Stéphanie Deschanel - Academia.edu (original) (raw)

Papers by Stéphanie Deschanel

HAL (Le Centre pour la Communication Scientifique Directe), Sep 13, 2022

La fatigue, c'est-a-dire la rupture de structures mecaniques sous l'effet de chargements ... more La fatigue, c'est-a-dire la rupture de structures mecaniques sous l'effet de chargements cycliques, demeure une gageure technologique considerable, car elle survient de facon inattendue lorsque la structure fonctionne apparemment dans un regime stabilise et sur, sans signe exterieur de deterioration mecanique. Apres avoir presente ce contexte et les methodes classiques de controle de la fatigue par emission acoustique (EA), une nouvelle methode non destructive de detection specifique de la propagation de fissures de fatigue est exposee a partir de multiplets acoustiques : signaux d’EA repetitifs, aux formes d'ondes quasi identiques. Des perspectives de controle en service sont ensuite proposees.

HAL (Le Centre pour la Communication Scientifique Directe), Jun 15, 2011

Cet article porte sur la caractérisation de l'endommagement par émission acoustique d'un matériau... more Cet article porte sur la caractérisation de l'endommagement par émission acoustique d'un matériau composite carbone / polyamide 12 (PA12) à fibres longues. L'objectif de ce travail est de discriminer en temps réel les différents types d'endommagement au sein du composite afin de mieux comprendre les mécanismes mis en jeu qui conduisent à la ruine de la structure. Une technique de classification supervisée a été utilisée. L'intérêt de cette technique est de pouvoir établir à partir d'une bibliothèque de signaux déjà labellisés une classification et une labellisation automatique des signaux en temps réel.

NUMERICAL ANALYSIS AND APPLIED MATHEMATICS ICNAAM 2011: International Conference on Numerical Analysis and Applied Mathematics, 2011

We present a study of collective dislocation dynamics and plasticity during fatigue of pure Alumi... more We present a study of collective dislocation dynamics and plasticity during fatigue of pure Aluminum from the analysis of continuous and discrete acoustic emission (AE). The three stages of macroscopic fatigue behavior (strainhardening, shakedown, and strain softening) are clearly differentiated in terms of AE. During the first loading cycles, collective dislocation dynamics consists in dislocation avalanches of various sizes and clustered in time. Once a microstructure of dislocation cells and walls is formed, the spreading of such avalanches is restrained, and the discrete AE strongly decreases. Instead, a symmetrical (tension-compression) continuous AE, maximal at plastic yield, is observed, likely associated to a superposition of numerous, small and uncorrelated motions such as dislocation loops initiation from cell walls. However, some discrete AE activity remains during shakedown, a possible signature of sudden rearrangements of the microstructure occurring at scales larger than its wavelength. Finally, the onset of strain softening is associated to a strong increase of discrete AE, in relation with microcracking. Our results suggest that collective dislocation instabilities and the emergence of a dislocation microstructure are interrelated, and challenge future numerical modeling developments of dislocation assemblies.

International Journal of Fracture, Jul 1, 2006

We report tensile failure experiments on polyurethane (PU) foams. Experiments have been performed... more We report tensile failure experiments on polyurethane (PU) foams. Experiments have been performed by imposing a constant strain rate. We work on heterogeneous materials for whom the failure does not occur suddenly and can develop as a multistep process through a succession of microcracks that end at pores. The acoustic energy and the waiting times between acoustic events follow power-law distributions. This remains true while the foam density is varied. However, experiments at low temperatures (PU foams more brittle) have not yielded power-laws for the waiting times. The cumulative acoustic energy has no power law divergence at the proximity of the failure point which is qualitatively in agreement with other experiments done at imposed strain. We notice a plateau in cumulative acoustic energy that seems to occur when a single crack starts to propagate.

