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Tue, 15 Oct 2019 04:32:08 GMT
20191015T04:32:08Z

Multiscale fatigue damage characterization in short glass fiber reinforced polyamide66
http://hdl.handle.net/10985/7934
Multiscale fatigue damage characterization in short glass fiber reinforced polyamide66
ARIF, Muhamad Fatikul; SAINTIER, Nicolas; MERAGHNI, Fodil; FITOUSSI, Joseph; CHEMISKY, Yves; ROBERT, Gilles
This paper aims at studying fatigue damage behavior of injection molded 30 wt% short glass fiber reinforced polyamide66 composite (PA66/GF30). The evolution of dynamic modulus, hysteresis area, cyclic creep and temperature during fatigue tests were analyzed and discussed. Damage analyses by Xray microcomputed tomography (lCT) technique on interrupted fatigue tests at several percentages of total fatigue life were performed to further understand the damage mechanisms and evolution during fatigue loading. It can be observed that experimental results related to the evolution of dynamic modulus, strain, temperature and energy dissipation are important and consistently complement each other for damage evaluation of PA66/GF30. During fatigue loading, diffuse damage occurs over the entire specimen though the damage does not necessarily exhibit the same level between different locations inside the specimen. The lCT analysis of voids characteristics demonstrates that the damage continuously increases during fatigue loading. The damage is developed notably along fiber interface in the form of fiber/matrix interfacial debonding.
Wed, 01 Jan 2014 00:00:00 GMT
http://hdl.handle.net/10985/7934
20140101T00:00:00Z
ARIF, Muhamad Fatikul
SAINTIER, Nicolas
MERAGHNI, Fodil
FITOUSSI, Joseph
CHEMISKY, Yves
ROBERT, Gilles
This paper aims at studying fatigue damage behavior of injection molded 30 wt% short glass fiber reinforced polyamide66 composite (PA66/GF30). The evolution of dynamic modulus, hysteresis area, cyclic creep and temperature during fatigue tests were analyzed and discussed. Damage analyses by Xray microcomputed tomography (lCT) technique on interrupted fatigue tests at several percentages of total fatigue life were performed to further understand the damage mechanisms and evolution during fatigue loading. It can be observed that experimental results related to the evolution of dynamic modulus, strain, temperature and energy dissipation are important and consistently complement each other for damage evaluation of PA66/GF30. During fatigue loading, diffuse damage occurs over the entire specimen though the damage does not necessarily exhibit the same level between different locations inside the specimen. The lCT analysis of voids characteristics demonstrates that the damage continuously increases during fatigue loading. The damage is developed notably along fiber interface in the form of fiber/matrix interfacial debonding.

Overload effects on a ferriticbaintic steel and a cast aluminium alloy: two very different behaviours
http://hdl.handle.net/10985/8814
Overload effects on a ferriticbaintic steel and a cast aluminium alloy: two very different behaviours
SAINTIER, Nicolas; PALINLUC, Thierry; EL DSOKI, Chalid; BIDOUARD, Hadrien; KAUFMANN, Heinz; DUMAS, C.; VÖLLMECKE, F.J.; SONSINO, Cetin Morris
Load controlled fatigue tests were performed up to 107 cycles on flat notched specimens (Kt =2.5) under constant amplitude and variable amplitude loadings with and without periodical overloads. Two materials are studied: a ferriticbainitic steel and a cast aluminium alloy. These materials have a very different cyclic behaviour: the steel exhibits cyclic strain oftening whereas the Al alloy shows cyclic strain hardening. The fatigue tests show that, for the steel, periodical overload applications reduce significantly the fatigue life for fully reversed load ratio (Rr = – 1), while they have no influence under pulsating loading (Rr = 0). For the Al alloy overloads have an effect (fatigue life decreasing) only for variable amplitude loadings. The detrimental effect of overloads on the steel is due to ratcheting at the notch root which evolution is overload's dependent.
Sat, 01 Jan 2011 00:00:00 GMT
http://hdl.handle.net/10985/8814
20110101T00:00:00Z
SAINTIER, Nicolas
PALINLUC, Thierry
EL DSOKI, Chalid
BIDOUARD, Hadrien
KAUFMANN, Heinz
DUMAS, C.
