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RESEARCH PAPERS

Large Strain Mechanical Behavior of Poly(methyl methacrylate) (PMMA) Near the Glass Transition Temperature

[+] Author and Article Information
G. Palm

Department of Mechanical Engineering,  The Ohio State University, Columbus, OH 43210

R. B. Dupaix1

Department of Mechanical Engineering,  The Ohio State University, Columbus, OH 43210dupaix.1@osu.edu

J. Castro

Department of Industrial, Welding, and Systems Engineering,  The Ohio State University, Columbus, OH 43210

True stress was calculated as force (from the load cell) divided by current area. Current area was approximated based on the initial area and the current specimen height, assuming no barreling or volume change.

At temperatures below θg, the material exhibits a definite yield stress followed by strain softening. This behavior is due to thermal aging, and is discussed thoroughly by Hasan et al. (4). At temperatures above θg, no softening is observed, as the material simply rolls over to flow.

To make the representation unique, either the plastic spin or the plastic or elastic components of rotation must be prescribed. Boyce et al. (19) demonstrated that the choice to prescribe the plastic spin for an isotropic material was arbitrary and had no effect on the solution. Thus, without loss of generality, we set the plastic spin to zero.

1

Corresponding author.

J. Eng. Mater. Technol 128(4), 559-563 (Jan 16, 2006) (5 pages) doi:10.1115/1.2345447 History: Received July 30, 2005; Revised January 16, 2006

The mechanical behavior of amorphous thermoplastics, such as poly(methyl methacrylate) (PMMA), strongly depends on temperature and strain rate. Understanding these dependencies is critical for many polymer processing applications and, in particular, for those occurring near the glass transition temperature, such as hot embossing. In this study, the large strain mechanical behavior of PMMA is investigated using uniaxial compression tests at varying temperatures and strain rates. In this study we capture the temperature and rate of deformation dependence of PMMA, and results correlate well to previous experimental work found in the literature for similar temperatures and strain rates. A three-dimensional constitutive model previously used to describe the mechanical behavior of another amorphous polymer, poly(ethylene terephthalate)-glycol (PETG), is applied to model the observed behavior of PMMA. A comparison with the experimental results reveals that the model is able to successfully capture the observed stress-strain behavior of PMMA, including the initial elastic modulus, flow stress, initial strain hardening, and final dramatic strain hardening behavior in uniaxial compression near the glass transition temperature.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

Undeformed and deformed uniaxial compression specimens at a final true strain of −1.5

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Figure 2

Stress-strain curves showing the temperature dependence at a true strain rate of −1.0∕min in uniaxial compression

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Figure 3

Stress-strain curves showing the temperature dependence at a true strain rate of −3.0∕min in uniaxial compression

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Figure 4

Stress-strain curves showing the strain rate dependence at T=107°C

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Figure 5

Stress-strain curves showing the strain rate dependence at T=112°C

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Figure 6

Schematic of the material model

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Figure 7

Modeling versus experimental results for a constant true strain rate of 1.0∕min

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Figure 8

Modeling versus experimental results for a constant temperature of T=112°C(Tg+10)

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