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 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

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

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

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

Study of the microstructure of the grade 91 steel after more than 100.000h of creep exposure at 600°C

International audienceThis paper presents results on the evolution of microstructure (both matrix and precipitates) of an ASME Grade 91 steel that has been creep tested for 113,431 h at 600 °C under a load of 80 MPa. The microstructure was investigated using transmission electron microscopy (TEM) and revealed chromium rich M23C6 carbides, MX-type precipitates, Laves phases and modified Z-phases. Only a small amount of modified Z-phase was found. In order to quantify coarsening of precipitates and growth of new phases during creep, the size distributions of the identified precipitates were determined by analysis of TEM images. In addition to this, the size distribution of Laves phases was determined by image analysis of scanning electron micrographs. Substructure modifications and creep damage were investigated on cross sections of the creep specimen using Electron Backscatter Diffraction and Scanning Electron Microscopy

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

Ensemble averaging stress-strain fields in polycrystalline aggregates with a constrained surface microstructure-Part 2 : Crystal plasticity

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, eight 3D random 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 continuum crystal plasticity framework is adopted and the resulting plastic strain fields at the free surface z=0 are analysed. Ensemble average and variance maps of the plastic strain field at the observed free surface are computed. In the case of elastoplastic copper grains, fluctuations ranging between 2% and 80% are found in the equivalent plastic slip 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. The presented results have important implications in the way of comparing finite element simulations and experimental strain or lattice rotation field measurements in metal polycrystals

Numerical generation and study of synthetic bainitic microstructures

International audienceModels classically used to describe the probability of brittle fracture in nuclear power plants are written on the macroscopic scale. Its is not easy to surely capture the physical phenomena in such a type of approach, so that the application of the models far from their identification domain (temperature history, loading path) may become questionable. To improve the quality of the prediction of resistance and life time, microstructural information, describing the heterogeneous character of the material and its deformation mechanisms has to be taken into consideration. This paper is devoted to 16MND5 bainitic steel. Bainitic packets grow in former austenitic grains, and are not randomly oriented. Knowing the macroscopic stress is thus not sufficient to describe the stress-strain state in ferrite. An accurate model must take into account the actual microstructure, in order to provide realistic local stress and strain fields, to be used as inputs of a new class of cleavage models based on the local behavior. The paper shows the first two steps of the study: mesh generation and finite element computations using crystal plasticity