Fatigue design of structures under thermomechanical loadings

Abstract

This paper presents a global approach to the design of structures that experience thermomechanical fatigue loading, which has been applied successfully in the case of cast-iron exhaust manifolds. After a presentation of the design context in the automotive industry, the important hypotheses and choices of this approach, based on a thermal 3D computation, an elastoviscoplastic constitutive law and the dissipated energy per cycle as a damage indicator associated with a failure criterion, are first pointed out. Two particular aspects are described in more detail: the viscoplastic constitutive models, which permit a finite element analysis of complex structures and the fatigue criterion based on the dissipated energy per cycle. The FEM results associated with this damage indicator permit the construction of a design curve independent of temperature; an agreement is observed between the predicted durability and the results of isothermal as well as non isothermal tests on specimens and thermomechanical fatigue tests on real components on an engine bench. These results show that thermomechanical fatigue design of complex structures can be performed in an industrial context

Temperature–stress–strain trajectory modelling during thermo‐mechanical fatigue

“…Fatigue life depends primarily on loads, material, geometry and environmental effects. Its evaluation is generally based on tests of three forms: 2,3 …”

“…The key idea is, therefore, to predict fatigue life by avoiding expensive TMF and thermal shock tests. The methodology is discussed comprehensively in Refs [3–9].…”

Fast time-scale average for a mesoscopic high cycle fatigue criterion

“…The hydrostatic stress is considered in order to account for the local heterogenous structure. This has only recently been justified using a precise homogenisation procedure [3,15].…”

Identification of heat source fields from infrared thermography: Determination of ‘self-heating’ in a dual-phase steel by using a dog bone sample

“…The measurement and analysis of temperature fields during mechanical tests are often used (i) to analyze a physical phenomenon (e.g., Portevin-Le Châtelier (PLC) bands in aluminum alloys Ranc and Wagner, 2005;Louche et al, 2005, phase transformations in shape memory alloys Balandraud et al, 2001), (ii) to validate hypotheses (e.g., self-heating localized in a structure to correlate with the region of fatigue initiation Charkaluk et al, 2002), (iii) to identify macroscopic parameters (e.g., velocity of PLC bands Ranc and Wagner, 2005;Louche et al, 2005, fatigue properties Luong and Dang Van, 1994;Krapez et al, 1999;La Rosa and Risitano, 2000;Liaw et al, 2000;Morabito et al, 2002;Doudard et al, 2005;Meneghetti, 2007;Ummenhofer and Medgenberg, 2009, the scatter of fatigue limits Doudard et al, 2004, multiaxial fatigue criteria Doudard et al, 2007;Poncelet et al, 2007), (iv) or to validate the thermodynamic framework of a macroscopic model (Vincent, 2008). As temperature variation is not totally intrinsic to the material behavior (it depends on the diffusion properties of the materials but also on thermal boundary conditions), the development of constitutive equations in a thermodynamic framework (Germain et al, 1983;Lemaitre and Chaboche, 1990) requires the determination of the heat source field accompanying these phenomena (e.g., PLC bands, fatigue properties).…”