Laser ultrasonic propagation imaging method in the frequency domain based on wavelet transformation Jung-Ryul Lee a,n , Chen Ciang Chia a , Hye Jin Shin a , Chan-Yik Park b , Dong Jin Yoon c a Department of Aerospace Engineering, Chonbuk National University, 664-14 Duckjin-dong, Jeonju, Chonbuk 561-756, Republic of Korea b Aeronautical Technology Directorate, Agency for Defense Development, Yuseong-gu, P.O. Box 35-7, Daejeon 305-600, Republic of Korea c Korea Research Institute of Standards and Science, P.O. Box 102, Yuseong, Daejon, 305-340, Republic of Korea article info Article history: Received 12 June 2010 Received in revised form 27 July 2010 Accepted 27 July 2010 Available online 1 September 2010 Keywords: Q-switched continuous wave laser Laser ultrasonics Ultrasonic propagation imaging Nondestructive evaluation Wavelet transformation abstract A wavelet-transformed ultrasonic propagation imaging method capable of ultrasonic propagation imaging in the frequency domain was developed and applied as a new structural damage or flaw visualization algorithm. Since the wavelet-transformed ultrasonic propagation imaging method has strong frequency selectivity, it can visualize the propagation of ultrasonic waves of a specific frequency (for example, to isolate ultrasonic mode of interest and a damage-related ultrasonic wave). The strong frequency selectivity of the wavelet-transformed ultrasonic propagation imaging method was demonstrated, isolating only the zeroth-order asymmetrical mode of the fundamental Lamb wave modes in an anisotropic carbon fiber-reinforced plastic plate with a thickness of 5 mm. The wavelet- transformed ultrasonic propagation imaging method can also convert a complex time domain multiple wavefield into a simple frequency domain single wavefield. This feature enables easy interpretation of the results, and facilitates the precise evaluation of the location and size of structural damage or flaws. We demonstrated this capability by detecting a disbond in a sandwich structure made of Al-alloy skins and a foam core. A disbond with a diameter of 20 mm, which is representative of a common manufacturing flaw, was successfully detected, localized, and evaluated. Since a method to determine the allowable maximum pulse repetition frequency depending on target materials and structures was found by investigating the residual wave caused from the previous laser impinging, our laser ultrasonic system can scan rapidly the target with an optimal pulse repetition rate. In addition, the proposed wavelet-transformed ultrasonic propagation imaging method can visualize damage or flaw without the need for reference data from the intact state of the structure. Hence, we propose the wavelet- transformed ultrasonic propagation imaging approach for automatic inspection of in-service engineering structures, or in-process quality inspection in manufacturing. & 2010 Elsevier Ltd. All rights reserved. 1. Introduction Damage and flaw evaluation of aerospace and naval engineer- ing structures is of great importance because these structures are usually life-critical. This is particularly true as structures become larger and support an increasing number of passengers and payloads, and at the same time become more complex as a result of integral production using advanced composite components for lower manufacturing and assembly cost [1]. Thus, the need to adopt advanced nondestructive evaluation (NDE), integrated structural health monitoring (SHM), and in-process quality control (IPQC) systems for these structures is justified. Further- more, these systems guarantee the designed lifetime of the structures, facilitate condition-based maintenance to minimize premature part changes and to prolong the lifetime. Effective NDE, SHM, and IPQC methods must also have high throughput, as well as the capability of generating intuitively understandable inspection results. High throughput is necessary due to the increasing number and size of structures that require inspection. The ability to generate intuitively understandable inspection results is preferred so that the cost of training skilled personnel can be reduced, and the possibility of human error can be minimized. Inspection methods with imaging capability have great potential to fulfill these two requirements because an image could cover a large inspection area, and reveal not only the existence of a single damage event, but also the location and size of multiple damage events. Information about the location and extent of damage is crucial in determining the most cost effective repair or replacement plan for the inspected structure. A broad range of acousto-ultrasonic-based inspection methods have imaging capability. Among them, the ultrasonic wavefield imaging and ultrasonic wave propagation imaging methods based on laser inteferometric and Q-switching technologies deserve extra attention. This is because ultrasonic waves are sensitive to Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optlaseng Optics and Lasers in Engineering 0143-8166/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2010.07.008 n Corresponding author. Tel.: + 82 63 270 4637; fax.: + 82 63 270 2472. E-mail address: leejrr@jbnu.ac.kr (J.-R. Lee). Optics and Lasers in Engineering 49 (2011) 167–175