Fatigue crack detection in orthotropic steel deck bridges applied by FMC/TFM using reflected waves of multiple paths (1)

We introduce the latest developed FMC/TFM (Full Matrix Capture and Total Focusing Method) and its application for fatigue crack detection in orthotropic steel deck bridge. This developed inspection system consists of the advanced high-speed ultrasonic inspection equipment and the remotely operated four-wheel scanner with a 32-channel ultrasonic array probe for angle beam inspection. We developed MP-FMC/TFM (Multiple Paths Phased-array FMC/TFM) in this project. MP-FMC/TFM enables to visualize outline image of cracks clearly by image synthesis based on the propagation route information of direct path and the other paths reflected on the bottom of an inspection object against clacks. This developed inspection system enables to generate clear B-scope images of deck plate crack and bead (in deck-to-rib weld) crack with high-resolution. Furthermore, it enables to synthesizes 3D images of crack propagation with high-resolution along the weld line in real time by integrating the B-scope images every time the scanner moves 2mm. As a result of verification test of fatigue crack propagation test specimens, it was confirmed that this inspection system enables to visualize crack shape clearly and to achieves measurement accuracy of ±0.8mm for depth of deck plate crack, and ±1.0mm for depth of bead crack. Currently, verification tests are being conducted on an actual bridge, and practical application is planned in near future.


Introduction
In orthotropic steel deck bridges on routes with heavy traffic volume of large vehicles, cracks may occur from deck plate and bead in rib-to-deck weld, as shown in Fig. 1.And high-precision inspection is required to detect the initial stage of crack propagation.There is a need for nondestructive testing technology that can do this.This time, we have developed a new ultrasonic image synthesis technology that is an advanced version of FMC/TFM technology, and This report shows the results of technological development that has improved visibility and accuracy in detecting deck plate crack and bead crack.The developed ultrasonic inspection system called "Steel Deck Plate Matrixeye consists of a portable inspection equipment, a self-propelled scanner and an analytical software as shown in Fig. 2. As shown in Fig. 3, the scanner can run along the weld line on the underside of deck plate using magnetic force and can inspect deck plate crack propagation.The self-propelled scanner is equipped with a removable array probe (10MHz, 32ch) with a wedge for angle beam inspection, and can scan while continuously supplying a small amount of water as an acoustic medium.When inspecting bead crack propagation, it can be inspected by replacing the array probe and running the scanner on the U rib surface.Fig. 4 shows an explanation of how to inspect the deck plate cack propagation.The developed inspection system collects ultrasonic echo data by scanning an ultrasonic beam in a fan shape (sector scan) from a linear array probe through a wedge shoe, and instantly synthesis BF-scope (cross-sectional image orthogonal to welding line) of deck plate crack with high-resolution by the newly developed MP-FMC/TFM.Furthermore, it enables to display BS-scope (crosssectional image along welding line) from the synthesizes 3D images of deck plate crack propagation with high-resolution in real time by overlaying the B-scope images every time the scanner moves 2mm.

Developed imaging method and performance test result
In this development, we newly developed MP-FMC/TFM (Multiple Paths Phased-array FMC/TFM) as a method that enables more precise imaging of crack propagation.MP-FMC/TFM enables to synthesize the outline of cracks clearly.Ultrasonic echo data is collected by the all elements of a linear-array probe in each focal laws of sector scan in FMC process.And image is synthesized from the acquired echo data based on information of the propagation path reflected at the bottom of the inspection target and the information of propagation path directly reflected from clacks within the inspection target in TFM process.The details of the developed method are explained below.

Development of P-FMC/TFM
FMC/TFM consists of FMC processing and TFM processing.FMC processing is a process that collects ultrasound echo data of all combinations of transmission and reception by a pair of elements in all (32) channel elements.TFM processing is a process to synthesize an image by distributing and adding the ultrasound echo data collected by FMC processing to pixels within the imaging area according to the transmission/reception propagation distance (time) to each pixel.FMC/TFM has the advantage of being able to synthesize uniform and detailed ultrasound images.However, it has the disadvantage of low transmission power because it is transmitted from a single element.Therefore, we have developed P-TFM/FMC (Phased array FMC/TFM) that uses a phased array method that simultaneously excites multiple elements as a transmission means and performs image synthesis using TFM processing.Data acquisition in P-FMC/TFM is a process of collecting ultrasound echoes using a combination of all focal laws and all receiving elements.The image synthesis process in P-FMC/TFM is basically the same as FMC/TFM, but the only difference is that the transmission and reception propagation distances (time) are calculated based on the focus positions as shown in Fig. 5. P-FMC/TFM maintains uniform and fine imaging performance while improving transmission flexibility and transmission strength by adding a phased array technology.Furthermore, as shown in Fig. 6, P-FMC/TFM enables to synthesize an image within the propagation range of a transmission beam at once with a single focal law.Normally use, an image is displayed that is an integrated image of all focal laws.And it also has the advantage of being able to generate a clear and uniform image even if it is a small number, for example 10 points or less, of focal laws.

