Stress analysis based on strain measurement in sheet metal laser bending

Abstract

Laser application in sheet metal bending represents a flexible forming method, which does not require specialized mechanical tools or large external forces. Whereas the required process parameters leading to simple bending geometries can be estimated to some extent by analytical models, for complex structures it is mostly not possible. A sufficient planning flexibility is generally provided by numerical simulation, yet particularly time-consuming if applied to incremental processes. To optimize this, an extended understanding of the material response during laser beam bending is required. Based on in situ strain and temperature measurements an analysis of incremental stress behavior is undertaken at multiple positions on the laser averted surface. For transient temperature recording a thermal camera is used. The surface strains are continuously detected by an ARAMIS-system. To calculate stress values an approach is proposed, which adapts Hook’s law on stress and strain for two-dimensions and which is further enhanced by plastic strain state and temperature changes. An analysis is provided for stainless steel (1.4301) and pure aluminum (EN AW-1050A), as well as for two cooling time periods in-between cycles. The incremental behavior of localized transversal stresses proves to be particularly complex. In contrast, longitudinal stresses approach a recurring incremental behavior pattern after very few initial laser scans. Furthermore, an indication on a material specific evolution of the plastic zone geometry is won.