Ultrasonic Testing: The What, Why, and How
This article explains the science behind ultrasonic testing, it’s application in industry and considerations when performing tests.
Prior to World War II, sonar – the technique of sending sound waves through water and observing the returning echoes to characterize submerged objects – inspired early ultrasound investigators to explore ways to apply the concept to medical diagnosis. In the early 1900s, ultrasound waves were studied for the possibility of detecting defects in metal objects. Patents were obtained for using two transducers to detect flaws in solids. In the mid-1900s, ultrasonic testing was developed using a pulse-echo technique.
What is Ultrasound?
Ultrasound is the same sound we hear every day except it is produced at a higher frequency – above the normal hearing range of approximately 20,000 Hz. Special equipment is needed to transmit and receive the response of this high frequency. Ultrasound has been a very valuable tool as one of the NDT (nondestructive testing) techniques having the ability to test a part without harming it. This can be used on a variety of materials – both metallic and nonmetallic – obtaining information about discontinuities without impairing the usefulness of the part. In order to obtain correct information, training and certification of the personnel doing the inspection is of the utmost importance.
Training and Certification
Guidelines have been set by the American Society for Nondestructive Testing (ASNT). ASNT has written a document giving the recommended practice for training and certification of NDT personnel (SNT-TC-1A). This document gives an employer the necessary guidelines to properly qualify and certify the NDT technician – not only in the ultrasonic method but other NDT methods as well.
To comply, a customer-approved, written practice must be established by the employer detailing how the technician will be trained, examined and certified. SNT-TC-1A also gives the number of classroom hours and months of experience necessary to be certified as an ultrasonic technician. There are three possible certification levels: Level I, II and III. Each level has additional responsibility for performing and interpreting the results of ultrasonic testing. Level I and II technicians must be examined in the following areas.
- General test – must have general knowledge of the inspection being performed
- Specific test – must be able to deal specifically with the inspection performed
- Practical test – must show evidence that he/she can perform the inspection and report the results
Even with the use of new computer-controlled equipment (Fig. 1), it is imperative that the technician and his supervisor have a basic understanding of the ultrasonic principle to effectively perform the inspection and report the results.
Everyone contemplating the use of ultrasound should know some of the basic facts of ultrasonic vibrations. Vibration is a back-and-forth movement – energy in motion. A displacement is a depression of a particle from its normal position. A succession of particle displacements is how vibration passes through a solid material. Many small particles or groups of atoms can be displaced from these normal positions by applying a force such as ultrasound. When the force is removed, the particles tend to move back to their original position. This is the basic principal behind all sound from everyday speech at low frequencies to ultrasound at high frequencies – above 20,000 Hz.
Velocity of sound will vary depending on the material that it is being transmitted through, but it will remain constant in any given material. For example, if you place a transducer in a tank of water and inspect a steel specimen, the sound will take longer to travel through the water than through the steel because the velocity of sound in steel is approximately four times greater than in the water. So even though the same energy is transmitted through the water to the steel, it will pass slower through the water, increase speed in the steel and return back to the transducer more slowly through the water.
Ultrasonic testing is the process of inducing ultrasound into a specimen to determine some physical property. This is accomplished by using a transducer (Fig. 2). A transducer is a device that converts energy from one form to another – the piezoelectric effect. Piezoelectric is the ability to convert electrical energy to mechanical energy and back from mechanical to electrical energy again so it can be displayed on the ultrasonic equipment. An ultrasonic unit utilizes a transducer that can be either pulsed or continuous. Pulse is short groups of transmitted vibration before and after which the transducer can act as a receiver.
Ultrasound can be transmitted through various materials such as steel, water and oil. At high frequencies, however, vibrating above 20,000 Hz sound does not transmit well through the air. Air is a poor transmitter of ultrasound because the ultrasound density is so low that it is difficult to transmit sound energy from particle to particle. That is why it is necessary to put oil, grease, water or some other couplant between the transducer and the specimen to dissipate air. Couplant allows the ultrasound vibrations to be transmitted from the transducer into the test specimen. Even though the ultrasound velocity does not change in a specimen, the sensitivity can be varied by changing the wavelength – the direct result of the velocity divided by the frequency.
l = v/f
The test is more sensitive with shorter wavelengths. Increasing the frequency can shorten the wavelength. In practical situations the smallest discontinuity you can detect is approximately ½ lambda (wavelength).
