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Research Papers

Tensile Stress Relaxation Behavior of Thermosetting Polyurethane Solid and Foams: Experiment and Model Prediction

[+] Author and Article Information
Bipul Barua

School of Aerospace and Mechanical Engineering,  University of Oklahoma, Norman, OK 73019

Mrinal C. Saha1

School of Aerospace and Mechanical Engineering,  University of Oklahoma, Norman, OK 73019msaha@ou.edu

1

Corresponding author.

J. Eng. Mater. Technol 133(4), 041007 (Oct 14, 2011) (7 pages) doi:10.1115/1.4004697 History: Received March 19, 2011; Revised July 13, 2011; Accepted July 22, 2011; Published October 14, 2011; Online October 14, 2011

Stress relaxation behavior of thermosetting polyurethane (PU) solid and foams were investigated in tensile mode using a dynamic mechanical analyzer (DMA). PU solid samples were manufactured in a closed mold to avoid any foam formation, whilst PU foam samples were manufactured inside a woven using a silicone mold. Samples with rectangular cross-section were subjected to a predetermined amount of tensile strain and the tensile force was recorded as a function of time. Relaxation modulus was determined for different temperatures up to near the glass transition temperature. It was found that the viscous part becomes more dominant with increasing test temperature. Although the stress relaxation behavior of PU solid and foam were found similar at lower temperature, the relaxation behavior of the foam was influenced by the cellular structure especially at higher temperature due to the combination of gas expansion and cell wall softening. Different stress relaxation models such as Maxwell model, Burgers model, Generalized Maxwell (GM) model, and Stretched exponential model were employed to predict the relaxation behavior of PU solid and foams. It was found that the GM model (with three or more elements) and the Stretched exponential model were in good agreement with the experimental data in predicting the stress relaxation behavior of both solid and foams. The predicted relaxation time and equilibrium modulus were found to decrease with increase in temperature.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Effect of temperature on stress relaxation modulus of PU solid as a function of time

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Figure 2

Normalized stress relaxation modulus of PU solid at different temperature

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Figure 3

Stress relaxation modulus of (a) PU415 foam and (b) PU404 foam as a function of time and temperature

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Figure 4

Normalized stress relaxation curves of PU solid and PU foams with different densities at (a) 25 °C, (b) 45 °C, (c) 60 °C, (d) 75 °C, (e) 90 °C, and (f) 100 °C

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Figure 5

Predicted and experimental stress relaxation curves for PU solid at 25 °C

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Figure 6

Stress relaxation curves predicted from the Stretched exponential model for PU solid. Symbols are experimental data and solid lines are the predicted curves.

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Figure 7

Stress relaxation curves predicted from the Stretched exponential model for (a) PU415 foam and (b) PU404. Symbols are experimental data and solid lines are the predicted curves.

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Figure 8

Predicted equilibrium stress relaxation modulus of PU solid as a function of test temperature

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Figure 9

Predicted equilibrium stress relaxation modulus of PU415 and PU404 foams as a function of test temperature

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