Study On Material Property Changes In Forming And Its Effects On Crashworthiness

In recent years, simulation methods have been used in the design of vehicle security. And improving the calculation precision has become the focal point of the crashworthiness simulation research.Most parts of a vehicle body are formed by sheet metal, whose properties would always be changed during the stamping process. Nevertheless, all of these changes have been ignored in the traditional numerical simulation. Therefore a large number of calculation errors would occur in simulation. Recently, more and more savants pay attention to the forming influence on crashworthiness. However, in most of the involved studies, the forming results were mapped into the collision directly and the materials’mechanical property changes which made by forming were often ignored.In order to solve the above problems, based on the experimental results, the author summarized the material properties under pre-strain condition, established a constitutive model which not only fully considered the effects of strain path changes on material properties, but also could identify its material parameters by reverse methods. This research improved the accuracy of the analysis in vehicle and parts crashworthiness. The main content and innovations of this paper are as follows:(1) Established the pre-stain path effect rules based on the proportional loading, provided important reference data to the correct constitutive models. A comparative research has been finished to seek the pre-strain influence on material’s properties by the pre-strain solo stretching test of high strength steel. In this research, some factors were considered, especially the vehicle’s body parts state after forming. Meanwhile, stretching experiment was carried under different strain rates. And the discipline of material’s rate sensitivity introduced by pre-strain has also been investigated.(2) Established a plastic constitutive model (Modified Voce-Johnson-Cook) based on multi-level loading to correctly descript the stain path changes effects on material. The M-V-JC model was modified from classic V-JC model by introducing the pre-strain coefficient. The model was applied to simulation the tensile test by compiling the user material subroutine library. The results indicated that the model was correct and practical.(3) Identified the parameters of modified JC model accurately by using the inverse strategy based on collision simulation, which combined optimization algorithm. To deal with the parameters increase of modified JC model and data deficiency of conventional tensile test, this paper presented the inverse method to adjust parameters by genetic algorithm. With certain parameters, the difference between calculation result and impact test result was the minimum in the least-squares sense. As an example, the inverse identification of material parameters and sled impact test for the cap beam validated the effectiveness of this method.(4) Established an approximate model, which was based on the difference compensation that stamping factors were considered or not. Furthermore, an available way was also provided for the design of whole vehicle body crashworthiness optimization which fully considered changes of the material’s properties. In order to cope with the problem of the inefficiency in considered forming model and the matter of low precision in non-considered forming model, a new difference compensation model between the above two physical models, including few samples, was set up. The method’s practicability was validated by following results:one was the multi-objective optimization of a vehicle frontal impact and weight lighting, another was the relative national certification test.