Creep failure model of a 9Cr1Mo-NvV (P91) steel integrating multiple deformation and damage mechanisms

International audienceP91 tempered martensitic stainless steels have recently been developed for boilers and turbines of supercritical and ultra supercritical power plants. Under creep loading conditions at high temperature, those steels exhibit changes in creep flow and damage mechanisms depending on the stress level. Several creep flow and damage mechanisms may also be simultaneously activated. The aim of this contribution is to present a new model integrating a coupling between constitutive equations and damage evolution, and considering multiple viscoplastic deformation mechanisms. The model formulation is first introduced. It is then applied to model creep flow and damage of a tempered martensitic stainless steel at 625°C for which creep tests on several kinds of specimen geometry were carried out

Damage of TWIP steels for automotive application

International audienceUltra high strength Fe-Mn-C austenitic steels with low Stacking Fault Energy (SFE) have been studied for more than fifty years [1]. They offer a great potential for reducing automotive body weight through their formability and their high mechanical properties. Tensile strength levels higher than 1000 MPa in combination with a high ductility (>50%) are reached by the optimization of the Twinning Induced Plasticity (TWIP) effect and the control of the microstructure

Effect of welding defects on plastic behaviour and fatigue lifetime of friction stir welded Al-Cu-Li alloy

International audienceThe effects of joint line remnant (JLR), kissing bond (KB), and clearance between the sheets (Gap) on tensile and fatigue properties of 2198-T851 friction stir welds have been quantitatively evaluated with respect to a reference weld made using one single sheet. The JLR has no significant influence in the investigated conditions. KB and Gap-induced defects do not significantly influence plastic yield but may induce premature crack initiation by ductile tearing and intergranular decohesions respectively. A critical value for KB opening (280 MPa), a threshold value for fatigue crack propagation from the KB (1 MPa√m) and crack growth rates consistent with literature data have been determined

Fracture toughness of the molten zone of resistance spot weld

International audienceA methodology for measuring the fracture toughness at crack initiation and the crack extension resistance of the molten zone of resistance spot welds under Mode I loading has been developed. The cross tensile test of U-shaped specimens was modified by crack growth monitoring and stress intensity factor determination. The resulting values of fracture toughness at crack initiation are independent of the nugget diameter and of the base material mechanical properties. The crack extension resistance seems to depend on base material mechanical properties and nugget diameter. Mixed cleavage + ductile mode associated to medium values of fracture toughness (54-90 MPa m 0.5 ) suggested a ductile to brittle transition behaviour. The relatively low fracture toughness (55-59 MPa m 0.5 ) associated to full interfacial ductile failure was quantitatively related to the high number density of small particles in the molten zone. This study opens the possibility to apply the local approach to fracture under monotonic loading to interfacial failure of resistance spot welds

Damage mechanisms of ultrahigh strength steels in bending application to a trip steel

International audienceIn order to optimize their metallurgical quality, the present study aims at understanding damage mechanisms involved in bending of Ultra High Strength Steels (UHSSs). It focuses on a TRansformation Induced Plasticity (TRIP)-aided steel. This work is based on three complementary approaches: first, instrumented V-bending and stretch bending tests that are usually performed to compare the bending behaviour of various steels, then, metallographic observations carried out to investigate damage initiation and finally, simulation of bending tests by finite element methods. Three-point-bending and stretch bending tests involve different crack initiation areas. Metallographic observations performed on V-bent specimens show crack initiation just below the outer surface, whereas in a stretch bending test, the crack clearly initiates from the central segregation (if any). V-bending tests were modelled with a finite element simulation approach to assess the stress and strain fields by comparison with experimental results. Modelling of stretch bending is currently in progress

