See Yenn Chong

Repeat scanning technology for laser ultrasonic propagation imaging

Laser ultrasonic scanning in combination with contact or non-contact sensors provides new paradigms in structural health management (SHM) and non-destructive in-process quality control (IPQC) for large composite structures. Wave propagation imaging technology based on laser ultrasonic scanning and fixed-point sensing shows remarkable advantages, such as minimal need for embedded sensors in SHM, minimum invasive defect visualization in IPQC and general capabilities of curved and complex target inspection, and temporal reference-free inspection. However, as with other SHM methods and non-destructive evaluation based on ultrasound, the signal-to-noise ratio (SNR) is a prevalent issue in real structural applications, especially with non-contact thin-composite sensing or with thick and heterogeneous composites. This study proposes a high-speed repeat scanning technique for laser ultrasonic propagation imaging (UPI) technology, which is realized with the scanning speed of 1 kHz of a Q-switched continuous wave laser, and precise control of the laser beam pulses for identical point scanning. As a result, the technique enables the achievement of significant improvement in the SNR to inspect real-world composite structures. The proposed technique provides enhanced results for impact damage detection in a 2 mm thick wing box made of carbon-fiber-reinforced plastic, despite the low sensitivity of non-contact laser ultrasonic sensing. A field-applicable pure laser UPI system has been developed using a laser Doppler vibrometer as the non-contact ultrasonic sensor. The proposed technique enables the visualization of the disbond defect in a 15 mm thick wind blade specimen made of glass-fiber-reinforced plastic, despite the high dissipation of ultrasound in the thick composite.

Design of Fiber Bragg Grating Acoustic Sensor for Structural Health Monitoring of Nuclear Power Plant

Nuclear power plant (NPP) is commonly categorized as a harsh environment that gives rise to age-related degradation on the structures of plant and eventually leads to radiation leakage that threatens human. Integrated structural health monitoring (ISHM) technology is a strong candidate for NPP accidents. The emergence of optical fiber technology into SHM system for NPP was greatly interested by researchers, and also prior research works have shown that the fiber Bragg grating (FBG) was able to retain its reflectivity under radiation exposure. In this paper, a metal-coated fiber was newly used to develop a FBG acoustic sensor for ISHM of NPP. The 7 mm length of aluminum, copper/carbon coatings were successfully removed with sodium hydroxide and nitric acid solutions. A 5 mm FBG was successfully inscribed in the silica core through the 7 mm long coating-removed silica section of copper/carbon-coated fiber and the initial reflectivity was 71%. Then, the FBG acoustic sensor was developed in one-end-free FBG configuration on the stainless steel vessel using a high temperature metallic adhesive. The reflective power of the sensor was stabilized at 345°C during the high temperature elevation cyclic process. The FBG acoustic sensor showed good response to the acousto-ultrasonic waves during pencil-lead breaking and laser ultrasonics tests. The high temperature FBG acoustic sensor written in the cost-effective metal/carbon is feasible to be used for ISHM of the NPP.

Shock Response Spectra Reconstruction of Pointwise Explosive-Induced Pyroshock Based on Signal Processing of Laser Shocks

Pyroshock has been an issue of great concern for aerospace and defense industrial applications. When pyroshock devices are detonated, they can easily cause failures in electronic, optical, relay, and magnetic components generally in mid- and far-fields which is not avoidable at the design level. Thus, many numerical and experimental pyroshock simulations have been widely studied to predict explosive-induced pyroshock effect quantitatively, especially the shock response spectrum (SRS). In this study, a laser shock-based pyroshock reconstruction method is proposed to simulate a pointwise explosive-induced pyroshock signal. The signal processing algorithm for the laser shock-based pyroshock reconstruction is developed in a LabVIEW platform and consists of subbands decomposition, SRS matching in decomposed bands, and wave synthesizing. Then, two experimental setups are configured to obtain pyroshock signals and laser shock signals at four points in an aluminum plate. The reconstructed pyroshock signals synthesized according to the signal processing of the laser shocks demonstrate high similarity to the real pyroshock signals, where the similarity is evaluated by the mean acceleration difference between the SRS curves. The optimized settings of the subband decomposition were obtained and can be in the future used in a pyroshock simulator based on laser shock for pyroshock simulation at any arbitrary point.

