Common Disadvantages to Ultrasonic Sensors
Standard conventional ultrasonic sensors for nondestructive testing (NDT) have a wide array of benefits, but they don’t have the same comprehensive scanning mechanisms available as more advanced Phased Array ultrasonic testing (PAUT). For example, conventional ultrasonic sensors are not agile enough to scan pipes efficiently, but many PAUT probes used with scanners can read pipes and other curved surfaces with ease. Sensors can determine potential defects, but there are times when additional testing measures are necessary to obtain complete data.
Ultrasonic NDT is superior to destructive testing due to its ability to detect flaws without operators dismantling the test object, saving companies time and money in the process. However, there are instances when conventional ultrasonic testing fails to detect underlying corrosion that could compromise operations, erode wall thickness, and create cavities. Some common disadvantages of conventional ultrasonic sensors include limited testing distance, inaccurate readings, and inflexible scanning methods. All of these drawbacks, however, can be mitigated and even overcome with the right NDT tools and techniques.
Limited Detection Ranges
Tests will only be as good as the reach of the probe. If the probe is unable to emit sound waves to certain areas of a test object, the end result will be skewed data that doesn’t paint a full picture of an item’s viability. When using an ultrasonic arm scanner, for instance, you will only get limited testing data based on the scanner arm’s size. Standard scanners also have the same confined reach due to the limited capacity of the X-Y interface.
Example: A certified analyst needs to test a welding sheet using standard X-Y ultrasound mapping. The problem is that the sheet is larger than the probe itself. Even though the probe can provide adequate thickness measurements in certain spots of the weld, it must be moved around continuously to get a complete idea of the weld’s integrity. For a more detailed and accurate analysis, the analyst can use an NDT PaintBrush, which encodes both scanner wheels for a more comprehensive scan. The wheels track the scanning position, allowing operators to easily differentiate between areas that have been scanned and those that have not.
For a more detailed and accurate analysis, the analyst can use an NDT PaintBrush, which encodes both scanner wheels for a more comprehensive scan. The wheels track the scanning position, allowing operators to easily differentiate between areas that have been scanned and those that have not.
If we’re talking about large infrastructure in the form of pipelines, long-range ultrasonic testing LRUT) is your best option, but LRUT is limited in the following ways:
- It can’t determine if the corrosion occurs on the pipe surface or within the pipe.
- It can’t provide wall thickness measurements, only thickness variations throughout the pipes.
- It can’t differentiate between active and passive corrosion.
Moreover, LRUT can give you false impressions if you fail to embark on supplementary examination strategies. The best solution would be to commence an LRUT and follow with phased array ultrasonic testing.
The Potential for Inaccurate Results
Non-homogenous items will be harder to read with conventional ultrasonic technology. The following factors tend to produce inaccurate data:
- Odd Formations: Rough surfaces, small objects, and curved items will lead to alignment issues. A wheel-probe scanner is a superior option for curved objects, but it can lead to misalignment issues, as the scanner cannot read the second part of the two axes.
- Accessibility: The surface of the test item must be accessible for probing. Coupling liquids must also be applied to the surface to facilitate sound transfer throughout the object.
- Material Composition: Waves cannot penetrate cast iron and grainy materials, leading to diminished sound transmission. For example, the larger grain found in austenitic steel can cause attenuation, which could cover defects during ultrasonic testing.
Furthermore, conventional ultrasound NDT probes are not malleable enough to properly process high-grain materials, resulting in incorrect data that could lead you astray.
Example: An analyst must inspect welds for downstream infrastructure. However, the coarse grain materials in the stainless steel make probing difficult. Instead, the inspector applies a high-functioning phased array system with 2D matrix array probes to combat the propagation problems. The analyst can also use a specialized scanner like a weld crawler to inspect different welding types without implementing different testing plans.
For conventional probes, irregular surface patterns may cause dead zones where the sensor probe reads nothing. But the phased array probes eliminate dead zones thanks to the phased array capabilities and versatile software technology. To counter dead zones on austenitic welds, 2D dual matrix array probes can be low-tuned between 1.5 MHz and 3 MHz. This method will also address excess noise generated by grain reflection in the weld.
Another problem is the rigidity of the probe itself. The inability to control wave emissions during the testing phase will produce a limited scale of an object’s structural profile. Additionally, conventional ultrasonic testing can be so rigid that it may overlook small flaws.
Example: An analyst is testing a solid, metallic cylinder using a standard sensor, and the results are clean. Not quite satisfied with the data, they decide to use phased array testing instead, moving the probe at different angles. The analyst soon notices a small flaw that the previous ultrasonic sensor failed to pick up. In this case, a portable phased array instrument will allow them to angle beams accordingly and detect the most difficult-to-find blemishes.
With standard probes, linear flaws that run parallel to the beam itself will go unnoticed. Standard UT tends to come with fixed angles that cannot be adjusted during testing. Without the ability to angle beams accordingly, there is a chance that certain awkwardly positioned flaws will escape the probe’s wave beams. With phased array scanning, analysts can customize the inspection criterion based on the object under observation.
Overcoming Ultrasonic Sensor Disadvantages
Conventional ultrasonic sensors have their disadvantages, to be sure, but the benefits of nondestructive testing outweigh the drawbacks, especially in regard to portable NDT UT technology. One of the best ways to stay ahead in your industry is to switch from standard sensors to phased array scanning technology. If you’re looking for newer ultrasound tools, you’ll discover that phased array ultrasound is a step above traditional wavelength testing, as phased array techniques overcome some common challenges associated with standard ultrasound NDT thanks to advanced capabilities such as linear scanning, utilizing numerous apertures, and scanning from multiple angles. Armed with cutting-edge technology and the latest NDT techniques, analysts will be well-equipped to handle inspections more accurately and efficiently than ever.
Zetec is a global leader in nondestructive testing (NDT) solutions. We offer high-performance solutions in the form of innovative eddy current and ultrasonic tools and techniques. Contact us today to learn more about our phased array technologies and corrosion scanners.