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Research Papers: Materials and Fabrication

# Rate Dependence and Short-Term Creep Behavior of a Thermoset Polymer at Elevated Temperature

[+] Author and Article Information
C. M. Falcone

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 45433-7765marian.ruggles-wrenn@afit.edu

1

Corresponding author.

J. Pressure Vessel Technol 131(1), 011403 (Nov 25, 2008) (8 pages) doi:10.1115/1.3027475 History: Received June 11, 2007; Revised October 26, 2007; Published November 25, 2008

## Abstract

The inelastic deformation behavior of PMR-15 neat resin, a high-temperature thermoset polymer, was investigated at $288°C$. The effect of loading rate on monotonic stress-strain behavior as well as the effect of prior stress rate on creep behavior were explored. Positive nonlinear rate sensitivity was observed in monotonic loading. Creep response was found to be significantly influenced by prior stress rate. The effect of loading history on creep was studied in stepwise creep tests, where specimens were subjected to a constant stress rate loading followed by unloading to zero stress with intermittent creep periods on both loading and unloading paths. The strain-time response was strongly influenced by prior deformation history. Negative creep was observed on the unloading path. In addition, the behavior of the material was characterized in terms of a nonlinear viscoelastic model by means of creep and recovery tests at $288°C$. The model was employed to predict the response of the material under monotonic loading/unloading and multistep load histories.

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## Figures

Figure 1

Freestanding postcure cycle for PMR-15 panels

Figure 2

Test specimen

Figure 3

Figure 4

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

Figure 5

Creep strain versus time at 20MPa and 288°C. Effect of prior stress rate on creep strain is apparent. Creep strain increases nonlinearly with prior loading rate.

Figure 6

Schematic illustrating the control (input) signal in the stress-controlled loading-unloading test with intermittent periods of creep. Segments AB and CD represent creep tests of equal duration conducted at the same creep stress level. Creep test AB is performed during loading, while creep test CD is performed during unloading.

Figure 7

Stress-controlled test with intermittent creep periods of 1h duration at 288°C. At the same stress level, the creep rate is different during loading and unloading. A reversal in the strain rate is observed in creep test during unloading. The gray line is an uninterrupted stress-strain curve.

Figure 8

Figure 9

Creep curves obtained at 20MPa in a stepwise creep test and a creep test preceded by uninterrupted loading. Strains at the beginning of the creep tests are 3.04% (stepwise creep test) and 1.30% (creep preceded by uninterrupted loading).

Figure 10

Recovery at zero stress following the stepwise creep test conducted with the stress rate magnitude of 1.0MPa∕s. A comparison between experimental data and predicted strain response.

Figure 11

Schematic of (a) the stress input for a creep test and (b) the associated strain response

Figure 12

Comparison between the linear prediction and experiment {σo=10MPa} for (a) creep and (b) recovery

Figure 13

Material properties as functions of stress: (a) g0 versus stress, (b) g1 versus stress, (c) g2 versus stress, and (d) aσ versus stress

Figure 14

A comparison between experimental and predicted stress-strain curves for PMR-15 neat resin at 288°C at constant stress rates of (a) 1.0MPa∕s and (b) 0.01MPa∕s

Figure 15

A comparison between experimental and predicted strain versus time curves obtained for PMR-15 neat resin at 288°C for various prior loading rates: (a) creep at 20MPa and (b) recovery at zero stress

Figure 16

A comparison between experimental and predicted creep curves obtained on (a) the loading path and (b) the unloading path of the stepwise creep test conducted with the stress rate magnitude of 1.0MPa∕s

## Errata

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