HAL (Le Centre pour la Communication Scientifique Directe), 2007

International audienceno abstrac

International Journal of Fatigue, Jun 1, 2023

Polymer, Apr 1, 2007

The large deformation stressestrain behavior of thermoplasticeelastomeric polyurethanes and elast... more The large deformation stressestrain behavior of thermoplasticeelastomeric polyurethanes and elastomeric-thermoset polyureas is strongly dependent on strain rate. Their mechanical behavior at very high strain rates is of particular interest due to their role as a protective coating on structures to enhance structural survivability during high rate loading events. Here we report on the uniaxial compression stressestrain behavior of a representative polyurea and a representative polyurethane over a wide range in strain rates, from 0.001 s À1 to 10,000 s À1 , successively marching through each order of magnitude in strain rate using equipment relevant for testing at each particular rate. These results are further analyzed in association with recently reported compressive data on the same materials by Yi et al. [Polymer 2006;47(1):319e29] and intermediate rate tensile data on the same polyurea by Roland et al. [Polymer 2007;48(2):574e8]. The polyurea tested is seen to undergo transition from a rubbery-regime behavior at low rates to a leathery-regime behavior at the highest rates, consistent with the earlier compression study as well as the recent tension study; the polyurethane tested is observed to undergo transition from a rubbery-regime behavior at the low rates to a glassy behavior at the highest rates. The uniaxial compression data for the polyurea are found to be fully consistent with the recently reported uniaxial tension data over the range of rates studied, demonstrating the consistency and complementary aspects of testing at high rates in both compression and tension.

e-Journal of Nondestructive Testing, 2023

Strength hardened aluminum alloys as 7075-T6 al are widely used in aeronautical industry due to t... more Strength hardened aluminum alloys as 7075-T6 al are widely used in aeronautical industry due to their low density and high mechanical properties. Their specific microstructural composition makes them sensitive to corrosion damage at intermetallic particles. Aeronautical parts are often submitted to corrosive environments and fatigue loadings. For safety and economic matters, it is important to be able to monitor the development of damages during application. In this context, Acoustic Emission (AE) is a useful tool to monitor the state of material damage and predict its remaining useful lifetime. This work, which is part of a larger European project (Early detection and progress monitoring and prediction of corrosion in aeronautic Al alloys through calibrated Ultrasonic-CorROSion Sensor application), has for main objective to understand, identify and quantify, via AE, how corrosion defects impact the fatigue behaviour of aluminum alloy 7075-T6 specimens and covered with different types of coatings (top coat, primer and a conversion coating obtained by anodizing process). Therefore, tensile-tensile fatigue tests (R = 0.1) monitored with AE are performed at room temperature on non-corroded samples and on corroded samples. Precorrosion defects are generated by the complete immersion of the samples in an NaCl bath (3.5% wt). Pre-corrosion defects tend to decrease the fatigue lifetime of the material tested and create damage in the substrate and coatings generating new AE sources. For all types of coated specimens, damage indicators based on the AE activity are studied in order to find characteristic damage times giving information on the remaining useful lifetime of the material during fatigue tests. Characteristic times linked to damage initiation in the substrate and propagation the main fatigue crack in the material are defined.

e-Journal of Nondestructive Testing, 2023

HAL (Le Centre pour la Communication Scientifique Directe), Aug 29, 2022

Des signaux d'émission acoustique (EA) spécifiques à l'avancée incrémentale de fissure de fatigue... more Des signaux d'émission acoustique (EA) spécifiques à l'avancée incrémentale de fissure de fatigue peuvent etre détectés, cycles après cycles lors d'essais de propagation de fissure dans les métaux : ce sont des multiplets acoustiques. Afin d'en savoir plus sur les mécanismes physiques à leur source, une analyse de corrélation d'images numériques (DIC) couplée à l'EA est menée dans cette étude. Il est montré que ce type d'analyse est efficace pour distinguer les multiplets issus de 2 catégories de sources : la propagation de fissure par rupture ou plasticité en pointe de fissure et le frottement des lèvres contre elles-meme.

