Ductile to brittle transition concept on fracture behavior of poly(vinylidene fluoride) / poly(methyl methacrylate) blends

International audienceThe fracture behavior of blends of poly(ninylidene fluoride) (PVDF) and poly(methyl methacrylate) (PMMA) was investigated by gradually increasing the PVDF content. The study focuses on semi-crystalline blends. The trends of net stress versus crack opening displacement curves were analyzed. From these plots, two fracture energies were defined: the fracture energy to crack initiation corresponding to the area under the curve up to the maximum net stress and the fracture energy to crack propagation considering the last part of the curve where the load continuously decreases. Fracture surface inspections confirmed typical semi-crystalline polymer features. Critical values of the degree of crystallinity corresponding to brittle to ductile transition were determined, depending on the selected fracture energ

Micromechanisms of fracture propagation in glassy polymers

While most glassy polymers are nominally brittle at macroscopic scales, they are known to exhibit plastic deformation in indentation, scratching, and microcutting when the loaded region is sufficiently small. The same applies to the micrometer size process zone at the tip of a propagating crack. While the presence and approximate size of this microscale plastic zone is well described by the Dugdale model, the prediction of the toughness of these materials is not possible without accounting for the details of the local large strain field and the work hardening behaviour of these polymers, which can be inferred from their response to compressive tests. Strain localization mechanisms such as crazing or shear banding should also be taken into account to properly model toughness. Finally, viscoplastic creep plays a major role in determining the dependence of the toughness on crack propagation velocity, as well as the important difference between the initiation and propagation toughness, which is responsible for the occurrence of a characteristic stick-slip propagation under some loading conditions. Please click Additional Files below to see the full abstract

Fracture propagation in glassy polymers: From nanometer to centimeter

Please click Additional Files below to see the full abstract

Multi-scale analysis of damage mechanisms of composite materials with complex morphology for aircraft

L’étude porte sur la propagation lente de fissure à température ambiante dans deux classes de polymères à l’état vitreux : le PMMA et la résine époxy-amine stœchiométrique DGEBA-IPD comme prototypes respectifs de thermoplastique (TP) et de thermodurcissable (TD). Des mélanges TD/TP présentant une séparation de phase de nano-domaines de thermoplastique dans une phase dominante de thermodurcissable ont été aussi explorés en vue de leur intérêt en tant que matrice à morphologie complexe utilisée dans les composites à renfort de fibre de carbone destinés à l’aéronautique. Un montage expérimental original a été développé pour le suivi in situ de propagation lente de fissures, combinant une caméra optique, un microscope à force atomique (AFM) et un échantillon sollicité en géométrie Double Cleavage Drilled Compression (DCDC). La possibilité d’accéder ainsi à une gamme de vitesses de propagation entre le pm/s et le nm/s a permis de mettre en évidence pour la toute première fois un régime de propagation stationnaire dans les thermodurcissables (résines pures et mélanges TD/TP) analogue au comportement établi pour les thermoplastiques, et de le caractériser en termes d’une loi cinétique reliant la vitesse de propagation au facteur d’intensité des contraintes (SIF). L’analyse in situ AFM a de plus permis de caractériser les différentes modalités de déformation à l’échelle de la zone de process en combinant l’imagerie topographique et des techniques de corrélation d’images numériques (DIC). Un modèle de prédiction des énergies de rupture en propagation stationnaire ainsi qu’en régime de stick-slip a été développé, en intégrant les champs de déformation visco-plastiques mesurés dans la zone de process, la vitesse de déformation locale, et la loi de comportement des matériaux mesurée à l’échelle macroscopique.This work deals with the slow crack growth in glass polymer materials at room temperature, namely PMMA and stoechiometric epoxy-amine DGEBA-IPD resin as archetypes of thermoplastic (TP) and thermoset (TS) respectively. TS/TP blends developing phases separation in nano-domains of thermoplastic during the cure process have also been studied since they are used as matrix with a complex morphology in carbon fibers reinforced composites materials for aircrafts. An innovative experimental setup has been developed for in situ investigation of slow crack growth by combining optical measurements, in situ atomic force microscopy (AFM) and a Double Cleavage Drilled Compression (DCDC) sample. By this way, a steady state crack propagation regime has been highlighted for a first time in thermosets (pure and TS/TP blends) with crack speed ranging from pm/s to nm/s. Thus crack speed and stress intensity factor (SIF) diagrams were established for each material. Furthermore, in situ AFM images allowed characterizing deformations at the process zone scale by combining topographic imaging with Digital Images Correlation (DIC) technique. Finally, we developed a model for predicting fracture energy for steady state propagation and stick-slip regime, based on the visco-plastic strain fields measured round the process zone, the local strain rate, and the constitutive laws of materials measured at the macroscopic scale

