Experimental study of the compression properties of Al/W/PTFE granular composites under elevated strain rates

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Abstract

Granular composites consisting of aluminium (Al), tungsten (W) and polytetraflouroethylene (PTFE) are typical energetic materials, which possess high density and strength along with other advantageous properties. To investigate the mechanical behaviour of Al/W/PTFE granular composites, compression tests of three Al/W/PTFE mixtures under quasi-static loading and high strain rate conditions were conducted on a CSS-44100 Materials Testing System and a Split-Hopkinson Pressure Bar (SHPB), respectively. By employing Al bars, the amplitude of the transmitted signal was significantly enhanced and a high signal-to-noise ratio was obtained. This enhancement was due to the decreased Young's modulus of the bars, which led to increased signal amplitude from the strain gauges. The Al/W/PTFE granular composites were processed using a cold isostatic pressing and vacuum sintering approach. In this work, the fracture modes and stress-strain relationships of Al/W/PTFE composites with mass ratios of Al:W:PTFE of 24:0:76, 12:50:38 and 5.5:77:17.5 were studied. A detailed discussion is provided to cover the effect that tungsten addition, strain rate and mass ratio have on the deformation behaviour of the composites. The results show that the mass ratio plays a significant role in determining the dynamic behaviour and failure modes of the composites. Both the Al/W/PTFE (24:0:76) and the Al/W/PTFE (12:50:38) composites are strain rate dependent, elasto-plastic materials characterised by increased yield stress with increased strain rate. However, the Al/W/PTFE (5.5:77:17.5) composite is a brittle material, which shows brittle fracture at a relatively low strain.

Introduction

Multifunctional energetic structural materials (MESMs) are a special category of energetic composites that consist of two or more non-explosive solid materials. These materials integrate a high energy density with rapid energy release and at least one other desirable designed functionality, e.g., mechanical strength. MESMs will undergo fast burning or explosion and release a large amount of chemical energy under impact loading. These materials can consist of combinations of thermites, intermetallics, metal-polymer mixtures, metastable intermolecular composites (MICs), matrix materials or hydrides [1], [2]. Among these, the granular metals and polymers are classes of composite materials, whose constituents can undergo reactions as a result of impact loading [3]. Granular composite mixtures of aluminium (Al) and polytetraflouroethylene (PTFE) are one such type of multifunctional energetic structural material. PTFE is a common polymer employed to make these mixtures, and it has many desirable properties, i.e., a low friction coefficient, high thermal stability, high electrical resistance, high chemical inertness, high melting point, high melting viscosity, and easy deformability [4]. Traditionally, Al/PTFE mixtures have relatively low densities and material strength characteristics compared with conventional metals or composites. To improve the structural strength and density of Al/PTFE granular composites, tungsten (W) can be used to keep the composition close to the stoichiometric value [5], [6]. A thorough understanding of the mechanical behaviour of Al/W/PTFE granular composites, especially their dynamic compression behaviours and their corresponding constitutive relationships, is vital for exploiting their multifunctional character.

Over the past several decades, many researchers have made notable progress in understanding the mechanical behaviour of granular/porous reactive materials from both an experimental and numerical perspective. In the case of the former, Herbold et al. [6] performed quasi-static and dynamic experiments on samples that had identical constituent mass fractions but differed in the size of their constituent W particles from which they were composed under a variety of pressing conditions. Cai et al. [7], [8], [9], [10] carried out both experiments and simulations on Al/W/PTFE granular mixtures by changing the morphology of the constituent particles and their porosity. Hopkinson bar techniques have been used to determine the rate dependent stress-strain relationship of composites at different strain rates. Various factors influencing the mechanical response of Al/W/PTFE granular mixtures have been studied by Olney et al. [11], [12], Zhao et al. [13], Xu et al. [14], Yang et al. [15] and Xu et al. [16], [17]. Raftenberg et al. [18] and Rae et al. [19] studied the deformation properties of solid rods of Al/PTFE mixtures using the Taylor experimental method. Casem [20] carried out a series of compressive tests with a servo-hydraulic load frame and a Split-Hopkinson Pressure Bar and obtained data on the mechanical response of Al/PTFE composites at different strain rates and temperatures. Mock et al. [21] investigated the shock initiation behaviour of rods by gas gun experiments, which involve high impact velocities. Parameters for the Pressure Shear Damage (PSDam) model and the Johnson-Cook model were determined from these experimental results. In addition, various methods were also used to investigate the equation of state (EOS) and shock induced chemical reaction (SICR) behaviour of composite mixtures. Qiao et al. [22] and Nesterenko et al. [23] carried out mesoscale modelling to determine the shock compression behaviour of Al/W/Binder mixtures. Ames et al. [24], [25], [26] and Wang et al. [27], [28] developed a vented chamber to evaluate the performance of impact initiation on Al/W/PTFE mixtures. Both experimental and numerical results show that the absolute and relative particle size, morphology and porosity of the various granules play a significant role in the mechanical and fracture behaviour of the mixtures. However, little attention has been paid to the influence of the mass ratio on the mechanical properties and the dynamic failure behaviour of Al/W/PTFE granular mixtures to date.