VÖLLMECKE, F.J.
SONSINO, Cetin Morris
Load controlled fatigue tests were performed up to 107 cycles on flat notched specimens (Kt =2.5) under constant amplitude and variable amplitude loadings with and without periodical overloads. Two materials are studied: a ferriticbainitic steel and a cast aluminium alloy. These materials have a very different cyclic behaviour: the steel exhibits cyclic strain oftening whereas the Al alloy shows cyclic strain hardening. The fatigue tests show that, for the steel, periodical overload applications reduce significantly the fatigue life for fully reversed load ratio (Rr = – 1), while they have no influence under pulsating loading (Rr = 0). For the Al alloy overloads have an effect (fatigue life decreasing) only for variable amplitude loadings. The detrimental effect of overloads on the steel is due to ratcheting at the notch root which evolution is overload's dependent.

Effects of microstructure and local mechanical fields on intergranular stress corrosion cracking of a friction stir welded aluminum–copper–lithium 2050 nugget
http://hdl.handle.net/10985/9125
Effects of microstructure and local mechanical fields on intergranular stress corrosion cracking of a friction stir welded aluminum–copper–lithium 2050 nugget
DHONDT, Matthieu; AUBERT, Isabelle; SAINTIER, Nicolas; OLIVE, JeanMarc
The effects of the microstructure and mechanical fields on intergranular stress corrosion cracking (IGSCC) of the nugget zone of heat treated welds obtained by friction stir welding in the AA2050 aluminum alloy have been investigated at different scales. At low strain rate, in 1.0 NaCl aqueous solution, IGSCC develops in the microstructure, whereas only pitting corrosion is observed without any mechanical stress. Based on surface observations, EBSD analysis and Xray tomography, the key role of submillimetric textured bands (induced by the welding process) on the IGSCC is demonstrated. Analyses at a more local scale show the grain boundary (low angle boundary, special coincident site lattice boundary or high angle boundary) do not have a significant effect on crack initiation. Crystal plasticity finite element calculations show that the threshold normal stress at grain boundaries for IGSCC development is about 80% of the macroscopic stress. It is also highlighted by crystal plasticity calculations that there is a drastic effect of the local stress field on the shape of cracks. Finally, it is shown that plasticity induced residual stresses are sufficient for the formation of IGSCC. © 2014 Elsevier Ltd.
Wed, 01 Jan 2014 00:00:00 GMT
http://hdl.handle.net/10985/9125
20140101T00:00:00Z
DHONDT, Matthieu
AUBERT, Isabelle
SAINTIER, Nicolas
OLIVE, JeanMarc
The effects of the microstructure and mechanical fields on intergranular stress corrosion cracking (IGSCC) of the nugget zone of heat treated welds obtained by friction stir welding in the AA2050 aluminum alloy have been investigated at different scales. At low strain rate, in 1.0 NaCl aqueous solution, IGSCC develops in the microstructure, whereas only pitting corrosion is observed without any mechanical stress. Based on surface observations, EBSD analysis and Xray tomography, the key role of submillimetric textured bands (induced by the welding process) on the IGSCC is demonstrated. Analyses at a more local scale show the grain boundary (low angle boundary, special coincident site lattice boundary or high angle boundary) do not have a significant effect on crack initiation. Crystal plasticity finite element calculations show that the threshold normal stress at grain boundaries for IGSCC development is about 80% of the macroscopic stress. It is also highlighted by crystal plasticity calculations that there is a drastic effect of the local stress field on the shape of cracks. Finally, it is shown that plasticity induced residual stresses are sufficient for the formation of IGSCC. © 2014 Elsevier Ltd.

