Statistical analysis of the material properties of selected structural carbon steels

• Strain hardening of carbon steels implicitly used in structural engineering design.
• Manufacturers do not guarantee this crucial parameter and little data exists on it.
• Extensive statistical analyses performed on a big data set of stress–strain curves.
• Confidence intervals obtained for strain hardening and yield plateau length.
• Academic community urged to apply novel characterisation method to own material data.

Modern design procedures for steel structures increasingly employ more realistic representations of the stress–strain behaviour of steel rather than a simple ideal elastic–plastic. In particular, for buckling failure modes in the plastic range, stresses in excess of the yield stress are always involved, together with a finite post-yield stiffness. Moreover, the ‘plastic plateau’ in buckling curves for stocky structural members cannot be predicted computationally without a significant strain hardening representation. If a good match is to be sought between experiments and computational predictions in the elastic–plastic zone, strain hardening must be included. Most studies have either used individual laboratory measured stress–strain curves or educated guesswork to achieve such a match, but it is not at all clear that such calculations can reliably be used for safe design since the same hardening properties may not exist in the next constructed structure, or even within a different batch of the same steel grade.A statistical exploration is presented here to assess the reliable magnitudes of post-yield properties in common structural grade steels. For simplicity, only two critically important parameters are sought: the length of the yield plateau and the initial strain hardening tangent modulus. These two are selected because they both affect the elastic–plastic buckling of stockier structural elements. The statistical analyses exploit proprietary data acquired over many years of third-party auditing at the Karlsruhe Institute of Technology to explore possible regressed relationships between the post-yield properties. Safe lower bounds for the selected properties are determined.