Influence of single hat crash box flange triggering and impactor top plate welding strategy on axial crash foldability of AHSS & UHSS sheets

• Published in: IOP conference series. Materials Science and Engineering 2019-11, Vol.651 (1), p.12023
• Main Authors: Larour, P, Lackner, J, Wagner, L
• Format: Article
• Language: English
• Subjects: Box columns; Crashworthiness; Folding; Gas welding; Homology; Load transfer; Materials failure; Metal active gas welding; Resistance spot welding; Sheets; Stiffness; Tensile strength
• ISSN: 1757-8981; 1757-899X
• DOI: 10.1088/1757-899X/651/1/012023

Axial crash tests are usually performed in order to assess the crash foldability of AHSS & UHSS sheets in the framework of a material homologation process. With a real component like single hat column geometry with RSW flange welded and MAG welded top/down head plates crash box design, it is getting more and more difficult to assess properly the crash performance of AHSS & UHSS steel grades with tensile strength Rm ⩾ 1000 MPa (RSW: resistance spot welded, MAG: metal active gas welded). The crash box flange is namely excessively stiff and prevents the initiation of a proper crash-folding pattern, resulting in a high amount of material failure, which makes some judgement about material crash ability difficult. Therefore the present experimental investigation focuses on the one hand on the triggering strategy in the top flange area to break the excessive stiffness of the spot-welded flange. On the other hand some effort has been put in the improved joining strategy of the top impactor head plate to the crash box itself. The top head plate is usually necessary in real components for load transfer to the crash box and fixation to the surrounding body in white structure. Some excessive welding of the top impactor plate, especially in the already largely stiffened spot-welded flange region, has however a strong detrimental effect on crash fold initiation. The welding of the top head plate in the flange and back crash box areas is avoided, thus increasing the degrees of freedom between top impactor plate and crash box column along with an effective reduction of the initial crash box bending stiffness. Some minimized welding of impactor plate to the crash box column, especially avoiding flange areas, allows a successful initiation of the crash folding process but only in combination with pressed round triggers in the flange area near the first spot weld. With these modifications it is first possible to assess and rank the intrinsic axial crash foldability of the material with Rm ⩾ 1000 MPa by means of a crack length based crash index.