Ductile fracture of an ultra-high strength steel under low to moderate stress triaxiality

Aeronautical structures such as landing gears, wheels or turbine shaft are designed to withstand predefined loadings corresponding to critical scenarios. This work addresses the definition of a ductile failure initiation criterion under multiaxial loading for ML340 (c)(TM) ultra high strength steel employed in such structures. The experimental testing campaign includes tension-torsion tests under proportional loading, standard tensile tests, tensile tests on notched round and flat specimens and plane strain tests. The experimental database covers a stress triaxiality range from 0 to 1.3 and various positive Lode parameter stress states. A von Mises quadratic yield criterion with a combined Voce/linear hardening function is employed to describe plastic behavior. The hardening function is identified using an inverse method to fit cross section reduction in order to take into account large strains. A simple non-linear damage cumulation rule based on Rice and Tracey/Johnson Cook formulation with a Lode-dependent term is proposed. Good agreement was found between predicted and observed failure initiation load steps and failure locations. (C) 2017 Elsevier Ltd. All rights reserved.