Multiaxial high cycle fatigue damage mechanisms associated with the different microstructural heterogeneities of cast aluminium alloys
http://hdl.handle.net/10985/10857
Multiaxial high cycle fatigue damage mechanisms associated with the different microstructural heterogeneities of cast aluminium alloys; Mechanismes d'endommagement en fatigue multiaxiale à grand nombre de cycles associés aux différentes hétérogénéités microstructurales des alliages d'aluminium de fonderie
LE, Viet Duc; MOREL, Franck; SAINTIER, Nicolas; BELLETT, Daniel; OSMOND, Pierre
This article is dedicated to the high cycle fatigue (HCF) behaviour of cast AlSi alloys. In particular, three similar alloys with different microstructural characteristics are investigated. The result of an experimental campaign are presented, in order to characterise the fatigue behaviour, and more specifically the fatigue damage mechanisms related to the different microstructural heterogeneities (i.e. casting porosity, dendrite size, SDAS, nonmetallic inclusions and silicon particles), observed under different multiaxial loading conditions: pure tension, plane bending, pure torsion and combined tensiontorsion with a load ratio R=1. It is shown that casting porosity has a very detrimental influence on the uniaxial and combined tensiontorsion fatigue strengths. However, a much lower influence is observed for the torsional fatigue strength. For the porosityfree alloy, it is observed that the formation of persistent slip bands (PSB) in the aluminium matrix is the major fatigue crack initiation mechanism regardless of the loading modes, at a load ratio of R=1. It is also shown that the aluminium matrix has a large role in the formation of PSB and that the Si particles facilitate the formation of PSB.
Fri, 01 Jan 2016 00:00:00 GMT
http://hdl.handle.net/10985/10857
20160101T00:00:00Z
LE, Viet Duc
MOREL, Franck
SAINTIER, Nicolas
BELLETT, Daniel
OSMOND, Pierre
This article is dedicated to the high cycle fatigue (HCF) behaviour of cast AlSi alloys. In particular, three similar alloys with different microstructural characteristics are investigated. The result of an experimental campaign are presented, in order to characterise the fatigue behaviour, and more specifically the fatigue damage mechanisms related to the different microstructural heterogeneities (i.e. casting porosity, dendrite size, SDAS, nonmetallic inclusions and silicon particles), observed under different multiaxial loading conditions: pure tension, plane bending, pure torsion and combined tensiontorsion with a load ratio R=1. It is shown that casting porosity has a very detrimental influence on the uniaxial and combined tensiontorsion fatigue strengths. However, a much lower influence is observed for the torsional fatigue strength. For the porosityfree alloy, it is observed that the formation of persistent slip bands (PSB) in the aluminium matrix is the major fatigue crack initiation mechanism regardless of the loading modes, at a load ratio of R=1. It is also shown that the aluminium matrix has a large role in the formation of PSB and that the Si particles facilitate the formation of PSB.

Micromechanical modelling of high cycle fatigue behaviour of metals under multiaxial loads
http://hdl.handle.net/10985/6796
Micromechanical modelling of high cycle fatigue behaviour of metals under multiaxial loads
ROBERT, Camille; SAINTIER, Nicolas; PALINLUC, Thierry; MOREL, Franck
An analysis of high cycle multiaxial fatigue behaviour is conducted through the numerical simulation of polycrystalline aggregates using the finite elementmethod. The metallicmaterial chosen for investigation is pure copper, which has a Face Centred Cubic (FCC) crystalline microstructure. The elementary volumes are modelled in 2D using an hypothesis of generalised plane strain and consist of 300 equiprobability, randomly oriented grains with equiaxed geometry. The aggregates are loaded at levels equivalent to the average macroscopic fatigue strength at 107 cycles. The goal is to compute the mechanical quantities at the mesoscopic scale (i.e. average within the grain) after stabilization of the local cyclic behaviour. The results show that the mesoscopic mechanical variables are characterised by high dispersion. A statistical analysis of the response of the aggregates is undertaken for different loading modes: fully reversed tensile loads, torsion and combined inphase tensiontorsion. Via the calculation of the local mechanical quantities for a sufficiently large number of different microstructures, a critical analysis of certain multiaxial endurance criteria (Crossland, Dang Van and Matake) is conducted. In terms of material behaviour models, it is shown that elastic anisotropy strongly affects the scatter of the mechanical parameters used in the different criteria and that its role is predominant compared to that of crystal plasticity. The analysis of multiaxial endurance criteria at both the macroscopic and mesoscopic scales clearly show that the critical plane type criteria (Dang Van and Matake) give an adequate estimation of the shear stress but badly reflect the scatter of the normal stress or the hydrostatic stress.
