Tech Briefs

The mechanical and physical properties of a composite depend on the mechanical and physical properties of the individual components.

Polymer composites comprised of metallic particles distributed throughout a contiguous polymer matrix can often be modified to produce advanced composites that exhibit multifunctional characteristics. The properties of the particulate composites often depend on varying particle size, loading fractions, particle type, and the adhesion between the particulate and the matrix. Studies have shown that particle size, shape, and concentration, and properties of the constituents can affect mechanical properties. This work compared aluminum and nickel particles in PMMA thermoplastic prepared by injection molding, with the same particles in epoxy prepared in a cast-cure process.

Stress-Strain Response of RXRW-4, PMMA-Al-Ni, MNML-4, and Epoxy- Al-Ni, at a strain rate of 5000/s.
The samples were prepared using a factorial design of experiments approach in order to maximize the number of variables tested with the minimum number of test specimens. The variables tested were aluminum particle size (5, 30, or 50 μm), volume percent of aluminum (20, 30, 40 vol.%), and the volume percent nickel (0, 5, 10 vol.%). Two levels and a centerpoint were chosen for the samples. The binder was PMMA. These samples compare directly with the epoxy-based samples used in previous studies, with the exception that the PMMA-based samples added a centerpoint to test for curvature.

Dynamic compression experiments were conducted using a split Hopkinson pressure bar (SHPB) system at a strain rate of approximately 5000/s. The bar system is comprised of 1524-mm-long, 12.7-mm-diameter incident and transmitted bars of 6061-T6 aluminum. The striker is 610 mm long and made of the same material as the other bars. The samples, which were nominally 5-mm-diameter by 2.5-mm thick, are positioned between the incident and transmitted bars. The bar faces were lightly lubricated with grease to reduce friction.

Both sets of samples, PMMA-based and epoxy-based, were tested in compression at 5000/s. The figure shows representative curves from both materials containing 20 vol.% of 5 μm Al and 10 vol.% Ni. The compressive response of these materials is very different. The epoxy-based composite shows a rise to a peak stress, followed by a small decrease, a region of perfect plasticity, and then work hardening, similar to other epoxybased composites. This is consistent with the deformation of the epoxy binder with the strain softening after the peak decreased potentially due to particle-particle interaction. In contrast, the PMMA-based material shows a rapid rise to a peak stress, followed by a rapid strain softening, and then perfect plasticity at a very low stress. This is consistent with behavior of particulate composites that fail at the peak stress, where the low residual stress is the loading of sample fragments.

There is good agreement between the previously acquired epoxy-based data and that measured in this study. For both materials, the small aluminum samples generally have a higher strength than the large aluminum samples. For the epoxybased, the strength of the composites increases with increasing volume fraction of particles. For the PMMA-based composites, the strength of the composites decreases with an increase in volume fraction of particles. This decrease is believed to be due to the failure of the samples at the peak stress, which could be caused at lower stresses with a higher fraction of particles. These samples were loaded to 50 vol.% total particles. At this point, both the PMMA-based materials and epoxy-based materials seem to be converging to a common peak stress. This stress may be similar to that for a collection of particles as the particle-particle interaction begins to dominate the stressstrain response of the composites, irrespective of the particular polymeric binder used.

Multi-constituent particulate composites consist of individual particles of more than one material dispersed throughout and held together by a polymer binder. The mechanical and physical properties of the composite depend on the mechanical and physical properties of the individual components; particularly the binder, their loading density, the shape and size of the particles, the interfacial adhesion, residual stresses, and matrix porosity. The PMMA-based materials show a rise to peak stress followed by a sharp strain softening, indicating failure of the samples. In contrast, the epoxy-based samples show a nearly perfect plastic stress after a rise to peak stress.

This work was done by Dr. Jennifer L. Jordan and Dr. Jonathan E. Spowart of the Air Force Research Laboratory. AFRL-0223