The main idea of composite material is to combine different materials to produce a new material with performance unattainable by the individual constituents. It gives flexibility to the designer to tailor the new material with properties to obtain peak performance for a particular application.

(Left) Time histories of the impact forces of GR Composites, and (right) time histories of the impact forces of GL/GR/GL composites. The impact force is the resistive force experienced by the impactor as it impacts the specimen. It can be seen that as the temperature decreases, the initial peak of the force increases.

Investigations into woven composites have been extensive over recent years mainly because of their excellent impact resistance, dimensional stability, ease of handling, etc. This work investigated the effect of temperature on hybrid and non-hybrid woven composite panels, drop-weight impacted at five different temperatures: -60 °C, -20 °C, room temperature, 75 °C, and 125 °C. The studies were conducted by combining experimental and 3D dynamic finite element approaches. The specimens tested were made of plain-weave hybrid S2 glass-IM7 graphite fibers/toughened epoxy.

The individual constituent materials combined to form the composite material studied in this research are: IM-7 graphite (IM7-GP 6000) and S2-glass (S2-4533 6000) woven fabrics in a SC-79 toughened epoxy matrix. The IM-7 graphite woven fabric and SC-79 epoxy matrix form the non-hybrid composite called GR. The GR specimen contained 28 layers of graphite fabrics. For the case of the hybrid composite, it consists of a GR core, which is made up of 16 layers. This core is sandwiched between two outer laminates. Each outer laminate is made up of 9 layers of S2-glass fabrics and SC-79 epoxy. The hybrid composite is called GL/GR/GL.

The composite was manufactured using the vacuum-assisted resin transfer molding (VARTM) technique to stack the plain woven fabrics together. The specimens were cured at 177 °C. Fiber volume fraction for all types was 55%. The final thickness of the specimens to be tested was 6.35 mm.

The composite panels were damaged using an instrumented drop-weight impact tester equipped with an environmental chamber for temperature control. The time-histories of impact-induced dynamic strains and impact forces were recorded. The damaged specimens were inspected visually, and using the ultrasonic C-scan method. A 3D dynamic finite element (FE) software package, with Chang-Chang composite damage model, was then used to simulate the experimental results of the drop-weight tests. Good agreement between experimental and FE results has been achieved.

At low temperatures, the material is more brittle so more of the kinetic energy of the impactor is dissipated in delaminating the specimen rather than elastically deforming the specimen. As the temperature increases, the delamination decreases because the material becomes softer and more ductile, and has better contact with the impactor. In this case, more energy of the impactor is dissipated in deforming the specimen rather than in delamination. At 125°C, the delamination area is larger than was expected, and also does not follow the delamination trend. It is believed that at this temperature, the material may become unstable, since the test temperature is very close to the curing temperature (177 °C) of the composite material.

It is observed that the variation of results obtained from the experiments for the hybrid composites was very small (about 8%) when compared to those of non-hybrid composites. Also, when looking at the hybrid or nonhybrid composite, the effect of temperature at -60 °C or -20 °C was not significant, whereas at 75 °C and 125 °C, the results were more distinct.

This work was done by Ramki Iyer and Basavaraju Raju of the U.S. Army – TARDEC; and Yougashwar Budhoo, Benjamin Liaw, and Feridun Delale of the The City College of New York. For more information, download the Technical Support Package (free white paper) at under the Materials category. ARL-0113.

This Brief includes a Technical Support Package (TSP).
Temperature Effect on Drop-Weight Impact of Hybrid Woven Composites

(reference ARL-0113) is currently available for download from the TSP library.

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This article first appeared in the December, 2010 issue of Defense Tech Briefs Magazine.

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