e-Journal of Nondestructive Testing, 2023

Acoustic emission signals specific to the incremental advance of fatigue cracks can be detected, ... more Acoustic emission signals specific to the incremental advance of fatigue cracks can be detected, cycle after cycle, during crack propagation tests. They were termed acoustic multiplets in reference to repeating earthquakes. Detecting such multiplets would give information about the fracturing process and provide early warnings when monitoring industrial parts in service. We developed a clustering method to extract multiplets signals from all signals and noise of fatigue tests. This method is based on the computation of the cross-correlation function between waveforms. On the distribution of the cross-correlation matrix, signals belonging to multiplets are clearly identified and permit to automatically clustered. This paper develops the steps of the method, presents results obtained on a dataset of a fatigue test and shows its noise robustness.

HAL (Le Centre pour la Communication Scientifique Directe), 2008

Matériaux et techniques, 2009

ABSTRACT In this article, we present the possibility to identify in real time the acoustic signat... more ABSTRACT In this article, we present the possibility to identify in real time the acoustic signature of damage mechanisms occurring in a structure from the monitoring of acoustic emission and a multivariate statistical analysis. Two examples of application are given, one to long-fiber-reinforced organic matrix composites, the other to metastable metallic alloys. Finally, we briefly discuss the access to the lifetime prediction of a structure from the acoustic emission data. Dans cet article, nous allons présenter la possibilité d'identifier en temps réel la signature acoustique des mécanismes endommageants se produisant dans une structure en service, à partir d'un suivi par émission acoustique et d'une analyse statistique multivariable. Nous donnerons deux exemples d'application, l'un aux composites à matrice organique renforcés par des fibres longues, l'autre aux alliages métallique métastables. Enfin, nous évoquerons brièvement l'approche de la prévision de la durée de vie d'une structure à partir des données d'émission acoustique.

Materialia, Dec 1, 2019

Crystalline plasticity can take place through numerous, small, uncorrelated dislocation motions (... more Crystalline plasticity can take place through numerous, small, uncorrelated dislocation motions (mild plasticity) or through collaborative events: dislocation avalanches (wild plasticity). Here, we study the correlation between dislocation patterning under cyclic loading and the nature of collective dislocation dynamics. The dislocation motion of a [110] oriented pure copper single crystal was dynamically followed using Acoustic Emission (AE) for different imposed stress amplitudes. The dislocation structure between each cyclic stress step was investigated using Electron BackScattered Diffraction (EBSD) and Rotational-Electron Channeling Contrast Imaging (R-ECCI) in a Scanning Electron Microscope (SEM). At low imposed stress, when the structure consists of dislocation cells, few dislocation avalanches are observed, while for a wall structure, at higher imposed stress, the contribution of avalanches is increased during the first cycles. For a given stress amplitude, the evolution of mild plasticity is synchronous with the plastic strain-rate, and rapidly vanishes after few cycles due to work hardening. The mean free path of the dislocations in this mild plasticity regime corresponds to the characteristic size of the dislocation structure (cell size, distance between walls). From one stress level to another, brutal rearrangements of the dislocation structure occur within a few numbers of cycles. Those rearrangements take place, at least partly, through dislocation avalanches. Upon reloading at a larger stress amplitude, dislocation avalanches can travel over distances much larger than the former dislocation mean free path. As the dislocation avalanches spread within the crystal, the memory of the previous dislocation structure is lost and a new dislocation structure emerges.

Polymer, Aug 1, 2017

Among the micro-mechanisms and damage that can simultaneously occur during the plastic deformatio... more Among the micro-mechanisms and damage that can simultaneously occur during the plastic deformation of Polyethylene one can quote shear crystallites, fragmentation of crystalline lamellae, cavitation or martensitic transformation. Distinctly, the determination of the initiation of plasticity and damage within this kind of polymers remains questionable. The aim of this study is to characterize the plasticity and damage of several Polyethylene (PE) during tensile tests using the ultrasonic (US) monitoring technique. The proposed methodology uses both guided and longitudinal waves in the ultrasonic frequency range and enables to separate the geometrical effects from those of the material. It is shown that the US attenuation increases when the degree of crystallinity decreases. Besides, the US attenuation appears to be higher in the amorphous phase than in the crystal. During a tensile test, a strong decrease of the transmitted energy is observed once the yield point is reached, due to the formation of the fibrillar microstructure, which breaks the crystalline percolation. Finally, the results evidence that the chain alignment during a tensile test favors the wave propagation; in addition, cavitation induces a significant attenuation, which is strongly anisotropic as the voids are aligned with the fibrils.

