Fracture Surface Energy Measurement at High Crack Speed Using a Strip Specimen : Application to Rubber Toughened PMMA

An experimental procedure is proposed to measure the lowest elastic stored energy density leading to a rapid crack propagation (RCP) in polymeric materials. A strip specimen with a blunt saw cut is uniformly loaded and a razor is pushed into the saw cut to initiate the crack. The very high crack velocity which is reached is measured with a resistive coating on the sample surface. Results obtained for a rubber toughened PMMA show that fracture energy for RCP at approximately 500 m/s is much smaller than that in a quasi static tests. The material exhibits a ductile-brittle transition under particular loading conditions.Une essai est proposé pour mesurer les densité minimales d'énergie élastique de déformation conduisant à des propagations rapides de fissures dans des matériaux polymères. La géométrie de l'éprovette est une bande ayant un trait de scie arondi chargée uniformément sur sa longueur. La fissure est amorcée par la percussion d'une lame de rasoir au fond du trait de scie. La vitesse de propagation de la fissure est mesurée par une jauge résisistive. Des résultats sont obtenus pour un PMMA "choc". Les énergies de rupture obtenues pour des vitesses de fissuration d'environ 500 m/s sont bien inferieures à celles obtenues lors d'essais quasi statiques et montrent un comportement fragile de ces matériaux sous certaines sollicitations

A Mechanical Model for the Onset of Damage in Rubber Modified Amorphous Polymers

At medium loading rates, void formation usually precedes plasticity in the matrix around the particles in rubber toughened polymers. Many authors have proposed models for the relationship between rubber surface energy, volume strain energy and void growth. In this paper, it is shown that another volume criterion must also be satisfied, arising from the fact that in all these models, no decohesion is allowed at the particle-matrix interface. A fracture mechanics approach, where linear and non linear elasticity are assumed for the matrix and the rubber particle respectively, is used to define a void formation criterion depending on the rubber fracture surface energy. After formation, the stability of the void is examined, taking into account the volume conservation between matrix and particle and the stress due to surface tension when the void size is very small. A size effect is observed, indicating that voids cannot grow in small particles. The required value of fracture energy in a particle on a microscopic scale is discussed

Analyses des sillons viscoélastiques lors d'un contact entre une pointe rigide et la surface d'un polymère

Les polymères possèdent comme spécificité un comportement viscoélastique prononcé même à température ambiante. Ainsi, sous certaines conditions, une rayure peut donc cicatriser. Cette aptitude est reliée aux déformations imposées lors du contact, fonction de la géométrie de la pointe, du frottement interfacial, et de la rhéologie du matériau. De plus, cette aptitude est conditionnée par l'existence d'un sillon ductile. Le sillon doit en effet présenter le moins de fissures ou de craquelures possible. À partir des observations in-situ offerte par notre dispositif expérimental de "microvisioscratch", nous avons analysé la recouvrance de surfaces de PMMA rayées par des pointes sphériques de rayon R = 200 μm. La forme du sillon laissée sur la surface a été relevée et le profil lissé par une fonction exponentielle définie par un temps caractéristiques. L'évolution des paramètres de lissage en fonction du frottement vrai local $\mu_{loc}$ et du rapport a/R (avec a, le rayon de contact) a été étudiée. Ces résultats expérimentaux complétés par des simulations numériques du test de rayure montrent que la cicatrisation est bien dépendante de la déformation plastique générée lors du contact

A surface flow line model of a scratching tip: apparent and true local friction coefficients

Abstract The apparent friction coefficient is the ratio between the tangential force and the normal load applied to a moving scratching tip. It includes a so-called ''true local'' friction coefficient, which is the scission at the interface between the tip and the surface being scratched, and a ''geometrical'' friction coefficient, which is the plough effect due to the wave front created ahead of the moving tip and depends on the shape of the tip. Like in any mechanical test, three basic types of behaviour of the material at the interface are observed: purely elastic, elastic-plastic and fully plastic. As is usual in polymers, the material behaviour is time and temperature dependent and may exhibit strain hardening. A surface flow line model is developed here to deduce the geometrical and true friction coefficients at the interface between a moving scratching tip and a surface from the apparent friction coefficient. Using this model, several situations may be simulated to predict the influence of the geometry of the tip on the scratch resistance of the material.

Fracture surface energy and craze fibril stress in PMMA in water

Polymethylmethacrylate (PMMA) is often used as a material for portholes in submarine applications. Therefore, the fracture properties of dry and wet PMMA in water and/or under hydrostatic pressure are of great importance. In this study compact tension specimens immersed in water have been used on a tensile machine and crack tips have been observed by means of optical interferometry during propagation. Fracture stress intensity factors, and plastic zone size have been measured as a function of loading time and crack speed in water. The results have been rationalized in terms of craze stress and craze life time at the crack tip, and in terms of a stress activated process of craze fibril growth and rupture. It has been shown that water increases the fracture toughness, in spite spite of the fact that the microscopic craze stress and craze fibril life time decrease at the crack tip

Influence de nano-rugosités sur le frottement d'un indenteur sphérique sur surface de polymère vitreux à faibles pressions de contact.

Colloque avec actes et comité de lecture. Internationale.National audienceDes rugosités contrôlées (4 à 100 nm) ont été obtenues par attaque chimique sur des billes de verre de différents rayons. Des essais de frottements ont ensuite été réalisés sur un polycarbonate. Pour les contacts élastiques le frottement décroît avec la rugosité et augmente avec la pression de contact. Pour les contacts plastiques le frottement atteint une valeur plateau. Les contributions locales de pression, fonction de la forme des rugosités, conduisent à une courbe maîtresse de frottement. Des essais en cours pour les très faibles pressions de contact visent à identifier la rugosité seuil

Mechanical analysis of the damage of a thin polymeric coating during scratching: role of the ratio of the coating thickness to the roughness of a scratching tip

Abstract It is generally accepted that the critical load generating the first damage in a scratch test is representative of the behaviour of a coating. As the properties of polymers are time and temperature dependent, a single value of the critical load cannot describe the overall mechanical behaviour. A new scratch apparatus has been designed which allows scratching over a wide range of velocities and temperatures and record real time photographs of the in situ contact area. It was observed that cracking appears within the contact area. In the case of thin solid films, the ratio of the contact radius to the radius of the grooving tip proved to be a pertinent parameter to predict the damage and did not depend on the scratching velocity or temperature. The ratio of the thickness of the coating to the roughness of the tip is another critical parameter: the coating prevents the roughness of the diamond tip from creating micro-scratches at the surface of the macro-groove. Therefore, since the absence of micro-scratches is a condition for relaxation of the macro-groove, the thickness of the coat must be greater than the roughness of the tip.