Acoustic Emission Monitoring of CFRP Laminated Composites ...

26 Chapter 3. Acoustic Emission
ized (surface SV-waves) and Love waves are horizontally polarized (surface
SH-waves). Both are dispersive and the amplitudes decreases exponentially
with depth. Flexural waves, or Lamb waves, are a special class of
waves which occur in plates which are bounded by two surfaces, e.g. laminated
composites. There are three fundamental classes of Lamb waves,
analogous to the body waves: LP, LSH, and LSV. 123
Two modes of Lamb
waves are observed in thin plates: extensional (symmetric) and exural
(antisymmetric) modes. The largest component of the extensional mode
lies in the plane of the plate, while the largest exural mode component is
out of the plane. 124
The wave velocity in isotropic and homogeneous materials depends on
the density and stiness of the material. Fiber-reinforced composites are
anisotropic and inhomogeneous; hence, the wave velocity is anisotropic.
Furthermore, the wave velocity is a function of frequency, 123
i.e. high and
low frequency waves will separate as they propagate. This separation is
referred to as dispersion and results in an amplitude decrease. A parameter
which describes the resistance of a material to transfer waves is the
acoustic impedance. This parameter is analogous to electrical impedance.
The acoustic impedance depends both on the wave type and the frequency.
It is dened as the product of the material density with the velocity of the
wave type. When the waves come across inhomogeneities such as defects,
interfaces between the constituent materials of the composite, and the surface
of the composite, they will reect and refract. The actual response
depends on the acoustic impedance of the two adjacent materials and the
angle of incidence between the wave and reecting boundary. Assuming
that the wave's energy is conserved, the energy per unit area of the wave
decreases as the surface area of the wave increases. Hence, the rate of decrease
depends on how the wave propagates, e.g. as plane-, spherical- or
cylindrical waves. The wave's energy, however, is not conserved. Due to
internal friction, part of the energy is converted into heat. Consequently,
when a stress wave nally arrives at the transducer its waveform parameters
will have changed. For these reasons, the analysis and interpretation
of AE signals is a complex subject.