|
In structural health monitoring (SHM) of composite members, the choice of sensor positions, frequency band and signal interpretations are directly affected by the attenuation levels. Hence, it is important to consider the influence of attenuation for quantitative interpretation of signals in SHM applications. In this paper, attenuation of the two fundamentals Lamb wave modes, namely the symmetric mode S0 and antisymmetric A0 were experimentally measured in Carbon Fiber Reinforced Polymer (CFRP) laminates. The stress waves were launched using piezoelectric wafers bonded to the center of the laminate. Symmetric and antisymmetric modes were excitation through appropriate combination of two piezoelectric wafers bonded on opposite surfaces of the plate at the same location. The out of plane displacements corresponding to both modes were measured using a scanning laser vibrometer (SLV), along different orientations starting from 00 to 900 at 150 increments. Attenuation coefficients were obtained from 100 kHz to 350 kHz for both cross-ply as well as quasi-isotropic laminates. The attenuation coefficients ranged from about 5 nepers/m to 40 nepers/m. The experimental results as obtained by the scanning laser vibrometer compared favorably with results available in the literature. The antisymmetric mode was found to undergo significantly higher attenuation compared to the symmetric mode. The attenuation of the two modes further depends on the direction of propagation and their frequency components. Because of the dependence of the attenuation level on the Lamb wave mode, frequency, and direction of propagation, a wide frequency content signal such as acoustic emission will undergo substantial changes before being detected by sensors. Successful acoustic emission monitoring will have to take such attenuation into account to get reliable results.
|