In this paper, the quasi-static strength and failure modes of Al/W/PTFE granular composites under dynamic loading are investigated. Three types of Al/W/PTFE granular composites were processed using a cold isostatic pressing and vacuum sintering approach. Experiments were then conducted to investigate the influence of the mass ratio of W on the corresponding mechanical strength of these granular composites. Quasi-static compression and Split-Hopkinson Pressure Bar (SHPB) experiments were performed to obtain stress-strain curves for these materials at high strain rates. The effects of adding tungsten powders and varying the strain rate and the mass ratio on the failure modes of the Al/W/PTFE granular composites are discussed in detail herein.

Section snippets

Material types

In this study, Al/W/PTFE granular composites with mass ratios of 24:0:76, 12:50:38 and 5.5:77:17.5 were considered. The mixtures consisted of a spherical aluminium powder with an average particle size of 2 μm and a platelet-shaped PTFE powder with a sub-micron particle size. Table 1 shows the wt % mass ratios of these three Al/W/PTFE granular composites and the corresponding theoretical material densities (TMD).

Fabrication of Al/W/PTFE specimens

A detailed description of the fabrication process for these Al/W/PTFE granular

Mesoscale properties of Al/W/PTFE granular composites

The mesostructure of a granular mixture, including the particle size, shape and morphology, plays a significant role in its mechanical behaviour and dynamic response. To gain insight into the mesostructure of the three composite mixtures, images were collected using optical and scanning electron microscopy (SEM) (Fig. 3, Fig. 4, Fig. 5). The W particles appear as bright features in the backscattered images because W has a high atomic number. Although the Al and PTFE particles cannot be

Quasi-static compression system

Quasi-static compression tests were conducted using a CSS-44100 Universal Materials Testing Machine (Changchun Machinery Research Institute Co., Ltd, Changchun, Jilin, China) with a 300 kN loading capacity. The tests were carried out under displacement control with a constant cross-head speed of 0.6 mm/min. This corresponds to a nominal strain rate of 10−3 s−1. The longitudinal strains were measured by a displacement gauge situated between two mechanical grips that were attached to the specimen.

SHPB systems

The Hopkinson bar technique was initially developed by Hopkinson [30] and later refined by Kolsky [31]. The Split-Hopkinson Pressure Bar (SHPB) is based on the classical mechanics of elastic wave propagation in the bars and the principles of wave superposition. Two assumptions are made in these experiments. The first assumption is that the elastic wave propagation in the bars occurs in only one dimension; the second assumption is that the stress state and deformation are homogenous and uniform

Conclusions

An experimental study of the mechanical properties of three Al/W/PTFE granular mixtures possessing different mass ratios was performed using quasi-static and SHPB compression techniques. This work provides new insight into the effect of the mass ratio of the reactive material on its mechanical behaviour. The following conclusions can be drawn from this study:

  • (1)

    Specimens with increasing W mass ratios were prepared. Then, the effect of the amount of W on the density and ultimate compressive

Acknowledgments

This research was sponsored by the National Natural Science Foundation of China (NSFC10902053), the Zijin Intelligent Program, the Nanjing University of Science and Technology (2013_ZJ_0101) and the Qing Lan Project of Jiangsu province.

References (33)

  • J. Cai et al.

    Mater. Sci. Eng.: A

    (2008)
  • J. Cai et al.

    Mater. Sci. Eng.: A

    (2008)
  • M.N. Raftenberg et al.

    Int. J. Impact Eng.

    (2008)
  • L. Qiao et al.

    Mater. Des.

    (2013)
  • N.N. Thadhani

    J. Appl. Phys.

    (1994)
  • National Research Council
  • R.J. Lee et al.
  • C.A. Sperati et al.

    Adv. Polym. Sci.

    (1961)
  • J. Cai et al.

    Appl. Phys. Lett.

    (2008)
  • E.B. Herbold et al.

    J. Appl. Phys.

    (2008)
  • J. Cai et al.
  • Cai J, Properties of Heterogeneous Energetic Materials under High Strain, High Strain Rate Deformation, Ph.D. Thesis,...
  • K.L. Olney et al.

    J. Appl. Phys.

    (2011)
  • K.L. Olney et al.

    AIP Conf. Proc.

    (2012)
  • P. Zhao et al.

    DYMAT

    (2009)
  • S.L. Xu et al.

    Chin. J. High Pressure Phys.

    (2009)
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