Defect Detection of a Thick Plate
Using Elastic Wave Propagation
Yong Han Kim
This paper presents a damage detection and localization method for a thick plate by using a two-dimensional elastic wave propagation. Generally, it is almost impossible to obtain analytic solutions for wave propagation problems of thick plates in which guided waves and bulk waves propagate simultaneously. Wave components sensitive to damage are selected for damage detection of a thick plate by using a semi-analytical method which incorporates finite element based numerical analysis into classical elasticity theories.
Detecting internal cracks and bottom through cracks in a thick plate is proposed by using bulk transverse waves. If a certain crack is located on the propagating path of bulk transverse waves, the wave energy received by a sensor at a distinct point will decrease because the propagating waves will be reflected on crack surface and scattered into other directions. Taking advantage of this phenomenon, damage index is defined as the ratio of the received energy at the presence of crack to that without crack. Damage detectability is investigated through parametric studies in terms of energy attenuation, reflections on crack surface, interference of reflected waves and damping effect.
Smart PZT sensor with a function of precision vibration and active actuation is used for damage detection of a thick plate. The elastic waves are generated by PZT sensor attached on a surface of a plate. Tone burst signal with narrow band is used as input for vibration control.
Damage localization is conducted by using wave signals received by a moving sensing window which consists of a group of sensors with good capability of signal classification and advances in the longitudinal direction of a plate. Probabilistic damage evaluation based on maximum likelihood is proposed to improve the reliability of damage assessment by analyzing the assessment results from each sensing block.
This paper also presents detection of a surface through crack on a thick plate and debonding of a thin plate attached on a thick host plate by using Rayleigh surface waves, respectively. The surface through crack on a plate can be detected through the identical moving sensing window that is used for detecting internal cracks in a plate., For detecting the debonding of a plate, it has been shown that the wave speed on the bonded plate is determined by the slowest one among those of Lamb waves on the bonded plate, Rayleigh waves on the thick host plate. The debonding size is calculated by using the difference of the arrival time between an original wave packet and a wave packet by mode conversion due to debonding.
The validity of the proposed method has been demonstrated through numerical examples in which different types of cracks in terms of location, size, and angles are successfully identified and the debonding size of a thin plate attached on the host plate is estimated accurately.
thick plate, damage detection, elastic waves, bulk transverse waves, Rayleigh surface waves, PZT sensor, moving sensing window, maximum likelihood, layered plate, debonding assessment