a b s t r a c tThis study is devoted to the necking phenomenon and to the kinematic aspects associated with this plastic instability. The material studied is a High Density PolyEthylene deformed in tension. Full-field 3D-DIC (Digital Image Correlation) strain measurements were carried out on one face of a tensile specimen. On the opposite face, the longitudinal strain was measured in the central section using video-traction Ò , an extensometer based on marker tracking. This last measurement was used with feedback to control the strain rate in the specimen center. Specific data processing was run on the 3D-DIC measurements to obtain strain and strain rate maps up to true strain levels greater than 1.8. Even with measurements made on a relatively small strain rate range (smaller than 1 decade), we highlighted how much the necking phenomenon depends on the strain rate. In particular, we found that, around the yield point, the strain localization is more pronounced with higher strain rates. By analyzing the evolution of the strain rates in the necking shoulder regions, we carried out direct and original measurements of the strain level corresponding to the onset of necking stabilization (Natural Draw Ratio). This was possible because our original experimental protocol enabled us to obtain such strain maps even when the true strain values were significantly greater than 1. From an experimental point of view, we found that a video-extensometer based on marker tracking cannot efficiently measure the strain rate in the specimen center at large strain levels when the markers are greatly very deformed. In addition, a wide range of new experimental results (2D and 1D strain-rate and velocity data) is presented in this paper and we consider that this data should be particularly useful for the validation of numerical simulations of necking in polymers.

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The deformation mechanisms of a semicrystalline polymer (SCP) subjected to tensile loading were experimentally studied from nano to micro and macro length scales. The paper's focus is on the development of anisotropic features at all scales during deformation. At the nanoscale, original results are presented based on in-situ SAXS (small-angle X-ray scattering) measurements. At the microscale, previously obtained results using in-situ ISLT (incoherent steady light transport) and postmortem SRXTM (synchrotron radiation X-ray tomographic microscopy) are briefly summarized. For all these techniques, original data treatments were carried out for quantifying the evolution of anisotropy in the material's microstructure. At the macroscopic scale, a 3D digital image correlation technique was used for determining all the components of the Hencky strain tensor in the center of the necking region. New results are presented in this field for the studied SCP in terms of the quantity D = ε L /|ε T |: ratio of the longitudinal strain (along the drawing axis) to the transverse strain (perpendicular to the drawing axis). The main result of this study lies in a plot of variable D as a function of ε L . This plot evidences the same three clearly distinct regimes as those obtained using SAXS, ISLT, and SRXTM experiments to measure anisotropy quantitatively at the microstructural level. This result proves that everything occurring at the microscale gives a signature at the macroscale and hence opens up new routes for the modeling of the mechanical behavior of such materials.

We present an experimental investigation of the mechanical properties of solid vitrimer samples obtained by incorporating a diepoxide into a commercial PBT in the presence of a Zn(II) catalyst using batch compounder and injection molding machines. Tensile experiments were carried out at different temperatures (80°C, 120°C, 160°C) below the melting point (220°C) and with various diepoxide concentrations (0, 1, 2wt%,). We found that vitrimer plastic deformation mechanisms differ drastically from those of the PBT precursor, and more generally from the regular behaviour of other semi-crystalline polymers. In PBT vitrimers, strain localization (necking) is very weak and can even be suppressed. This type of behaviour is due to the increase in strain hardening caused by vitrimer crosslinking. The creep resistance and stiffness of PBT vitrimers are significantly higher than that those of the pristine PBT. We found that vitrimer modification of PBT at moderate levels improves dimensional stability without inducing brittleness. These promising results highlight the need to find processes which enable significant 2 production levels of these new materials. In the perspective section, we briefly present a way to produce PBTbased vitrimers using CRE (Continuous Reactive Extrusion).'' G ) moduli measured by rheometry with temperature or frequency sweeps. 6,13,27,30,37,[40][41] In particular, it is possible to roughly estimate whether a vitrimer has predominantly "rubber" ( ' '' GG  ) or "viscous" behaviour ( '' ' GG  ) for a given temperature and time scale. During vitrimer synthesis, the measurement of the temporal increase of ' G and '' G also makes it possible to monitor how the crosslinking reaction progresses and find the gelation point, which occurs approximately when ' '' GG . 13,[40][41] After the gelation point, the crosslinking reaction ends and the storage and loss moduli reach constant values with ' '' GG  . The molten polymer remains then in the "rubbery" state except when destruction of cross-links takes place by thermal degradation. 27 At service temperatures, i.e. in the solid state or below m T , the published results concern mainly the viscoelastic characteristics of vitrimers, or more specifically the creep properties of these materials. Röttger et al. and Caffy et

In this work, we used two techniques to study the deformation‐induced whitening phenomenon that occurs when certain semicrystalline polymers (SCPs) are subjected to tensile drawing: (1) IPLST (Incoherent Polarized Steady Light Transport) was used for characterizing the light scatterers and in particular for determining their size. (2) SRXTM (Synchrotron Radiation X‐Ray Tomographic Microscopy) was used to visualize the internal structure of the deformed SCPs. In particular, with this technique the possible presence of micrometric cavities can be detected. In the early whitening stage of a cavitating polypropylene (PP), the IPLST technique was found to show that the size of the light scatterers is larger than 1 μm. At the same time, the SRXTM measurements showed that no void larger than 1 μm was present in the material. The micrometric light scatterers responsible for the whitening phenomenon may thus not be simple cavities. In fact, this experimental study suggests that they correspond to areas where smaller objects (possibly nanovoids) are highly confined. At the scale of visible wavelengths, these regions could scatter visible light like individual entities of micrometric size. The study also showed that the size of cavities observable using SRXTM for a very deformed PP is dependent on the initial dimensions of the spherulites. Results previously obtained for a non‐cavitating high density polyethylene are also briefly presented in this article to confirm the theory that deformation‐induced‐whitening phenomenon may have various origins for such complex microstructuring. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys., 2013

This work is devoted to the study of the deformation mechanisms of a high-density polyethylene deformed in tension. Specific treatments were applied to synchrotron wideangle X-ray scattering patterns obtained in situ with the aim of quantifying: (i) the evolution of the apparent crystal sizes during the deformation process, (ii) the reorientation dynamics of the fragmented crystals while aligning their chains along the drawing axis during the establishment of the fibrillar morphology, and (iii) the reorientation dynamics of the amorphous chains. In addition, the volume strain evolution was measured using 3D digital image correlation. The cavitation phenomenon was found to mainly occur during the lamellae fragmentation phase. At the end of the deformation process, when the lamellar structure is destroyed, the fragmented crystals have new degrees of freedom and become free to rotate to align their chains along the drawing axis. Crystal fragmentation is then no longer needed to allow material deformation, and there is no further volume strain increase.

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