Physical Review Materials, Feb 25, 2019

In metallic materials subjected to cyclic loading, strain hardening as well as fatigue crack init... more In metallic materials subjected to cyclic loading, strain hardening as well as fatigue crack initiation have been linked for a long time with the evolution of dislocation patterns and structures. In particular, the development of low-energy dislocation configurations such as persistent slip bands (PSBs) is considered as a precursor to crack initiation. However, the associated scenarios have been elaborated mainly from postmortem observations capturing only static pictures of dislocation patterns, while the dynamics of the problem has been somewhat overlooked. Here we analyze collective dislocation dynamics during cycling loading of aluminum using acoustic emission (AE). A strong link is revealed between dislocation patterning, cyclic hardening/softening, and the intermittency of plasticity: Plastic intermittency and dislocation avalanches rapidly decay during the initial hardening stage, in conjunction with the reduction of an internal length scale characterizing the dislocation structure. However, in nonannealed samples, a transient softening stage ensues, associated with a brutal reorganization of this structure. These initial stages of cyclic deformation illustrate the competition between two phenomena: collective dislocation dynamics, governed by long-ranged elastic interactions among dislocations, and the emergence of a self-organizing network controlled by short-range interactions and progressively inhibiting collective effects. Later on, the emergence of PSBs is accompanied by a reincrease of the AE intermittent activity. We propose that the associated AE bursts may be the signature of collective and coordinated dislocation motions along PSBs leading to the formation of incipient microcracks.

HAL (Le Centre pour la Communication Scientifique Directe), Sep 13, 2022

La fatigue, c'est-a-dire la rupture de structures mecaniques sous l'effet de chargements ... more La fatigue, c'est-a-dire la rupture de structures mecaniques sous l'effet de chargements cycliques, demeure une gageure technologique considerable, car elle survient de facon inattendue lorsque la structure fonctionne apparemment dans un regime stabilise et sur, sans signe exterieur de deterioration mecanique. Apres avoir presente ce contexte et les methodes classiques de controle de la fatigue par emission acoustique (EA), une nouvelle methode non destructive de detection specifique de la propagation de fissures de fatigue est exposee a partir de multiplets acoustiques : signaux d’EA repetitifs, aux formes d'ondes quasi identiques. Des perspectives de controle en service sont ensuite proposees.

HAL (Le Centre pour la Communication Scientifique Directe), Jun 15, 2011

Cet article porte sur la caractérisation de l'endommagement par émission acoustique d'un matériau... more Cet article porte sur la caractérisation de l'endommagement par émission acoustique d'un matériau composite carbone / polyamide 12 (PA12) à fibres longues. L'objectif de ce travail est de discriminer en temps réel les différents types d'endommagement au sein du composite afin de mieux comprendre les mécanismes mis en jeu qui conduisent à la ruine de la structure. Une technique de classification supervisée a été utilisée. L'intérêt de cette technique est de pouvoir établir à partir d'une bibliothèque de signaux déjà labellisés une classification et une labellisation automatique des signaux en temps réel.

