Prediction of Failure with or without Strain Localization of Advanced High Strength Hot-rolled Steel Sheets

Abstract

Numerical analysis is commonly applied to predict failure and optimize forming process in the industrial sheet metal forming. In most of typical sheet forming processes at room temperature, the metal sheets fail to deform after strain localization in the thinning mode. This strain localization is a result of the boundary value problem of the force equilibrium condition, which is strongly affected by the following three mechanical properties: the hardening behavior, yield function and strain-rate sensitivity. However, some rather brittle metals fail without strain localization. It would be determined by the fracture criterion in principle, which is one of mechanical properties. Ultimately, failure is a result of competition between two sets of properties, the first set is the fracture criterion, which directly controls failure and the other consists of the three properties, which affect strain localization. The main object of this work is to predict failure of rather thick advanced high strength hot-rolled steel sheets depending on the existence of strain localization. As for the failure with strain localization, a new numerical procedure to evaluate the formability of rather thick advanced high strength hot-rolled steel sheets was developed in this work. The new procedure differs from the practice commonly applied for rather thin cold-rolled metal sheets with four main features: employing 3-D continuum elements, non-quadratic yield functions such as Yoshida and Hosford yield functions and hardening with its deterioration (or ultimate softening) beyond uniform deformation limit as well as directly monitoring strain localization to determine failure without employing any forming limit criterion (unlike the common practice for cold-rolled sheets, which typically employs shell elements, Hills quadratic yield function, extrapolated hardening and forming limit criterion). The characterization of material properties for the new procedure involves the simple tension, disk compression and hemispherical dome stretching tests. In case of failure without localization, to properly predict the formability, the strain rate effect and fracture criterion, which is dependent on stress triaxiality and strain rate, were considered. The stress triaxiality and strain rate dependent fracture criterion was characterized based on tensile tests of four different shape specimens to cover the wide range of deformation mode with various strain rates. From the experiments, the numerical and experimental punch force and displacement results were compared and the effective fracture strain was determined. For application and validation, advanced high strength hot-rolled steel sheets, HB780 and DP780 with the thickness of 2.9 mm for failure with strain localization and TWIP980 with the thickness of 3.1mm for failure without strain localization, were considered for the circular cup drawing test.