Characterization of Cracks in Thick Plates by Lamb Wave Diffraction Studies

Abstract

INTRODCCTIONThe object of the work is a combined theroretical and experimental study of diffraction of Lamb waves by two-and three-dimensional cracks in a plate. An important motivation for this project is the desire to develop a technique capable of detecting and characterizing cracks in weldments in plates. We want to analyze the dependence of the scattered signals on the dimension, angle of inclination and depth of burial of the crack. The basic measurement techniques requires the accurate determination of minute normal surface displacements at ultrasonic frequencies. Such measurements have recently been made possible by the development in our laboratory of a new surface displacement capacitive tranducer Ilj.The measured displacements will be compared with the theoretical predictions. EXPERIMENT ConfigurationThe first generation of measurements is being made on a 2 dimensional model. The specimen is a glass piece of 6.35 mm thickness, 25.4 mm height, and 2286 mm length. The signal in the specimen is generated by a steeel ball bearing with a diameter of 1.59 mm diameter being dropped from a height of 165 mm onto the smallest dimension edge of the specimen. Normal displacements are measured along the impacted surface of the specimen at various locations. For these idealized tests the specimen is mechanically decoupled from its supports and from the receiver transducer housing. This provides isolation from floor vibrations and prevents loss of the signal into the support structure. 1521Aluminium-coated coverslides of 0.18 mm thickness are glued on the plate to provide electrically conductive and smooth surfaces required by our capacitive transducer. The receiver output is recorded on a 20 MHz digital oscilloscope with pretriggering capability and can be transferred to a computer for analysis. Capacitive TransducerThe principle of our capacitance transducer is simple. The grounded conducting sample surface constitutes one electrode of a capacitor and the capacitance probe the other. A surface displacement changes the gap between the sample and the capacitance probe. The resulting change in capacitance constitutes the signal. A detailed description of the instrument is given in [1].There are several advantages of the capacitive transducer compared with the registration of ultrasonic waves via piezoelectric transducers:1.) The transducer is non contacting and does not interfere with the displacements of the free surface of the sample.2.) This eliminates problems with the reproducibility of the transducer-sample contact as found with piezoelectric transducers.3.) Because the transducer has no mechanical resonances and the detection circuits are suitably broad-band the response function is flat over a wide frequency range.4.) Due to the small probe diameter of 0.3 mm the transducer acts as a true point receiver over the entire frequency spectrum of interest. Thus problems with incoherent arrival of energy, which occur with larger sensing areas are avoided. In it's present configuration, it measures only that componen...