The ultrasonic equipment is the heart of ultrasound testing. This equipment ranges from relatively affordable portable equipment to very expensive stationary equipment. Newer ultrasonic digital equipment has many capabilities to aid the field technician (Fig. 3). Previous portable equipment could range from 50-100 pounds whereas some of the newer digital equipment may be less than five pounds. This digital equipment provides the technician with features that were previously only found on the more expensive equipment such as thickness measurement, angle beam, weld calculations and others. Equipment ranges from A-, B- and C-scan displays to phase-array equipment that is capable of producing internal 3-D graphic images of the part as well as color images depicting different flaws of varying degrees in different colors.
Reference blocks are important to the ultrasonic test as all UT is based on some sizing of reflections. Since we cannot have perfect shape defects in a product we must make artificial defects in reference standards to have something to compare to the inherent defects in the tested material. Usually, standards are made of a similar material as the product being tested (Fig. 4). These test blocks typically have a flat-bottom hole, a side-drilled hole or “V”-notches manufactured at a distance corresponding to the critical depth. Also, the acceptable area of the artificial defects is determined for the part being inspected.
The acceptable flaw size is usually determined by an engineering department and/or in conjunction with the customer and manufacturing. The acceptance standards might be comprised of a set of distance/amplitude blocks with areas of acceptable flaw sizes that correspond to similar test distances in the part. Acceptance standards are based on the customer and the manufacturer working together to determine the final acceptable limits in conjunction with several written standards (ASTM, AWS, etc.). Without realistic collaboration it is possible to impose standards that would be much tighter than necessary, resulting in an inability to produce the part.
Uses of Ultrasound
Ultrasound has been used for years to find defects in various products such as steel forgings and welds in structural-steel buildings, bridges and aerospace. The use of the new lightweight digital equipment has been very helpful in inspecting these types of welds. Traditionally, they have been inspected utilizing the “A” scan display (Fig. 5). This display relies heavily on the technician’s interpretation of the indications. The new digital equipment has aided the technician by also having the ability to store calibration in memory and recall this calibration while on top of bridges, in power plants or wherever the test is being conducted.
Ultrasound is very effective in detecting discontinuities. The main limitation is the technician’s expertise of interpreting the defect and the equipment being used. In this fast-changing world, however, the use of more computer-enhanced and smaller handheld ultrasonic equipment has helped technicians to interpret different types of flaws, resulting in better determination of the type of indications detected.
Accuracy is mainly dependent on the calibration of the ultrasonic instrument, the qualification of the technician and the sophistication of the equipment being used. Again, the technician must be trained to properly calibrate the equipment, interpret the results and have knowledge of the material he is inspecting. The technician, as well as his supervisor, must recognize the difference between relevant and non-relevant indications such as indications from the geometry or indications produced by the equipment itself.
There are many things to be considered when applying ultrasound to verify the integrity of the product:
- Wave propagation
- Straight beam or angle beam
- Surface wave
- Immersion or phase array
This is dependent on the product under inspection and the configuration of the product. All of these should be taken into consideration, but the technician is one of the most important variables in the ultrasonic inspection process. Without the proper training/qualification of the technician, other considerations are less important.
A quality report is important for the proper documentation of the test. The test document should include the following to be able to properly identify the test part with the inspection report.
- Description of the part
- Equipment used
- Indications found
- Other pertinent information such as serial number
ASNT is a valuable resource for information regarding ultrasound as well as other nondestructive testing (NDT) methods that can be used to inspect various parts. IH
For more information: Hartley Johnston is the past chairman of the Pittsburgh section of ASNT. His contact information is 3443 Old Pittsburgh Rd., New Castle, PA 16101; e-mail: firstname.lastname@example.org. The national ASNT organization can be located at www.asnt.org and the local Pittsburgh section at www.asntpgh.org.
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: ultrasonic testing, wavelength, frequency, nondestructive test, wave propagation