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- 1. Introduction
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The method utilizes the high performance of eddy current testing without the known problem of the edge effect, especially for components of complex geometry. It provides a reliable and efficient method for defect detection and characterization over a relatively large area. It is efficient for the effective detection and measurement of multiple, natural defects inside in-service components, even hidden subsurface defects.
Electromagnetic-acoustic EM-A techniques emerged in the middle-late part of the last century as a new ultrasonic testing method and were then applied to nondestructive testing [ 29 ]. As the non-contact ultrasonic transmitting and receiving device [ 30 ] used for the nondestructive inspection and materials characterization of conductive materials, EMATs generate and detect ultrasonic waves via electromagnetic coupling between the transducer and the samples.
Usually, the EM-A system consists of a magnet, a coil and a specimen, in which the coil and magnet are usually regarded as an EMAT probe. This technique has many advantages, such as being free of a couplant, having no need of surface preconditioning of the test piece, non-contact operation and high temperature operation. What is more, various ultrasound waves can be used in this technique, like surface waves, plane waves, bulk waves, etc.
Finkel and Godinez demonstrated the ability and advantages of the EM-induced defect stimulation to identify small cracks and ferromagnetic inclusions in thin-walled aluminum structures [ 33 ].
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A method based on electromagnetic modulation of the ultrasonic signal was also proposed in the paper. The method could increase the detection ability of small fatigue cracks compared with the crack closure technique proven by Nagy [ 34 ]. The experimental setup and schematic of the method is illustrated in Figure It shows that an EM pulse was able to induce and stimulate elastic waves by the defect itself and can used for modulation of an ultrasonic signal.
MacLauchlan et al. A 32 active channel phased array system was used to investigate the inspection of submerged arc welds SAW during welding and showed good performance. It is widely known that EMATs operate on ferromagnetic materials via two different transduction mechanisms: the Lorentz force and magnetostriction. The Lorentz force is in the position of the dominant transduction effect and is not significantly sensitive to the typical range of the physical properties of steel.
Experimental tests and numerical simulations undertaken by Ribichini et al. The magnetostrictive sensor technology based on magnetostriction was developed by Southwest Research Institute SwRI for long-range inspection and structural health monitoring of pipes, plates, bridge cables and tubes.
It was fast and cost-effective. Considering the poor transduction efficiency of EMATs and its high sensitivity to surrounding electrical noise, an electromagnetic ultrasonic inspection system must possess the strong ability of weak signal detection, so as to extract defect information from received signals. In , Lei et al. The schematic diagram of the proposed technique is shown in Figure 12 , combining both the cross-correlation and self-correlation method. The feasibility and efficiency of the technique was verified by experimental results with high reproducibility.
Although EMATs have several distinct advantages, the transduction efficiency of the transducers is very poor, because of the shortcoming of design theories of EMATs.
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Considering that the relationship between the parameters of an electromagnetic surface acoustic transducer and its transduction efficiency has been seldom reported, Wang et al. Figure 13 shows the diagram of the experiment aimed at verifying the relationship between lift-off distance and transduction efficiency The experimental results indicated that the decrease of lift-off distance, coil conductor width, magnet length and width and the increase of magnet thickness could effectively improve the transduction efficiency of the EMAT.
Schematic of the experiment for verifying the relationship between lift-off distance changed by stepping motor and transduction efficiency.
In , another paper dealt with the design of electrical parameters, and a geometric parameters of electromagnetic acoustic surface wave detection system was published by Yang et al. The directory information provided by this design could serve as a key basis for an automatic flaw detection system for moving-wheels. Previous application of EMATs took advantage of small and compact permanent magnets to provide the high magnetic field required for detection, but the low Curie point of the permanent magnet made a water cooling device necessary at the EMAT head and impeded the application of the permanent magnet.
In , Palmer et al. The experimental set-up is illustrated in Figure The results demonstrated a significant enhancement in the generated ultrasonic signal amplitude for operation on mild steel samples. The paper published by Zhai et al.
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According to the equation, the excitation of the electromagnetic ultrasonic Lamb wave has something to do with three parameters: the meander-line coil spacing interval between adjacent wires, the frequency of the pulse current and the thickness of the specimen. They found the key to the optimization and proposed a method for optimizing excitation by removing the effect of multi-modes and dispersion [ 39 ]. Two types of sensor techniques—the eddy current EC sensor technique and the electromagnetic ultrasonic EMUS sensor technique—were employed, respectively, to detect the surface defects and longitudinal cracks in this inspection procedure.
A combination of both sensors was a good choice for detecting defects in hostile environments. The EMATs can also be used in railway structural health monitoring. Later, in , Zhu et al.
Both the cumulative average method and cross-correlation detection method were used to realize the processing of the echo signals. The detection system achieved a standout denoising effect with high speed and realized the overall detection of the rails. This method is unique compared to other thermal methods, since the excitation source is not a heat source, but a sonic one.
The principle of crack detection by the sonic-IR technique [ 49 ] is shown in Figure 17 , in which a short burst of high power acoustical energy is launched by an ultrasonic emitter. If there is a defect, such as a crack, inside the sample, the acoustical energy will induce vibrations and cause the crack interfaces to rub, then frictional heating is generated with the localized temperature increase.