Sun, 01 Jan 2012 00:00:00 GMT
http://hdl.handle.net/10985/6796
20120101T00:00:00Z
ROBERT, Camille
SAINTIER, Nicolas
PALINLUC, Thierry
MOREL, Franck
An analysis of high cycle multiaxial fatigue behaviour is conducted through the numerical simulation of polycrystalline aggregates using the finite elementmethod. The metallicmaterial chosen for investigation is pure copper, which has a Face Centred Cubic (FCC) crystalline microstructure. The elementary volumes are modelled in 2D using an hypothesis of generalised plane strain and consist of 300 equiprobability, randomly oriented grains with equiaxed geometry. The aggregates are loaded at levels equivalent to the average macroscopic fatigue strength at 107 cycles. The goal is to compute the mechanical quantities at the mesoscopic scale (i.e. average within the grain) after stabilization of the local cyclic behaviour. The results show that the mesoscopic mechanical variables are characterised by high dispersion. A statistical analysis of the response of the aggregates is undertaken for different loading modes: fully reversed tensile loads, torsion and combined inphase tensiontorsion. Via the calculation of the local mechanical quantities for a sufficiently large number of different microstructures, a critical analysis of certain multiaxial endurance criteria (Crossland, Dang Van and Matake) is conducted. In terms of material behaviour models, it is shown that elastic anisotropy strongly affects the scatter of the mechanical parameters used in the different criteria and that its role is predominant compared to that of crystal plasticity. The analysis of multiaxial endurance criteria at both the macroscopic and mesoscopic scales clearly show that the critical plane type criteria (Dang Van and Matake) give an adequate estimation of the shear stress but badly reflect the scatter of the normal stress or the hydrostatic stress.

Competition between microstructure and defect in multiaxial high cycle fatigue
http://hdl.handle.net/10985/10058
Competition between microstructure and defect in multiaxial high cycle fatigue
MOREL, Franck; GUERCHAIS, Raphaël; SAINTIER, Nicolas
This study aims at providing a better understanding of the effects of both microstructure and defect on the high cycle fatigue behavior of metallic alloys using finite element simulations of polycrystalline aggregates. It is well known that the microstructure strongly affects the average fatigue strength and when the cyclic stress level is close to the fatigue limit, it is often seen as the main source of the huge scatter generally observed in this fatigue regime. The presence of geometrical defects in a material can also strongly alter the fatigue behavior. Nonetheless, when the defect size is small enough, i.e. under a critical value, the fatigue strength is no more affected by the defect. The socalled Kitagawa effect can be interpreted as a competition between the crack initiation mechanisms governed either by the microstructure or by the defect. Surprisingly, only few studies have been done to date to explain the Kitagawa effect from the point of view of this competition, even though this effect has been extensively investigated in the literature. The primary focus of this paper is hence on the use of both FE simulations and explicit descriptions of the microstructure to get insight into how the competition between defect and microstructure operates in HCF. In order to account for the variability of the microstructure in the predictions of the macroscopic fatigue limits, several configurations of crystalline orientations, crystal aggregates and defects are studied. The results of each individual FE simulation are used to assess the response at the macroscopic scale thanks to a probabilistic fatigue criterion proposed by the authors in previous works. The ability of this criterion to predict the influence of defects on the average and the scatter of macroscopic fatigue limits is evaluated. In this paper, particular emphasis is also placed on the effect of different loading modes (pure tension, pure torsion and combined tension and torsion) on the experimental and predicted fatigue strength of a 316 stainless steel containing artificial defect.
