Hogeon Seo

Noncontact Evaluation of Acoustic Nonlinearity of a Laser-Generated Surface Wave in a Plastically Deformed Aluminum Alloy

A noncontact method to evaluate the acoustic nonlinearity of surface waves in a plastically deformed aluminum alloy is proposed. Line-arrayed laser beams modulated with slit masks were used for the generation of narrowband surface waves. A laser-ultrasonic detector using a two-wave mixing (TWM) approach was also employed to detect the surface waves. The specimens were deformed by a stroke-controlled tensile tester so as to generate various degrees of tensile deformation. The experimental results showed that the acoustic nonlinearity of the laser-generated surface waves increased according to the level of tensile deformation. This tendency was in good agreement with our previous results obtained using a contact piezoelectric (PZT)-transducer as the receiver. These results imply that our noncontact technique is suitable for the evaluation of acoustic nonlinearity and can be applied to practical damage assessment.

Frequency Characteristics of Surface Wave Generated by Single-Line Pulsed Laser Beam with Two Kinds of Spatial Energy Profile Models: Gaussian and Square-Like

Using a single-line pulsed laser beam is well known as a useful noncontact method to generate a directional surface acoustic wave. In this method, different laser beam energy profiles produce different waveforms and frequency characteristics. In this paper, we considered two typical kinds of laser beam energy profiles, Gaussian and square-like, to find out a difference in the frequency characteristics. To achieve this, mathematical models were proposed first for Gaussian laser beam profile and square-like respectively, both of which depended on the laser beam width. To verify the theoretical models, experimental setups with a cylindrical lens and a line-slit mask were respectively designed to produce a line laser beam with Gaussian spatial energy profile and square-like. The frequency responses of the theoretical models showed good agreement with experimental results in terms of the existence of harmonic frequency components and the shift of the first peak frequencies to low.

Nonlinear ultrasonic technique for closed crack detection

The detection of cracks at the early stage of fracture is important in industrial structures in order to guarantee their structural safety. Ultrasound has been widely utilized in the field of nondestructive testing of materials. However, most of these conventional methods using ultrasonic characteristics in the linear elastic region are mostly sensitive to opened cracks but much less sensitive to such closed cracks. The nonlinear ultrasonic technique (NUT) based on the contact acoustic nonlinearity (CAN) has been considered as a promising method for the closed crack detection. However, most of the previous studies were limited to the modeling of the second-order harmonic wave generation at contacted interfaces and its verification by testing artificially contacted interfaces in the through-transmission method. In this study, we investigated experimentally the contact acoustic nonlinearity at a real crack by using the measurement system constructed in the reflection mode that permits the transducers to acc...

Feasibility of MFC (Macro-Fiber Composite) Transducers for Guided Wave Technique

Since MFC(macro-fiber composite) transducer has been developed, many researchers have tried to apply this transducer on SHM(structural health monitoring), because it is so flexible and durable that it can be easily embedded to various kinds of structures. The objective of this paper is to figure out the benefits and feasibility of applying MFC transducers to guided wave technique. For this, we have experimentally tested the performance of MFC patches as transmitter and sensors for excitation and reception of guided waves on the thin aluminum alloy plate. In order to enhance the signal accuracy, we applied the FIR filter for noise reduction as well as used STFT(short-time Fourier transform) algorithm to image the guided wave characteristics clearly. From the results, the guided wave generated based on MFC showed good agreement with its theoretical dispersion curves. Moreover, the ultrasonic Lamb wave techniques based on MFC patches in pitch-catch manner was tested for detection of surface notch defects of which depths are 10%, 20%, 30% and 40% of the aluminum plate thickness. Results showed that the notch was detectable well when the notch depth was 10% of the thickness or greater.

Assessment of Thermal Aging of Aluminum Alloy by Acoustic Nonlinearity Measurement of Surface Acoustic Waves

ABSTRACT Acoustic nonlinearity measurements via contact and noncontact generations of surface acoustic waves (SAWs) were performed in order to characterize the thermal aging of aluminum alloy. The experiments were conducted on aluminum alloy samples (Al6061-T6) that were heat-treated at 220°C for different times (0 min, 20 min, 40 min, 1 h, 2 h, 10 h, 100 h, 1,000 h) and thus had the different levels of thermal aging. The acoustic nonlinearity of the specimens in two types of SAWs was observed according to the thermal aging. The fractional changes in the acoustic nonlinearity exhibited similar trends in both contact and noncontact SAWs, showing that the acoustic nonlinearity measurement via SAWs is independent of the SAW-excitation method. Furthermore, the fractional changes agreed well with the variation in the yield strength, which was a minimum when the acoustic nonlinearity reached its first peak. Then, the acoustic nonlinearity drastically dropped while the yield strength increased to its highest value. Thus, the variation in the acoustic nonlinearity can be perceived as an indicator of the aging level. These results demonstrate the potential feasibility of acoustic nonlinearity measurements via SAWs for the nondestructive evaluation of material degradations.

