On the constitutive behaviour of polycarbonate under large deformation

doi:10.1016/0032-3861(90)90078-D

Polymer

Volume 31, Issue 11, November 1990, Pages 2071-2076

https://doi.org/10.1016/0032-3861(90)90078-D Get rights and content

Abstract

The uniaxial stress-strain behaviour of polycarbonate (Lexan) is examined. A grid technique is used to determine the inhomogeneous strain field associated with a steadily growing neck. An extension of photoelasticity is used in obtaining the stress variation along the line of symmetry in the neck. Comparing the grid and photoelastic measurements, the true-stress-stretch curve is established for polycarbonate.

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Effects of cold rolling on the tensile response of glassy polymers: Experiments and modeling
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Cold rolling can significantly affect the mechanical response of amorphous glassy polymers. Traditionally, uniaxial compression tests are commonly used to evaluate the mechanical response of oriented polymers. In contrast, the tensile response of predeformed polymers is still not well characterized since the specimens typically do not exhibit a homogenous deformation in the subsequent reloading process. To address this limitation, in this work we first perform cold rolling on poly(ethylene terephthalate)-glycol (PETG), an amorphous glassy polymer, followed by a subsequent reloading test in uniaxial tension mode. Based on the digital image correlation technique, we are able to extract the stress–strain relationship of rolled specimens. The influence of rolling ratio and direction on stress response is then obtained, which shows a significant increase of hardening response for specimens rolled in the length direction while much less pronounced strain hardening for polymers rolled in the width direction. The yield strength and strain softening also depend on the rolling conditions. We then apply a viscoplastic model to predict the above experiments. The results show that the model can capture the hardening response of cold rolled specimens, while it fails to describe the yield strength. This indicates that the constitutive model needs to be improved for the future study. Overall, our work can deepen the fundamental understanding on the effects of cold rolling on mechanical properties of amorphous glassy polymers.
Deformation and failure maps for PMMA in uniaxial tension
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Typically, yield is accompanied by a load drop and by the development of a neck. High strain rate sensitivity reduces the degree of neck formation and orientation hardening leads to neck propagation along the length of the specimen [13–15]. After the neck has propagated along the entire length of the sample, the load increases again until failure intervenes.
Uniaxial tensile tests are performed on a polymethyl methacrylate (PMMA) grade over a range of temperatures near the glass transition and over two decades of strain rate. Deformation maps are constructed for Young's modulus, flow strength, and failure strain as a function of temperature for selected strain rates. The glassy, glass transition and rubbery regimes are identified, and constitutive relations are calibrated for the modulus and flow strength within each regime.
A simple nonlinear model to simulate the localized necking and neck propagation
2016, International Journal of Non-Linear Mechanics
Citation Excerpt :
The necking is so explained as a mechanical melting of the folded chain blocks. Experimental aspects of the constitutive behavior of a polycarbonate (Lexan) have been investigated accurately in the work by Buisson and Ravi-Chandar [22], where a grid technique has been used to determinate the non-homogeneous deformation field associated with a steadily growing neck. However, in these experimental works there has been no attempt to derive a mechanical constitutive relation to describe the necking.
This paper deals with the equilibrium problem in nonlinear dissipative inelasticity of damaged bodies subject to uniaxial loading and its main purpose is to show the interesting potentialities offered by the damage theory in modeling the necking and neck propagation phenomena in polymeric materials. In detail, the proposed mechanical model is a two-phase system, with the same constitutive law but with different levels of damage for each phase. Despite its simplicity, it is shown that the model can straightforwardly reproduce the overall load–elongation curve provided by experimental tensile tests by involving only five parameters of clear physical meaning.
Measurement of multi-scale deformation of polycarbonate using X-ray scattering with in-situ loading and digital image correlation
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Citation Excerpt :
The design process for such applications requires accurate understanding of the material properties and, importantly, how the material responds and evolves under mechanical load. However, PC (as many other polymers) exhibits highly inhomogeneous deformation phenomena, e.g., necking and strain localisation, such that local deformation differs from the overall macroscopic response [2–4]. Such deformation responses are generally attributed to mechanisms occurring at the molecular-scale.
Deformation of amorphous, glassy polycarbonate (PC) is characterised over a wide range of length-scales using spatially-resolved wide angle X-ray scattering (WAXS) with simultaneous digital image correlation and in-situ uni-axial loading. Novel observations are presented on the correlation of the evolution of the molecular structure linked to the relevant local macroscopic measures of strain as the sample deforms. This provides new insights into the mechanisms of deformation in amorphous, glassy polymers. Strains and evolving anisotropy are observed, and quantified, at different length scales identified from the WAXS measurements connected to inter- and intramolecular structures. The inter-molecular structures show permanent deformations in the form of unrecoverable stretches and reorientations, whilst intra-molecular structures show recoverable strains and unrecoverable reorientations. Such insights pave the way for development of enhanced constitutive models for amorphous glassy polymers with a correct micro-mechanical basis and calibrated evolution of anisotropy.
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Three methods for the determination of the large-strain behavior of ductile polymers are compared in both tension and compression. Each method relies on some (non-contact) measurement of the strain and some approximations in the calculation of stress. The strain measurement techniques include digital image correlation (DIC) and two techniques of video-based extensometry: marker tracking and area variation monitoring. Since the specimens are inevitably subject to structural plastic instabilities (necking in tension, barreling in compression) the strain and stress states are no longer uniform in the gauge section after the peak load. Under such circumstances, it is demonstrated that the three experimental methods can lead to significant differences. It is inferred from the comparative analysis that the method based on vertical marker tracking is not reliable. Validated by DIC, video-based area variation is shown to be a simple alternative way to obtain an excellent estimate of the intrinsic true stress–strain behavior of the polymer.

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Polymer paperOn the constitutive behaviour of polycarbonate under large deformation

Abstract

J. Mech. Phys. Solids

J. Mech. Phys. Solids

Int. J. Solids Struct.

Polymer paper
On the constitutive behaviour of polycarbonate under large deformation