Generalized multistage mechanical model for nonlinear metallic materials

Metallic alloys have a significant role in thin-walled engineering structures due to their unique properties such as corrosion resistance, low density or durability. Their mechanical behaviour is usually nonlinear, and this nonlinearity can be further increased during the work-hardening process. In such cases, designers have to take the proper stress-strain relationship into account to obtain reliable prediction of deformations or internal forces. In this paper, a theoretical model is proposed to match different kinds of measured data or already existing stress-strain models. It is flexible to accommodate any number of measured or recommended material parameters, and therefore makes design rules independent on testing standards. It is particularly suitable for computer code implementation. The approximate inversion of the multistage model is also included in the presented study. The general formula is applied on the set of parameters typically available for structural stainless steels in Europe (0.2% and 1.0% proof strength and ultimate strength) and compared to the existing models by curve-fitting of analytical equations to measured stresses and strains of austenitic, duplex and ferritic stainless steels. The comparisons clearly showed that this three-stage application of the generalized multistage model yields more accurate results compared to the existing material models both in direct and inverse form.