A continuum damage model applied to high‐temperature fatigue lifetime prediction of a martensitic tool steel

Abstract

A B S T R A C T High-temperature operational conditions of hot work tool steels induce several thermomechanical loads. Depending on the processes, (i.e. forging, die casting or extrusion), stress, strain, strain rate and temperature levels applied on the material are nevertheless very different. Thus, lifetime prediction models need to be able to take into account a broad range of working conditions. In this paper, a non-isothermal continuum damage model is identified for a widely used hot work tool steel AISI H11 (X38CrMoV5) with a nominal hardness of 47 HRc. This investigation is based on an extensive high-temperature, lowcycle fatigue database performed under strain rate controlled conditions with and without dwell times in the temperature range 300-600 • C . As analysis of experimental results does not reveal significant time-dependent damage mechanisms, only a fatigue damage component was activated in the model formulation. After normalization, all fatigue results are defined on a master Woehler curve defined by a nonlinear damage model, which allows the parameter identification. Last, a validation stage of the model is performed from thermomechanical fatigue tests.Keywords continuum damage mechanics; fatigue life prediction; high-temperature fatigue; tempered martensitic steels; Woehler curve. N O M E N C L A T U R E D = fatigue damage value a, β, M, α, b = fatigue model parameters < u > = u.H(u), the McCauley brackets with H the Heaviside function N R = number of cycles to failure σ M = mid-life maximal stress σ m = mid-life minimal stress ε p = mid-life strain range σ = mid-life mean stress σ = mid-life stress range S m = reduced mid-life minimal stress S M = reduced mid-life maximal stress S = reduced mid-life stress range σ u = ultimate stress σ l = fatigue limit σ l 0 = fatigue limit for zero mean stress I N T R O D U C T I O NHot work tool steels undergo very critical thermomechanical loads that are usually very hard to evaluate from an Correspondence: V. Velay. experimental point of view and whose levels strongly depend on the location on the structure. Numerical simulation seems to be adequate to reach this information in order to optimize the tool design and to improve their lifetime. For that purpose, several preliminary stages are necessary. First of all, a cyclic elasto-viscoplastic behaviour model has to be identified in order to provide stress-strain

The role of creep and fatigue in determining the high-temperature behaviour of AISI H11 tempered steel for aluminium extrusion dies

“…The test at 800 MPa and 580 • C showed an unexpected trend; this was probably related to a lack of the stability at the tool-specimen contact under fatigue loads at very high specimen displacement rates. Velay (2003) and Velay et al (2005) investigated the cyclic behaviour of tempered martensitic hot-work tool steels with particular reference to the AISI H11 tool steel. Cyclic softening of the material was observed, which could be divided into three stages: initial strong softening followed by slow, steady softening that took the major part of the material life, and finally drastic softening driven by crack propagation.…”

Creep-Fatigue Interaction in the AISI H11 Tool Steel

“…from 0.702 mm to 1.260 mm at 400 MPa). Velay, Bernhart et al investigated the cyclic behavior of tempered martensitic hot-work tool steels with particular reference to the AISI H11 tool steel [10][11][12]. Cyclic softening of the material was observed, which could be divided into three stages: an initial strong softening followed by a slow steady softening that took the major part of the material life, and finally a drastic softening driven by crack propagation.…”

Thermo-Mechanical Analysis and Lifing of Ni-Cr SPF Tool

“…In addition, cyclic mechanical loads result from self-weight, clamping and pressurization (due to the forming cycle). A significant amount of work has been carried out by Bernhart and co-workers at Ecoles des Mines in Albi on hot working tools [1][2][3][4].…”