Abdesselam Dahoun

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.

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.

Identification of LDPE Grades Focusing on Specific CH2 Raman Vibration Modes

The possibilities of applications of vibrational spectroscopy techniques (Raman spectroscopy) in the analysis and characterization of polymers are more and more used and accurate. In this paper, our purpose is to characterize Low Density Poly(Ethylene) (LDPE) grades by Raman spectroscopy and in particular with CH2 Raman vibration modes. With temperature measurements, we determine different amorphous and crystalline Raman assignments. From these results and on the basis of the evolution of CH2 bending Raman vibration modes, we develop a phenomenological model in correlation with Differential Scanning Calorimetry and in particular with crystalline lamella thickness determination.

Quantification of Cavitation in Neat and Calcium Carbonate-Filled High-Density Polyethylene Subjected to Tension

Cavitation-induced deformation mechanisms in neat semicrystalline polymers, i.e., crazing, and in the derived composites, i.e., particle-matrix debonding, are generally activated during the transition between viscoelastic and viscoplastic deformation stages. However, little quantitative information about the void evolution with the drawing level is to date available in the literature. The objective of this work is to quantify cavitation mechanisms in neat and calcium carbonate-filled high-density polyethylene (HDPE) subjected to tensile deformation. Attention was first focused on the properties of the materials that were assessed by means of a thermogravimetric analyzer, a differential scanning calorimeter, a scanning electron microscope (SEM), and a dynamic mechanical analyzer. In a second step, macroscopic aspects of cavitation were studied by quantifying volume variation of the materials subjected to tension using an accurate optical extensometer (VideoTraction). Attention was then turned to microscopic features of cavitation through a careful quantification of void density and shape factor by means of a method coupling a SEM with an image analysis procedure. At the two scales of interest, the results demonstrate that (i) the void density generated by crazing in neat HDPE or particle-matrix debonding in the composites gradually increases with the deformation state, (ii) void density induced by debonding is higher than that generated by crazing, and (iii) decreasing particles size causes an increase of void density. We also estimated the void shape factor, that is, ratio between the height and the width of the cavities. In all the studied materials, this parameter starts from a value that is below 1 and increases by a factor of 2 with increasing deformation. Moreover, in the case of the composites, one notes a higher void shape factor compared with the neat material, and particle size does not influence this parameter. The results provided by this paper can be the basis of a physically based model predicting cavitation mechanisms in semicrystalline polymers.

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.

Shrinkage Study of Polypropylene Films Laminated on Steel‐Influence of the Conformation Processes

Nowadays, thermoplastic polymers do not cease to attract the interest of the industrialists as steel / polymer composites for various applications in several domains, such as the automotive and the packaging. The ratio between their wide range of thermo‐mechanical properties and their low weight density make these materials a real alternative for the current solutions for the lightening and the reinforcement of structural pieces. Likewise, their working facility is a major asset for performing parts of complex geometry. In this paper, we highlight the narrow relationship between the microstructure of a small impact isotactic polypropylene film, either filled or not by mineral particles (calcite), and its behaviour towards shrinkage which can occur during thermal treatments above its melting temperature. This phenomenon of shrinkage is characterized by dimensional instabilities which can in particular, affect the life cycle of the material. Indeed, they may induce the partial delamination of the steel shee...

Characterization of cavitation processes in filled semi-crystalline polymers

Modification with rigid filler particles of semi-crystalline polymer has received considerable attention in recent years because it is an easy and cheap method to enhance impact toughness of pure matrix at low temperature. It is generally admitted that improvement of toughness is linked to the formation of voids by matrix/particle interface debonding, which facilitates molecular rearrangement of interparticle ligaments under stress [1]. But, characterization of these debonding mechanisms is qualitative to date. This is firstly due to the difficulties encountered to estimate volume change induced by void development. Indeed, quantification of volume variation during mechanical tests was a challenge from necking initiation that causes a localization of mechanical variables. Thanks to recent progress in optical extensometers (VideoTraction [2]), it is now possible to record volume strain up to large deformation. Secondly, effect of sample preparation on scanning electron microscope (SEM) observation has been neglected. Attention has systematically been focused on one preparation mode (for example: cryofractured sample or polished specimen + chemical etching [3], followed or not by metal coating). Morphological and/or chemical artifacts may arise from preparation mode and consequently influence the interpretation.