The Effect of Complex Stress State and Material Strength on the Ductile Failure Loci of Steel

  • Nareg Baghous

Student thesis: Master's Thesis


One of the most important and widely used standards that requires steels to resist local ductile failure, specifically in pressure vessels, is the ASME Boiler and Pressure Vessel Code Section VIII, which allows the calculation of multiaxial strain in terms of the stress triaxiality and other material coefficients. Recent work has shown that the local ductile failure highly depends on the stress state characterized by both stress triaxiality and Lode parameter. In this study, the effect of stress state is studied on six different steel grades with various strengths. To account for different Lode parameter values, two different specimen designs were considered. One design represents the axisymmetric loading conditions (L = -1) while the other design represents the plane strain loading conditions (L = 0). To account for a range of stress triaxialities, four different notch sizes were considered for each design. In order to construct the failure locus of each material, finite element models were developed for each specimen to extract the average stress triaxiality and the plastic strain at the moment of failure, at the center of the specimen. Based on the failure loci, all materials showed dependence on Lode parameter, with different sensitivity levels to Lode parameter. Thus, the Lode sensitivity of each material was studied and was found to be a function of the material strength (either yield or ultimate strengths). These functions were able to accurately predict the Lode sensitivity of four different literature materials. Using all these findings, two new local ductile failure criteria were developed, which take into consideration the stress triaxiality, Lode parameter and the Lode sensitivity in terms of a material strength parameter.
Date of AwardDec 2019
Original languageAmerican English
SupervisorImad Barsoum (Supervisor)


  • Ductile failure
  • Lode parameter
  • Lode sensitivity
  • Stress triaxiality
  • Failure Locus
  • Finite element modelling.

Cite this