Bendability of advanced high strength steels

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Bendability of advanced high strength steels - A new evaluation procedure

Kaupper, Markus / Merklein, Marion

The significant lightweight engineering potential of advanced high strength steels (AHSS) is limited by its moderate formabihty and altered failure behavior compared to mild steel grades. Especially bendability is crucial in automotive applications. Small curvature bending of AHSS is governed by significant ductile damage before final fracture. Several major damaged zones evolve, where localization and stable defect growth is observed leading to fracture in the most dominant damaged zone. A new approach to assess the bending limits based on optical strain measurement was developed and implemented. The influence of local defects on effective strain can be captured by optical strain measurement. Extraction of mean summarized strains and the introduction of characteristic deformation paths offer the opportunity to reflect the experimentally observed states of bending deformation and ductile damage. It enables a comprehensive judge of material damage within bending by means of critical strains and bending angle in a material- and user-independent manner. It is a broadly applicable approach to assess the bendability of various sheet metals as a prerequisite for safe process design and to fully realize the potential of new high strength materials. In future work, the mathematical algorithm could be more refined in terms of material and microstructure specific thresholds. Moreover, the new evaluation procedure could well be applied to in-plane shear-dominated failure as well.