Effect of friction stir weld defects on fatigue lifetime of an Al-Cu-Li alloy

International audienceThe effect of Joint Line Remnant (JLR) on the fatigue lifetime of friction stir welds of a 2198Al-alloy in T851 condition has been assessed experimentally by investigating "JLR-free"welds (welded in one sheet) and "JLR-bearing" welds (produced by welding 2 sheets with anatural oxide layer). A strong decrease in Microhardness is found for the weakest weld zonetogether with a reduction in tensile properties compared to the base material, namely, 45% inyield strength and 22% in ultimate tensile strength. The fatigue strength of JLR-free and JLRbearingwelds at 105 cycles (R=0.1) is reduced by 10% and 15% respectively compared tothe base material. No obvious effect of JLR was evidenced concerning crack initiationmechanisms

Brittle fracture in heat-affected zones of girth welds of modern line pipe steel (X100)

International audienceGirth welds of modern line pipe steel, such as X100, issued from a pulsed automatic gas metal arc welding, were tested to check their performance in artic temperature conditions. It is shown that an impact specimen at -20 °C with a notch placed in the middle of the fusion line could break at low energy (<40 J). The brittle zone is located in the coarse-grained heat-affected zone of the weld. The reproduction of two heat-affected zones with a thermal-mechanical simulator, Gleeble 1500, allows to determine the mechanical behaviour of representative microstructures of the welded joint. Tension tests with or without notch and impact tests are performed between -196 °C and 20 °C. This experimental database is used to fit materials constitutive equations which are used in a finite element code to predict the fracture of the welded joint

Ensemble averaging stress-strain fields in polycrystalline aggregates with a constrained surface microstructure-Part 1: Computational tools and application to anisotropic elastic behaviour

International audienceThe effect of three-dimensional grain morphology on the deformation at a free surface in polycrystalline aggregates is investigated by means of a large scale finite element and statistical approach. For a given 2D surface at z=0 containing 39 grains with given lattice orientations, 17 random 3D polycrystalline aggregates are constructed having different 3D grain shapes and orientations except at z=0, based on an original 3D image analysis procedure. They are subjected to overall tensile loading conditions. The resulting stress-strain fields at the free surface z=0 are analysed. Ensemble average and variance maps of the stress field at the observed surface are computed. In the case of an anisotropic elastic behaviour of the grains, fluctuations ranging between 5% and 60% are found in the equivalent stress level at a given material point of the observed surface from one realization of the microstructure to another. The obtained fields are compared to the prediction based on the associated columnar grain microstructure, often used in literature. These results have important implications in the way of comparing finite element simulations and surface strain field measurements in metal polycrystals

Quantitative investigation of brittle out-of-plane fracture in X70 pipeline steel

International audienceBrittle out-of-plane cracking (by delamination or brittle tilted fracture) affects the impact toughness of pipeline steels. It has been investigated on notched tensile specimens using a local approach to fracture. Smooth and notched bars taken along four directions (including the short transverse direction, ND) have been tested in tension at temperatures between 20 °C and -196 °C. Delamination partly results from the lower value of the critical cleavage stress along ND, linked to the microtexture anisotropy, but also from the presence of ductile microcracks acting as stress concentrators triggering fracture along the rolling plane. Brittle tilted fracture was associated to a relatively lower value of the critical cleavage stress in that plane, but prior delamination was necessary to trigger it. The relevance of a macroscopic critical stress criterion for delamination is finally discussed

Local approach applied to the fracture toughness of resistance spot welds

International audienceA recently developed methodology for measuring the nugget fracture toughness in mode I was applied to three high-strength steel resistance spot welds, exhibiting either mixed cleavage/ductile or ductile fracture at room temperature. Fracture toughness tests revealed a difference in ductile-to-brittle transition temperature between the welds. Constitutive equations of weld nuggets were determined and implemented in a finite element model of the fracture toughness test, revealing both in-plane bending and in-plane stretching in front of the crack tip. Brittle cleavage fracture of the nuggets is not conveniently described using a simple Ritchie-Knott-Rice approach but the Rice-Tracey model allowed quantitatively relating the high number density of small particles to the nugget fracture toughness in the ductile regime