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Research Papers: Materials and Fabrication

The Rate (Time)-Dependent Mechanical Behavior of the PMR-15 Thermoset Polymer at 316°C: Experiments and Modeling

[+] Author and Article Information
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

O. Ozmen

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

1

Corresponding author.

J. Pressure Vessel Technol 132(4), 041403 (Jul 23, 2010) (6 pages) doi:10.1115/1.4000730 History: Received June 11, 2009; Revised October 14, 2009; Published July 23, 2010; Online July 23, 2010

The inelastic deformation behavior of PMR-15 neat resin, a high-temperature thermoset polymer, was investigated at 316°C. The experimental program was designed to explore the influence of strain rate on tensile loading, unloading, and strain recovery behaviors. In addition, the effect of the prior strain rate on the relaxation response of the material, as well as on the creep behavior following strain-controlled loading were examined. Positive, nonlinear strain rate sensitivity is observed in monotonic loading. The material exhibits nonlinear, “curved” stress-strain behavior during unloading at all strain rates. The recovery of strain at zero stress is strongly influenced by the prior strain rate. The prior strain rate also has a profound effect on relaxation behavior. Likewise, creep response is significantly influenced by the prior strain rate. The experimental data are modeled with the viscoplasticity theory based on overstress (VBO). The comparison with experimental data demonstrates that the VBO successfully predicts the inelastic deformation behavior of the PMR-15 polymer under various test histories at 316°C.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Stress-strain curves obtained for the PMR-15 polymer in tensile tests to failure and in loading/unloading tests conducted at 316°C with constant strain rates ranging from 10−6 s−1 to 10−3 s−1. The dependence of the stress-strain behavior on the strain rate is evident.

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Figure 2

Recovery at zero stress at 316°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.

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Figure 3

Stress-strain curves obtained for PMR-15 polymer in a strain rate jump test and in constant strain rate tests at 316°C. Upon a change of the strain rate, the material “returns” to the stress-strain curve characteristic for that particular strain rate.

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Figure 4

Stress-strain curves obtained for PMR-15 polymer in constant strain rate tests with an intermittent period of relaxation at 316°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.

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Figure 5

Stress decrease versus relaxation time for the PMR-15 polymer at 316°C. Influence of prior strain rate on the stress drop during relaxation is evident.

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Figure 6

Creep strain versus time curves obtained at 12 MPa and 316°C for the PMR-15 polymer. Effect of prior strain rate on creep is apparent. Creep strain increases nonlinearly with prior strain rate.

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Figure 7

Schematic of a stress-strain path generated by the VBO. The equilibrium stress (g) and kinematic stress (f) curves are also shown.

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Figure 8

A comparison between experimental and predicted stress-strain curves obtained for PMR-15 polymer at constant strain rates of 10−6, 10−5, 10−4, and 10−3 s−1 at 316°C. The model successfully represents the strain rate dependence.

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Figure 9

A comparison between experimental and predicted stress-strain curves obtained for PMR-15 polymer in loading and unloading at two constant strain rates at 316°C. The model successfully represents the strain rate dependence on the unloading.

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Figure 10

A comparison between experimental and predicted stress-strain curves obtained for PMR-15 polymer in the strain rate jump test at 316°C. The model successfully represents the behavior upon a change in strain rate.

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