Material parameters and defects in anisotropic plates determined by hologram interferometry

Abstract: Material parameters and defects in composite plates are determined in a non-contacting and nondestructive way by use of optical methods. Material parameters are determined from TV-holography images, and defects are detected by holographic interferometry with a double pulsed laser as the light source. A plate excited into harmonic vibration will be in resonance for certain frequencies. At each such frequency the plate vibrates in a certain manner, called a mode. Experimentally determined frequencies and shapes of these modes of vibration for rectangular orthotropic and anisotropic plates are compared with the results of FE-calculations or calculations using Rayleigh's method. In both cases it is possible to estimate the two Young's moduli, the in-plane shear modulus and Poisson's ratio of the orthotropic and the anisotropic plates. Defects are detected by impacting the plates with a small pendulum. This creates propagating transient bending waves in the plates. These waves are recorded using holographic interferometry. A defect gives rise to anomalies in the interference pattern. Large defects can be examined in the interferograms with the naked eye. Small defects, such as delaminations with a diameter of 15 mm, could be discovered using a proposal method. The impacted plate problem is solved analytically for an isotropic plate. A similarity parameter depending upon thickness, density and effective material parameters is found. The existence of this variable brings new understanding to the importance of specific parameters for wave propagation in plates.