Ultrasonic testing (UT) is used for detecting flaws and discontinuity in the internal structure of materials like steel, concrete, wood-castings, forged materials, welds, and alloys. During the process, no damage is caused to the materials inspected, thus this technology falls in the non-destructive testing group (NDT).
Short ultrasonic wave pulses are used in the process of ultrasonic testing. These wave pulses are in a frequency range of 0.1-15 MHz, though they can even reach 50 MHz. The testing process makes use of different pieces of equipment, like pulsers or receivers, instrumentation transducers, calibration standards, and display devices. Ultrasonic inspection finds its application in evaluating materials used in aerospace, automotive, and transportation industries.
Principles
In this type of testing, ultrasonic waves are introduced via the transducer into the materials to be tested. The frequency range of these waves is beyond the human audible range, and the waves propagate in the form of particle vibration. These waves traverse in a straight line with a uniform speed, until their path is obstructed by any discontinuity on the surface. Upon hitting the flaw, reflection and transmission of the wave energy takes place at the same time. The transducer converts the reflected portion of the wave into an electrical signal, which is fed into a diagnostic device to display the results. This process is used for measuring the size of the reflector and the time interval between the transmission and reception of the signal. The signal travel time enables the calculation of distance that the wave has traveled, and the location and orientation of the reflector.
The magnitude of frequency depends on the nature of the test material. Low-frequency beams have greater penetration effect due to less attenuation, and are used for rough surfaces, since they are scattered less. However, the results for detection of smaller flaws or changes in the material is not efficient as the divergence becomes larger with low-frequency ultrasonic waves. High-frequency beams produce more efficient results in case of small imperfections. In case of ultrasonic waves producing small divergence, a concentrated beam can be launched into the test materials. However, because of higher attenuation (attenuation is defined as the gradual fall in the intensity of any kind of flux through a medium), high-frequency waves cannot penetrate greater depths.
The transducer is nothing but a sensor that converts energy into high-frequency sound waves. The transducer works on a principle similar to sonar or radar. This principle is also in used in anemometers, humidifiers, medical ultrasonography, etc.
Advantages of Ultrasonic Testing Over Other NDT Methods
- It permits detection of extremely small imperfections in a material.
- It provides accurate results for the dimensional measurements and characteristics of testing objects.
- The testing can be carried out on a single side with the reflection or the pulse-echo mode.
- Through the use of automated and electronic-based procedures, instant and immediate inspection results and interpretation of detailed images can be achieved.
- The use of ultrasonic waves for testing is a non-hazardous operation.
Limitations of Ultrasonic Testing
- The testing surface must be accessible for the transducer to launch the sound energy or wave into the object.
- Testing specimens which are way too irregular in their dimensional characteristics make the inspection difficult, or in some cases impossible as they may cause more than a few geometric echoes.
- The testing should be followed by a proper cleaning of the testing surface and removal of any loose scale or paint, so that the required contact can be made between the probe and the object.
- To ensure effective movement of ultrasonic waves or energy between the transducer and the surface being inspected, a coupling medium (a thin layer of a uniform or homogenized mixture of one polysaccharide or diassharide and water) has to be applied between the transmitting and receiving surface of the transducer and the testing specimen. So, use of water-based couplants will require items that are water-resistant.
- If a discontinuity in the item has been oriented in parallel direction to the ultrasonic sound, there are chances that it may go undetected.
The above content presents a simplified version of the procedure. However, to carry out the operation effectively, skilled and extensively trained technicians are required.
