True Stress-True Strain Models for Structural Steel Elements

A standard uniaxial tensile test, which establishes the engineering stress-strain relationship, in general, provides the basic mechanical properties of steel required by a structural designer. Modern numerical analysis techniques used for analysis of large strain problems such as failure analysis of steel structures and elements metal forming, metal cutting, and so forth, will require implementation and use of true stress-true strain material characterization. This paper establishes a five stage true stress-strain model for A992 and 350W steel grades, which can capture the behavior of structural steel, including the postultimate behavior of steel, until fracture. The proposed model uses a power law in strain hardening range and a weighted power law in the postultimate range. The true stress-true strain model parameters were established through matching of numerical analysis results with the corresponding standard uniaxial tensile test experimental results. The material constitutive relationship so derived was then applied to predict the load-deformation behavior of coupons with a hole in the middle region subjected to direct tension loading. The predicted load-deformation behavior of perforated tension coupons agreed well with the corresponding test results validating the proposed characterization of the true stress-true strain relationship for structural steel. 1. Introduction The finite-element- (FE-) method-based numerical analysis and other numerical analysis techniques are widely used in research involving structural steel and in the analysis and design of steel structures and elements. In research, numerical modeling techniques are often used to effectively expand the limited experimental results and used to investigate the influence of relevant parameters associated with a problem. Such simulations models for structural steel, however, require the use of realistic material stress-strain relationships, often extending up to fracture. Mechanical behavior of metallic type material, such as that of steel, is generally established by means of uniaxial tension test. Such tension test protocol [1], which was primarily created only for use in comparison of different steels, establishes the engineering stress and the engineering strain. Figure 1 shows a typical engineering stress-strain relationship for steel (solid line), where the stress was calculated as load divided by the original cross-section area of the tension coupon, and the engineering strain was calculated as change in length divided by the original gauge length. Such calculations, which do