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Materials and Fabrication

The Rate (Time)-Dependent Mechanical Behavior of the PMR-15 Thermoset Polymer at Temperatures in the 274–316 °C Range: Experiments and Modeling

[+] Author and Article Information
M. B. Ruggles-Wrenn

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

1Corresponding author.

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received June 9, 2011; final manuscript received February 6, 2012; published online October 18, 2012. Assoc. Editor: Young W. Kwon.

This material is declared a work of the US Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited.

J. Pressure Vessel Technol 134(6), 061404 (Oct 18, 2012) (8 pages) doi:10.1115/1.4006122 History: Received June 09, 2011; Revised February 06, 2012

The inelastic deformation behavior of the PMR-15 neat resin, a high-temperature thermoset polymer, was investigated at temperatures in the 274–316 °C range. The experimental program was designed to explore the influence of strain rate on monotonic loading at various temperatures. In addition, the effects of prior strain rate on relaxation response and on creep behavior following strain-controlled loading were examined at temperatures in the range of interest. Positive, nonlinear strain rate sensitivity is observed in monotonic loading at all temperatures investigated. Both relaxation behavior and creep are profoundly influenced by prior strain rate at all temperatures. The time-dependent mechanical behavior of the PMR-15 polymer is also strongly affected by temperature. The elastic modulus decreases and the departure from quasi-linear behavior is accelerated with increasing temperature. Stress levels in the region of inelastic flow decrease as the temperature increases. The relaxation behavior as well as the creep response is strongly influenced by temperature. The viscoplasticity theory based on overstress for polymers (VBOP) is augmented to model the effects of temperature on the inelastic deformation behavior of PMR-15. VBOP is a unified state variable theory with growth laws for three state variables: the equilibrium stress, the kinematic stress, and the isotropic stress. Based on the experimental findings several VBOP model parameters are developed as functions of temperature. The augmented model is employed to predict the response of the material under both strain- and stress-controlled loading histories at temperatures in the range of interest. Comparison with experimental data demonstrates that the augmented VBOP successfully predicts the inelastic deformation behavior of PMR-15 polymer under various loading histories at temperatures between 274 and 316 °C.

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Figures

Grahic Jump Location
Fig. 1

Stress–strain curves obtained for the PMR-15 polymer in tensile tests to failure conducted at constant strain rates of 10−6, 10−5, 10−4, and 10−3 s−1 at 274 °C. The dependence of the stress–strain behavior on the strain rate is evident.

Grahic Jump Location
Fig. 2

Stress–strain curves obtained for the PMR-15 polymer in tensile tests to failure conducted at constant strain rates of 10−6, 10−4, and 10−3 s−1 at: (a) 288 °C, (b) 302 °C, and (c) 316 °C. The dependence of the stress–strain behavior on the strain rate is evident. Data at 288 °C from Ref. [33] are also included.

Grahic Jump Location
Fig. 3

Stress–strain curves obtained for the PMR-15 polymer in strain rate jump tests and in constant strain rate tests at: (a) 274 °C and (b) 302 °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
Fig. 4

Stress decrease versus relaxation time for the PMR-15 polymer at: (a) 274 °C, (b) 288 °C, (c) 302 °C, and (d) 316 °C. The influence of prior strain rate on the stress drop during relaxation is evident. Data at 288 °C from Ref. [33].

Grahic Jump Location
Fig. 5

Creep strain versus time at 21 MPa and: (a) 274 °C, (b) 288 °C, (c) 302 °C, and (d) 316 °C. Effect of prior strain rate on creep is apparent. Creep strain increases nonlinearly with prior strain rate.

Grahic Jump Location
Fig. 6

Creep strain versus time at 12 MPa and 316 °C. Effect of prior strain rate on creep is apparent. Creep strain increases nonlinearly with prior strain rate. Data from Refs. [35] and [40].

Grahic Jump Location
Fig. 7

Elastic modulus versus temperature for PMR-15 neat resin

Grahic Jump Location
Fig. 8

A comparison between experimental and predicted stress–strain curves obtained for the PMR-15 polymer in strain rate jump tests conducted at: (a) 274 °C and (b) 302 °C. The augmented VBOP model successfully represents the behavior upon a change in strain rate.

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