Reactive materials can be loosely categorized as composites of inert solid materials which, when subjected to a violent mechanical stimulus such as an impact, react exothermally with a rapid release of energy. This reaction, while aptly described as “explosive,” differs from a true detonation or deflagration in that it requires a mechanical stimulus to not only initiate the reaction but also to sustain it. Such materials can also be fairly robust mechanically and can serve as substantial structural components. Because of these properties, reactive materials have a number of potential ordnance applications. Various compositions have been investigated to tailor properties of reactivity, strength, and density to suit particular needs.

Figure 1. Engineering Stress vs. Engineering Strain for Al/PTFE (compression). All specimens were initially at room temperature (22 °C).
An aluminum/ polytetrafluoroethylene (Al/PTFE) formulation serves as a benchmark for current reactive material development. In addition to material development, work is underway to develop physics-based modeling capabilities for reactive materials. Basic constitutive models were developed for the inert behavior of this material. Compression tests were performed over a range of strain rates and temperatures relevant to the conditions present during low-speed impact. This data, presented in the following sections, is used to generate parameters for both the Johnson-Cook (JC) and Modified Johnson-Cook (MJC) constitutive models. Although not ideally suited to represent this material, these parameters are given primarily due to the widespread use of these models and their availability in the current suite of hydrocodes. An additional, and more appropriate, fit is given for the Zerilli-Armstrong (ZA) model for polymers.

Figure 2. Lateral Strain as measured by a laser extensometer in the SHPB tests plotted in Figure 1 (thin black curves). The heavy red curve shows that which would be expected from incompressible behavior.
The samples tested were supplied by General Sciences, Inc. (GSI) and were made from a material designated GSI-0017. It is a pressed and sintered mixture of aluminum and PTFE powders, 26.5% and 73.5% by weight, respectively. The initial powder sizes are 44 and 31 μm, respectively. The density of the compacted material, as measured by a buoyancy method based on Archimedes principle, is 2.29 g/cm3

Low-rate tests were performed with an Instron Model 1331 servo-hydraulic load frame. The load applied to the specimen was measured with a load cell, and the specimen deformation was measured using a linear variable differential transformer (LVDT) measurement of the cross-head displacement, and includes a correction for machine compliance. The specimens were cylindrical, nominally 6.35 mm in both diameter and length. Contact surfaces were lubricated with a heat-stable silicone lubricant. All of the low-rate tests were performed at room temperature (22 °C).

A 6.35-mm-diameter 7075-T6 aluminum Split Hopkinson Pressure Bar (SHPB) was used for the high-rate tests. A series of six tests was performed at rates from 600 to 8000/s, all initially at room temperature (22 °C). The specimens were cylindrical, 3.18 mm in diameter and length, and contact surfaces were lubricated with the same silicone lubricant used in the low-rate tests.

A final set of experiments was performed at elevated temperatures to quantify the thermal softening behavior of the material. These were performed at a consistent strain rate of 4000/s using the SHPB. Heating was accomplished by circulating heated air into a chamber that enclosed the specimen and the adjacent ~65 mm sections of the bars. Ideally, specimen temperature would have been monitored directly with a thermocouple glued to each specimen. However, this proved impractical because of the small sample size and also because of the difficulty in adhering gages to the specimen. Instead, specimen temperature was measured with a thermocouple probe placed within 1 cm of the specimen; i.e., the probe measures ambient air temperature and not the specimen temperature directly. The temperature in the chamber was allowed to equilibrate over a 20-minute period prior to each test to ensure that the specimen and relevant sections of the bars were allowed to reach the ambient temperature. The temperature gradient in the bars is believed to have negligible effects on the bar wave propagation and the strain gage measurements. Temp erature measurements made in this way are estimated to be accurate to within ±2 °C.

Figure 1 shows the engineering stress-strain curves obtained from the room-temperature tests. Because of the brief duration of the deformation, the high rate curves (600/s and beyond) are considered adiabatic. In contrast, the lowest rate curves, 0.001 and 0.01/s, are considered isothermal. The intermediate rate, at 0.1/s, is probably somewhere between the two limits.

Figure 2 shows the tensile radial engineering strain as measured by the laser extensometer as a function of compressive axial engineering strain for the six SHPB experiments shown in Figure 1. These are plotted as thin black curves. The heavy red curve is the relationship that would result from incompressible deformation. The agreement is good, and what deviation can be measured could easily be explained by barreling.

Data was fitted to the JC and MJC constitutive equations. These models were chosen primarily because of their widespread use, and not necessarily because they are particularly well suited to this material.

Zerilli and Armstrong have developed constitutive models for metals based on thermally activated dislocation motion. Recently, they have adapted the basic framework to apply to viscoplastic deformation of polymers. They have used it to model PTFE, and Cai et al. have used it to model a composite mixture of PTFE-Al-W. Based on the success of these applications, it was decided to use this model to represent the present data. Although the model is physically based, the approach taken here is to treat it as a curve fit; i.e., the values presented below may not necessarily be realistic beyond the observation that they match the existing data.

This work was done by Daniel T. Casem of the Army Research Laboratory. ARL-0051

This Brief includes a Technical Support Package (TSP).
Mechanical Response of an Al-PTFE Composite to Uniaxial Compression

(reference ARL-0051) is currently available for download from the TSP library.

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