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High Strain Rate Mechanical Properties of Glassy Polymers

Tension testing of brittle, glassy polymers like epoxy is challenging due to the low strains to failure.

Arange of experimental data has been generated describing the response of glassy polymers to high strain rate loading in compression. Recently, research programs that study the combined effects of temperature and strain rate have made significant steps in providing better understanding of the physics behind the observed response, and also in modeling this response. However, limited data are available in tension, and even more limited are data describing both the compressive and tensile response of the same polymer. In those studies that do examine tensile response, often there are large gaps in the strain rate dependence. These gaps are due to the relative difficulty of performing characterization experiments in tension, especially on polymers and especially at high rates.

Tension testing of brittle, glassy polymers, like epoxy, is even more challenging due to the low strains to failure. This brittleness can result in invalid tests due to failure outside the gauge length and susceptibility to bending. In order to achieve valid tension tests on epoxy at high strain rates, pulse shaping techniques have been developed. Add itionally, digital image correlation coupled with high-speed photography has been used to measure the full field strain state in situ.

Polymers exhibit pressure-dependent yield, which has been measured in the past using complex loading apparatus. However, comparison of the tensile and compressive yield stresses of individual polymers can also result in the determination of the hydrostatic pressure depen dence in these materials.

Impact-resistant PVC (Type II) in the form of 25.4-mm diameter extruded rod was machined into specimens of the appropriate dimensions. Right circular cylinders were used for all compression experiments, with the quasi-static experiments using 8 x 8 mm samples and the medium rate and dynamic experiments using 8-mm diameter by 3.5-mm samples. The samples for tensile experiments were designed with a shortened gauge length and reduced radius of curvature in order to promote sample failure within the gauge length.

Dynamic Mechanical Analysis (DMA) samples were tested in dual cantilever configuration in a TA Instruments Q800 at frequencies of 1, 10, and 100 Hz; displacements of 5, 10, 15, and 25 μm; and a temperature range of -100 °C to 190 °C.

The dynamic mechanical analysis of PVC showed that the α, or glass, transition varies from 79.5 °C at 1 Hz to 83.7 °C at 100 Hz. The lower temperature β phase transition, attributed to restrictions in secondary chain motions, moves from -44 °C at 1 Hz to -30.8 °C at 100 Hz. The β transition changes more than the glass transition over the same frequency range due to the lower activation energy for the β transition.

The strain rate is determined from the test frequency, displacement, and gauge length. Extrapolation of the β phase transition to room temperature results in a strain rate of ~7000 s-1. The stress increases with strain rate in both cases. In both sets of experiments, the stressstrain response is typical for a glassy polymer, with an initial elastic region followed by a non-linear elastic region and yield, then strain softening followed by strain hardening. At higher strain rates (>0.06 s-1), the strain hardening is masked by thermal softening due to the transition between isothermal and adiabatic test conditions.

There are oscillations in the high strain rate tensile experiments, which are believed to be an experimental artifact. The real stress-strain curve is believed to be an average line fitted through the oscillations. The regular noise on this signal is due to the camera recording a picture at set intervals. For many glassy polymers, the beta phase transition results in increased yield strength under high strain rate loading. For PVC, the DMA results predict this transition at ~7000 s-1. The yield strength as a function of strain rate is approximately linear, at perhaps other than the highest strain rate compression experiments.

The compressive and tensile stress-strain curves for PVC across a range of strain rates are typical for glassy polymers, and the yield strength increases with strain rate. The compressive yield stress is consistently higher than the tensile yield stress, and both are linearly dependent on strain rate within the regime tested. The relationship between the compressive and tensile yield stress will be linear when a single, simply activated flow process is driving the behavior of the polymer, which is in agreement with the extrapolation of the β transition strain rate from DMA data.

This work was done by J.L. Jordan and B.T. Woodworth of the Air Force Research Laboratory, and C.R. Siviour of the University of Oxford. AFRL-0219

Topics:
Engine components Engine cylinders Engine mechanical components Engines Glass Hardening Materials Materials properties Metallurgy Polymers Powertrains Research and development Resins Tensile strength Thermoplastics
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    Defense Tech Briefs Magazine

    This article first appeared in the December, 2012 issue of Defense Tech Briefs Magazine (Vol. 6 No. 6).

    Read more articles from this issue here.

    Read more articles from the archives here.

    Overview

    The document titled "High Strain Rate Mechanical Properties of Glassy Polymers" is an interim report authored by Jennifer L. Jordan, C.R. Siviour, and B.T. Woodworth, published by the Air Force Research Laboratory (AFRL) on July 25, 2012. It focuses on the mechanical properties of glassy polymers when subjected to high strain rates, which is crucial for applications in defense and aerospace where materials are often exposed to extreme conditions.

    The report is part of the AFRL's efforts to advance the understanding of material behavior under dynamic loading conditions. Glassy polymers are widely used in various applications due to their favorable properties, such as lightweight and high strength. However, their performance under high strain rates—such as those encountered during impacts or explosive events—requires thorough investigation to ensure reliability and safety.

    The document outlines the methodology used in the research, including experimental setups and testing procedures designed to measure the mechanical properties of these materials under high strain rates. The findings are expected to contribute to the development of better materials for military applications, enhancing the performance and durability of systems that rely on these polymers.

    The report is approved for public release, indicating that the information can be shared without restrictions, which is significant for fostering scientific and technical exchange. It also includes a distribution list for interested parties, such as the Defense Technical Information Center (DTIC), where qualified requestors can obtain copies of the report.

    Additionally, the document emphasizes the legal considerations regarding the use of government data and the rights associated with the findings. It clarifies that while the U.S. Government holds rights to the work due to the involvement of government employees, any other use of the findings may be subject to copyright restrictions.

    In summary, this interim report serves as a valuable resource for researchers and engineers interested in the mechanical properties of glassy polymers under high strain rates, providing insights that could lead to advancements in material science and engineering, particularly in defense-related applications.

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