Mechanical loss in a glass-epoxy composite
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Stuttgart,
Hassel Ledbetter Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Boulder, Colorado 80303 (Received 30 August 1989; accepted 29 January 1990)
Using a computer-controlled inverted torsion pendulum at frequencies near 1 Hz, we determined the mechanical losses in a uniaxially fiber-reinforced composite. The composite comprised glass fibers in an epoxy-resin matrix. We studied three fiber contents: 0,41, and 49 vol. %. Three mechanical-loss peaks appeared: above 300 K, near 200 K, and near 130 K. They correspond closely to a, /3,andy peaks found previously in many polymers. We failed to see a mechanical-loss peak for either the glass or the glass-resin interface. Between 300 and 4 K, the torsion modulus increased in the resin by a factor of 3.30 and in the 0.49 glass-epoxy by a factor of 2.37.
In composites, mechanical loss arises from three sources: the occluded phase, the matrix phase, and the interface. In structural composites, the reinforcing phase usually possesses high elastic stiffness and low mechanical loss. Thus, composites with good (lowmechanical-loss) interfaces possess a lower mechanical loss than the matrix phase. Whether usual composite-material interfaces contribute measurably to mechanical loss remains uncertain. Some studies report an interface contribution.1 Others found none.2"4 This question assumes importance for several reasons. First, mechanical loss is a fundamental physical property whose understanding pays many dividends.5 Second, mechanical loss (or damping) enters many composite-material engineering applications.6 Third, we need to know whether mechanical loss provides a useful tool for studying interfaces. A recent review on composite-material plastic deformation concluded that "interfaces play the most important role in the behavior of a composite. "7 Fourth, current theoretical studies8'9 on interface damping would profit enormously from experimental studies. This study used a torsion pendulum to determine the low-frequency mechanical loss in glass-fiberreinforced epoxy-matrix composites. Previously, for these composites, Ledbetter and coworkers4 used kilohertz standing-wave methods to measure the Young's modulus and mechanical loss at ambient temperature. Composites were made at NIST by using vacuumimpregnation methods described by Kasen.10 The manufacturing method yielded 3-mm-diameter cylinders up to 30 cm long. These were cut to 5-6-cm lengths and turned on a lathe to 2-mm diameter. The matrix mate-
rial consisted of a commercial epoxide resin formulated for use in a low-temperature radiation environment. The reinforcing phase consisted of rows of commercial glass fibers 7 /xm in diameter with a reported mass density of 2.60 g/cm3 and a Young's modulus of 72.4 GPa. Figure 1 shows a photomicrograph of the 41%-glass composite. Table I contains the mass densities of all three studied materials. Fiber volume fraction was determined from glass mass, glass mass density, and mold volume. We measured mechanical loss with an invert
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