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.
This research aims to study and describe the effect of microstructure on shape of distance amplitude correction (DAC) curve and ultrasonic inspectability of stainless steel weld joint. Two calibration blocks (side drilled hole block) were prepared from AISI 316 stainless steel plate according with ASME section V version 2013. One calibration block was varied the grain size by annealing process. The annealing temperature and holding time were 1,200 °C and 4 hours, respectively and then cooled down in furnace. AISI 316 Butt joint welding specimens were prepared to establish the artificial discontinuities. Lack of fusion and drill hole, diameter 1 and 2.5 mm, were selected to establish as discontinuities. Specimens were welded by gas tungsten arc welding and shielded metal arc welding process. Then, the gain size and microstructure of two calibration blocks were analyzed by microscope. The macrostructure, gain size and microstructure of weld joint were determined by microscope. Longitudinal and transverse wave with probe frequency 2.25 and 4 MHz and angle probe 45, 60 and 70 degrees were used to describe the effect of microstructure on shape of DAC curve and investigate the ultrasonic inspectability in stainless steel weld. The experiment results found that the ultrasonic energy of longitudinal and transverse wave in calibration blocks decreased as the gain size increased. The attenuation due to gain size affected to the shape of DAC curve. The grain size in heat effected zone (HAZ) of weld specimen is larger than base material and the ultrasonic transverse wave can detect lack of fusion and drill hole diameter 2.5 mm but cannot detect drill hole diameter 1 mm. The usefulness of this research is utilizable for searching the discontinuities in the weld zone of stainless steel by ultrasonic. The attenuation of ultrasonic energy in the weld zone of AISI 316 is usually high and the amplitude displayed at the screen is very low. This reason can be made the operator wrong result interpretation, if they did not consider about the attenuation from microstructure.
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