Micro-indentation based study on steel sheet degradation through forming and flattening: Toward a predictive model to assess cold recyclability

• Experimental damage characterization through thickness of sheet
• Evaluation of material load-bearing capacity after forming and after flattening
• Employing submodelling with solid elements to predict the through thickness gradient of plastic strain more accurately

Sheet metal forming has always been an important sector of the metal industry thanks to work hardening, however, a complex set of deformation entails evolution of damage in the material through the forming stages. With an outlook to cold recycling of sheet metals, this paper focuses on experimental quantification of damage and degradation in load carrying capacity due to the forming process. Assuming that cold recycling of sheet metals involves an intermediate flattening prior to the secondary forming process, the adverse effects of flattening on the material was also investigated. An industrial cold roll forming process was taken as case study. An experimental investigation using microhardness mapping on the cross-section of fold zones, in conjunction with 3D global-local FE modelling, were the basis of through-thickness damage analysis. Taking advantage of strain-hardness correlation a new method was established to extrapolate hardness for ductile damage characterization. Investigation on sensitivity of the measured microhardness to crystallographic texture and sample surface preparation backed the experimental results. The results particularly outlined the progressive decrease in load carrying capacity of material after forming and after flattening. The possibility of a secondary manufacturing process is discussed.

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