Pas d'embargo connu sur Sherpa Romeo
Thu, 01 Jan 2015 00:00:00 GMT
http://hdl.handle.net/10985/10058
20150101T00:00:00Z
MOREL, Franck
GUERCHAIS, Raphaël
SAINTIER, Nicolas
This study aims at providing a better understanding of the effects of both microstructure and defect on the high cycle fatigue behavior of metallic alloys using finite element simulations of polycrystalline aggregates. It is well known that the microstructure strongly affects the average fatigue strength and when the cyclic stress level is close to the fatigue limit, it is often seen as the main source of the huge scatter generally observed in this fatigue regime. The presence of geometrical defects in a material can also strongly alter the fatigue behavior. Nonetheless, when the defect size is small enough, i.e. under a critical value, the fatigue strength is no more affected by the defect. The socalled Kitagawa effect can be interpreted as a competition between the crack initiation mechanisms governed either by the microstructure or by the defect. Surprisingly, only few studies have been done to date to explain the Kitagawa effect from the point of view of this competition, even though this effect has been extensively investigated in the literature. The primary focus of this paper is hence on the use of both FE simulations and explicit descriptions of the microstructure to get insight into how the competition between defect and microstructure operates in HCF. In order to account for the variability of the microstructure in the predictions of the macroscopic fatigue limits, several configurations of crystalline orientations, crystal aggregates and defects are studied. The results of each individual FE simulation are used to assess the response at the macroscopic scale thanks to a probabilistic fatigue criterion proposed by the authors in previous works. The ability of this criterion to predict the influence of defects on the average and the scatter of macroscopic fatigue limits is evaluated. In this paper, particular emphasis is also placed on the effect of different loading modes (pure tension, pure torsion and combined tension and torsion) on the experimental and predicted fatigue strength of a 316 stainless steel containing artificial defect.

Effet de surface libre dans les agrégats polycristallins en fatigue à grand nombre de cycles.
http://hdl.handle.net/10985/7383
Effet de surface libre dans les agrégats polycristallins en fatigue à grand nombre de cycles.
ROBERT, Camille; HOR, Anis; MOREL, Franck; SAINTIER, Nicolas; PALINLUC, Thierry
An analysis of high cycle fatigue behavior is done via the numerical simulation of polycrystalline aggregates. Different metallic materials with a FCC crystalline structure, but different cubic elastic coefficients, are investigated. Several statistical elementary volumes (SEV), consisting of 300 grains with isotropic texture and equiaxed geometries, are loaded at the median macroscopic fatigue limit for 107 cycles. Three different models are studied: 2D generalized plane strain, 3D periodic and 3D periodic with a free surface. Different mesoscopic variables are analyzed using extreme value statistics. The results show a detrimental e ect on the fatigue strength of the modeled aggregates with a free surface, if the crystalline elastic anisotropy is sufficient.
Tue, 01 Jan 2013 00:00:00 GMT
http://hdl.handle.net/10985/7383
20130101T00:00:00Z
ROBERT, Camille
HOR, Anis
MOREL, Franck
SAINTIER, Nicolas
PALINLUC, Thierry
An analysis of high cycle fatigue behavior is done via the numerical simulation of polycrystalline aggregates. Different metallic materials with a FCC crystalline structure, but different cubic elastic coefficients, are investigated. Several statistical elementary volumes (SEV), consisting of 300 grains with isotropic texture and equiaxed geometries, are loaded at the median macroscopic fatigue limit for 107 cycles. Three different models are studied: 2D generalized plane strain, 3D periodic and 3D periodic with a free surface. Different mesoscopic variables are analyzed using extreme value statistics. The results show a detrimental e ect on the fatigue strength of the modeled aggregates with a free surface, if the crystalline elastic anisotropy is sufficient.

