Ultrasonic nondestructive testing, also known as ultrasonic NDT or simply UT, is a method of characterizing the thickness or internal structure of a test piece using high-frequency sound waves. The frequencies, or pitch, used for ultrasonic testing typically range from 500 kHz to 20 MHz, many times higher than the limit of human hearing.
High-frequency sound waves are very directional and will travel through a medium, such as steel or plastic, until they encounter a boundary with another medium, such as air, and reflect back to their source. By analyzing these reflections, it is possible to measure the thickness of a test piece or find evidence of cracks or other hidden internal flaws.
In industrial applications, ultrasonic testing is widely used on metals, plastics, composites, and ceramics. The only common engineering materials that are not suitable for ultrasonic testing with conventional equipment are wood and paper products.
Ultrasonic testing is completely nondestructive. The test piece does not have to be cut or sectioned, or exposed to damaging chemicals. Access to only one side is required, unlike measurements with mechanical thickness tools like calipers and micrometers. There are no potential health hazards associated with ultrasonic testing, unlike radiography. When a test has been properly set up, results are highly repeatable and reliable.
How Ultrasonic Testing Works
Ultrasonic flaw detection requires a trained operator who can set up a test using the appropriate reference standards, inspect equipment or structures that sometimes feature challenging complex geometries, and properly interpret the results. Ultrasonic thickness gages (or gauges) must be calibrated with respect to the material being measured, and applications requiring a wide range of thickness measurements or measurement of acoustically diverse materials may require multiple setups. In addition, ultrasonic thickness gages are more expensive than mechanical measurement devices.
1. An ultrasonic transducer, typically a small, handheld assembly, converts electrical energy into sound waves and sound waves into electrical energy. Courtesy: Olympus
A transducer is any device that converts one form of energy into another. An ultrasonic transducer (Figure 1) converts electrical energy into mechanical vibrations (sound waves), and sound waves into electrical energy. Typically, ultrasonic transducers are small, handheld assemblies that come in a wide variety of frequencies and styles to accommodate specific test needs.
An ultrasonic thickness gage is an instrument that generates sound pulses in a test piece using an ultrasonic transducer and then precisely measures the time interval until the echoes are received. Having been programmed with the speed of sound in the test material, the gage uses that sound velocity information and the measured time interval to calculate thickness through the simple relationship of distance equals velocity multiplied by time.
Under optimum conditions, commercial ultrasonic gages can achieve accuracies as high as±0.001 millimeter (mm,±0.00004 inch), with a possibility of±0.025 mm (±0.001 inch) or better in most common engineering materials. Factors affecting accuracy include the uniformity of the sound velocity in the test material, the degree of sound scattering or absorption, the surface condition, and the level of precision and care with which the instrument has been calibrated for the application at hand.
2. Ultrasonic thickness gages, such as the Olympus 38DL Plus being used here, can measure remaining wall thickness of corroded process piping, tubing, and pressure vessels. Courtesy: Olympus
In the power generation industry, ultrasonic thickness gages (Figure 2) are often used to measure the remaining wall thickness of corroded process piping, tubing, and pressure vessels. The measurements can be made quickly and easily without needing access to the inside or requiring the pipe or tank to be emptied. Other applications include thickness measurements of molded plastic bottles and similar containers, turbine blades and other precision machined or cast parts, small-diameter medical tubing, rubber tires and conveyor belts, fiberglass boat hulls, and even contact lenses.
Sound waves traveling through a material will reflect in predictable ways off flaws such as cracks and voids. An ultrasonic flaw detector is an instrument that generates and processes ultrasonic signals to create a waveform display that can be used by a trained operator to identify hidden flaws in a test piece. The operator identifies the characteristic reflection pattern from a good part and then looks for changes in that reflection pattern that may indicate flaws.
A wide variety of cracks, voids, disbonds, inclusions, and similar problems that affect structural integrity can all be located and measured with ultrasonic flaw detectors. The minimum detectable flaw size in a given application depends on the type of material being tested and the type of flaw under consideration.
Flaw Detection Applications
Power generation flaw-detection applications include turbine component inspections; inspecting for delaminations, porosity, cracks, and flaws in composite wind blades; wind tower seam weld inspections; and bond testing of silicone coatings on high-voltage power line insulators. Ultrasonic flaw detectors (Figure 3) are also widely used in critical safety-related and quality-related applications involving structural welds, steel beams, forgings, pipelines and tanks, aircraft engines and frames, automobile frames, railroad rails, power turbines and other heavy machinery, ship hulls, castings, and many other applications.
3. The EPOCH 650 Flaw Detector shown in use here has a variety of applications in the power generation space. It can be used to inspect wind turbine components and bond-test silicone coatings on high-voltage power lines. Courtesy: Olympus
Ultrasonic imaging systems are used to generate highly detailed pictures similar to X-rays, mapping the internal structure of a part with sound waves. Phased array technology, originally developed for medical diagnostic imaging, is used in industrial situations to create cross-sectional pictures. Large scanning systems are used by the aerospace industry and metalworking suppliers to check for hidden flaws in both raw materials and finished parts. Ultrasonic pulser/receivers and signal analyzers are used in a variety of materials research applications.
—Calvin Jory is the senior product marketing specialist at Olympus for portable nondestructive testing equipment.