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Research Papers: Materials and Fabrication

Strain Rate Dependence and Short-Term Relaxation Behavior of a Thermoset Polymer at Elevated Temperature: Experiment and Modeling

[+] Author and Article Information
A. J. W. McClung

Department of Aeronautics and Astronautics, Air Force Institute of Technology, Wright-Patterson Air Force Base, OH 45433-7765

M. B. Ruggles-Wrenn1

Department of Aeronautics and Astronautics, Air Force Institute of Technology, Wright-Patterson Air Force Base, OH 45433-7765marina.ruggles-wrenn@afit.edu

1

Corresponding author.

J. Pressure Vessel Technol 131(3), 031405 (Apr 21, 2009) (8 pages) doi:10.1115/1.3110025 History: Received April 24, 2008; Revised July 15, 2008; Published April 21, 2009

The inelastic deformation behavior of polymerization of monomeric reactants-15 (PMR-15) neat resin, a high-temperature thermoset polymer, was investigated at 288°C. The experimental program was designed to explore the influence of strain rate changes in the 106103s1 range on tensile loading, unloading, and strain recovery behavior, as well as on the relaxation response of the material. The material exhibits positive, nonlinear strain rate sensitivity in monotonic loading. Nonlinear, “curved” stress-strain behavior during unloading is observed at all strain rates. The strain recovery at zero stress is profoundly affected by prior strain rate. The prior strain rate is also found to have a strong influence on relaxation behavior. The rest stresses measured at the termination of relaxation tests form the relaxation boundary, which resembles a nonlinear stress-strain curve. The results suggest that the inelastic behavior of the PMR-15 solid polymer at 288°C can be represented using a unified constitutive model with an overstress dependence of the inelastic rate of deformation. The experimental data are modeled with the viscoplasticity theory based on overstress. A systematic procedure for determining model parameters is presented and the model is employed to predict the response of the material under various test histories.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 1

Stress-strain curves obtained for the PMR-15 polymer in tensile tests to failure and in loading/unloading tests conducted at constant strain rates of 10−6 s−1, 10−5 s−1, 10−4 s−1, and 10−3 s−1 at 288°C. The dependence of the stress-strain behavior on the strain rate is evident.

Grahic Jump Location
Figure 2

Recovery at zero stress at 288°C (following loading and unloading in strain control). Recovered strain is shown as a percentage of the initial value (inelastic strain value measured immediately after reaching zero stress). The effect of the prior strain rate on the recovered strain is apparent.

Grahic Jump Location
Figure 3

Stress-strain curves obtained for the PMR-15 polymer in constant strain rate tests with intermittent periods of relaxation at 288°C. When loading at a constant strain rate is resumed after the relaxation period, the material reaches the flow stress characteristic for that particular strain rate.

Grahic Jump Location
Figure 4

Stress decrease versus relaxation time for the PMR-15 polymer at 288°C. The influence of prior strain rate on the stress drop during relaxation is evident. Stress drop during relaxation of a fixed duration is independent of the stress and strain at the beginning of relaxation.

Grahic Jump Location
Figure 5

Stress-strain curves obtained for the PMR-15 polymer in strain rate jump tests and in constant strain rate tests at 288°C. Upon a change in the strain rate, the material returns to the stress-strain curve characteristic for that particular strain rate.

Grahic Jump Location
Figure 6

Schematic of a standard linear solid, which forms the basis for VBO

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Figure 7

A comparison between experimental and predicted stress-strain curves obtained for the PMR-15 polymer at constant strain rates of 10−6 s−1, 10−5 s−1, 10−4 s−1, and 10−3 s−1 at 288°C. The model successfully represents the strain rate dependence.

Grahic Jump Location
Figure 8

A comparison between experimental and predicted stress-strain curves obtained for the PMR-15 polymer in loading and unloading at two constant strain rates at 288°C. The model successfully represents the strain rate dependence on the unloading.

Grahic Jump Location
Figure 9

A comparison between experimental and predicted stress-strain curves obtained for the PMR-15 polymer in the strain rate jump test at 288°C. The model successfully represents the behavior upon a change in strain rate.

Grahic Jump Location
Figure 10

Comparison between the experimental and predicted stress-strain curves obtained for the PMR-15 polymer at 288°C at constant stress rates of 0.01 MPa/s and 1 MPa/s

Grahic Jump Location
Figure 11

Comparison between the experimental and predicted strain versus time curves obtained for the PMR-15 polymer at 288°C in creep at 21 MPa. Prior strain rates of 10−6 s−1 and 10−4 s−1.

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