ULTRASONIC TESTING (UT)

Wall-thickness measurements are often used for maintenance tasks performed on pipelines and in ships. For instance, they serve to detect general erosive corrosion or selective corrosion, cavitation or erosion. Staff carrying out these tasks often have sound knowledge of how to assess the findings thus brought to light without having any further skills or knowledge of ultrasonic testing.

TRAINING CONTENTS

Scientific fundamentals of ultrasonic testing
Generation of ultrasound, ultrasonic probes and testing devices
Device calibration using calibration blocks
Fundamentals of perpendicular wave incidence
Defects due to corrosion and erosion
The use of ultrasound for wall-thickness measurement
NDT Lab Reporting

Another way to couple the sound from transducer to a test object is coupling the sound with water. This can be done with squirters where the sound travels through a jet of water or by immersing the transducer and test object in a tank of water. Both techniques are called immersion testing. In immersion testing, the transducer is placed in the water, above the test object, and a beam of sound is projected. The graph of peaks using the immersion method is slightly different. Between the initial pulse and the back wall peaks there will be an additional peak caused by the sound wave going from the water to the test material. This additional peak is called the front wall peak. The ultrasonic tester can be adjusted to ignore the initial pulse peak, so the first peak it will show is the front wall peak. Some energy is lost when the waves hit the test material, so the front wall peak is slightly lower than the peak of the initial pulse.

TRAINING CONTENTS
Scientific fundamentals of ultrasonic testing
Generation of ultrasound, ultrasonic probes and testing devices
Device calibration using calibration blocks
Fundamentals of perpendicular wave incidence
Defects due to corrosion and erosion
The use of ultrasound for wall-thickness measurement
NDT Lab Reporting

Ultrasonic phased arrays use a multiple element probe whereby the output pulse from each element is time delayed in such a way so as produce constructive interference at a specific angle and a specific depth. These time delays can be incremented over a range of angles to sweep the beam over the desired angular range. For example, 40 to 75 degree beam sweep would be produced by calculating the time delays to produce constructive interference at 40, 41, 42 …75 degs.

This NDT technology is also referred as Swept Beam Ultrasonic testing. The main advantages of phased array in NDE are:

TRAINING CONTENTS

Scientific fundamentals
Probe design, sonic field and device engineering
Device calibration using standardized calibration blocks
Inspection techniques
Specimen and flaw analysis
NDT Lab Reporting

Time of Flight Diffraction (TOFD) method of Ultrasonic inspection is a very sensitive and accurate method for non destructive testing of welds for defects. TOFD originated from tip diffraction techniques. Measuring the amplitude of reflected signal is a relatively unreliable method of sizing defects because the amplitude strongly depends on the orientation of the crack. Instead of amplitude, TOFD uses the time of flight of an ultrasonic pulse to determine the position of a reflector.

In a TOFD system, a pair of probes sits on opposite sides of a weld. One of the probes emits an ultrasonic pulse that is picked up by the probe on the other side. In undamaged pipe, the signals picked up by the receiver probe are from two waves: one that travels along the surface and one that reflects off the far wall. When a crack is present, there is a diffraction of the ultrasonic wave from the tip(s) of the crack. Using the measured time of flight of the pulse, the depth of a crack tip can be calculated automatically by simple trigonometry. This method is even more reliable than Radiographic testing of a weld.

TRAINING CONTENTS

Scientific fundamentals of TOFD ultrasonic testing
Probe design, sonic field and device engineering
Device calibration using standardized calibration blocks
Inspection techniques
Specimen and flaw analysis
NDT Lab Reporting