Nonlocal energy based fatigue life calculation method under multiaxial variable amplitude loadings
http://hdl.handle.net/10985/7414
Nonlocal energy based fatigue life calculation method under multiaxial variable amplitude loadings
SAINTIER, Nicolas; PALINLUC, Thierry; BÉNABÈS, Jérôme; COCHETEUX, Francis
Reliable design of industrial components against high cycle multiaxial fatigue requires a model capable of predicting both stress gradient and load type effects. Indeed, taking into account gradient effects is of prior importance for the applicability of fatigue models to real structures. In this paper, a fatigue life assessment method is proposed for proportional and nonproportional multiaxial variable amplitude loadings in the range 104 –107 cycles. This method derives from the fatigue criterion initially proposed by PalinLuc and Lasserre (1998) [2] and revisited by Banvillet et al. (2003) [16] for multiaxial constant amplitude loading. The new proposal consists of a complete reformulation and extension of the previ ously cited energy based fatigue strength criteria. It includes two major improvements of the existing criteria. The first one consists in a fatigue criterion for multiaxial variable amplitude loadings while only constant amplitude loadings were considered in the above cited works. The second one is an extension to an incremental fatigue life assessment method for proportional and nonproportional multiaxial variable amplitude loadings. No cycle counting technique is needed whatever the variable amplitude load ings type considered (uniaxial or multiaxial). The predictions of the method for constant and variable amplitude multiaxial loadings are compared with experimental results on specimens from literature and from new experiments on a ferritoperlitic steel. The above mentioned method has been implemented as a postprocessor of a finite element software. An application to a railway wheel is finally presented.
Tue, 01 Jan 2013 00:00:00 GMT
http://hdl.handle.net/10985/7414
20130101T00:00:00Z
SAINTIER, Nicolas
PALINLUC, Thierry
BÉNABÈS, Jérôme
COCHETEUX, Francis
Reliable design of industrial components against high cycle multiaxial fatigue requires a model capable of predicting both stress gradient and load type effects. Indeed, taking into account gradient effects is of prior importance for the applicability of fatigue models to real structures. In this paper, a fatigue life assessment method is proposed for proportional and nonproportional multiaxial variable amplitude loadings in the range 104 –107 cycles. This method derives from the fatigue criterion initially proposed by PalinLuc and Lasserre (1998) [2] and revisited by Banvillet et al. (2003) [16] for multiaxial constant amplitude loading. The new proposal consists of a complete reformulation and extension of the previ ously cited energy based fatigue strength criteria. It includes two major improvements of the existing criteria. The first one consists in a fatigue criterion for multiaxial variable amplitude loadings while only constant amplitude loadings were considered in the above cited works. The second one is an extension to an incremental fatigue life assessment method for proportional and nonproportional multiaxial variable amplitude loadings. No cycle counting technique is needed whatever the variable amplitude load ings type considered (uniaxial or multiaxial). The predictions of the method for constant and variable amplitude multiaxial loadings are compared with experimental results on specimens from literature and from new experiments on a ferritoperlitic steel. The above mentioned method has been implemented as a postprocessor of a finite element software. An application to a railway wheel is finally presented.

Statistical assessment of multiaxial HCF criteria at the grain scale
http://hdl.handle.net/10985/8394
Statistical assessment of multiaxial HCF criteria at the grain scale
HOR, Anis; SAINTIER, Nicolas; ROBERT, Camille; PALINLUC, Thierry; MOREL, Franck
Multiaxial high cycle fatigue modeling of materials is an issue that concerns many industrial domains (automotive, aerospace, nuclear, etc.) and in which many progress still remains to be achieved. Several approaches exist in the literature: invariants, energy, integral and critical plane approaches all of them having their advantages and drawbacks. These different formulations are usually based on mechanical quantities at the micro or mesoscales using localization schemes and strong assumptions to propose simple analytical forms. This study aims to revisit these formulations using a numerical approach based on crystal plasticity modeling coupled with explicit description of microstructure (morphology and texture) and proposes a statistical procedure for the analyses of numerical results in the HCF context. This work has three steps: First, 2.5D periodic digital microstructures based on a random grain sizes distribution are generated. Second, multiaxial cyclic loading conditions corresponding to the fatigue strength at 106 cycles are applied to these microstructures. Third, the mesoscopic Fatigue Indicator Parameters (FIPs), formulated from the different criteria existing in the literature, are identified using the finite element calculations of the mechanical fields. These mesoscopic FIP show the limits of the original criteria when it comes to applying them at the grain scale. A statistical method based on extreme value probability is used to redefine the thresholds of these criteria. These new thresholds contain the sensitivity of the HCF behavior to microstructure attributes. Finally, the biaxiality and phase shift effects are discussed at the grain scale and the loading paths of some critical grains are analyzed.
