The variation of metallic particle size and sample porosity significantly alters the dynamic mechanical properties of high density granular composites processed using a cold isostatically pressed mixture of polytetrafluoroethylene (PTFE), aluminum (Al) and tungsten (W) powders. Quasi-static and dynamic experiments are performed with identical constituent mass fractions with variations in the size of the W particles and pressing conditions. The relatively weak polymer matrix allows the strength and fracture modes of this material to be governed by the granular type behavior of agglomerated metal particles. A higher ultimate compressive strength was observed in relatively high porosity samples with small W particles compared to those with coarse W particles in all experiments. Mesoscale granular force chains comprised of the metallic particles explain 2 this unusual phenomenon as observed in a hydrocode simulation of a drop-weight test.Macrocracks forming below the critical failure strain for the matrix and unusual behavior due to a competition between densification and fracture in dynamic tests of porous samples were also observed. Shock loading of this granular composite resulted in higher fraction of total internal energy deposition in the soft PTFE matrix, specifically thermal energy, which can be tailored by the W particle size distribution.Porous PTFE-Al-W specimens containing coarse W particles (denoted "porous PTFE-Al-coarse W") were processed with a significantly reduced CIPing pressure (20 MPa) to investigate the behavior of materials with different porosity and different particle sizes of W powder. This resulted in a similar porosity to samples with fine W particles (compare the Porous PTFE-Al-fine W sample with the Porous PTFE-Al-coarse W sample

The dynamic mechanical properties of reactive materials (e.g., high density mixtures of polytetraflouroethylene (PTFE), aluminum (Al) and tungsten (W) powders) can be tailored by changing the morphology of the particles and porosity. Cold isostatically pressed PTFE-Al-W powder composites with fine metallic particles and a higher porosity exhibited higher ultimate compressive strength than less porous composites having equivalent mass ratios with coarse W particles. The mesoscale force chains between the fine metallic particles are responsible for this unusual phenomenon. We observed macrocracks below the critical failure strain for the matrix and a competition between densification and fracture in some porous samples in dynamic tests. 2Mixtures containing polytetrafluoroethylene (PTFE) and aluminum (Al) are known to be energetic under dynamic and/or thermal loading [1][2][3][4][5][6] . They are similar in composition to thermites 7 , a subgroup of the class of pyrotechnics, and are formulated to generate a large quantity of heat during the reaction driven by mechanical deformation in the bulk material. This paper describes the mechanical behavior of the PTFE-Al-W composites with varying W particle size and porosity. Varying cold isostatic pressing conditions introduces different porosities and component configurations within the samples. The quasi-static and dynamic ultimate compressive strength of the composites were measured to identify influence of their mesostructure on mechanical properties and fracture. The unusual phenomenon of increased strength of porous composites with reducing size of metallic particles was observed. A two-dimensional Eulerian hydrocode is used to numerically model the composite systems to explain experimental phenomenon.Tailoring the mechanical and chemical properties of reactive materials is important for various applications. For example, varying particle size and morphology in pressed explosives (HNS) 8 or layer thicknesses in laminate 9 can be used for tailoring of shocksensitivity and the rate of energy release. The stress/force chain formation in granular energetic materials can be related to ignition sites within composite energetic materials under a compressive load 10 . Bardenhagen, Brackbill 11 and Roessig, Foster, and Bardenhagen 12 examined the localized stress propagation due to force chains and effects of binder in two dimensional particle bed under static and dynamic loading.Cold Isostatic Pressing (CIPing) was used to prepare specimens from a mixture of 17.5 wt% PTFE, 5.5 wt% Al, and 77 wt% W powders with different porosities. The initial powders had the following average sizes: Al: 2 μm (Valimet H-2); coarse W powder: < 44 μm (Teledyne, -325 mesh) and fine W powder with particle sizes < 1μm 3 (Alfa-Aesar); PTFE: 100 nm (DuPont, PTFE 9002-84-0, type MP 1500J). The mixed powders were ball milled in the SPEX 800 mill for 2-10 minutes using alumina balls with a 1:5 mass ratio of balls to powder to break down the agglomeration of powders. Table I shows the density of vari...

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Conventional drop-weight techniques were modified to accommodate low-amplitude force transducer signals from low-strength, cold isostatically pressed 'heavy' composites of polytetrafluoroethylene, aluminum and tungsten. The failure strength, strain and the post-critical behavior of failed samples were measured for samples of different porosity and tungsten grain size. Unusual phenomenon of significantly higher strength (55 MPa) of porous composites (density 5.9 g/cm 3 ) with small W particles (<1 μm) in comparison with strength (32 MPa) of dense composites (7.1 g/cm 3 ) with larger W particles (44 μm) at the same volume content of components was observed. This is attributed to force chains created by a network of small W particles. Interrupted tests at different levels of strain revealed the mechanisms of fracture under dynamic compression.

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