Particle size effect on strength, failure, and shock behavior in polytetrafluoroethylene-Al-W granular composite materials

Abstract

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

Role of material properties and mesostructure on dynamic deformation and shear instability in Al-W granular composites

“…This behavior is similar to observations in the PTFE-Al-W mixtures and in the experiments with the Al-W powder mixture. 11,12,[16][17][18] The global shear localized zones and the subsequent cracks developed at an approximate 45 degree angle but was kinked by the rigid W particles resulting in the initial shear band forming in a range of angles from 36 to 50 degrees (see Fig. 9).…”

“…A similar effect of rigid and heavy W particles is instrumental also in the shock loading of corresponding mixtures allowing tailored redistribution of internal energy between components. 11,12 G. The role of the constitutive behavior of Al…”

“…This behavior is similar to the behavior observed in. 11,12,[16][17][18] The two-dimensional simulation of the unbonded material, due to the material rearrangement, had near zero engineering stress on the top and bottom boundary and was not included. The comparable strength of bonded and unbounded samples in experiments is mostly due to interconnected W wires that cannot be accounted in these two-dimensional simulations.…”

“…6 Additionally, mesoscale features like force-chains in granular energetic materials may also serve as ignition sites. [7][8][9] Quasi-static, Hopkinson bar, and drop-weight experiments were performed for PTFE-Al-W and for Al-W composites [10][11][12][13][14][15][16][17][18] with different particle sizes of W, porosity, and morphology. W particles were used to increase sample density and generate the desirable mode of disintegration of the sample into small sized debris.…”

Mechanics of particulate composites with glassy polymer binders in compression

“…These studies have shown that particle size [15,19,21,22], shape [23] and concentration [24] and properties of the constituents can affect mechanical properties. In Al 2 O 3 particle-filled epoxy (Epon 828/Z), increasing the particle concentration and decreasing the particle size is found to increase the stress corresponding to 4% plastic strain [25].…”