Abstract (Invited)

Nonlinear Vibro-Acoustic Imaging for Non-Destructive Flaw Detection

N.Krohn, G.Busse (Institute for Polymer Testing and Polymer Science (IKP) Nondestructive Testing (ZFP), University of Stuttgart, Stuttgart, Germany); I.Yu.Solodov (Department of Acoustics, Faculty of Physics, Moscow State University, Moscow, Russia)

e-mail: krohn@ikp.uni-stuttgart.de

Nonlinear dynamics of non-bonded interfaces in solids was observed by various ultrasound techniques but mostly for idealized (specially prepared) surfaces in contact. However, the opportunities for applications of those results to a practical context are still not obvious. The matter is that much larger gap and roughness for realistic boundaries of cracked defects may prevent development of nonlinear effects for small vibration amplitudes in high frequency range. By reducing the frequency, much higher amplitudes can be generated to solve this problem. In the experiment, vibro-acoustic excitation of fundamental frequency 20 kHz is performed by a piezoelectric stack actuator clamped to a sample. A high power diffuse like sound field is generated in the specimen, so that a single point excitation is sufficient for subsequent nonlinear raster imaging. A scanning laser vibrometer is used for measuring the local vibration spectra. Resonant field patterns of the sample have to be avoided because local vibration knots appear as "dark areas" in both linear and nonlinear images. The vibro-acoustic imaging using the higher harmonic (up to 10) and nonlinear frequency mixing modes demonstrates an excellent sensitivity to delicate features of local nonlinearity associated with damaged areas. The results of extensive investigations of the nonlinear imaging applied to a variety of industrial component-like samples with material specific damages will be presented.

Section : 3