NUMERICAL ANALYSIS AND APPLIED MATHEMATICS ICNAAM 2011: International Conference on Numerical Analysis and Applied Mathematics, 2011

We present a study of collective dislocation dynamics and plasticity during fatigue of pure Alumi... more We present a study of collective dislocation dynamics and plasticity during fatigue of pure Aluminum from the analysis of continuous and discrete acoustic emission (AE). The three stages of macroscopic fatigue behavior (strainhardening, shakedown, and strain softening) are clearly differentiated in terms of AE. During the first loading cycles, collective dislocation dynamics consists in dislocation avalanches of various sizes and clustered in time. Once a microstructure of dislocation cells and walls is formed, the spreading of such avalanches is restrained, and the discrete AE strongly decreases. Instead, a symmetrical (tension-compression) continuous AE, maximal at plastic yield, is observed, likely associated to a superposition of numerous, small and uncorrelated motions such as dislocation loops initiation from cell walls. However, some discrete AE activity remains during shakedown, a possible signature of sudden rearrangements of the microstructure occurring at scales larger than its wavelength. Finally, the onset of strain softening is associated to a strong increase of discrete AE, in relation with microcracking. Our results suggest that collective dislocation instabilities and the emergence of a dislocation microstructure are interrelated, and challenge future numerical modeling developments of dislocation assemblies.

International Journal of Fracture, Jul 1, 2006

We report tensile failure experiments on polyurethane (PU) foams. Experiments have been performed... more We report tensile failure experiments on polyurethane (PU) foams. Experiments have been performed by imposing a constant strain rate. We work on heterogeneous materials for whom the failure does not occur suddenly and can develop as a multistep process through a succession of microcracks that end at pores. The acoustic energy and the waiting times between acoustic events follow power-law distributions. This remains true while the foam density is varied. However, experiments at low temperatures (PU foams more brittle) have not yielded power-laws for the waiting times. The cumulative acoustic energy has no power law divergence at the proximity of the failure point which is qualitatively in agreement with other experiments done at imposed strain. We notice a plateau in cumulative acoustic energy that seems to occur when a single crack starts to propagate.

HAL (Le Centre pour la Communication Scientifique Directe), 2007

International audienceno abstrac

International Journal of Fatigue, Jun 1, 2023

Polymer, Apr 1, 2007

The large deformation stressestrain behavior of thermoplasticeelastomeric polyurethanes and elast... more The large deformation stressestrain behavior of thermoplasticeelastomeric polyurethanes and elastomeric-thermoset polyureas is strongly dependent on strain rate. Their mechanical behavior at very high strain rates is of particular interest due to their role as a protective coating on structures to enhance structural survivability during high rate loading events. Here we report on the uniaxial compression stressestrain behavior of a representative polyurea and a representative polyurethane over a wide range in strain rates, from 0.001 s À1 to 10,000 s À1 , successively marching through each order of magnitude in strain rate using equipment relevant for testing at each particular rate. These results are further analyzed in association with recently reported compressive data on the same materials by Yi et al. [Polymer 2006;47(1):319e29] and intermediate rate tensile data on the same polyurea by Roland et al. [Polymer 2007;48(2):574e8]. The polyurea tested is seen to undergo transition from a rubbery-regime behavior at low rates to a leathery-regime behavior at the highest rates, consistent with the earlier compression study as well as the recent tension study; the polyurethane tested is observed to undergo transition from a rubbery-regime behavior at the low rates to a glassy behavior at the highest rates. The uniaxial compression data for the polyurea are found to be fully consistent with the recently reported uniaxial tension data over the range of rates studied, demonstrating the consistency and complementary aspects of testing at high rates in both compression and tension.