Measurement of Elastic Constants by Simultaneously Sensing Longitudinal and Shear Waves as an Overlapped Signal

Measurement of elastic constants is crucial for engineering aspects of predicting the behavior of materials under load as well as structural health monitoring of material degradation. Ultrasonic velocity measurement for material properties has been broadly used as a nondestructive evaluation method for material characterization. In particular, pulse-echo method has been extensively utilized as it is not only simple but also effective when only one side of the inspected objects is accessible. However, the conventional technique in this approach measures longitudinal and shear waves individually to obtain their velocities. This produces a set of two data for each measurement. This paper proposes a simultaneous sensing system of longitudinal waves and shear waves for elastic constant measurement. The proposed system senses both these waves simultaneously as a single overlapped signal, which is then analyzed to calculate both the ultrasonic velocities for obtaining elastic constants. Therefore, this system requires just half the number of data to obtain elastic constants compared to the conventional individual measurement. The results of the proposed simultaneous measurement had smaller standard deviations than those in the individual measurement. These results validate that the proposed approach improves the efficiency and reliability of ultrasonic elastic constant measurement by reducing the complexity of the measurement system, its operating procedures, and the number of data.

Relative measurement of acoustic nonlinear parameters and comparison of sensitivity to thermal aging

The acoustic nonlinearity measurement of ultrasonic waves are being extensively researched as a promising nondestructive evaluation element. In the condition of constant propagation distance and wave number, many researchers have measured the second-order relative acoustic nonlinear parameter, β′, that can be simply defined as the ratio of the amplitude of the second harmonic frequency component to the amplitude squared of the fundamental frequency component and compared them in order to identify the acoustic nonlinearity variation according to material degradation. In this study, we extended this concept to the third-order relative acoustic nonlinear parameter, γ′, by defining it as the ratio of the amplitude of the third harmonic frequency component to the amplitude cubed of the fundamental frequency component. To investigate its effectiveness as a nondestructive evaluation element for the material property degradation, both the second-order acoustic relative nonlinear parameter and the third-order rela...

In-Line Ultrasonic Monitoring for Sediments Stuck on Inner Wall of a Polyvinyl Chloride Pipe

This research verified the applicability and effectiveness of the ultrasonic monitoring of sediments stuck on the inner wall of polyvinyl chloride (PVC) pipes. For identifying the transmittance of acoustic energy and the speed of sound in the PVC material, the pulse-echo ultrasonic testing was conducted for PVC sheets of different thicknesses. To simulate the solidified sediment, the hot melt adhesive (HMA) was covered on the inner wall of the PVC pipe in different heights. From the experiment, the speeds of sound in the PVC and the HMA materials were obtained as about 2258 and 2000 m/s, respectively. The thickness of the materials was calculated through the signal processing such as taking the absolute value and low pass filtering, the echo detection, and the measurement of the time of flight. The errors between actual and measured thicknesses of PVC sheets were below 5%. In the case of the substance stuck on the inner wall, the errors were below 2.5%. Since the pulse-echo ultrasonic inspection is available on the outer surface and its measurement accuracy was over 95%, it can be an efficient and effective in-service structural health monitoring for the sediment on the wall of PVC pipes.

Influence of laser beam profiles on the frequency bandwidth of laser-generated surface acoustic waves

The frequency characteristics of laser-generated surface acoustic waves (SAWs) were investigated depending on laser beam profiles. The laser-generated SAWs were analytically and experimentally compared when using the single-line laser beams of Gaussian and square-like energy profiles. Also, the line-arrayed laser beam in the square-like energy distribution was introduced to generate the SAW that had a narrow frequency bandwidth. The results from both the experiments and the simulations showed the similar distinction of the frequency characteristics in between using Gaussian and square-like laser energy profiles. The SAW by the Gaussian laser beam showed only single peak in the frequency spectrum, whereas the odd harmonic frequency components appeared in the case of using the square-like laser beams. Moreover, the SAW by the line-arrayed laser beam had narrower frequency bandwidth than the SAW by the single-line laser beam. These results were in good agreement with the theoretical predictions, which supports that laser-generated SAWs have the advantage of frequency modulation by modifying the spatial energy profile of a laser beam.