Research and Development of Energy Absorption System Using Nanostructured Metallic Materials

使用納米金屬材料的能量吸收結構的研發

Student thesis: Doctoral Thesis

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Award date11 Sep 2020

Abstract

Lightweight vehicles, for example electric motor cars, have attracted more attention because of their advantages of environmental friendliness and energy saving properties over the past decades. Recently, the most important technology of the light-weighted vehicles has been crashworthiness enhancement because the capability of energy dissipation of light-weighted vehicles has significantly decreased. The conventional structure-based design has become more difficult with the development of such technology. Moreover, the cost of the light-weighted vehicles has significantly increased as a result of the development of the technology. Therefore, structure optimization and material enhancement are utilized in the energy absorber design which can realize a large safety margin and significantly decrease the cost of light-weighted vehicles. The surface mechanical attrition treatment (SMAT) can enhance the strength of the metals by modifying nanostructures without sacrificing ductility. Therefore, SMAT is a good method for thin-walled frames, which are widely used in light-weight structures. SMAT technology was well developed according to the structure of energy absorbers, which can fully utilize excellent characteristics of the advanced steel material and satisfy the requirements of industry application. Structure optimization is proposed in this paper and performed through numerical simulations and extensive experiments. Experiments are carried out that evaluate the performance of the final products. The result shows the proposed method has designed a stronger and lighter material with high-safety impact levels compared to the traditional method.

Delayed fracture of TWIP steel easily occurs after deep drawing or the large amount deformation process. Most automotive components need to be produced by deep drawing or large amount deformation. The problem of delayed fraction of TWIP steel is a critical factor that impedes its commercial application in the auto industry. Sensitivity to delayed fracture increases with the mechanical strength of steel, especially after certain cold-forming operations since high residual stress is liable to remain after deformation. The intensity of the residual stress concentration field will accelerate the content of hydrogen to attain a critical level that induces delayed cracking. SMAT is found to be a promising, effective way to decrease the residual stress of cold deformed products; hence, it will enhance delayed fracture resistance.

In this study, SMAT technology is applied in the lightweight structural development and solves the delayed fracture problem of TWIP steel. The fundamental mechanisms regarding the influence of SMAT on the resistance to the delayed fracture were investigated. Such integration has the purpose of developing the function of the localized improvement of SMAT to induce the non-uniform material strength distribution in order to optimize the energy absorption structure.

To confirm that SMAT can deliver these desired improvements to TWIP steel, the critical residual stress for triggering delayed fracture and stress corrosion cracking were investigated. Finite Element Modelling (FEM) of a high manganese TWIP steel are verified by deep drawing (cup forming and stamping) tests. A database platform was built based on the obtained results for development into a controllable processing procedure. The procedure will enable the mass commercial application of TWIP steel in automotive and other industries, such as construction application and infrastructure repair.

In this research, the theoretical were converted into a practical industrial application, which can employ use on the industry directly.

    Research areas

  • Surface mechanical attrition treatment, Nanomaterials, Energy absorption system, Delayed fracture