Statistical properties of microcracking in polyurethane foams under tensile test, influence of temperature and density

Abstract 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

Dynamique des précurseurs de la rupture des matériaux hétérogènes : application aux mousses polymères vitreuses

De nouvelle approches physiques concernant les mécanismes d\u27endommagement consistent à considérer le phénomène de rupture comme le point critique d\u27une transition de phase. La rupture peut alors se traduire pour certains matériaux par une percolation de microfissures. Cette multi-fissuration implique le choix de matériaux hétérogènes. Des essais mécaniques sur mousses polymères solides sont conduits jusqu\u27à rupture, couplés au suivi de l\u27activité acoustique. Les distributions des énergies révèlent des lois de puissance indépendamment de la densité du matériau, du mode de chargement ou des lois de comportement. En revanche, la conformité à une loi de puissance des intervalles de temps semble exiger une contrainte quasi constante sur la plus grande partie de l\u27essai. l\u27allure de l\u27énergie cumulée dans le cas d\u27essais de fluage semblerait présenter une loi de puissance sur une plage de temps restreinte. En revanche, pour les essais de traction, aucune loi de puissance n\u27est observée

Acoustic Emission: identification of the acoustic signature of damage mechanisms and lifetime prediction

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Acoustic Emission: Identification of the acoustic signature of damage mechanisms and lifetime prediction [Émission acoustique: Identification de la signature acoustique des mécanismes d'endommagement et prévision de la durée de vie]

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Émission acoustique : identification de la signature acoustique des mécanismes d'endommagement et prévision de la durée de vie

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Collective Dislocation Dynamics and Avalanches during Fatigue of Aluminum

International audienceWe 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 (strain-hardening, 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

Acoustic emission multiplets as early warnings of fatigue failure in metallic materials

cited By 0International audienceFatigue, i.e. the failure of mechanical structures under cycling loading, remains a considerable technological challenge as it occurs unexpectedly when the structure is operating apparently in a safe and steady state regime, without external signs of mechanical deterioration. Here we report for the first time, in different metallic materials, the detection of acoustic emissions specific of fatigue crack growth. These so-called acoustic multiplets are characterized by nearly identical waveforms, signature of a unique source, are repeatedly triggered over many successive loading cycles at the same stress level, and originate from a single location. They mark the slow, incremental propagation of a fatigue crack at each cycle, or the rubbing along its faces. Being specific to fatigue cracking, they can be used as early warnings of crack propagation, which will ultimately lead to structural failure. Their detection and characterization thus open the way towards a new, reliable monitoring of the onset of fatigue cracking during mechanical tests or within structures in service. © 2017 The Author(s)

Etude par émission acoustique de la plasticité et de l'endommagement de l'aluminium en fatigue oligocyclique