Imaging and Characterizing Structural Defects through the Estimation of Local Dispersion Curves

We describe a method to effectively image and characterize structural features and defects using local estimates of wavenumber for propagating guided Lamb waves at a fine grid of spatial sampling points. The guided waves are rapidly excited at each grid point using a scanning Q-switched laser system and sensed by a single fixed ultrasonic transducer. Through reciprocity, this produces a full-wave-field time history of a virtual wave being excited from the transducer.We first demonstrate the unique capability of the measurement system by showing frequency-wavenumber intensity diagrams that resolve up to 5th order guided Lamb wave modes. Well then show how, using frequency-wavenumber processing, localized wavelength estimates can be obtained by isolating each guided Lamb wave mode, extracting a specific frequency bin, and identifying the center-wavelength of a sliding wavenumber band-pass filter that maximizes the energy at each grid point. This procedure is repeated across all frequency bins in order generate estimates of the local dispersion curves at each spatial sample point on the structure. The approach was capable of producing detailed images of hidden wall-thinning in an aluminum plate and local impact delamination in a complicated composite component.

Development of Wireless Ultrasonic Propagation Imaging System

In recent years, the laser ultrasonic propagation imaging (UPI) system has attracted many researcher. In this paper, we present the laser ultrasonic propagation imaging system with the wireless sensing system to realize wireless UPI system. The wireless laser UPI system consist of a very ground structural health monitoring (SHM) system, preamplifier-integrated piezoelectric (PZT) sensor, and wireless ultrasonic device (WUD). The ground SHM system has laser system and Laptop/Personal Computer. The computer initiates the WUD board to acquire ultrasonic wave and laser scanning system to start scanning. The computer uses Wi-Fi communication to communicate with WUD. The WUD, on detecting ultrasonic wave generated by impinging of the laser, collects the wave, digitizes it, and send it wirelessly to the ground SHM system. To find the efficiency of WUD, the ultrasonic wave data was collected using both wired and wireless connection and compared them with each other.

Effect of Laser Pulse Fatigue on the Mechanical Characteristics of a CFRP Plate

A laser ultrasonic based nondestructive evaluation (NDE) technique has been widely used in aerospace industries for inspecting parts and structures made of composite materials. The thermoelastic regime is used for the ultrasonic generation, so no plasma is formed on the surface of composite structure. Generally, the service lifetime for an aircraft could be more than 25 years. Thus, the composite structures of the aircraft could be susceptible to laser pulse fatigue damage caused by the laser pulse energy of a laser ultrasonic generator in the long-term periodic maintenance inspection. In this paper, the effect of laser pulse fatigue on the mechanical characteristics of a carbon-fiber-reinforced polymer (CFRP) plate (USN175BX Carbon UD preprag) with the stacking sequence of [0/45/-45/90]s is investigated to verify the reliability of the use of a laser ultrasonic based NDE technique on the CFRP plate specimen inspection. A high-speed laser ultrasonic scanning system (400mm/s at the intervals of 0.4 mm) was setup to perform repeat scanning of 1300 times on a CFRP plate specimen with the scanning area of 70 mm x 60 mm. These repeat scanning times were set in consideration of the periodic maintenance inspection scheduled to be 1 time/week x 52 weeks/year x 25 years. A 532nm Q-switched continuous wave laser (QL) was used and set at the laser pulse energy levels of 0.6 mJ and 1.2 mJ. Lamb wave assessment based on pitch-catch method was proposed in this paper to monitor the mechanical characteristics of a composite specimen. In each completion of 100 times repeat scanning, the Young’s modulus of the scanning area was evaluated based on the group velocity of S0 Lamb wave mode. In addition, the surface condition of the scanning area was investigated by using a microscope.