Material Spread and Local Failure in Breakage Modeling for Steel Safety Components

Abstract

Many load-bearing components in automotive seat-belt systems are stamped from high-strength steel. To achieve no-compromise quality and to determine the “distance to failure”, components are tested up to and including breakage and the results are statistically analyzed. Spread in tensile test mechanical properties, ranges defined by industry steel norms, is a major reason for variations in the breaking load of components. In component design-for-robustness, FEA plays a key role: It is possible to generate an elastic-plastic FEA material model including breakage from tensile test values alone [1]. In [1], it is shown how a set of serial production certificates are used as input for breaking load simulations and the prediction of the statistical performance of a component. The material model generation requires the local failure strain as input. This is not part of a steel material certificate, using DIC measurements to obtain the value is impractical in a serial production scenario. Therefore, [1] proposed to identify the local failure strain by reverse engineering FEA analysis of the tensile test. However, this paper shows that a tensile test FEA is insensitive to variations of the local failure strain, making a reverse method to estimate local failure strain inherently unreliable. In strong contrast, variations of the local failure strain have the largest influence on the calculated breaking load of components, especially when a coupled forming and breaking load simulation is performed. The impact of the local failure strain overshadows variations in strength and elongation. Steels with high local formability, such as complex phase- and nano-steels, are in the focus of recent developments of the steel industry. However, test methods to determine the local failure strain are still under debate. Linking test methods, such as DIC and hole expansion, back to FEA procedures is the next step to improve the robustness steel components for safety applications.