Dynamic tests on composites - High-frequency characterization

Organic matrix composites have damping properties that are essentially due to the viscoelastic nature of the matrix and the properties of the fiber-matrix interface. These properties are more or less strongly dependent on frequency and temperature. In composites, these properties have a directional dependency, which implies that this type of composite material is an anisotropic damping medium.

While the elastic properties of composites can be determined using quasistatic tests, damping properties inherently require the use of dynamic tests. In the first part of the analysis devoted to dynamic tests on composites we have examined the principles and devices for obtaining the technical moduli of a composite material (except for Poisson's ratio) at frequencies of up to ten kilohertz. This second part, devoted to wave propagation testing on composites, completes the "dynamic testing" set-up by acting in two directions: to complete the stiffness (or flexibility) matrix, and to get to know the material at frequencies well in excess of ten thousand hertz.

Devices based on wave propagation must avoid standing wave phenomena, which means testing in semi-infinite media. In other words, the wavelength must be much smaller than the size of the material sample. Given the speed of waves in composites (2,000 to 5,000 m/s), tests must be carried out at ultrasonic frequencies. We will illustrate dynamic testing by characterizing a viscoelastic composite with unidirectional fibers and a laminated composite in the form of a cylindrical tube.

This article is the second in a series devoted to dynamic testing of composites:

• Dynamic tests on composites. Characterization at low frequencies ;

• Dynamic tests on composites. Characterization at high frequencies ;

• Dynamic tests on composites. Find out more.