Ultrasonic Testing (UT) is a versatile volumetric nondestructive testing (NDT) method that enables the NDT technician to investigate the full thickness of the material being examined. The method is extensively used for the inspection of critical welds. It is a preferred NDT method of inspecting welds because it is fast, relatively inexpensive, requires access to only one side of the weld, needs only one person to perform the test, and it presents no health risks to the NDT technician or the public.
The basic principles of ultrasonic testing are relatively simple. A pulse of high-frequency sound is generated by a piezoelectric crystal when it is energized by an electrical current. The electrical current induces a vibration of the crystal. The vibrations of the piezoelectric crystal intern vibrate the material being inspected. The vibrations are well above the audible range, so a human beings are not able to hear the sound. The sound waves travel from the transducer containing the piezoelectric crystal through a coupling material and into the test object at a constant speed until a reflective surface is encountered. Some of the sound is returned to the piezo-electric crystal making it vibrate. The piezo-electric crystal transforms the vibrations into a weak electrical current that is transmitted to the UT machine (better known as a scope), where it is amplified and displayed so the NDT technician can see the distance the soundwave traveled and how much sound returned amongst other data.
The Importance of Calibration
The ultrasonic test method is a comparative type of test, which means the test equipment must be calibrated on a calibration standard which contains an artificial defect of a known size. The test instrument is also calibrated to ensure the sound wave signal received is accurately displaying the correct distance it travelled. Generally, an International Institute of Welding or IIW calibration block is used for calibrating the UT machine when testing welds, per the AWS structural welding codes. It is well worth the time and effort needed to calibrate the test instrument correctly. Even newer digital test instruments must to be calibrated per the requirements of the structural welding code.
The code requires the test instrument to be recalibrated every time the transducer is changed, each time the machine is connected and disconnected from the power supply, each time the battery is changed, when the coaxial cable is changed, or there is a power outage. Granted, the calibration process is much easier and faster with the new digital machines than it was with the old analog machines, but calibration and recalibration is a requirement.
Once the UT machine is calibrated for distance and the sensitivity is set based on a known defect (typically the side drilled hole in the IIW calibration block), the machine can be used to inspect welds per the structural welding code. The reflected signal is compared to the signal produced by the known defect. Generally, the amplitude of the reflected signal is compared to the amplitude of the known defect. Consideration is given to the sound path distance and the attenuation of the material being tested. The AWS structural welding code uses a rating system to determine whether the discontinuity producing the reflected signal is accepted or rejected.
For many people, the examination of welds is their only exposure to ultrasonic testing, but there are many more applications that can be utilized with this NDT method.
Real-Life Applications
Pulse-echo ultrasonic testing can be used to map corrosion locating areas with thickness reductions. An engineering assessment can be made to determine whether it is safe to continue using the item as is, remove the item from service, or repair the affected area. When performing a corrosion survey, a straight beam (longitudinal wave) is typically used. In many cases, a duel element (transducer) is used. One piezoelectric crystal generates the sound wave and sends it into the material being tested, and a second piezo-electric crystal detects the reflected signal and generates the weak voltage signal that is sent back to the machine for amplification and processing.
Ultrasonic testing can be used to detect cracks in machine components such as motor shafts, bolts, and tie rods. A straight beam transducer producing a longitudinal soundwave can be used to inspect the component from one end of the part. If there is a crack, some of the sound energy is reflected back to the transducer. Newer ultrasonic test machines can have a range of 500” in steel, meaning it can send the sound wave through 500” of steel and detect defect. It can be interesting if the shaft has a threaded end because some of the sound energy will be reflected by the surface of the threads. Now the NDT technician has to determine whether the signal is due to a crack or simply reflecting off the threads.
The motor shafts used in large electric motors for pumping coolant in power plants are routinely inspected using ultrasonic testing when the motors are rebuilt. Some of them are monstrous 6000-hp motors. They are critical for the continued safe operation of the power plants. A combination of longitudinal wave and shear wave transducers are used to optimize the probability of detecting cracks in the shaft. The NDT technician has to understand how the geometry of the shaft can produce a reflected signal and distinguish between signals produced by part geometry and a crack.
Many jobs performed using ultrasonic testing are not governed by a construction code. The NDT technician collects the data and provides it to the client. The client then must determine what that data means to the safe operation of the equipment. The goal is to avoid unscheduled shut downs due to the failure of a key component. These jobs can be challenging and interesting.
While many NDT technicians perform ultrasonic testing to inspect welds, the ultrasonic testing method can be used for many other tasks. It is a very versatile technology, but the NDT technician has to have a thorough understanding of the principle of ultrasonics to fully utilize the test method to its full potential. Many of the inspections may not be covered by a code, so your company’s NDT ultrasonic testing procedures have to be developed and tested to verify it will provide the information necessary to make an informed decision. In some cases, a special calibration standard may be necessary. Training and experience are required to fully utilize the full potential of the ultrasonic test method.