e-Journal of Nondestructive Testing, 2023

Strength hardened aluminum alloys as 7075-T6 al are widely used in aeronautical industry due to t... more Strength hardened aluminum alloys as 7075-T6 al are widely used in aeronautical industry due to their low density and high mechanical properties. Their specific microstructural composition makes them sensitive to corrosion damage at intermetallic particles. Aeronautical parts are often submitted to corrosive environments and fatigue loadings. For safety and economic matters, it is important to be able to monitor the development of damages during application. In this context, Acoustic Emission (AE) is a useful tool to monitor the state of material damage and predict its remaining useful lifetime. This work, which is part of a larger European project (Early detection and progress monitoring and prediction of corrosion in aeronautic Al alloys through calibrated Ultrasonic-CorROSion Sensor application), has for main objective to understand, identify and quantify, via AE, how corrosion defects impact the fatigue behaviour of aluminum alloy 7075-T6 specimens and covered with different types of coatings (top coat, primer and a conversion coating obtained by anodizing process). Therefore, tensile-tensile fatigue tests (R = 0.1) monitored with AE are performed at room temperature on non-corroded samples and on corroded samples. Precorrosion defects are generated by the complete immersion of the samples in an NaCl bath (3.5% wt). Pre-corrosion defects tend to decrease the fatigue lifetime of the material tested and create damage in the substrate and coatings generating new AE sources. For all types of coated specimens, damage indicators based on the AE activity are studied in order to find characteristic damage times giving information on the remaining useful lifetime of the material during fatigue tests. Characteristic times linked to damage initiation in the substrate and propagation the main fatigue crack in the material are defined.

e-Journal of Nondestructive Testing, 2023

HAL (Le Centre pour la Communication Scientifique Directe), Aug 29, 2022

Des signaux d'émission acoustique (EA) spécifiques à l'avancée incrémentale de fissure de fatigue... more Des signaux d'émission acoustique (EA) spécifiques à l'avancée incrémentale de fissure de fatigue peuvent etre détectés, cycles après cycles lors d'essais de propagation de fissure dans les métaux : ce sont des multiplets acoustiques. Afin d'en savoir plus sur les mécanismes physiques à leur source, une analyse de corrélation d'images numériques (DIC) couplée à l'EA est menée dans cette étude. Il est montré que ce type d'analyse est efficace pour distinguer les multiplets issus de 2 catégories de sources : la propagation de fissure par rupture ou plasticité en pointe de fissure et le frottement des lèvres contre elles-meme.

e-Journal of Nondestructive Testing, 2023

Acoustic emission signals specific to the incremental advance of fatigue cracks can be detected, ... more Acoustic emission signals specific to the incremental advance of fatigue cracks can be detected, cycle after cycle, during crack propagation tests. They were termed acoustic multiplets in reference to repeating earthquakes. Detecting such multiplets would give information about the fracturing process and provide early warnings when monitoring industrial parts in service. We developed a clustering method to extract multiplets signals from all signals and noise of fatigue tests. This method is based on the computation of the cross-correlation function between waveforms. On the distribution of the cross-correlation matrix, signals belonging to multiplets are clearly identified and permit to automatically clustered. This paper develops the steps of the method, presents results obtained on a dataset of a fatigue test and shows its noise robustness.

HAL (Le Centre pour la Communication Scientifique Directe), 2008

Matériaux et techniques, 2009

ABSTRACT In this article, we present the possibility to identify in real time the acoustic signat... more ABSTRACT In this article, we present the possibility to identify in real time the acoustic signature of damage mechanisms occurring in a structure from the monitoring of acoustic emission and a multivariate statistical analysis. Two examples of application are given, one to long-fiber-reinforced organic matrix composites, the other to metastable metallic alloys. Finally, we briefly discuss the access to the lifetime prediction of a structure from the acoustic emission data. Dans cet article, nous allons présenter la possibilité d'identifier en temps réel la signature acoustique des mécanismes endommageants se produisant dans une structure en service, à partir d'un suivi par émission acoustique et d'une analyse statistique multivariable. Nous donnerons deux exemples d'application, l'un aux composites à matrice organique renforcés par des fibres longues, l'autre aux alliages métallique métastables. Enfin, nous évoquerons brièvement l'approche de la prévision de la durée de vie d'une structure à partir des données d'émission acoustique.

