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A study of methodological nature demonstrates the efficiency of a probation test allowing for the intrinsic character of a rheological constitutive law to be assessed. Such a law is considered here for Semi-Crystalline Polymers exhibiting necking and for large deformation. In the framework of a (, ,,) behavior's law, tensile experiments conducted at an imposed constant strain rate 0 bring true stress responses from which constitutive (material) parameters can be identified from Model-Based Metrology concepts. The same experiment repeated at various strain rates gives then access to the dependence of the non-elastic parameters on the strain rate. Then the intrinsic law is tested severely by considering a new set of experiments carried out for constant displacement rates of the grips. In that case, the specimens show local strain rates which evolve strongly during the test (by a factor of 5-10 here). The parameter identification process requires then the introduction of the exact realized input strain and strain-rate command into the model. Accounting for strain rate dependency requires additionally the knowledge of the preliminary identified strain rate dependence of the non-elastic constitutive parameters for good predictions of the experimental response directly. This is what is proven here. The conclusion speaks in favor of a possible upgrade of international standards for the mechanical characterization of polymers based on constant strain-rate tensile tests and properly applied model-based metrology.

Colloque avec actes et comité de lecture. Internationale.; International audience; We show in this study the high benefits that can be retrieved from the SAXS technique (Small Angle X-ray Scattering) performed with synchrotron lightsource (beamline cSAXS at PSI-SLS) to follow the initiation and the development of a necking (plastic instability) phenomenon underwent by a semicrystalline polymer under tension. High sensitivity and fast detectors used at this beamline as well as motorized fast displacement systems enable to realize 1D and even 2D scans of the specimen during the test. The integration time (33ms) and read out time (3ms) make a spatial resolution of the scanning of 360microm at the displacement rate of 10mm/s ensured by the displacement OWIS stages. At the same time, the tensile machine proceeds with a displacement rate of 20mm/s which means a negligible displacement of the material points on the specimen during the scan. The methodology to handle the data is quite tedious also not complicated. In order to study quantitatively the microstructure evolution during tension for different material points, a precise calibration procedure has been set up thanks to 3-D stereo Digital Image Correlation (DIC system ARAMIS from GOM instruments). The same tensile tests have been performed under DIC to provide the relationship between the running time of the test, the material points coordinates in the reference frame, their respective eulerian positions according to time and their measured local true strain. This gives rise to the following Fig.1 linking all of these variables. The points give the position of the recorded scans along the specimen (Z screaning). The red lines figure out the trajectory of initial material points and allow to find the patterns taken for a same material points at different times. The blue curves are isostrain curves and allow to find all patterns corresponding to a given strain, although for different material points. This figure is the key to be able now to determine whether or not the microstructure state is only governed by the true strain. As a result of the [0.017-1] nm-1 q-range of our experiments, the microstructure is probed at the length scale of scatterers ranging approximately in the 10-350 nm. Its evolution is monitored through a quantitative observable raised from the SAXS pattern analysis. It is an anisotropy index [1] defined as where correspond to the intensities measured along the horizontal and vertical axis of the image frames, respectively the tensile and transverse directions. The local anisotropy is then measured at different locations for different true strain values ranging from 0 to 2 (about 700% of extension ratio). We cover the range of viscoelasticity followed by a long stage of plastic development and a plastic propagation accompanying the hardening rubbery state. The anisotropy index measured for different material points as a function of true strain produces identical curves (within the uncertainty of the measurements). This new result proves that the microstructure is fully governed by the sole strain. Such results enable to consider that this observable gives a true signature of the kinetics of internal reorganization during the tensile deformation of the polymer. It is worthwhile to mention that next efforts made in the modeling of semicrystalline polymer should pretend to recover such signature. References [1] L. Farge, S. André, F. Meneau, J. Dillet, C. Cunat, A common multiscale feature of the deformation mechanisms of a semi-crystalline polymer, Macromolecules, 46(24), (2013), 9659-9668.

In the study presented in this paper, we analyzed the mechanical response of a glass fiber plain weave/polymer composite at the fabric millimetric mesoscale. The detail of the stress and strain fields in a fabric repeating unit cell was numerically calculated using CraFT (Composite response and Fourier Transforms), a code specifically conceived for simulating the mechanical behaviour of materials with complex microstructure. The local strain fields obtained by simulation were found to be in very good agreement with measurements carried out using 3D Digital Image Correlation (3D DIC). From numerical stress fields calculated with the CraFT solver, we also highlighted the subregions inside the periodic mesostructure where there is maximum stress. Furthermore, with X-ray tomography post mortem measurements, we were able to confirm that certain damage modes were well initiated in these microstructure subregions of stress concentration.

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In the experimental part, the displacement field on the edges of a [0, 554, -554]s GF/EP laminate and [302,-602]s GF/EP laminate specimens with multiple transverse cracks is visualized and analyzed on dependence of the applied mechanical load by using ESPI(Electronic Speckle Pattern Interferometry).The different displacement profiles along the tensile-axis are drawn along the specimen edges at several distances from the mid-plane corresponding to the different plies. The measurements out-of-plane deformations are effected to determine and to compare the crack opening displacement (COD) with the crack face sliding displacement (CSD). In the numerical part, the strong dependence of the average normalized (COD) on the relative stiffness and thickness of the surrounding layers, based on FE results, is described by a simple empirical expression for non-interactive cracks. The FEM simulation results obtained by ABAQUS/CAE are compared with the general expressions for calculation of elastic constants of the damaged laminate developed by Lundmark and Varna [1]. The effect of crack interaction on the crack opening displacement (COD) is also discussed in this study. The effect on (COD) is described by the introduced “interaction function” which is determined using FE analysis.

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