Un suivi des processus microstructuraux prenant place au cours de la fatigue oligocyclique de l aluminium pur est assuré par la technique d émission acoustique EA par ces deux types: émission continue et discrète. Cette technique est intéressante car elle permet de suivre l évolution dynamique de la structure tout le long de l essai. Les différents stades du comportement macroscopique du matériau au cours des sollicitations cycliques sont clairement différenciés par l activité acoustique. Nous distinguons cinq stades : écrouissage primaire, adoucissement primaire, écrouissage secondaire, adoucissement secondaire et rupture. Les trois premiers stades mettent en jeu des phénomènes microstructuraux liés à la plasticité du matériau tandis que des phénomènes relatifs à l endommagement (micro et macro-fissuration) dominent les derniers stades. L EA continue résulte de l effet cumulatif de nombreux mouvements de dislocations de faible amplitude et décorrélés entre eux. Cette plasticité continue diminue au cours du 1er stade mais copie l évolution de la réponse macroscopique de l échantillon au cours des stades suivants. Ce comportement est lié aux structures de dislocations établies à travers les différents stades de fatigue. En revanche, l EA de type discret enregistrée lors des trois premiers stades est associée à un autre type de plasticité : la plasticité intermittente, se manifestant à travers des mouvements coopératifs de grande ampleur, les avalanches de dislocations. Ces avalanches de dislocations génèrent des signaux acoustiques de tailles variables, distribuées en loi de puissance. La plasticité intermittente est alors invariante d échelle tandis que la plasticité continue met en jeu des mouvements ayant une taille caractéristique. Nous mettons ainsi en évidence pour la première fois la coexistence de ces deux types de plasticité dans un matériau cubique à faces centrées CFC, qui ne sont donc pas incompatibles. Au cours des deux derniers stades de fatigue, les signaux acoustiques enregistrés se catégorisent également en deux groupes: l un est caractérisé par des invariances d échelle, l autre associé à une taille caractéristique. La première catégorie comprend des signaux acoustiques indépendants, apparaissant aléatoirement au cours des cycles. Ces signaux sont générés par des phénomènes de microfissuration au sein du volume de l échantillon (nucléation, percolation ). Le second groupe, réunit des signaux acoustiques générés quasiment au même niveau de contrainte sur plusieurs cycles successifs et ayant une signature acoustique quasi identique. Nous nommons ces signaux multiplets en référence à la sismologie. Nous émettons l'hypothèse que de tels multiplets d EA sont la signature de la propagation, cycle après cycle, d'une fissure de fatigue dont la trace peut être vu post-mortem avec les stries de fatigue sur une surface de fracture, ou encore la signature de frottements entre les aspérités présentes de part et d autre des lèvres de fissures.An analysis of microstructural processes taking place during low-cycle fatigue of pure aluminum is performed by the Acoustic Emission technique (AE) with its two types: continuous and discrete. The main interest of this technique is that it enables the following of the dynamic evolution of the microstructure during the fatigue test. We distinguished five fatigue stages: primary hardening, primary softening, secondary hardening, secondary softening and failure. The various stages of the material s macroscopic behavior during cyclic loading are clearly differentiated by the acoustic activity. During the first three stages, mainly microstructural phenomena related to plasticity of material are taking place, whereas damage (micro and macro-cracking) dominate the last two stages. The continuous AE results from the cumulative effect of many uncorrelated dislocations movements of low amplitude. This continuous plasticity decreases during the 1st stage but reproduces the evolution of the macroscopic behavior of the sample during following stages. This behavior is related to the dislocation structure established during the various fatigue stages. On the other hand, the discrete AE recorded at the time of the first three stages is associated to another type of plasticity: intermittent plasticity. This plasticity is associated to co-operative dislocation movements of great amplitude; dislocation avalanches. These dislocation avalanches generate acoustic signals power law distributed in amplitude and energies. Intermittent plasticity is then scale invariant while continuous plasticity is associated to dislocation movements with a characteristic size. We highlight for the first time the coexistence of these two types of plasticity in FCC materials, which are therefore not incompatible. During the last two stages of fatigue, the recorded acoustic signals are categorized in two groups: the first one is characterized by scale invariance whereas the other is associated to a characteristic size. The first category comprises independent acoustic signals, appearing randomly during cycles. These signals are generated by micro-cracking events within the volume of the sample (nucleation, percolation ). The second group contains acoustic signals generated almost at the same stress level during several successive cycles and having a nearly identical acoustic signature. We name these signals multiplets in reference to seismology. We put forth the hypothesis that such AE multiplets are the signature of fatigue crack propagation, one cycle after the other, whose trace can be observed post-mortem with fatigue striations on fracture surface, or a signature of frictions between the asperities present on both sides of the crack.VILLEURBANNE-DOC'INSA-Bib. elec. (692669901) / SudocSudocFranceF

RATE DEPENDENT MECHANICAL PERFORMANCE OF ETHYLENE METHACRYLIC ACID(EMAA) COPOLYMERS AND POSS‐ENHANCED EMAA NANOCOMPOSITES

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