Analyse multi-échelle des mécanismes d'endommagement des matériaux composites à morphologie complexe destinés à l'aéronautique

This work deals with the slow crack growth in glass polymer materials at room temperature, namely PMMA and stoechiometric epoxy-amine DGEBA-IPD resin as archetypes of thermoplastic (TP) and thermoset (TS) respectively. TS/TP blends developing phases separation in nano-domains of thermoplastic during the cure process have also been studied since they are used as matrix with a complex morphology in carbon fibers reinforced composites materials for aircrafts. An innovative experimental setup has been developed for in situ investigation of slow crack growth by combining optical measurements, in situ atomic force microscopy (AFM) and a Double Cleavage Drilled Compression (DCDC) sample. By this way, a steady state crack propagation regime has been highlighted for a first time in thermosets (pure and TS/TP blends) with crack speed ranging from pm/s to nm/s. Thus crack speed and stress intensity factor (SIF) diagrams were established for each material. Furthermore, in situ AFM images allowed characterizing deformations at the process zone scale by combining topographic imaging with Digital Images Correlation (DIC) technique. Finally, we developed a model for predicting fracture energy for steady state propagation and stick-slip regime, based on the visco-plastic strain fields measured round the process zone, the local strain rate, and the constitutive laws of materials measured at the macroscopic scale.L’étude porte sur la propagation lente de fissure à température ambiante dans deux classes de polymères à l’état vitreux : le PMMA et la résine époxy-amine stœchiométrique DGEBA-IPD comme prototypes respectifs de thermoplastique (TP) et de thermodurcissable (TD). Des mélanges TD/TP présentant une séparation de phase de nano-domaines de thermoplastique dans une phase dominante de thermodurcissable ont été aussi explorés en vue de leur intérêt en tant que matrice à morphologie complexe utilisée dans les composites à renfort de fibre de carbone destinés à l’aéronautique. Un montage expérimental original a été développé pour le suivi in situ de propagation lente de fissures, combinant une caméra optique, un microscope à force atomique (AFM) et un échantillon sollicité en géométrie Double Cleavage Drilled Compression (DCDC). La possibilité d’accéder ainsi à une gamme de vitesses de propagation entre le pm/s et le nm/s a permis de mettre en évidence pour la toute première fois un régime de propagation stationnaire dans les thermodurcissables (résines pures et mélanges TD/TP) analogue au comportement établi pour les thermoplastiques, et de le caractériser en termes d’une loi cinétique reliant la vitesse de propagation au facteur d’intensité des contraintes (SIF). L’analyse in situ AFM a de plus permis de caractériser les différentes modalités de déformation à l’échelle de la zone de process en combinant l’imagerie topographique et des techniques de corrélation d’images numériques (DIC). Un modèle de prédiction des énergies de rupture en propagation stationnaire ainsi qu’en régime de stick-slip a été développé, en intégrant les champs de déformation visco-plastiques mesurés dans la zone de process, la vitesse de déformation locale, et la loi de comportement des matériaux mesurée à l’échelle macroscopique

Fracture behavior of amorphous and semicrystalline blends of poly(vinylidene fluoride) and poly(methyl methacrylate)

International audienceThe fracture behavior of blends of poly(vinylidene fluoride) and poly(methyl methacrylate) was investigated all over the composition range. A detailed analysis of the net stress versus crack opening displacement curves was performed. Fracture surface observations allowed statements on the process zone characteristics ahead of the crack tip. For the amorphous blends, the crack initiation energy is well related to the glass transition temperature. For the semicrystalline blends, the fracture energy is correlated with the degree of crystallinity

Bridging steady-state and stick-slip fracture propagation in glassy polymers

International audienceBoth an experimental and a theoretical investigation of the fracture propagation mechanisms acting at the process zone scale in glassy polymers are presented. The main aim is to establish a common modeling for different kinds of glassy polymers presenting either steady-state fracture propagation or stick-slip fracture propagation or both, depending on loading conditions and sample shape. On the experimental point of view, new insights are provided by in-situ AFM measurements of the viscoplastic strain fields acting within the micrometric process zone in a brittle epoxy resin, which highlight an extremely slow unexpected steadystate regime with finite plastic strains of about 30% around a blunt crack tip, and accompanied by propagating shear lips. On the theoretical point of view, we apply to glassy polymers some recently developed models for describing soft dissipative fracture that are pertinent with the observed finite strains. We propose a unified modeling of the fracture energy for both the steady-state and stick-slip fracture propagation based on the evaluation of the energy dissipation density at a characteristic strain rate induced in the process zone by the competition between the crack propagation velocity and the macroscopic sample loading rate