Yun-Kyu An

Complete noncontact laser ultrasonic imaging for automated crack visualization in a plate

This paper presents an automated crack visualization technique using ultrasonic wavefield images obtained by a complete noncontact laser scanning system. First, the complete noncontact laser scanning system is built by integrating and synchronizing a Q-switched Nd:YAG laser for ultrasonic generation, a laser Doppler vibrometer for ultrasonic measurement and galvanometers for scanning. Then, four different laser scanning schemes are compared to find the most effective and practical ultrasonic scanning strategy for the presented application. Second, a novel image processing technique is developed to isolate and visualize crack-induced standing wave energy from the constructed ultrasonic propagation images. Finally, the effectiveness of the proposed laser ultrasonic scanning system and imaging processing technique is experimentally verified using ultrasonic scanning images obtained from an aluminum plate. The test results confirmed that a hidden notch invisible from the scanned surface was successfully detected and visualized, while no false positive alarm was triggered for an intact specimen.

Lamb Wave Line Sensing for Crack Detection in a Welded Stiffener

This paper proposes a novel Lamb wave line sensing technique for crack detection in a welded stiffener. The proposed technique overcomes one of the biggest technical challenges of Lamb wave crack detection for real structure applications: crack-induced Lamb waves are often mixed with multiple reflections from complex waveguides. In particular, crack detection in a welded joint, one of the structural hot spots due to stress concentration, is accompanied by reflections from the welded joint as well as a crack. Extracting and highlighting crack-induced Lamb wave modes from Lamb wave responses measured at multi-spatial points along a single line can be accomplished through a frequency-wavenumber domain analysis. The advantages of the proposed technique enable us not only to enhance the crack detectability in the welded joint but also to minimize false alarms caused by environmental and operational variations by avoiding the direct comparison with the baseline data previously accumulated from the pristine condition of a target structure. The proposed technique is experimentally and numerically validated in vertically stiffened metallic structures, revealing that it successfully identifies and localizes subsurface cracks, regardless of the coexistence with the vertical stiffener.

Noncontact laser ultrasonic crack detection for plates with additional structural complexities

This article presents a new noncontact laser ultrasonic wavefield imaging technique for detecting subsurface cracks in metallic plates with additional structural complexities. The proposed technique offers noncontact, automated, and baseline-free crack diagnosis for complex metal structures with potential to field structural health monitoring applications. First, a complete noncontact laser ultrasonic wavefield imaging system is introduced, and its working principle is presented. Then, a self-referencing frequency–wavenumber (f-k) filter is developed for instantaneous crack detection. The self-referencing f-k filter isolates crack-induced features from the ultrasonic wavefield images obtained only from the current state of the target structure using the noncontact laser ultrasonic wavefield imaging system. Finite element analyses are employed to investigate the characteristics of laser-generated ultrasonic waves and validate the proposed self-referencing f-k filter. Finally, the proposed technique is experimentally validated using asymmetrically tapered and vertically stiffened aluminum plates. The numerical and experimental results confirm that subsurface cracks are well identified and localized. The uniqueness of this study lies in that crack damage in plates even with additional structural features can be autonomously detected without using baseline data from the pristine condition of a target structure and with no sensor placement.

Integrated impedance and guided wave based damage detection under temperature variation

Electro-mechanical impedances and guided waves have been widely studied for detecting localized structural damages due to their sensitivity to small structural changes. In this paper, an integrated impedance and guided wave (IIG) based monitoring system is developed to improve the detectability of various damage types under varying temperature. First, a hardware system, called the IIG system, was designed to achieve simultaneous measurements of electro-mechanical impedance and guided wave signals. Then, the effects of temperature on guided waves and electro-mechanical impedances were compensated using the passive imaginary part of the impedance signal. Finally, an automated damage classification algorithm which incorporates temperature compensation was developed. To validate the proposed algorithm, experimental investigations were performed for the detection of bolt loosening and crack in metallic structures subjected to the temperature varying condition, in the range of -20 to 70°C.

Visualization of non-propagating Lamb wave modes for fatigue crack evaluation

This article develops a non-propagating Lamb wave mode (NPL) imaging technique for fatigue crack visualization. NPL has a great potential for crack evaluation in that it significantly contributes local mode amplitudes in the vicinity of a crack without spatial propagation. Such unique physical phenomenon is theoretically proven and experimentally measured through laser scanning. Although its measurement is a quite challenging work due to the fact that it is quite localized and coexists with complex propagating Lamb wave modes, a NPL filter proposed in this article overcomes the technical challenge by eliminating all propagating Lamb modes from laser scanned full Lamb wavefields. Through the NPL filtering process, only fatigue crack-induced NPLs can be measured and retained. To verify such physical observation and the corresponding NPL filter, a real micro fatigue crack is created by applying repeated tensile loading, and its detectability is tested using a surface-mounted piezoelectric transducer for generating Lamb waves and a laser Doppler vibrometer for measuring the corresponding responses. The experimental results confirm that even an invisible fatigue crack can be instantaneously visualized and effectively evaluated through the proposed NPL measurement and filtering processes.

