Acoustic emission testing can be used to detect the energy emitted from material fracture and the advantage of this method is the real time monitoring, however the weld metal discontinuities are normally inspected by using conventional NDT methods such as Penetrant Testing (PT), Magnetic particle Testing (MT), Ultrasonic Testing (UT) and Radiographic Testing (RT) after the completion of welding. The weld defect must be repaired, which involves the cost and consumes a lot of time as well as reduce the reliability of manufactures. This paper presents the application of acoustic emission (AE) technique for monitoring and detecting the discontinuities during welding. In this study, gas tungsten arc welding (GTAW) was selected as test process. Carbon steel plate and autogenous welding technique were used to simulate the hot crack. The data acquisition (DAQ) and AE sensor were used to capture the acoustic signal generated during welding. The AE signals were amplified and filtered by using preamplifier. Then, signals were modified by wavelet transforms (WT) technique and classified by Fast Fourier Transform (FFT) technique. The results showed the possibility to use AE technique for monitoring and detecting the low signal amplitude generated from crack by using frequency domain. The advantage of this research is to propose the technique for monitoring the weld metal discontinuities during welding.
We investigated the effect of laser-irradiating conditions-specifically laser spot size, laser power density and laser weaving. Three laser spot widths of 10, 4 and 2 mm were applied by changing the optical-lens and fiber-cable combination to investigate the effects of the laser power density and laser spot width. The weaving-irradiating method was applied with narrow laser widths of 4 and 2 mm. The effects of the laserirradiating condition were obtained based on high-speed imaging during welding and cross-sectional observation. Stable laser irradiation by a 10-mm laser spot width provided a lower power density than the critical value of 35 W/mm 2 and a lack of fusion. Weaving laser irradiation by a 4-or 2-mm laser spot width provided a higher power density, reduced the large lack of fusion and achieved a large penetration of base metal. The ratio between the laser beam-spot width and gap width (W L / W G ratio) affect the base-metal fusion significantly. A sound W L / W G ratio promoted base-metal fusion by providing a uniform and stable molten-pool temperature, whereas a small W L / W G ratio maintained a smaller fusion area because of the sudden temperature drop and temperature fluctuation of the molten pool.
This research is aimed to compare the influence of backing materials on characteristics of ultrasonic probe for residual stress measurement. A disk-shaped piezoelectric ceramic (PZT) acts as the active element to generate the ultrasonic wave. An optimization composition of backing material was investigated in order to produce suitable acoustic impedance which matches to the active element. In this study, the backing material has been successfully fabricated from tungsten powder combined with epoxy resin (T-ER) and bronze powder associated with epoxy resin (B-ER). The influences of metal powder content on sound velocity and acoustic impedance of the backing materials were measured by using Pulser/Receiver and displayed by an oscilloscope. Each ultrasonic probe was then implemented to measure residual stress obtained from static tension load below yield point. The results shown that the highest acoustic impedance of T-ER and B-ER was 14.7 106 and 7.2 106 kg/m2s, respectively. The fabricated ultrasonic probe using B-ER as a backing material exhibited lower noise and higher amplitude than ultrasonic probe using T-ER backing material.
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