Ultrasonic technology has been a standard inspection technology for decades; it is a reliable means to nondestructively gage the thickness of materials. With advances in electronics and digital signal processing software, as well as innovations in terms of wave and frequencies, ultrasonics can do much more and its use is flourishing in a range of applications.

Conventional ultrasonic thickness gages work by measuring the round-trip transit time of a high frequency sound pulse as it travels through a material. An ultrasonic transducer generates bursts of sound energy that is coupled to one side of the test piece. Sound waves traveling through the material reflect off the far side and return to the transducer. The gage measures the time it takes for the echo to return and calculates the thickness of the part.

While the theory remains the same, as technology has improved, ultrasonic equipment’s uses and capabilities have expanded. Portable ultrasonic thickness gages, for instance, are digital units that generally offer everything from basic measurement to extended memory and RS232 output capabilities. They can measure a range of materials. The Phase II+ (Carlstadt, NJ) ultrasonic thickness gage can measure metallic and non-metallic materials such as aluminum, titanium, plastics ceramics, glass and other materials as long as it has parallel top and bottom surfaces.

Another example is the PosiTector 200 from DeFelsko Corp. (Ogdensburg, NJ), which has a range of tools with advanced features in the PosiTector series, that can measure coating thicknesses on wood, concrete and other materials. This easy-to-use tool, which operates on two AA batteries, has a two-button operation and is ready to measure-no adjustment is required to measure most coatings. It can switch from mils to microns, store up to 1,000 readings and continually display and update average, standard deviation, min/max thickness and other readings while measuring the part’s coatings. This tool conforms to ASTM standards and offers a certificate of calibration showing traceability to NIST included with every gage.

StressTel (State College, PA), which is part of the GE Inspection Technologies (Hurth, Germany), offers a general purpose thickness gage, which it calls the PocketMIKE. The tool combines the thickness gage and transducer into a single package. It features four-button operation with automatic on-block probe zero and automatic gain control. What it does not require is a separate transducer.

Signal processing has been improved on many systems. The EPOCH 4PLUS flaw detector from Panametrics-NDT, a Business of R/D Tech Instruments Inc. incorporates advanced signal processing features including a 25MHz RF bandwidth to permit testing of thin materials, narrowband filters to improve signal-to-noise in high gain applications, a negative spike pulser for applications requiring higher frequencies and a selectable square wave pulser to optimize penetration on thick or highly attenuating materials.

Krautkramer, a GE Inspection Technologies Co., has a line of handheld corrosion gages that measure remaining wall thicknesses of internally corroded structures such as pipes, tubes, tanks, plate, pressure vessels. Measurements can be made from one side of a part.

Another product is from the Sonotron NDT company. The tool, the ISONIC 2001 portable ultrasonic inspection system, was recently awarded the 2004 Frost & Sullivan Award for Product Differentiation Innovation. Frost & Sullivan is a consulting firm that presents an annual award to companies that it says has developed products with innovative technology that is expected to contribute to the industry in terms of product performance and rate of technical change.

Sonotron offers four variants of the ISONIC 2001 to do basic ultrasonic inspection and imaging, weld inspection and imaging, straight beam inspection and imaging, and large area inspection and imaging.

Ultrasonic use is diverse

Ultrasonic inspection use can be diverse. Constellation Energy (Baltimore), an electricity supplier, used ultrasonic testing on key piping and welds as part of a turbine replacement at its Calvert Cliffs Nuclear Power Plant. The turbine replacement was accomplished in 20 days and marks a new U.S. nuclear industry record for a three-section replacement.

In another example, Lockheed Martin (Ft. Worth, TX) inspected the carbon-fiber wing skins and other composite components for the F-35 Joint Strike Fighter with its Laser Ultrasonic (Laser UT) technology. The laser-based ultrasonic technology can inspect parts at a rate that is 10 times faster than current water-coupled ultrasonic inspection machines, the company says. Also, unlike conventional inspection methods that require access to both sides of a composite surface for inspection, Laser UT only requires access to one side of the part’s surface, enabling more complex-contoured parts to be tested.

The inspection tool was just one of a series of new production equipment, including a flexible overhead gantry that will mill the inside surface of the F-35’s composite skin to a 50 micron accuracy level to help ensure that the aircraft’s outer form is exact, helping to ensure stealth performance.

A new sound

Ultrasonic use is also diverging from the traditional thickness coating measurement. For instance, electromagnetic acoustic transducer (EMAT) use is beginning to accelerate. Innerspec (Lynchburg, VA) uses ultrasonic EMAT technology for seam inspection, tube and pipe inspection, plate inspection and gage measurement.

Ultrasonic EMAT differs from piezoelectric ultrasonics in the way the sound is generated. Traditionally, a piezoelectric crystal is used to transduce electrical energy into mechanical energy and the vibration makes its way into the test piece via the couplant.

An EMAT, however, is made up of a magnet and a coil of wire and relies on electro-magnetic acoustic interaction for elastic wave generation. The EMAT and the metal test surface interact and generate an acoustic wave within the material. The material being inspected is its own transducer, eliminating the need for liquid couplant.

Researchers at the University of Warwick (Coventry, England) are making use of ultrasonic EMAT technology. Researchers are using a low frequency wide band Rayleigh wave, which is a surface wave that decreases in motion as it goes deeper beneath the surface, to produce a crack testing technique that works at high speed and could transform every train into a network of rail crack detectors.

The researchers have taken pairs of the electromagnetic acoustic transducers to generate and detect the Rayleigh wave on the rail without touching the rail. The wave travels along the surface of the rail head, penetrating down to a depth of several millimeters. The technique uses a range of frequencies within a Rayleigh wave pulse because different frequencies allow penetration of the rail to a range of precise measurable depths.

When the wave, which travels along the surface of the length of the rail at 3,000 meters per second, interacts with crack the different frequency components of the signal are blocked to differing degrees, or are reflected from the crack. The researchers can determine the exact location of a crack by observing the loss of signal as it is blocked by the crack or, at lower speeds, by observing the sudden enhancement in signal created by the interference of waves reflected back from the crack with fresh waves generated by the first EMAT. With this technology, the location of the crack can be pinpointed and the depth of the crack ascertained by observing how the frequency content of the Rayleigh wave changes as it moves through a region containing the crack.

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