C. G'Sell

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.

In situ observation of the plastic deformation of polypropylene spherulites under uniaxial tension and simple shear in the scanning electron microscope

Abstract The α- and β-spherulites in polypropylene (PP) were identified by direct observation in the scanning electron microscope after appropriate etching. The α-phase has a dark contrast while the β-phase is brighter. Results concerning the individual behaviour of α- and β-spherulites in polypropylene samples which have been subjected to tensile and shear loading are reported. Under tensile loading, the α-spherulites exhibit a brittle behaviour, while the β-phase deforms plastically up to high deformations. The brittle behaviour of the monoclinic structure is characterized by cavitation at an early stage of deformation at the spherulites boundaries or at their equatorial region perpendicular to the tensile axis. Under shear loading, the α-phase cavitation disappears and both phases are then capable of undergoing large strains. However, quantitative characterization of the local deformation in each spherulite species showed that the α-structure deforms less than the global deformation while the β-phase compensates for this lack of plastic deformation of the other phase. The interlocked structure of the α-spherulites is discussed as being the leading contributing factor towards their brittleness, since it makes the plastic glide of this phase very difficult. In contrast, the radial lamellae of the β-spherulites allow the initiation and propagation of plastic glide more easily. The presence of a β-phase in PP with coarse spherulites considerably improves the mechanical properties at room temperature.

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.

Scanning force microscopic investigation of plasticity and damage mechanisms in polypropylene spherulites under simple shear

Abstract Plasticity and damage mechanisms have been investigated at the lamellar level by scanning force microscopy in bulk isotactic polypropylene under moderate applied shear stresses. The polypropylene under study contains both α and β spherulites. The SFM micrographs show clearly that the local mechanisms are basically the same for both α and β spherulites, but their intensity is much less lower in the α spherulites than in the β ones due to the presence of the daughter lamellae in the former case. The main deformation mechanisms which have been observed are: (i) kinking of the lamellae which lie along the principal compressive axis; (ii) nanocracks across the lamellae lying along the principal tensile axis; (iii) fragmentations of the lamellae which are parallel to the shear axis; (iv) fragmentations of the lamellae which are perpendicular to the shear axis.

Rheology of polypropylene in the solid state

The tensile behaviour of a commercial grade of isotactic polypropylene was tested in a temperature range between 20 and 150 °C with a video-controlled testing system which is capable of imposing a constant true strain-rate within the neck automatically. The results are displayed in the form of effective stress-strain curves and modelled by a constitutive equation in a multiplicative form. It is thus shown that, for each temperature, the plastic response can be described up to very large strains (ɛ ≃ 2.0) by a set of four parameters. The assumptions introduced in this modelling are critically discussed in order to check the validity of the simplifying hypotheses (strain homogeneity, isochoric deformation, etc.). The constitutive equation thus obtained was utilized in a finite difference code in order to predict the development of stretching instabilities of polypropylene. The simulation gives access to the engineering stress-strain response of the stretched test piece and to the detailed kinetics of the incipient neck. It is found that the severity of the instabilities is less at room temperature than near the melting point because of the decrease of the strain-hardening and of the strain-rate sensitivity with temperature.

Spherulitic morphology of isotactic polypropylene investigated by scanning electron microscopy

Abstract The aim of this work is to investigate the complex spherulitic structure of bulk polypropylene samples from direct scanning electron microscopy (SEM) observations of etched surfaces. Thick plates of isotactic polypropylene were moulded by intrusion. Preliminary characterization, involving X-ray diffraction, microdensitometry and differential scanning calorimetry (d.s.c.), showed that the slow solidification process develops a variable proportion of the monoclinic (α) and hexagonal (β) phases, ranging from 0% of β-crystals at the surface to 60 vol% of this phase at the core. In addition, samples were cut across the thickness of the plates, finely polished and then etched with an appropriate reagent which preferentially attacks the amorphous fraction of the polymer. SEM examination of such samples revealed two populations of spherulites with quite different contrasts, which were unambiguously associated with the two crystalline structures. The α-spherulites have a dark aspect while the β-ones are very bright. These contrast effects are discussed in terms of the topology of the etched surface. For the α-spherulites, whose lamellae are straight and finely interlocked along the radial and tangential directions, the etched sections are very smooth and consequently the lateral diffusion of normal incident electrons is weak. In contrast, the β-spherulites are characterized by curved lamellae and sheaf-like structures, thus making the surface rougher after etching, and which contribute to the emission of more secondary electrons to the detector. This interpretation is confirmed by the corresponding contrast observed in metallographic microscopy using low-angle illumination.

Experimental investigation of neck propagation in polymers

Abstract Neck propagation is investigated for two different thermoplastic polymers (high density polyethylene and polyethylene terephthalate) at room temperature and at several elongation rates. Two types of tensile test were performed: axisymmetric and plane-strain. The tests were performed under both isothermal and non-isothermal conditions. The results are displayed using a description developed by Neale et al. where the specimen neck profiles are simulated by hyperbolic tangent functions.

Evaluation of temporary restorations' microleakage by means of electrochemical impedance measurements

The aim of this study was to quantify the changes in the watertightness of three temporary filling materials over 1 wk with a new electrochemical technique: the impedance a measurement technique. Forty sound extracted human maxillary teeth were selected and prepared for the measurements. They were divided into three groups in addition to positive and negative controls. The resistance, and therefore the watertightness, of the intact crown and the resistance after preparation of an endodontic access cavity were registered. After a randomization procedure, 12 teeth were obturated with Cavit G, 12 teeth with Fermit-N, and 12 teeth with Intermediate Restorative Material (IRM). The changes in the resistance were measured first just after obturation (time 0), then after days 1, 2, 3, 4, and 7. The results showed that the IRM group was significantly more watertight than the Fermit-N group (p < 0.05) and much more than the Cavit G group (p < 0.005).

Effect of a plastic deformation on the nanostructure of polycarbonate: study by low-frequency Raman scattering

Abstract The effect of a plastic deformation, induced by a simple shear, on the nanostructure of amorphous polycarbonate (PC), is studied by low-frequency Raman scattering (LFRS). As previously observed for poly(methyl methacrylate) (PMMA), an excess appears in the low-frequency part of LFRS spectra after plastic deformation. The excess is stronger in the case of PC than in the case of PMMA, and its light polarization, relative to the molecular orientation, is different. In the frame of a non-continuous glassy polymer network, the experimental results are interpreted by an orientation of macromolecular strands in the less cohesive zones of the polymer matrix, between the more cohesive regions. When the deformation is carried out at a temperature lower than the glass transition temperature, the more cohesive regions are not affected by the plastic deformation. It is no more the case when the polymer is deformed at a temperature higher than T g .

Determination of the intrinsic stress/strain equation of thin polymer films from stretching experiments under plane-strain tension

Abstract Thin films of amorphous poly(ethylene terephthalate) were stretched under plane-strain tension at temperatures from 20°C to 100°C and at various elongation rates. It was observed that under such a deformation regime a neck is likely to form perpendicular to the tensile axis and to propagate steadily towards the gripped ends of the sample. The kinetics of the neck propagation was recorded carefully during the course of the tests by means of a photographic technique and the profile of the neck was analysed by quantitative microscopy during the propagation stage. It is shown that, under simple assumptions, the plastic constitutive equation of the material can be determined from this procedure in terms of the local effective stress versus the local effective strain and effective strain-rate at each of the temperatures investigated. It is demonstrated that the method employed is generally applicable to most ductile polymers.