Materialia, Dec 1, 2019

Crystalline plasticity can take place through numerous, small, uncorrelated dislocation motions (... more Crystalline plasticity can take place through numerous, small, uncorrelated dislocation motions (mild plasticity) or through collaborative events: dislocation avalanches (wild plasticity). Here, we study the correlation between dislocation patterning under cyclic loading and the nature of collective dislocation dynamics. The dislocation motion of a [110] oriented pure copper single crystal was dynamically followed using Acoustic Emission (AE) for different imposed stress amplitudes. The dislocation structure between each cyclic stress step was investigated using Electron BackScattered Diffraction (EBSD) and Rotational-Electron Channeling Contrast Imaging (R-ECCI) in a Scanning Electron Microscope (SEM). At low imposed stress, when the structure consists of dislocation cells, few dislocation avalanches are observed, while for a wall structure, at higher imposed stress, the contribution of avalanches is increased during the first cycles. For a given stress amplitude, the evolution of mild plasticity is synchronous with the plastic strain-rate, and rapidly vanishes after few cycles due to work hardening. The mean free path of the dislocations in this mild plasticity regime corresponds to the characteristic size of the dislocation structure (cell size, distance between walls). From one stress level to another, brutal rearrangements of the dislocation structure occur within a few numbers of cycles. Those rearrangements take place, at least partly, through dislocation avalanches. Upon reloading at a larger stress amplitude, dislocation avalanches can travel over distances much larger than the former dislocation mean free path. As the dislocation avalanches spread within the crystal, the memory of the previous dislocation structure is lost and a new dislocation structure emerges.

Polymer, Aug 1, 2017

Among the micro-mechanisms and damage that can simultaneously occur during the plastic deformatio... more Among the micro-mechanisms and damage that can simultaneously occur during the plastic deformation of Polyethylene one can quote shear crystallites, fragmentation of crystalline lamellae, cavitation or martensitic transformation. Distinctly, the determination of the initiation of plasticity and damage within this kind of polymers remains questionable. The aim of this study is to characterize the plasticity and damage of several Polyethylene (PE) during tensile tests using the ultrasonic (US) monitoring technique. The proposed methodology uses both guided and longitudinal waves in the ultrasonic frequency range and enables to separate the geometrical effects from those of the material. It is shown that the US attenuation increases when the degree of crystallinity decreases. Besides, the US attenuation appears to be higher in the amorphous phase than in the crystal. During a tensile test, a strong decrease of the transmitted energy is observed once the yield point is reached, due to the formation of the fibrillar microstructure, which breaks the crystalline percolation. Finally, the results evidence that the chain alignment during a tensile test favors the wave propagation; in addition, cavitation induces a significant attenuation, which is strongly anisotropic as the voids are aligned with the fibrils.

Physical Review Materials, Feb 25, 2019

In metallic materials subjected to cyclic loading, strain hardening as well as fatigue crack init... more In metallic materials subjected to cyclic loading, strain hardening as well as fatigue crack initiation have been linked for a long time with the evolution of dislocation patterns and structures. In particular, the development of low-energy dislocation configurations such as persistent slip bands (PSBs) is considered as a precursor to crack initiation. However, the associated scenarios have been elaborated mainly from postmortem observations capturing only static pictures of dislocation patterns, while the dynamics of the problem has been somewhat overlooked. Here we analyze collective dislocation dynamics during cycling loading of aluminum using acoustic emission (AE). A strong link is revealed between dislocation patterning, cyclic hardening/softening, and the intermittency of plasticity: Plastic intermittency and dislocation avalanches rapidly decay during the initial hardening stage, in conjunction with the reduction of an internal length scale characterizing the dislocation structure. However, in nonannealed samples, a transient softening stage ensues, associated with a brutal reorganization of this structure. These initial stages of cyclic deformation illustrate the competition between two phenomena: collective dislocation dynamics, governed by long-ranged elastic interactions among dislocations, and the emergence of a self-organizing network controlled by short-range interactions and progressively inhibiting collective effects. Later on, the emergence of PSBs is accompanied by a reincrease of the AE intermittent activity. We propose that the associated AE bursts may be the signature of collective and coordinated dislocation motions along PSBs leading to the formation of incipient microcracks.