Mechanical Behavior and Fracture of High-Strength Metallic Alloy – MARS 240: Experiments and Modelling.

The aim of this thesis is to investigate the fracture behavior of Mars® 240, a high-strength steel alloy with properties that lie between purely brittle and fully ductile regimes. This intermediate behavior presents challenges for standard failure models, which are often optimized for materials at the extremes of the mechanical spectrum. While criteria such as Maximum Principal Stress and Strain Energy Density are commonly applied to brittle materials, and models like Johnson-Cook or Gurson-Tvergaard-Needleman are well-established for ductile metals, fewer models effectively predict fracture in this intermediate behavior between brittle and ductile like the one exhibited by Mars 240. The tests conducted in this thesis were carried out on both notched and un-notched specimens under quasi-static loading conditions. The experimental data served as the basis for calibrating a constitutive model incorporating Voce-type hardening. Finite element simulations were then performed using LS-DYNA to replicate the mechanical response observed in the experiments, including fracture, and to analyze stress distribution near critical regions, particularly around the notches. The final objective of the study is to evaluate the suitability of various failure criteria - particularly in the context of notched geometries and different levels of stress triaxiality, in predicting fracture initiation and propagation in Mars 240 steel. In this thesis, we focus just in one, the Cocroft-Latham fracture criterion. The study aims to propose a modelling approach that balances physical accuracy with computational efficiency. The results are expected to support safer and more reliable design strategies for components made from high-strength steels used in defense, automotive, and structural applications.