Specimen geometry effects on 316L(N) creep crack growth analyses

International audienceThe ASTM E1457-98 standard [1] describes the procedure allowing to determine the master curve da/dt versus C* parameter, for creeping solids. However, the methodology is only to be applied on CT specimens. The European collaborative program CRETE aims at extending the application of the ASTM E1457-98 standard to other types of laboratory specimens. In this paper, existing database of creep crack growth on 316L(N) stainless steel is utilised, concerning three types of specimens: Circumferentially Cracked Round Bars (CCRB) and Double Edge Notched Tensile specimens (DENT) for tensile mechanical loading whereas classical CT specimen combines tensile and bending loading modes. The geometry effect is first investigated by introducing the Q* parameter by analogy to Q parameter in the elastic-plastic J-Q approach. A modified procedure based on the ASTM E1457-98 standard has been applied to the database, allowing to plot a unique master curve da/dt vs C*

Effect of notch root radius on the creep failure mechanisms on polyamide 6 : application of facture mechanics approaches

International audienceThe polyamide 6 (PA6) under study exhibits non-linear time dependent (viscoplastic)deformation when submitted to steady load (creep) at room temperature. An experimentaldatabase consisting of creep tests performed on notched round bars with various notch rootradii was built. Failure mechanisms of the PA6 material were investigated thanks tocomprehensive microscopic examinations of deformed and broken specimens. Globalapproach of fracture mechanics for creeping solids was applied and showed its limits.Alternatively, the local approach of fracture mechanics was attempted, with the help of finiteelement analysis. It was shown that this approach could successfully capture main featuresof creep damage mechanisms of the PA6 material, which lead to complete failure

Fracture mechanics of PVDF polymeric material : specimen geometry effects

International audiencePolyvinylidene fluoride (PVDF) is a semi-crystalline polymer that has been widely studied for structural applications, because it exhibits good mechanical properties and chemical resistance. During viscoplastic deformation, the material whitens after the onset of necking due to nucleation and growth of voids. Mechanical tests on cracked specimens show the crack instability on DENT specimens whereas stable crack growth on SENB specimens is also encountered. In the global approach of non linear fracture mechanics framework, the two-parameter approach indicates that according to the opening stress state in the remaining ligament, the crack growth can be more or less stable. Namely, tensile crack specimens such as DENT favour instabilities whereas bending specimens (like SENB) are proved to present stable cracking. This work deals with computing Q-stresses on DENT and SENB specimens with the help of FE modelling. The material toughness JIC is then determined by means of experimental data gathered with both specimens. JIC depends on the specimen geometry. A JIC-Q curve is then plotted for the PVDF material at 20°C

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

Multiaxial fatigue crack initiation on filled rubbers : statistical aspects

International audienceWhite reinforcement fillers such as precipitated ...

Toughness improvement of polyamide 11 assessed via quasistatic tensile tests on notched round bars

International audienceToughening a polymeric material by adding rubber particles is a common technique used forglassy polymers and measured by means of Charpy or Izod impact tests. These latter aredetermined under dynamic conditions, thus at high strain rate or equivalently at very lowtemperature for polymeric materials.This work is devoted to three grades of semi-crystalline PolyAmide 11 (PA11), used in alarge number of engineering components. Service conditions are close to the glass transitiontemperature. Therefore, tests with quasi-static loading are expected to be more appropriate.The aim of the study is to find a methodology allowing the quantification of toughnessimprovement. To this end, an experimental database was constituted. Three grades of PA11were involved, that enable to analyze effects of: i) aging, ii) addition of a plasticizer iii)addition of rubber particles. Tensile tests were then carried out on notched round bars.Trends of load versus notch opening displacement curves are investigated. Longitudinalcross-sections on specimens issued from interrupted tests and fracture surfaces wereexamined in order to study deformation and damage mechanisms. An attempt is made to linktoughness improvement to the increase in global fracture energy

Analyse expérimentale d'un composite textile en vue d'une modélisation multiéchelle = Experimental analysis of a textile composite for multiscale modelling purposes

National audienceOn propose dans cette étude de comprendre le comportement mécanique d'un composite textile. Pour ce faire, l'étude se scinde en deux parties distinctes. Dans cet article, nous présentons les essais expérimentaux. Ces essais ont été réalisés sur le matériau composite (pour laquelle l'échelle de travail est appelée l'échelle macroscopique) mais également sur les constituants des composites (pour laquelle l'échelle de travail est appelée l'échelle microscopique). L'étude de modélisation numérique du matériau fait l'objet d'un second article

Characterisation of aged HDPE pipes from drinking water distribution : investigation of crack depth by Nol ring tests under creep loading

International audienceHDPE pipes are used for the transport of drinking water. However, disinfectants in waterseem to have a strong impact on their mechanical behaviour, limiting their lifetime inoperation. Indeed, oxidation occurs when they are in contact with disinfectants leading to theformation of a thin oxidised layer coupled to the cracks initiation of cracks of different lengthsfrom the inner wall surface. An original method is proposed here to characterise the ageingeffect of the pipe mechanical behaviour. Inspired from the ASTM D 2290-04 standard, NolRing tests have been performed under tensile and creep loadings on smooth rings. Aconstitutive equation has been determined from these tests using a finite element (FE)modelling. FE simulations have been performed to study the influence of the thin oxidised PElayer. Precracked specimens with different crack depth ratio have also been modelled. Thecrack depth ratio is an important parameter to quantify pipe ageing

3D Stress Fields Versus Void Distributions Ahead of a Notch Tip for Semi-crystalline Polymers

The creep durability of engineering structures relies on the theory of Fracture Mechanics for Creeping Solids (FMCS). The studied material is a semi-crystalline polymer. The lifespan of plastic pipes being generally specified in terms of years of service, its prediction requires reliable constitutive models accounting for time dependent deformation under multiaxial stress states and failure criteria based on the mechanisms of damage and failure. Here, an experimental approach was developed so as to analyze the mechanisms of deformation and cavitation at the microstructural scale by using 3D imaging (tomography/laminography). Three stress triaxiality ratios were addressed using various notched specimen geometries. The void characteristic dimensions (volume fraction, height and diameter) were then measured by defining a volume of interest. The spatial distributions of these characteristics at a prescribed creep time were observed to be dependent on the stress triaxiality ratio. A finite element constitutive model using the porosity as an internal variable, was selected. Comparison of the multiscale experimental database with those simulated at the macroscopic scale as well as at the microstructure level was satisfactory. In the light of the finite element results, the principal stress singularities were in good agreement with the void characteristic lengths

Characterization and statistical analysis of single fiber strength of fibers at various processing stages

International audienceFibre properties are used for estimating the damage and strength behaviour of composite materials and structures, as the mechanical properties of fibre reinforced composites are primarily dependent on those of fibres. Fibre reinforced composites owe their outstanding properties to their constituents, mainly the fibres which reinforce matrices in composites. The accuracy of the predictions made by composite strength models would be highly dependent on the accuracy of the fibre strength distribution which is used as input. Any error in input fibre strength properties would lead to inaccurate model predictions. It has however been highlighted by many studies that there is discrepancy in experimental data even for the same type of fibres. The fibre strength data used to determine fibre strength distributions may be inaccurate. Due to the fibre preselection effect, experimental data is usually incomplete and does not appropriately represent the true fibre strength variation. An adaptation to the Weibull analysis has been presented to predict a more accurate statistical distribution for the representation of the strength behaviour of fibres inside composites. This analysis considers the preselection effect, which is used in modelling the fibre strength behaviour. The described analysis has the potential to significantly improve the accuracy of fibre strength distribution determined using experimentally generated fibre strength data