Abstract
There is a growing need to reduce vehicle mass as a means of improving fuel economy and addressing environmental concerns. At the same time, it is also important to continue to en-hance vehicle performance such as collision deformation. Ultra High Strength Steel (UHSS) is one means of addressing this issue and its adoption is expanding. Weight saving by substi-tution to UHSS is achieved by reducing the thickness of Body in White (BIW) parts. One method of deciding a thickness ratio required for equal structural characteristics in material substitution is described in the former paper.1) However decrease in thickness simply may change the buckling characteristics of the crash components, reducing performance due to the influence of elastic buckling.2) Therefore, when substituting material, it is important to con-sider not only changes in material strength, but also changes in section geometry. Section ge-ometry, which is the ratio of a thickness to a side length of a cross section, can have a signifi-cant effect on the buckling mode.3) From this point of view, a bending buckling analysis of beams is conducted. Six kinds of cross section size which are representative of BIW parts are chosen for analysis. Then the influence of section geometry and strength of the material on the buckling strength is investigated. The steel grades used in this research range from 270MPa to 1500MPa.
As an index of bending strength efficiency, the ratio of the collapse moment to the full plastic moment of the beam is used. In this study, a correlation between the strength efficiency and section geometry has been confirmed. And the efficiency is insufficient when section geome-try is small. The section geometry for the optimized mass efficiency is investigated for each steel grade. Through this investigation, the proper combination of steel grade and section ge-ometry for square tubes is determined. Following these results, a weight saving study is con-ducted for some components of BIW. Furthermore, the results of this study are incorporated into the BIW and their effectiveness is confirmed through CAE simulation and testing. This research identifies the appropriate application of the UHSS to the BIW to achieve weight sav-ing effectively. This study is limited to the crash components of thin walled structures in BIW, and also limited to crash performance without consideration of other BIW requirements.
Keywords: body design, weight saving, high strength steel, crashworthiness, bending strength of beam