Laser lock-in thermography for fatigue crack detection in an uncoated metallic structure

This paper presents a noncontact laser lock-in thermography (LLT) technique for surface-breaking fatigue crack detection. LLT utilizes a modulated continuous wave (CW) laser as a heat source for lock-in thermography instead of commonly used flash and halogen lamps. LLT has following merits compared to conventional active thermography techniques: (1) the laser heat source can be precisely positioned at a long distance from a target structure thank to its directionality and low energy loss, (2) a large target structure can be inspected using a scanning laser heat source, (3) no special surface treatment is necessary to measure thermal wavefields and (4) background noises reflected from arbitrary surrounding heat sources can be eliminated. The LLT system is developed by integrating and synchronizing a modulated CW laser, galvanometer and infrared camera. Then, a holder exponent filter is proposed for crack identification, localization and quantification. Test results confirm that a surface-breaking fatigue crack on a steel plate is successfully evaluated using the proposed technique without any special surface treatment.

Stripe-PZT Sensor-Based Baseline-Free Crack Diagnosis in a Structure with a Welded Stiffener

This paper proposes a stripe-PZT sensor-based baseline-free crack diagnosis technique in the heat affected zone (HAZ) of a structure with a welded stiffener. The proposed technique enables one to identify and localize a crack in the HAZ using only current data measured using a stripe-PZT sensor. The use of the stripe-PZT sensor makes it possible to significantly improve the applicability to real structures and minimize man-made errors associated with the installation process by embedding multiple piezoelectric sensors onto a printed circuit board. Moreover, a new frequency-wavenumber analysis-based baseline-free crack diagnosis algorithm minimizes false alarms caused by environmental variations by avoiding simple comparison with the baseline data accumulated from the pristine condition of a target structure. The proposed technique is numerically as well as experimentally validated using a plate-like structure with a welded stiffener, reveling that it successfully identifies and localizes a crack in HAZ.

Wireless ultrasonic wavefield imaging via laser for hidden damage detection inside a steel box girder bridge

This paper presents a wireless ultrasonic wavefield imaging (WUWI) technique for detecting hidden damage inside a steel box girder bridge. The proposed technique allows (1) complete wireless excitation of piezoelectric transducers and noncontact sensing of the corresponding responses using laser beams, (2) autonomous damage visualization without comparing against baseline data previously accumulated from the pristine condition of a target structure and (3) robust damage diagnosis even for real structures with complex structural geometries. First, a new WUWI hardware system was developed by integrating optoelectronic-based signal transmitting and receiving devices and a scanning laser Doppler vibrometer. Next, a damage visualization algorithm, self-referencing f-k filter (SRF), was introduced to isolate and visualize only crack-induced ultrasonic modes from measured ultrasonic wavefield images. Finally, the performance of the proposed technique was validated through hidden crack visualization at a decommissioned Ramp-G Bridge in South Korea. The experimental results reveal that the proposed technique instantaneously detects and successfully visualizes hidden cracks even in the complex structure of a real bridge.

A reference-free micro defect visualization using pulse laser scanning thermography and image processing

As quality control of micro devices and early detection of micro defects in these devices are becoming increasingly important, the demand for a fast and automated online inspection technique to detect micro defects with high spatial resolution is increasing. In this study, a reference-free micro defect visualization algorithm is developed based on laser scanning thermography to detect micro defects in devices instantaneously and automatically with high spatial resolution. A pulse modulated continuous wave laser generates thermal waves in a target device, and the corresponding thermal responses are recorded by an infrared (IR) camera. When the thermal wave encounters a micro defect, the propagation of the thermal wave is blocked at the interface of the micro defect. The blockage of the thermal wave is detected by the proposed reference-free micro defect visualization algorithm. First, an edge detection algorithm is applied to a raw thermal image obtained at a specific time point to extract the thermal discontinuities formed at the boundaries of the defect. The edge images obtained from all time sequences are then assembled into a single accumulated edge image to accentuate defect-induced thermal disturbances in the form of edge features. Finally, the accumulated edge image is automatically processed using a binary imaging algorithm to visualize the micro defect in the target device. The performance of the proposed reference-free micro defect visualization algorithm is examined using two types of specimens, semiconductor chips and ceramic-epoxy composites. The proposed algorithm successfully diagnoses micro defects ranging from 4 μm to 40 μm in width.

Experimental validations of a baseline-free crack detection technique using dual-PZTs

Guided wave-based structural health monitoring (SHM) techniques have been widely studied by many researchers. Recently, a new damage detection technique without comparison with baseline data is developed by the authors group. Since the baseline-free technique does not require the baseline data obtained from the healthy condition of a structure, this technique may reduce false alarms due to operational and environmental variations of the structure. However, the previously developed technique requires the placements of two pairs of collocated lead zirconate titanate transducers (PZTs), and each pair of PZTs should be properly collocated and identical. These constraints make the previous technique susceptible to varying PZT conditions, and they cannot be applied to structures such as pipelines and aircrafts where access to both surfaces of the structures is limited. In this study, a new PZT called dual-PZT is designed so that the limitations of the previous baseline-free technique can be overcome. To validate the applicability and robustness against undesirable variations in the system, experimental studies with an aluminum plate subjected to varying temperature as well as loading conditions are performed. Furthermore, the proposed baseline-free technique is applied to a decommissioned bridge (Ramp-G Bridge in Korea) to verify the feasibility of the proposed techniques in a real bridge structure.