Recently, the longitudinal, shear, and surface waves have been very widely used as ultrasonic wave-based exploration methods to identify internal defects of host structures. In this context, a noncontact nondestructive testing (NDT) method is proposed to detect the damage of plate-like structures and to identify the location of the damage. To achieve this goal, a scanning laser source actuation technique is utilized to generate a guided wave and scans a specific area to find damage location more precisely. The ND:YAG pulsed laser is used to generate Lamb wave and a piezoelectric sensor is installed to measure the structural responses. The measured responses are analyzed using 3-dimensional Fourier transformation (3D FT). The damage-sensitive features are extracted by wavenumber filtering based on the 3D FT. Then, flaw imaging techniques of a plate-like structure are conducted using the damage-sensitive features. Finally, the plates with notches are investigated to verify the effectiveness and the robustness of the proposed NDT approach.
Recently, there have been increasing demands on structural 2 health monitoring (SHM) and nondestructive testing (NDT) in the fields of civil, mechanical, and aerospace engineering and so on to prevent losses of life and property by continuously monitoring the systems. Especially, local monitoring methodologies have been studied to overcome the limitation of global monitoring techniques [
In this paper, ultrasonic wave propagation is visualized using ND:YAG pulsed laser and a flaw image is constructed based on wavenumber filtering and root mean square (RMS). The ND:YAG pulsed laser is used to generate Lamb wave and a piezoelectric sensor is installed to measure the structural responses. The measured responses are analyzed using 3D FT and then the damage-sensitive features are extracted by wavenumber filtering and RMS [
Ultrasonic wave propagation imaging (UWPI) system consists of a Q-switched laser system, a laser mirror scanner based on galvanometer, an ultrasonic sensor, a high-speed digitizer, and an image processor, which was rearranged referring the previous works [
A scheme of an ultrasonic wave propagation imaging system based on a UWPI laser system.
When the pulsed laser beams impact the target structure, ultrasonic waves are generated by thermoelastic mechanism and propagated. The multiple wave responses can be measured by only single PZT attached to the backside or the front side of the structure. The measured time signals are placed at each laser impinging point and the UWPI is obtained as shown in Figure
A scheme of an ultrasonic wave propagation imaging algorithm.
The flaw images can be easily obtained by observing reflected waves from damage after eliminating strong incident waves. To filter out the incident waves, wavenumber filtering concept is applied in this study. First, the wave propagation image is transformed from time/space domain to frequency/wavenumber domain by 3D Fourier transform as described in [
The incident waves can be filtered out by eliminating positive or negative side of the frequency/wavenumber domain signals using window function as shown in (
The filtered UWPI data is inversely transformed from wavenumber and frequency domain to spatial and time domain. In the next section, this process is described using some figures. Finally, damage can be quantified by calculating root mean square (RMS) values using the filtered signals as shown in [
First, an intact aluminum plate was scanned to verify the feasibility of the UWPI system. A 6061T aluminum alloy was used for the test as shown in Figure
An aluminum plate and scanning information.
Using the UWPI system, the snapshot of the UWPI at 20
UWPI snapshot at 20
Next, wavenumber filtering process based on 3D Fourier transform is shown in Figures
Elimination of upward incident waves by wavenumber filtering.
Incident waves before filtering
3D FT before filtering
Incident waves after filtering
3D FT after filtering
Elimination of downward incident waves by wavenumber filtering.
Incident waves before filtering
3D FT before filtering
Incident waves after filtering
3D FT after filtering
A steel coupon with a notch was used to verify the proposed method as shown in Figure
Configuration of a specimen with a notch (target to detect).
The UWPI image is complex before wavenumber filtering due to incident waves, reflections from side boundaries, and the notch as shown in Figure
Visualization of the guided wave propagation and flaw image.
Before filtering
After filtering
RMS image after filtering
For next damage detection test, a 6061T aluminum plate was used again, which had same size as in Figure
Damage cases: case 1: tangential to wave front, case 2: tangential to wave front (two damages), case 3: an angle of 45 degrees with wave front, and case 4: perpendicular to wave front; dotted line indicates the scan area.
