Jean-Marie Hiver

Experimental characterization of deformation damage in solid polymers under tension, and its interrelation with necking

In many polymers, including glassy thermoplastics and reinforced blends, it has been shown qualitatively that damage processes (crazing and cavitation) contribute to the apparent plastic deformation in addition to shear yielding. The aim of this paper is to determine more quantitatively their influence on the constitutive equation and/or on the kinetics of plastic instability. By using a novel video-controlled testing system, the evolution of volume strain is determined in polyethylene terephtalate (PET) and high-impact polystyrene (HIPS) by measuring in real time the three principal strain components in a small volume element, while the specimens are deformed under uniaxial tension at constant true strain rate. The contribution of volume strain to the overall true strain is 50% in the case of PET and nearly 100% for HIPS. Observation of sample geometry during complementary stretching tests at constant elongation rate show that necking is moderate in PET and completely absent in HIPS, although both polymers undergo stress drop at yield and nearly no strain hardening. This unexpected plastic stability is shown to be due to damage. In this scope, the classical theory of diffuse necking in polymers is revisited in order to take explicitly into account the damage rate, D, which expresses the slope of the volume strain vs. true strain curve.

Video-controlled tensile testing of polymers and metals beyond the necking point

A novel technique has been developed to record the intrinsic plastic behaviour of ductile materials by monitoring the effective strain and the effective stress in the mid-plane of hour-glass-shaped tensile specimens. The method utilizes a computer-aided video extensometer which analyses the sample profile in real time. The effective strain is computed automatically from the minimum diameter, and the effective stress is deduced from the applied load by taking into account the stress triaxiality corresponding to the local radius of curvature of the sample profile. Furthermore, a digital closed-loop system controls the ram speed of the hydraulic tensile testing machine in such a way that the local effective strain rate is maintained at a constant value. It is shown that most polymeric and metallic materials are entitled to be investigated by this method, which gives access in real time to the constitutive plastic equation, up to strains far beyond the necking point. The capabilities of the technique are illustrated and discussed critically, with more details for two polymers of different structures: polyethylene and polycarbonate.

Combination of EBSP measurements and SIMS to study crystallographic orientation dependence of diffusivities in a polycrystalline material: oxygen tracer diffusion in La2 − xSrxCuO4 ± δ

Abstract In the present work we demonstrate that electron back scattering pattern (EBSP) measurements together with secondary ion mass spectrometry (SIMS) can be used to study the crystallographic orientation dependence of tracer diffusivities in polycrystalline materials. The crystal orientation of single grains was determined with a dedicated scanning electron microscope in the EBSP mode. 18 O isotope depth profiles obtained in a subsequent tracer diffusion experiment were measured by SIMS on single grains with known orientation. The experiments yielded oxygen tracer diffusivities as a function of crystal orientation, temperature and strontium concentration for La 2 − x Sr x CuO 4 ± δ ( x = 0 and 0.15) between 600 and 900 °C. A strong anisotropy of the oxygen diffusivity for the investigated dopant concentrations was found.

Creep and Yield Behaviour of Semi-Crystalline Polyethylene in Uniaxial Tension

After being long used for commodity applications, polymeric materials are now selected for structural applications where their mechanical performances must fulfil drastic requirements. Among other parameters, the creep resistance is often of major importance if the material is to be subjected to long-term loading, as in the case of pressurised tubes. Such an artefact may be critical in semi-crystalline polymers whose glass-transition temperature (Tg) is lower than the service temperature, e.g. polyethylene at room temperature. The problem is then to predict the kinetics of creep deformation for loading times much larger than the duration of the normalised creep tests and eventually to take into account the ultimate creep-rupture phenomenon which may involve large plastic strains localised in a neck. The aim of this paper is triple: i) to present a novel method for recording the creep plastic behaviour of high-density polyethylene (HDPE for short) under constant true stress, ii) to connect this behaviour to the conventional response under constant load and, iii) to interpret the results obtained on the basis of microstructural models. This approach was made possible by means of videometric testing techniques developed in this laboratory, and the experience gained during research by the authors on various semi-crystalline polymers.

Anisotropic Plastic Hardening in Semi-Crystalline Polymers

The stress-strain behaviour of polyethylene was determined under uniaxial tension and simple shear. It was shown that the yield stress is correctly modelled by the J2 flow theory, but that the subsequent plastic flow follows a more complex behaviour, with a gradual strain hardening in tension and a marked plastic softening under simple shear. The X-ray diffraction analysis of plastically deformed specimens show that uniaxial tension induces a very acute orientation of the cristalline lamellae with the c axis (chain axis) along the tensile direction. Unlikely, simple shear gives rise to the rotation of the crystalline domains along a bimodal path which promotes a strain-softening. It is shown that these results agree with a microscopic deformation model based on the highly anisotropic pencil glide of polymeric chains and on the practical absence of intrinsic plastic hardening for the active glide system.