A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 2 2 2 7 Ultrasound has long been used for inspecting parts without damaging them. Like a sonogram at the doctor’s office, a gel-like couplant is applied to the surface and sound waves pulsed through a component to see in detail what is happening on the inside without the need for intrusive surgery. As advances in this technology enable physicians to see with greater detail, so has ultrasonic testing in the nondestructive evaluation (NDE) field improved drastically since its beginnings. PCI FOR DEFECT DETECTION A new evolving ultrasonic technique that has shown tremendous benefit is phase coherence imaging (PCI). PCI can provide finer detail on exceptionally small indications caused by harmful damage mechanisms. Phase coherence imaging (PCI) is a novel approach to processing ultrasonic signals. Other common techniques such as phased array ultrasound (PAUT) or conventional total focusing method (TFM) typically use the amplitude of the ultrasonic signal to detect a reflector. With amplitude-based techniques, when sound hits a reflector, it is reflected back to the sensor, and the intensity of the returned sound is a tell-tale sign of the significance of the flaw. For example, if the signal from a defect in a weld presents with a high amplitude, the inspector can differentiate between a severe flaw, such as a lack of fusion, and a smaller flaw, such as porosity, which would reflect at a much lower amplitude. However, certain damage mechanisms can be critically compromising regardless of their actual size. For example, small microfissures in a catalytic reformer could be a sign of a damage mechanism called high-temperature hydrogen attack (HTHA). HTHA starts as small individual flaws, but with time, they slowly join together and could result in catastrophic failure. Creep damage is another insidious yet small defect that affects catalyst tubes and can also cause cascading system failure in reformers. For HTHA microfissures and creep damage, an amplitude-based technique may not be able to reflect the early indications, and they could be missed entirely. ADVANTAGES OF PCI As its name suggests, PCI is not an amplitude-based technique. Unlike PAUT or TFM, PCI’s processing is based on the phase changes of the ultrasonic signal. The phase of a signal can be altered by a change of medium or simply a small change in the regularity of the part. This phenomenon means that PCI is highly sensitive to small-scale defects, such as the HTHA in the carbon steel sample shown in Fig. 1 and the creep damage in the sample in Fig. 2. Comparing the images generated of these samples using conventional TFM (Figs. 1b and 2b) and PCI (Figs. 1c and 2c) shows that the phase-based PCI was more effective at detecting the micro- fissures and in reducing the background noise in the creep damage sample than the amplitude-based TFM. In addition to its increased sensitivity, PCI also offers an advantage for another challenging ultrasonic inspection: coarse-grained and noisy materials. Phase coherence imaging is obtained using a statistical approach. Statistics in this context help differentiate what is coherent from what is noncoherent in the phase of the signals (thus the “phase coherence” in its name). In a nutshell, the PCI algorithm can, by computing a large number of samples, make sense of what is randomized noise in the part from what is reoccurring signals. In other words, it uses inherent noise from the inspected component to distinguish background noise from relevant indications. For this reason, PCI performs better in a grainy, noisy material than in a clean component. Figures 3a and 3b show the results on a stainless-steel component. Stainless steels are notorious in the ultrasonic inspection world as very grainy and hard-to-penetrate materials with noisy ultrasonic signatures. Figure 3a shows a TFM image (an amplitude-based technique) where a large porosity void was detected in a stainless-steel weld. Figure 3b shows the same weld inspection using the PCI technique. The image generated based on the phase changes reveals several smaller porosity pockets that were undetected with conventional TFM as well as the larger porosity void that was caught by the TFM. Additionally, the signal-to-noise ratio for these indications is greatly improved in the PCI image, thanks to the noise in the part helping PHASE COHERENCE IMAGING: ADVANTAGES OF A NEW ULTRASONIC TECHNIQUE A phase coherence imaging method allows inspection of attenuative, noisy, and thick parts, and early detection of small critical defects. TECHNICAL SPOTLIGHT
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