Comparison of UWPI snapshots at 55
Intact condition
Damage condition
Next, areas near the damages are zoomed in and flaw imaging process is conducted in those areas. First, intact and damaged images at 48
Comparison of UWPI snapshots at 48
Intact image before filtering
Damaged image before filtering
Intact image after filtering
Damaged image after filtering
Then, RMS values were calculated at 48
Comparison of RMS snapshots at 48
Intact RMS image before filtering
Damaged RMS image before filtering
Intact RMS image after filtering
Damaged RMS image after filtering
Next, the UWPI and flaw images for damage case 2 are considered. The patterns of the images are similar to damage case 1. In this case, two notches were created and hence the transmitted waves through the first damage were reflected at the second damage. Therefore, the shape of the reflections is more complicated than those from damage case 1 as shown in Figure
Comparison of UWPI snapshots at 48
Intact image before filtering
Damaged image before filtering
Intact image after filtering
Damaged image after filtering
In Figure
Comparison of RMS snapshots at 48
Intact RMS image before filtering
Damaged RMS image before filtering
Intact RMS image after filtering
Damaged RMS image after filtering
In the case of damage 3, reflected waves from the damage are observed, but the energy level of those is much low because the damage is inclined to the incident wave front as shown in Figure
Comparison of UWPI snapshots at 46
Intact image before filtering
Damaged image before filtering
Intact image after filtering
Damaged image after filtering
In this case, RMS images were obtained at relatively longer period than the previous two cases to use reflections from boundary. As results, the damage is visualized as shown in Figure
Comparison of RMS snapshots at 130
Intact RMS image before filtering
Damaged RMS image before filtering
Intact RMS image after filtering
Damaged RMS image after filtering
The final case is on the perpendicular damage to the incident wave front. In Figure
Comparison of UWPI snapshots at 35
Intact image before filtering
Damaged image before filtering
Intact image after filtering
Damaged image after filtering
RMS images are also obtained at relatively longer period so that the reflected waves from the corner of the plate can arrive at the damage as show in Figure
Comparison of RMS snapshots at 155
Intact RMS image before filtering
Damaged RMS image before filtering
Intact RMS image after filtering
Damaged RMS image after filtering
In this case, the damage could not be also clearly shown in a similar manner with case 3. Additionally, the damage was perpendicular to the wave front, the filter is applied horizontally, and hence the damage could be observed at the longer period of the data to consider the effect of the reflected waves from the boundaries. However, the damage could not be clearly detected because the reflections from the boundaries were attenuated as depicted in Figure
In this paper, the ultrasonic wave propagation imaging system and flaw imaging technique were proposed based on ND:YAG pulsed laser system and wavenumber filtering. To verify the feasibility of the proposed technique, two steps of damage detection test were conducted. First, a notch in a steel coupon was detected and then various types of damages were identified in an aluminum plate. The notch in the steel coupon was successfully detected by eliminating incident waves and by calculating root mean square, although the shape of the propagating waves was complicated due to the narrow width of the specimen. On the other hand, the damages in the aluminum plate were partially detected. The tangential damages to the incident wave front were successfully identified, while the inclined and perpendicular damages to the incident wave front were visible when the flaw images were compared to the intact images. It could be caused by the sparse scanning grid, the window for the wavenumber filtering, and the filtering direction. Therefore, further research is now ongoing to investigate the detectable size of the flaw and to detect various damages clearly. Additionally, other types of structures will be scanned to verify the applicability of the proposed method.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This study was supported by Basic Science Research Program (2010-0023404) through the National Research Foundation (NRF) of Korea, the Nuclear Research & Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant funded by the Ministry of Knowledge Economy of Korea government (no. 2011T100200161), the research project Development of Patch/Implant System based on IT Technology for Safe Management of Large Scale-Structure funded by the Ministry of Knowledge Economy of Korea Government. This all-out support is greatly appreciated.