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

Plane–strain condition in plane–strain grooved tensile (PSGT) specimens during traction and creep loading at room and high temperature

International audienceAbstract ISO 23228:2011 proposed a testing method in which the plastic material, experimental resins or compounds for pipes and fittings, can be exposed to stress conditions that mimic internally pressurised end‐capped pipes. The stress conditions are mimicked by the use of a plaque specimen having a grooved reduced section called plane–strain grooved tensile (PSGT) specimens producing a biaxial state of stress under uniaxial loading. In this study, PSGT specimens were cut out from high‐density polyethylene (HDPE) pipes. Two shape ratios, ratio between the width and the groove thickness, were used. Both the axial and transverse displacements and strain fields were followed by a digital image correlation (DIC) camera during tensile and creep loading, both at room and high temperature; furthermore, DIC images were used to estimate the notch opening displacement. The increasing effect of the temperature in both the axial and transverse displacement and strain was highlighted. No significant effect of the width was noticed. The results have evidenced that, as the plane–strain condition in the width is assured during the tests, PSGT specimens can be used to mimic internally pressurised pipes under monotonic increasing or constant‐in‐time loading at both room and high temperature, but it must be better to use specimens with a higher shape ratio, that is, higher width. The results contribute to the 9th Sustainable Development Goal: Industry, Innovation and Infrastructure by promoting a sustainable industrialisation and fostering innovation

Characterisation of the dilatational stress relaxation for viscoelastic Polyurethane materials

ICEM 14 - 14th International Conference on Experimental MechanicsInternational audienceThis contribution deals with three grades of polyurethane (PU) devoted to ensure sealing of filter cartridge endplate. PU is an amorphous polymer whose microstructure depends on the supplier processing. During service, the endplates are submitted to complex thermo-mechanical loading consisting of an applied pressure with a continuous increase of the temperature. The inner diametrical dilatation is the key parameter that has to be controlled. The main loading deals then with transient thermal expansion as well as hydrostatic pressure

Effects of stress triaxiality ratio on the heat build-up of polyamide 11 under loading

International audienceAdiabatic heat build-up in polymers attributed to the conversion of plastic work into dissipative heat is a wellknown phenomenon. The temperature in the vicinity of a crack tip due to heat build-up may be exceptionallyhigh so that it can locally reach the glass transition temperature T g , even though the ambient testing temperatureis lower than T g . A significant alteration of the local material response and damage mechanisms is then induced.By simultaneous measurement of temperature during experimental tests under quasi-static loading and for awide range of stress triaxiality ratios, a Gurson-Tvergaard-Needleman based thermo-mechanical constitutivemodel, integrating temperature-dependent coefficients, has been developed. Predictive capabilities of the pro-posed thermo-mechanical model to simulate the isothermal behaviour of PolyAmide 11 (PA11) have led toadiabatic simulations, to account for the heat build-up highlighted experimentally, of ductile crack extension.The model parameters were identified using experimental data obtained from PA11 samples with a given stresstriaxiality ratio. Predicted evolutions given by the proposed constitutive model for other stress triaxiality ratiosand geometries are found to be in good agreement with experimental data

On the use of the material plastic deformation function in fracture testing of ductile polymers

The plastic deformation behaviour of a material in a fracture process can be described by a plastic deformation function (H function), which is established by the load separation criterion. H function is the basis of single-specimen methodologies for the construction of the material crack growth resistance (JR) curve, but it can be given further significant roles in the study of the fracture behaviour of materials. This work describes further applications of H function in fracture characterization of ductile polymers. By referring to results obtained from single edge notched in bending, SE(B), specimen tests on different polymeric materials, H function is shown as an effective tool for: i. guiding specimen preparation for fracture testing, through its character of local property and the capability to reflect directly the geometry constraint degree; ii. studying the effects of the testing conditions on the plastic deformational mechanisms involved in the fracture process. Industrial grades of both amorphous and semi-crystalline polymers were examined [acrylonitrile-butadiene- styrene resin, high-impact polystyrene, polycarbonate, rubber toughened poly(methylmethacrylate), rubber toughened poly(lactic acid); linear low-density polyethylene, rubber toughened polyamide 66, rubber toughened poly(butylene terephtalate)]. It is worth noting, with reference to point ii., that the approach here employed for the evaluation of the H function, which is based on the execution of stationary crack experiments on blunt notched specimens, does not require the measurement of the crack extension produced in the test. This makes the approach suitable to be used at different testing conditions (temperature and loading rate) and, in particular, in impact tests

Constitutive Models for rubbers V

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Study of the plastic deformation behaviour of ductile polymers: Use of the material key curves

Material key curves are functions providing a relationship among load, displacement and crack size, and are used in fracture mechanics methods for ductile polymers. This work shows that the material key curves provide also a tool to distinguish among different deformational mechanisms involved in the fracture process and they reflect the constraint degree. Tests are carried out on an acrylonitrile-butadiene-styrene resin (ABS) as a model material, and different deformation mechanisms are induced by different temperatures and loading rates, up to impact conditions. Deformation mechanisms are investigated, for the first time on ABS, by tomography and the results are compared to more traditional imaging techniques as scanning and transmission electron microscopy

Determination of the fracture resistance of ductile polymers: The ESIS TC4 recent experience

Round-robin (RR) tests carried out under the direction of the Technical Committee 4, "Polymers, Polymer Composites and Adhesives," of the European Structural Integrity Society (ESIS TC4) showed that the multispecimen methodology employed for the construction of the crack growth resistance curve (J vs crack extension, Δa) of polymers often does not provide reliable data because of the uncertainties associated with the measurement of Δa. With this in mind, the ESIS TC4 attention has been more recently focused on the analysis of a testing scheme based on the load separation criterion, which does not require the measurement of Δa.With the aim to employ this new approach into a standardized procedure, the degree of reproducibility of the results obtainable with the application of this testing scheme to ductile polymers has been assessed by means of multilaboratory RR testing exercises that started in 2011. An ESIS TC4 reference draft protocol was prepared and 10 laboratories participated in the RR activities. The present work describes the load separation criterion-based testing procedure recently examined by ESIS TC4 and gives a summary of the results obtained in the RR activities, which appear encouraging