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Effect of surface roughness on ultrasonic testing

İşleyici, Umut
This study investigates the effect of front surface roughness on ultrasonic echo amplitude. Experiments were carried out on specimens whose front surfaces are machined by milling machine. Machining parameters were changed in milling process in order to obtain desired roughness values and milling head was tilted to a very small angle to obtain periodic rough surfaces. Experiments were performed with these specimens having roughness value of 0.5, 4.5, 11, 26.5 æm. Ra. The back surface roughness of all specimens was kept constant at 1.5 æm Ra by grinding operation. 1.5, 2, 3, 4 mm. holes were drilled at constant depth and to same side of each specimen to represent reference discontinuities. Ultrasonic tests, using pulse echo technique were carried out to monitor echo amplitudes corresponding to different roughness values. The tests were also repeated by using different ultrasonic probes having different frequencies. For additional comparison, different couplants were used through the tests. The results showed that there was a significant increase in the reduction of the sound pressure level with the increase in the surface roughness. Although there was no uncertainty observed about not being able to detect discontinuity because of roughness but correct couplant and frequency selection has a positive effect on correctly sizing the discontinuity and at attenuation measurements. The results obtained with this work can be used as a guide for testing rough surfaces, predicting the effect on ultrasonic examination before testing and discontinuity detecting capability under rough surface conditions.