Micromechanical studies and modelling of toughness in high strength aluminium alloys

In this thesis the influence of microstructure on fracture toughness is investigated for twodifferent medium/high strength Al-alloys for aerospace application. In weldable AA6156(Al-Mg-Si-Cu) alloy sheet, the quench sensitivity in toughness is assessed via enhancedKahn tear tests. Toughness was seen to be reduced for both 60°C water quenched and aircooled materials cf. 20°C water quench material. Fractography via scanning electronmicroscopy (SEM) and synchrotron radiation computed tomography (SRCT), as well asDifferential Scanning Calorimetry (DSC) and Transmission Electron Microscopy (TEM)studies, have clarified the mechanisms of the quench sensitivity with respect to toughness.Both the coverage of grain boundary decoration and precipitate free zone (PFZ) widthincrease with reduced quench rates. The failure morphology of the air cooled materialappears consistent with classical intergranular ductile failure. Coarse voiding and sheardecohesion was prevalent in 20°C water quenched material (depending on localtriaxiality), whilst the 60°C water quenched material showed a mixture of transgranularand intergranular fracture modes. The experimental toughness trends are compared tomodels in the literature and a simple new model is suggested.Fracture toughness anisotropy of AA2139 (Al-Cu-Mg), a candidate alloy for age forming,in T351 and T8 conditions has been investigated via mechanical testing of smooth andnotched specimens of different geometries, loaded in the rolling direction (L) or in thetransverse direction (T). Fracture mechanisms are again investigated via SEM and SRCT.Fracture toughness is seen to be anisotropic for both heat treatment conditions tested, butis substantially reduced for the T8 condition compared to the T351. Contributions tofailure behaviour have been identified with: (i) anisotropic initial void shape and growth,(ii) plastic behaviour, including isotropic/kinematic hardening and plastic anisotropy, and(iii) nucleation at a 2nd population of 2nd phase particles leading to coalescence via narrowcrack regions. SRCT analysis of arrested cracks revealed alignment of voids in the crackduring propagation in the rolling direction, resulting in shorter intervoid ligaments than forcrack propagation in the transverse direction. Coalescence through shear decohesion in thecrack initiation and propagation region was found indicating the necessity to investigateand account for this mechanism. A model based in part on the Gurson-Tvergaard-Needleman approach is constructed to describe and predict deformation behaviour, crackpropagation and, in particular, toughness anisotropy. Model parameters are fitted usingmicrostructural data and data on deformation and crack propagation for a range of smalltest samples. The model accounts for the material features found in the experimental studyand its transferability has been shown by simulating tests of large M(T) samples showingstrong fracture toughness anisotropy. A parametric study shows that nucleation of smallvoids at different strains for different loading directions is crucial for a correct model oftoughness anisotropy; the combined effects of kinematic hardening and void growthanisotropy can not fully describe fracture toughness anisotropy

Study of the flat to slant crack transition in ductile thin sheet material : simulations and experiments

International audienceFlat to slant crack transition can typically be observed in ductile thin sheet materials. The crack initiates perpendicularly to the loading direction from the notch and then turns to 45° with respect to the loading direction during crack propagation. This phenomenon is, however, still not well understood and, so far, attempts to simulate the transition in three dimensions often fail to predict macroscopic loads correctly. In this study an initial attempt has been made to reproduce the flat to slant transition performing an implicit 3D Finite Element simulation via adapting a Gurson-type model. A second void nucleation term for deformation under shear was introduced.The Lode parameter was used here to identify shear deformation. Using this modification the flat to slant transition has been reproduced successfully at loads similar to the experimental results. Further experimental investigations of void growth in the flat and slant crack propagation regime have been carried out. Cracks in Kahn tear test specimens have been arrested in the three regimes and subsequently been observed via Synchrotron Radiation Tomography of the crack tips 3D quantitative void growth analyses ahead of the crack tip in the flat and slant regimes have confirmed the change in fracture mechanisms: void growth in the flat region is substantially higher as compared to the slant crack propagation region

Etude du comportement mécanique d'un liner de titane

National audienceIl s'agit dans ce travail d'étudier le comportement mécanique d'un liner constitué de titane T35, fourni sous la forme de feuillard de 300 μm d'épaisseur. Ce travail porte notamment sur la caractérisation du métal de base mais aussi sur celui des cordons de soudure.. Pour le matériau de base, les observations microstructurales ont révélé des grains équiaxes avec une texture classique de tôle laminée pour les matériaux hexagonaux. Les soudures montrent une microstructure moins régulière et une texture plus isotrope. D'un point de vue comportement mécanique, nous avons pu identifier au moyen d'essais de traction un comportement élastoviscoplastique anisotrope des tôles. Ces résultats expérimentaux ont permis l'identification de 3 modèles issus de la littérature, (Mises, Hill et Bron-Besson) donnant lieu à des comparaisons de simulation. Enfin une analyse tomographique réalisée à l'ESRF sur une éprouvette fissurée a révélée la présence de peu de cavités à proximité de la fissure