Wed, 01 Jan 2014 00:00:00 GMT
http://hdl.handle.net/10985/8394
20140101T00:00:00Z
HOR, Anis
SAINTIER, Nicolas
ROBERT, Camille
PALINLUC, Thierry
MOREL, Franck
Multiaxial high cycle fatigue modeling of materials is an issue that concerns many industrial domains (automotive, aerospace, nuclear, etc.) and in which many progress still remains to be achieved. Several approaches exist in the literature: invariants, energy, integral and critical plane approaches all of them having their advantages and drawbacks. These different formulations are usually based on mechanical quantities at the micro or mesoscales using localization schemes and strong assumptions to propose simple analytical forms. This study aims to revisit these formulations using a numerical approach based on crystal plasticity modeling coupled with explicit description of microstructure (morphology and texture) and proposes a statistical procedure for the analyses of numerical results in the HCF context. This work has three steps: First, 2.5D periodic digital microstructures based on a random grain sizes distribution are generated. Second, multiaxial cyclic loading conditions corresponding to the fatigue strength at 106 cycles are applied to these microstructures. Third, the mesoscopic Fatigue Indicator Parameters (FIPs), formulated from the different criteria existing in the literature, are identified using the finite element calculations of the mechanical fields. These mesoscopic FIP show the limits of the original criteria when it comes to applying them at the grain scale. A statistical method based on extreme value probability is used to redefine the thresholds of these criteria. These new thresholds contain the sensitivity of the HCF behavior to microstructure attributes. Finally, the biaxiality and phase shift effects are discussed at the grain scale and the loading paths of some critical grains are analyzed.

HighCycle Fatigue Behaviour of Pure Tantalum under Multiaxial and Variable Amplitude Loadings
http://hdl.handle.net/10985/8397
HighCycle Fatigue Behaviour of Pure Tantalum under Multiaxial and Variable Amplitude Loadings
MARECHAL, David; SAINTIER, Nicolas; PALINLUC, Thierry; NADAL, François
Due to its specific mechanical properties, tantalum is often used in strengthdemanding military applications. Highcycle fatigue (HCF) behaviour of pure tantalum, however, has been rarely reported and the mechanisms at stake to account for deformation under cyclic loadings are still badly understood. This paper aims at better understanding the fatigue behaviour of tantalum and at clarifying the mechanisms of damage formation encountered under such loadings. HCF experiments performed at room temperature on commerciallypure tantalum are presented. Mean stress effects were investigated in the aim of clarifying the interaction between fatigue and creep. Fracture mechanisms were observed to vary from intergranular to transgranular depending on applied stress amplitude and mean stress. Damage mechanisms were investigated under tension and torsion. Results are analyzed in the light of existing fatigue criteria, the limitations of which are discussed. Finally, complex sequential loadings, representative of inservice loadings, were applied to tantalum smooth specimens. The contribution of each loading sequence to the overall damage was quantified and analyzed in terms of linear or nonlinear cumulative damage rule
Wed, 01 Jan 2014 00:00:00 GMT
http://hdl.handle.net/10985/8397
20140101T00:00:00Z
MARECHAL, David
SAINTIER, Nicolas
PALINLUC, Thierry
NADAL, François
Due to its specific mechanical properties, tantalum is often used in strengthdemanding military applications. Highcycle fatigue (HCF) behaviour of pure tantalum, however, has been rarely reported and the mechanisms at stake to account for deformation under cyclic loadings are still badly understood. This paper aims at better understanding the fatigue behaviour of tantalum and at clarifying the mechanisms of damage formation encountered under such loadings. HCF experiments performed at room temperature on commerciallypure tantalum are presented. Mean stress effects were investigated in the aim of clarifying the interaction between fatigue and creep. Fracture mechanisms were observed to vary from intergranular to transgranular depending on applied stress amplitude and mean stress. Damage mechanisms were investigated under tension and torsion. Results are analyzed in the light of existing fatigue criteria, the limitations of which are discussed. Finally, complex sequential loadings, representative of inservice loadings, were applied to tantalum smooth specimens. The contribution of each loading sequence to the overall damage was quantified and analyzed in terms of linear or nonlinear cumulative damage rule