Development of MP-FMC/TFM
In this development, we have developed MP-FMC/TFM (Multiple paths Phased array FMC/TFM) that integrate additional images synthesized from the information of propagation paths via bottom reflection and the image synthesized from the information of direct propagation path.The latter imaging method is P-FMC/TFM."①-②" and "②-②" in Fig. 7 show the propagation paths via the newly added bottom reflection.①-①" in Fig. 7 shows the direct propagation path used in P-FMC/TFM imaging.Both processes perform image synthesis from the same received echo data, but three different images can be synthesized from information of the above three sets of propagation paths.MP-FMC/TFM can clearly display the outline of a crack by integrating multiple images obtained from various directions through these multiple paths.Figure 8 shows the processing flow of MP-FMC/TFM.

Imaging performance test result of MP-FMC/TFM
We conducted a comparative evaluation of the imaging performance of the developed PA-FMC/TFM and the conventional P-FMC/TFM.The test piece used was a flat steel plate with 12 mm thick as same as an usual deck plate and slits cut at angles of 90°, 70°, and 50° with 6mm long in each.Fig. 9 shows the comparison results of images synthesized by PA-FMC/TFM and P-FMC/TFM.The lower images of P-FMC/TFM enable to visualize only the corner and tip of each slit can be seen.On the other hand, the upper images of MP-FMC/TFM enable to visualize the entire shape of the slit, confirming that it has a possibility to clearly image the contour shape of the defect.

User interface of the developed system
We fabricated two types of fatigue test specimens simulating deck plate crack propagations and bead crack propagations to evaluate the ability of the developed inspection system to evaluate the accuracy of crack propagation depth.Figure 10 shows an example of an inspection result screen when inspecting the deck plate fatigue specimen.During measurement, the BF-scope image (the screen that is perpendicular to the welding line) and the BS-scope image (the screen that is parallel to the welding line) are updated in real time, and the measurement is stopped at the set scanning distance and the image data is automatically saved.Detailed analysis including sizing can be performed offline using dedicated analysis software after copying the saved data to the saved PC.

Offline analysis software for evaluating sizing of crack propagation
Figure 11 shows an example of analysis of a deck plate specimen using the dedicated analysis software.On the BS-scope in the upper part of the screen, it can be seen whole crack propagation in a fan shape along the weld line.On the BF-scope in the lower left of the screen displays a cross-sectional image at the scanning position indicated by the arrow "(A)", it makes possible to confirm the crack shape and precise sizing of the crack.In both BF-scope and BS-scope display areas the MP-FMC/TFM image is displayed in the upper part of the area, and the P-FMC/TFM mirror image is displayed in the lower part of the area.The shape of the crack can be observed in the upper MP-FMC/TFM image, and the position of the crack tip can be observed precisely in the lower P-FMC/TFM mirror image on the BF-scope.

Verification result for inspection performance
The actual crack depths of the fatigue test specimens were measured destructively.The destructively measured results were compared with the measured results by using the developed inspection system.As a result of this verification the two measurement results agree well, indicating that fan-shaped crack growth can be measured with high accuracy to detect depth of deck plate crack and depth of bead crack.The accuracy to detect depth of deck plate crack propagation was ±0.8 mm (±2σ) over depth of 1.3 mm.The accuracy to detect depth of bead crack propagation was ±1.0mm (±2σ) over depth of 1.9mm.Both results were confirmed to be good.Furthermore, the measurement accuracy was almost uniform regardless of the crack depth, which is thought to be due to the feature of FMC/TFM that the depth is constant over the entire region.

Conclusions
We developed the ultrasonic inspection system "Steel Deck Plate Matrixeye" to detect crack propagating in steel deck plates and bead with high precision.As a result of measuring fatigue specimens of deck plate and bead by the developed inspection system, it was confirmed that crack shapes could be clearly displayed in both specimens.Furthermore, we evaluated the measurement accuracy of the developed system by comparing it with the destructive measured results of fatigue test specimens.As a result, it was confirmed that the depth of deck plate crack propagation was ±0.8 mm, and the depth of bead crack propagation was ±1.0 mm, making it possible to measure with good accuracy.Currently, verification tests are being conducted on an actual bridge, and practical application is planned in near future.

Fig. 1 Fig. 2 Fig. 3
Fig. 1 Explanation of inspection targets for steel deck crack and bead (in rib-to-deck weld) crack

Fig. 4
Fig. 4 Explanation of image integration and display inspection result of deck plate crack propagation

Fig. 10
Fig.10 Flaw detection screen on the equipment during and after scanning fatigue test specimen.

Fig. 11
Fig. 11 An example of analysis results of crack propagation of the deck plate fatigue test specimen