Dynamic tests on composites - Low-frequency characterization

Like all conventional materials, composite materials exhibit variable thermomechanical behavior depending on their field of application: dependence on stress, effect of temperature or environment. The mechanical tests carried out on this type of material are the classic ones, adapted to the specific environment and including destructive testing. However, the experimenter must keep in mind three concepts specific to composite materials.

• The first is anisotropy, which relates to the nature of the material.

  The composite material is macroscopically homogeneous, but exhibits more or less pronounced anisotropy depending on the nature of the reinforcements. A pure and simple transposition of the techniques applied to isotropic materials must be examined with the greatest care: the most classic example is the off-axis tensile test, which generates couplings of various stresses (tension, bending, torsion) and is a source of great dispersion if it is not taken into account.

• The second concept is heterogeneity, which influences testing to a lesser degree than anisotropy. If heterogeneity must be taken into account in fracture testing (crack propagation), it must also be taken into account in wave propagation characterization techniques, where inclusions can act as waveguides (in the case of fiber materials, for example).

• The third parameter to be taken into account during a test campaign is the shape of the specimens. The number of characteristics to be determined, and therefore the number of measurements, increases with the degree of anisotropy. A composite structure is a fixed structure from which characterization specimens are extracted. These specimens are of an imposed shape, which can give rise to a number of difficulties. For example, how do you determine the Young's modulus in the thickness direction of a thin, anisotropic plate? How can curvature be taken into account when specimens are extracted from a tube or composite tank?

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 mechanical properties depend to a greater or lesser extent on excitation frequency and temperature. In the case of composite materials, these properties are directionally dependent, making them an anisotropic damping medium.

While the elasticity properties of composite materials...