During the last two decades, significant advances in the field of laser generated ultrasounds have been made. In the conventional pulse echo technique, a discontinuity in the specimen can be detected only if an ultrasonic detector is properly oriented to the discontinuity. The 2-D Scanning Laser Source (SLS) technique allows detection of surface-breaking cracks of arbitrary orientation with respect to the direction. This technique also makes detection possible on rough and curved surfaces by scanning changes in ultrasonic amplitude and frequency when the laser source scans over flaws. Conventional ultrasonic scanning technique requires immersion of specimen or water jet for transferring ultrasonic wave between transducer and specimen. However, this SLS technique does not need contacting and couplant to generate surface wave and to get flaw images. Therefore, this SLS technique has several advantages, for complicated production inspection, high speed scanning, non-contact, remote from specimen, and high resolution. In this study, ultrasonic SLS images of fatigue cracks on turbine blade of aircraft and steel plates were obtained to describe shapes of defects by SLS technique.
Pipelines of nuclear power plants undergo high pressure and temperature. Thermal
stratification typically occurs in the surge line and the main feed water lines by flow and this
stratification will initiate and propagate thermal fatigue cracks. This may cause rupture and leakage
and it is a serious problem to nuclear power plants operation. Therefore it is very important to detect
and measure thermal fatigue cracks. In this study, thermal fatigue cracks were generated in
austenitic stainless steel specimens by a thermal cycle in notched pipes and weld jointed pipes.
Ultrasonic techniques were used to evaluate the thermal fatigue crack depth. When ultrasonic waves
propagate from an angle beam probe to thermal fatigue cracks, waves are reflected and diffracted.
Crack depth was evaluated by the reflected signals from back wall and diffracted signals from the
crack tip, but diffracted signals were too weak to detect so the reflected signals were more useful.
The TOFD and dB drop methods were used in this study. The TOFD method is uses a time delay of
diffracted signal from the crack tip. The dB drop method is an application of an amplitude
decreasing rate by a probe moving distance.
In carbon steel pipes of nuclear power plants, local wall thinning may result from
erosion-corrosion or flow-accelerated corrosion(FAC) damage. Local wall thinning is one of the
major causes for the structural fracture of these pipes. Therefore, assessment of local wall thinning
due to corrosion is an important issue in nondestructive evaluation for the integrity of nuclear power
plants. In this study, laser-generated ultrasound technique was employed to evaluate local wall
thinning due to corrosion. Guided waves were generated in the thermoelastic regime using a
Q-switched pulsed Nd:YAG laser with a linear slit array. . In this paper, time-frequency analysis of
ultrasonic waveforms using wavelet transform allowed the identification of generated guided wave
modes by comparison with the theoretical dispersion curves. Modes conversion and group velocity
were employed to detect thickness reduction.
Damage in nuclear facilities during operation is caused by cyclic loadings due to mechanical or thermal fatigue. Fatigue damage is often related with loads, which were not taken into account in the design, e.g. temperature cycling arising from unexpected stratified flow conditions. Thermal stratification typically occurs in the surge line or the main feed water lines. The large number of thermal cycles with great amplitude produced by stratification raises some concerns about the damage induced by fatigue in these lines. Therefore, nondestructive technique should be employed for the evaluation of residual life to guarantee its integrity. In this study, an ultrasonic technique was applied for evaluation of thermal fatigue cracks in stainless steel used for piping systems in the nuclear power plant. Quantitative evaluation of thermal fatigue cracks is available by the employment of advanced ultrasonic techniques equipped with digital signal processing techniques such as wavelet transform and neural network method in ultrasonic testing. In this investigation, fatigue cracks were generated in austenitic stainless steel specimen by mechanical load and thermal cycle, and the thermal fatigue cracks were quantitatively evaluated by the ultrasonic technique with the digital signal processing method.
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