Ultrasonic testing

http://www.google.co.uk/search?hl=en&sa=X&oi=spell&resnum=0&ct=result&cd=1&q=ultrasonic+testing+in+Longitudinal+welded+steel+pipes&spell=1

"A brief explanation of the principles..." (as opposed to sending you thru scads of applicable Codes, Standards, Techniques, etc.) -----

Assuming that you are interested in the inspection of the longitudinal weld seam: A pulse of acoustic energy is coupled into the pipe on one side of the weld. The pulse is introduced *into* the steel at an angle, such that refraction of the sound beam occurs, allowing the beam/wave to propagate through the body of the weld at a predetermined angle (commensurate with the angle of incidence, and the difference in acoustic velocities between the steel pipe and the probe-wedge...[Snell's Law]). When the acoustic wave impinges on a discontinuity within the weldment, an amount of the wave's energy is reflected back to the probe/transducer. This provides an "echo", which can be amplified and displayed for interpretation by the operator. There are various different "modes" that can be employed, and various ways of displaying the echo-data ; most frequently it is displayed as a function of the amplitude of the reflected echo with respect to the "Time-of-Flight" of the acoustic wave to-and-from the reflector(discontinuity).

One key element in the successful location of weld flaws (particularly small pores and slag inclusions), is using the correct frequency of wave. "Smaller waves see smaller stuff" is an aphorism in this field. The rule of thumb formula is to use a frequency whose wavelength (in that material being tested) is no longer than 2 times the diameter of those flaws which you hope to find.

Hope this helps ~

ndt tom has it perfect. Similar methods can be used for internal material defects in the pipe material itself, not just the weld zone.

milo

I have done a lot of UT on circumferential joints of thk. upto 60 mm. in Chemical Industries. But for longitudinal weld joints you may have to take into consideration the geometry of the curved pipe and also use special attachments along with the probe to achieve proper coupling on the curved surface. The Ultrasonic wave is sent and reflection received by the same probe. Sometimes 2 probes are used, one for sending and the other for receiving the reflection. Generally, on C.S. pipes one can use 2.5 MHz transverse wave Angle probes of various angles such as 45⁰. An Oscilloscope is used to detect the faults. You will need a Calibration block containing holes of about 1/8" dia. at different depths of t/4, t/2 and 3t/4 and obtain a 'Distance Amplitude Curve' or called DAC curve. this gives the amplitude of refected wave at standard path distances. This curve is used to compare the depth and size of defect. Also the defects are categorized as 'Volumetric' (such as slag inclusions, porosity etc.) and 'Non Volumetric' (such as cracks or lack of fusion). Non Volumetric are considered more serious defects and need to be eliminated. X-axis gives the path distance (the length of wave travel) and Y-axis shows the amplitude on Oscilloscope screen. From the distance of refection on X-axis and amplitude of defect one can conclude the seriousness of defect. A lot depends on the experience of the operator and the interpretation of defects based on the nature and behavior of the defect by small movements of the probe. The probe is moved in an oscillatory motion while moving forward in order not to miss defects. Non volumetric defects change the amplitude of refection at sudden rates whle moving the probe, while those of Non-volumetric do so slower. The DAC curve is taken as the acceptance standard. When there is no defect you will not see a reflection with angle probes. You will see only the initial wave of the probe on Oscilloscope.

For Stainless steels you have to use special probes which generate longitudinal waves as their grain size at the welds is too large and will not give adequate reflection with transverse waves.