Mechanical behaviour of high strength structural steel under high loading rates

Abstract

Despite offering significant strength-to-weight advantages, high-strength structural steels with high yield-to-tensile ratio >0.90, such as S690QL and S960QL, are used only in limited offshore applications. This is due to the lack of material characterisation in regard to their mechanical behaviour (tensile and fracture behaviour), with little data available on the loading rates other than those typically experienced offshore when compared to the dataset available on low strength structural steels with a yield-to-tensile ratio <0.85. The concern is that high-strength structural steels with high yield-to-tensile ratio obtain their strength at the expense of ductility and strain hardening capacity; properties which provide a sense of extra safety in avoidance of failure should service loads exceed yield. Owing to the fact that, the mechanical behaviour and performance of low strength structural steel is well known and established in the design codes and international standards, where most of the design codes relate the design formulae to low strength structural steel with Y/T ratio below 0.85, and yield strength up to 500 MPa for offshore design requirements. So, design codes that utilise these properties to deliver safety when using low strength structural steel with a yield-to-tensile ratio <0.85, may not currently be applicable for modern high strength structural steels. In this research, a programme of mechanical testing combining the tensile and fracture toughness properties of modern HSS (S690QL and S960QL) with high yield-to-tensile ratio under high loading rates applicable to offshore scenarios is proposed and investigated. This is supported by finite element analysis on the fracture toughness of S690QL in order to determine the crack driving force and the effect of loading rates on the crack mouth opening displacement which cannot be estimated experimentally using rate dependent material model developed for S690QL. Material model for S690QL is developed at a range of strain rates using a rate-dependent method available in ABAQUS code in order to allow for the prediction of the flow stress at elevated loading based on the quasi-static test data. The loading rates considered are those anticipated in offshore in-service conditions, up to 100 s-1 strain rates and K-rates up to the order of magnitude of 106 MPa√m/s. Results from the experimental tensile tests show that the strengths of the structural steel grades under consideration are relatively unaffected by the effect of loading rate when compared to low strength structural steel (mild steel), and this is predicted to be material dependent which decreases as the nominal yield strength increases. Also, like other ferritic steel, a shift to a higher ductile-to-brittle transition temperature was observed as the loading rate increases with S690QL and S960QL, associated with a reduction in the fracture toughness value on the lower transition region. Finally, the structural implication of dynamic loading on the mechanical behaviour of HSS, combining the tensile properties and fracture toughness data generated for S690QL on the FAD-based fracture engineering critical assessment (ECA) is assessed using CrackWISE® software in line with BS7910 and the results are presented in this research. The results from the assessment shows that proximity to failure by plastic collapse and fracture on the upper shelf decreases when the loading rate is increased, whereas on the lower shelf, the proximity to failure by fracture is increased for S690QL assessed. From these results, confidence and requirements regarding structural performance can be developed and re-evaluated in relevant codes and standards for these steel materials with high yield-to-tensile ratio, and high strength structural steel can exploit its strength, but not rely on its ability to deform or locally yield under extreme loading for offshore and marine applications.