Methodology for the definition of Failure Assessment Diagram of AHSS exposed to aggressive environments

摘要

The increasing demand of weight and cost reduction, and higher safety levels of the automotive industry is favouring the spread of Advanced High Strength Steels (AHSS) as structural materials. This class of steels is promising owing to their very good strength properties combined to ductility and formability. Although these very attractive properties, AHSS can suffer from phenomena induced by service environment which can jeopardize the material strength such as Hydrogen Embrittlement (HE) and Stress Corrosion Cracking (SCC). Experimental works have confirmed that AHSS mechanical resistance also depends on the applied stress and plastic strain states. Design and manufacturing practices have to take into account these issues for which traditional know-how may be no longer applicable. The knowledge of the dependence of the material strength on the stress-strain states, and environmental conditions, represents a crucial problem to be challenged. The present work is aimed at defining a methodology for identifying these kind of relationships. The research has been structured into three phases. Material selection and aggressive environment definition were firstly faced. The preliminary analysis led to three representative AHSS (M1200, HS1500 and TRIP800), for which standard characterization has been carried out. Salt aqueous solution (3.5% NaCl), with the application of different values of potential (0, -1 and -1.5 V SCE), was selected to reproduce service life conditions. Consistent stress and strain states were introduced adopting U-bend configuration. Specimens were bent by three points bending process, according to the prescriptions of VDEh SEP 1970 - 2011. Stress and strain distributions were estimated by an ad-hoc Finite Element Model (FEM) simulating the used process for specimen drawing. The proposed methodology allows to establish a direct correlation between the stress-strain states and the occurrence of failure and time for each combination of considered material and environment. This methodology appears to be successfully used for the definition of useful failure assessment diagrams.