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RESEARCH PAPERS: Special Issue on Time-Dependent Behaviors of Polymer Matrix Composites and Polymers

A Constitutive Model for Strain-Induced Crystallization in Poly(ethylene terephthalate) (PET) during Finite Strain Load-Hold Simulations

[+] Author and Article Information
Rebecca B. Dupaix1

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

Dwarak Krishnan

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

No summation on i is implied in this or subsequent equations.

Ahzi (2) also develops a rate-dependent equation for crystallinity, but as we are going to limit crystallization to only occur during periods of very slow deformation, it seems appropriate as a first approximation to begin with a rate-independent expression.

A chain is defined as the portion of a polymer molecule between two entanglements.

1

Corresponding author.

J. Eng. Mater. Technol 128(1), 28-33 (Mar 29, 2005) (6 pages) doi:10.1115/1.1924564 History: Received August 13, 2004; Revised March 29, 2005

Recently, a hyperelastic-viscoplastic constitutive model was developed for PET and the noncrystallizing copolymer PETG (R. B. Dupaix, Ph.D. thesis, MIT, 2003). The materials were found to behave very similarly under monotonic loading conditions and the single constitutive model was able to capture both materials’ behavior. However, differences were observed upon unloading, and it is expected that additional differences would be observed under more complex loading conditions. Here their behavior is investigated under nonmonotonic loading conditions, specifically under load-hold conditions. The model of Dupaix and Boyce (R. B. Dupaix, Ph.D. thesis, MIT, 2003) is modified to include Ahzi’s upper-bound model for strain-induced crystallization [Ahzi, Mech. Mater., 35(12), pp. 1139–1148 (2003)]. The crystallization model is adapted to include criteria for the onset of strain-induced crystallization which depend on strain rate and level of deformation. The strain-rate condition prevents crystallization from beginning prior to the deformation process slowing significantly. The level-of-deformation condition delays crystallization until the material has deformed beyond a critical level. The combined model demonstrates differences in behavior between PET and PETG during complex loading situations, indicating its ability to capture the fundamental criteria for the onset of strain-induced crystallization.

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Copyright © 2006 by American Society of Mechanical Engineers
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References

Figures

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Figure 2

Description of initial modulus curve fit parameters

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Figure 3

Illustration of the orientation parameter

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Figure 4

PETG Uniaxial compression, comparing simulation results with experimental data. Temperature=90°C.

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Figure 5

PET Uniaxial compression, comparing simulation results with experimental data. Temperature=90°C.

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Figure 6

(a) Strain history for load-hold experiment. (b) Evolution of plastic strain rate during load-hold experiment.

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Figure 7

Simulation results for PETG under monotonic loading and under load-hold conditions

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Figure 8

Simulation results for PET under monotonic loading and under load-hold conditions

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Figure 9

(a) Crystallinity as a function of time during load-hold simulation for PET. (b) Enlarged view of crystallization period.

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Figure 10

Crystallinity as a function of time during load-hold simulation for PET (slower crystallization rate)

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Figure 1

Schematic representation of the constitutive model

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