of functionally graded metal sheets disposed by surface mechanical attrition treatment(SMAT) under dynamic double shear tests and penetration tests. This thesis includes the following contents:(1)The experimental technique of double shearing on functionally graded multi-layered sheet metals has been studied. Firstly, in order to realize the double shearing tests on functionally graded multi-layered sheet metals, we systematically analyzed the errors and the related solutions using the large diameter Hopkinson bar and we designed the double shearing specimens and clamping fixture for the tests. In the next, the functionally graded specimens were fabricated by surface mechanical attrition treatment. The double shearing tests have been implemented by using the universal testing machine and large diameter Hopkinson bar under quasi-static loading and dynamic loading and the equivalent stress and equivalent strain for plane shear have been analyzed. Finally, the experimental results under quasi-static loading and dynamic loading have been compared.(2)The numerical study on double shearing has been presented. To discribe the double shearing tests in more detail and obtain more accurate experimental data, the stress state and strain state of the shear zone has been systematically investigated, the influence of legth of shear zone on the numerical results has been analyzed and the effect law of clamping fixture on the deformation uniformity of the shear zone has been obtained. Furtheremore, the definition of strain in the finite element software under large shear deformation has been analyzed. It is ultimately found that the cumulative equivalent strain and the equivalent stress could be calculated by Euler strain method and Cauchy stress method respectively.(3)The study on determining constitutive relations of each graded layer for the functionally graded multi-layered sheet metals has been implemented. The gradient distribution along the thickness direction has been founded through the indentation tests at first. At the same time, a reverse method to extract the elasto-plastic constitutive parameters of each gradient layer has been used. Then, an elastic-plastic ductile model has been built to obtain the failure strain of each graded layer latterly.(4)The study on the penetration resistance of the functionally graded multi-layered sheetmetals has been carried on. The penetration tests under quasi-static loading and dynamic loading on the graded circular specimen fabricated by SMAT with the help of clamping fixture have been implemented using MTS810 testing machine and improved Hopkinson bar,respectively. Combined the penetration tests, the numerical study for the penetration has been investigated as well. At that time, the piercing displacement and piercing force curves have been obtained under the two loading conditions and the fracture strains of each graded material layer have been ultimately verified through the numerical simulation and experimental results. At last, the strain rate sensitivity and penetration resistance of the treated by surface mechanical attrition material and material as received have been compared and analyzed.(5)The optimal design of material gradient has been premilary conducted by using ABAQUS/Explicit profiles from two sides including the gradient distribution and the stacking sequence of the graded layers. Finally, the energy absorption and the penetration resistance capability of the targets have been evaluated.