More attention has been drawn to ultrasonic guided waves (UGW) based damage detection method for its advantages of wide range inspection of large-scale structures. However, complex propagation characteristics of guided waves as well as traditional contact ultrasonic transducers limit its application for the practical damage detection. In this work, A fully noncontact laser ultrasonic detection system is designed by combining YAG pulsed laser with Scanning Laser Doppler Vibrometer (SLDV) to achieve high resolution sensing of UGW field in the structure. A Temporal filter method with multiple frequency bands is proposed to extract the scattering signal without reference signal. The f-k time reversal imaging method is introduced to obtain the reconstructed incident wavefield and scattered wavefield, while the cross-correlation imaging condition is used to identify the damage location in plate structures. Finally, the imaging results of different frequency bands are integrated to achieve accurate imaging with high-precision and high signal-to-noise ratio.
Total Focusing Method (TFM), as a kind of post-processing imaging method, has attracted researchers’ attention due to its better resolution and high signal-to-noise ratio (SNR) in comparison to traditional imaging techniques. However, without analyzing the properties of damage scattering, the TFM algorithm fails to realize the quantitative evaluation of the damage. On the purpose of improving imaging sensitivity and SNR, an ultrasonic scattering model is developed which takes into account the interaction between the incident ultrasonic fields and the damage, then the reflectivity of the damage surface can be obtained. Finally, the imaging of the reflectivity of the damage is formed by using this inverse scattering model in frequency domain. Because of the advantages of non-contact, non-destructive and couplant free, laser-generated ultrasound is used as an excitation method in the model. In this paper, the finite element models of ultrasonic propagation in damaged structures are carried out. The damage types are circular holes and cracks of different sizes. The simulation results show that the TFM algorithm combined with the inverse scattering model can locate the damages accurately, and the size as well as the orientation of the cracks can also be identified quantitatively. The proposed model obviously enhances the image sensitivity and SNR, which proves its ability of small damage location and characterization.
Mie scattering theory was shown in this paper to be suitable for analyzing the forward scattered light intensity distribution of micro-sized air bubble defects in glass, shining by a monochrome laser with a wavelength of 532um. The scattered light was measured by a high definition CCD camera. The scattering process can be classified as uncorrelated single scattering according to the properties of optical media. After calculating and smoothing the gray value of split rings of picture, Chahine algorithm was applied to reverse the size of defects. This technique was accurate to within 5% for defects with radii of <50um.
In this paper, a method to detect internal pocks and bubbles of optical elements based on laser line
source scanning is proposed. In dark field environment, a laser line source is used to illuminate from
one side of the glass under test, a high-resolution CCD camera is used to take pictures in front of the
glass sample. Images which contain information of defects are acquired through rough scanning and
accurate scanning. Accurate three-dimensional coordinates of the internal defects are acquired after
image processing, which descript the characteristic information of internal defects quantificationally.
Compared with the microscope imaging measurement, this proposed detection of defects in optics
based on laser line source scanning has a relative aberration smaller than 2%. In addition, the detection
time is approximately reduced to 20 minutes from 1 hour dramatically. The analysis indicates that the
error of the position of defects is much smaller than the size of them, which means the position of the
defects can be acquired accurately by this approach.