Frédéric Addiego

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.

Specimen Geometry Effect on the Deformation Mechanisms of Polypropylene-Based Composites Under Impact Loading at Different Temperatures

Dynamic behaviors of polypropylene (PP), PP–ethylene propylene rubber (PP–EPR) and PP–nanoclay (PP–Nanocor) composites were studied by using split Hopkinson pressure bar at different temperatures and under different strain rates. Samples with two different geometries and with or without petroleum jelly lubricant were tested under identical testing conditions to compare the dynamic responses with underlying deformation mechanisms. For all the test temperatures and strain rates, the dynamic responses of neat PP and PP–Nanocor showed post-yield strain-softening, whereas those of PP–EPR showed strain-hardening after the yield point. PP–Nanocor showed more strain-softening compared with neat PP at room temperature due to the more important localized shearing deformation at the nanofiller/matrix interfaces. Friction between the sample and the bars affected the dynamic response of the materials at room temperature as well as high temperatures seen as differences in behavior between the dynamic behaviors of the non-lubricated thin samples and the non-lubricated thick samples under the same testing conditions. At room temperature and without lubrication, the thicker specimens of neat PP and PP–Nanocor failed during dynamic testing due to the barreling-induced crack that propagated in the specimen and led to the formation of a peripheral fragment. Although petroleum jelly provided a satisfactory lubrication condition for both neat PP and PP-based composites at room temperature and at high temperatures, by reducing the friction effect on the yield behavior, the lubrication did not have a significant effect on the post-yield behavior of neat PP and PP–Nanocor, particularly for the room temperature testing and with the thick specimens.

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.