3D Digital Volume Correlation of Synchrotron Radiation Laminography images of ductile crack initiation: An initial feasibility study

International audienceA feasibility study of measuring 3D displacement fields in the bulk during ductile crack initiation via combined Synchrotron Radiation Computed Laminography (SRCL) and Digital Volume Correlation (DVC) is performed. In contrast to Tomography, SRCL is a technique that is particularly adapted to obtain in 3D reconstructed volumes of objects that are laterally extended (i.e., in 2 directions) and thin in the third direction, i.e. sheet-like objects. In-situ laminography data of an initiating crack ahead of a machined notch are used with a voxel size of 0.7 μm. The natural contrast of the observed 2XXX Al-alloy caused by intermetallic particles and initial porosity is used to measure displacement fields via a global DVC technique assuming a continuous displacement field. An initial performance study is carried out on data of the same undeformed material but after a substantial shift of the laminography rotation axis with respect to the imaged specimen. Volume correlations between different loading steps provide displacement fields that are qualitatively consistent with the remote loading conditions. Computed strain fields display a strain concentration close to the notch tip

Ductile rupture of prestrained X100 pipeline steel sheets

International audienceThe mechanical and damage behavior of a X100 steel after prestrain is studied in thiswork. Experimental results show both a plastic and rupture anisotropy: the T direction exhibits ahigher ultimate stress but the lowest ductility and toughness. Prestrain reduces ductility and crackgrowth resistance. A model able to represent the plastic and damage behaviour of the material beforeand after prestrain is proposed. The model incorporates plastic anisotropy, kinematic hardening, voidgrowth of the primary cavities, nucleation of secondary voids on carbides. Using the model afterimplementation in a FE software, allows to reproduce experimental trends

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

Localized strain field measurement on laminography data with mechanical regularization

International audienceFor an in-depth understanding of the failure of structural materials the study of deformation mechanisms in the material bulk is fundamental. In situ synchrotron computed laminography provides 3D images of sheet samples and digital volume correlation yields the displacement and strain fields between each step of experimental loading by using the natural contrast of the material. Difficulties arise from the lack of data, which is intrinsic to laminography and leads to several artifacts, and the little absorption contrast in the 3D image texture of the studied aluminum alloy. To lower the uncertainty level and to have a better mechanical admissibility of the measured displacement field, a regularized digital volume correlation procedure is introduced and applied to measure localized displacement and strain fields

Experimental and computational analysis of toughness anisotropy in an AA2139 Al-alloy for aerospace applications

International audienceFracture toughness anisotropy of AA2139 (Al-Cu-Mg) in T351 and T8 conditions has been investigated via mechanical testing of smooth and notched specimens of different geometries, loaded in the rolling direction (L) or in the transverse direction (T). Fracture mechanisms were investigated via SEM and synchrotron radiation computed tomography (SRCT). Contributions to failure anisotropy are identified with: (i) anisotropic initial void shape and growth, (ii) plastic behaviour including isotropic/kinematic hardening and plastic anisotropy, and (iii) nucleation at a 2nd population of 2nd phase particles leading to coalescence via narrow crack regions. A model based in part on the Gurson-Tvergaard-Needleman approach is constructed to describe and predict deformation behaviour, crack propagation and, in particular, toughness anisotropy. Model parameters are fitted using microstructural data and data on deformation and crack propagation for a range of small test samples. Its transferability has been shown by simulating tests of large M(T) samples

Local approach of fracture on semi-crystalline polymers : contribution of X-ray laminography technique

International audienceDamage mechanisms in a PolyAmide 6 semi-crystalline polymer were characterized by using Synchrotron Radiation Laminography technique on CT like specimen. Damage appeared as multiple penny shaped crazes. The maximum damage occurred at mid-thickness and located at a small distance from the notch root. An attempt was made to apply local approach of fracture concept thanks to finite element analysis using damage-based constitutive model. FE simulation successfully captured the aforementioned micro-mechanisms of crack initiation, by void coalescence. Further work is carried out to determine the crack dimension corresponding to the maximum net stress: the criterion being used for the global approach of fracture

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