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An infrared camera images the returning thermal wave reflections from the sample for further study to characterize the cracks. This technique has significant advantages and improvements over traditional NDE techniques, such as ultrasonic testing, liquid penetrant testing and eddy current testing. Both surface and subsurface cracks can be detected, especially small cracks, like stress corrosion cracking. The method used a single short pulse of low frequency sound to serve as the excitation. The image of the crack appeared at the millisecond ms level, namely, the crack was visible in real time in the raw IR images, so that image processing or averaging was not necessary [ 47 ].
Other examples of cracks in aluminum and titanium fatigue specimens were carried in the same year [ 46 ]. Experimental results showed that the sonic IR technique could inspect fatigue cracks as short as 20 micrometers in metal samples.
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Meanwhile, the Lawrence Livermore National Laboratory LLNL carried out experiments to observe the capability of sonic IR to detect small cracks on several materials and flaw types [ 43 ]. Experimental results showed that the method and equipment used here were effective only in certain circumstances, but not in others.
Excellent noticeable thermal images were produced only in the notched beam coupon specimens, while surface ground and Vickers coupons showed the inability to detect the flaws despite that man-made damage was plainly evident. Several parts smashed during testing, probably by being forced at resonance by the 20 kHz acoustic probe, as illustrated in Figure Moreover, the response was discovered to be modestly dependent on the contact location of the acoustic probe, as well as on the method of support used for the test objects in one case.
Photograph of a the thermal signature from the cracked beam couponand b the eventual fracture of the cracked beam coupon reproduced from [ 43 ] with permission. Although the potential benefits of sonic IR for practical NDE testing has been known to the NDE community for over a decade, many uncertainties with sonic IR still exist, such as the minimum sonic power and the effect of backing material. Attempts to explore the relationship between sonic energy and the amount of crack growth within a test sample were made by Chen et al.
They subjected the samples to the sonic IR inspection technique under various conditions to see if there existed some conditions that could lead to further damage propagation in the form of crack extension [ 44 ]. Experimental results proved that sonic IR could cause cracks to extend under particular testing conditions, but the mechanism for crack growth under sonic IR conditions remained unknown. Various materials for use as backing materials and other inspection parameters were also explored in this paper.
Results showed that the choice of backing material was important for sonic IR tests, and that high-density polyethylene HDPE had the potential to become the best choice to give consistent and repeatable results. Based on further research, they found that the extent of crack propagation strongly relied on the conditions under which the cracks were created [ 51 ]. Experimental results showed that cracks created under increasing stress intensity factors tended to grow less, and two hypotheses about the cause of this were discussed. In order to find the minimum vibration demands for the detection of smaller cracks of more practical relevance, Morbidini and Cawley introduced a method to investigate the detection ability of fatigue cracks in metallic components using sonic IR [ 52 ].
The method relied on the validation of simple finite-element thermal models of the cracks. The experiment was accomplished on two beams: mild steel beams with two-dimensional cracks obtained in the low-cycle fatigue regime, as well as nickel-based super alloy beams with three-dimensional thumbnail cracks generated in the high-cycle fatigue regime. The strain required increased as the crack size decreased, and the desired temperature increased.
For specimens with partially opened cracks, the predictions consistently overestimated the measured temperature profiles. In the paper published by Xu et al [ 53 ], they examined a steel plate with fatigue crack and a juncture of carbon fiber composite that has been used in a space probe; a ceramic plate with a visible crack on the edge of the face was also tested, and the results were satisfying.
The high speed, non-contact nature, the large imaging area and the sensitivity of the technique, especially the fact that it is suitable for cracks vertical to the structure's surface, made ultrasonic infrared thermal wave imaging an attractive NDE technique. It was significative for nondestructive testing in manufacturing and has application in aviation, cosmography and optoelectronics. For data processing, Sakagami et al. They developed a sonic-IR system applied for the detection of artificially introduced stress corrosion cracking SCC flaws with a compact hand-held ultrasonic excitation unit and a micro-bolometer infrared camera.
Results of sonic-IR testing for the stainless steel plate reproduced from [ 49 ] with permission , a Raw infrared image; b self-reference lock-in image. In the project Investigation of Hybrid Acoustic-Infrared NDE Imaging Mechanisms in [ 54 ], Han and Islam presented their research findings and studied several essential issues related to sonic infrared imaging: the relative motions between crack faces, the non-linear vibration behavior induced in the target materials and structures, via both experimental study and simulation.
The aluminum samples fabricated with fatigue cracks were used for studying the heating mechanism related to different vibration modes in the samples. The zoom-in IR image with two spots and their corresponding temperature-time plots in a ms-long ultrasound pulse showed that the temperature at the crack tip red spot was always higher than that at the open end of the crack blue spot during the excitation period before the temperatures dropped to the equilibrium value. The result verified that a highly non-linear situation existed indeed in the engaged system and that the frequency components and the number varied with time for open-closing or out-of-plane motion.
It was illustrated that the amplitude of out-of-plane motion was typically bigger than that in open-closing motion. Besides those above-mentioned efforts, in , Gonzalez et al. Pulse-echo and transmission methods, pulse echo lag techniques and a Cartesian scanner were used in this study.