Advanced non-destructive testing (NDT) techniques like alternating current field measurement (ACFM) and phased array ultrasonic testing (PAUT) are being deployed on hydropower and water navigation structures more and more frequent as these structures age. This article presents a dual approach using ACFM and PAUT for detection and sizing of fatigue cracks in discharge tube fillet welds that connect a variety of different stiffener types to large diameter stiffener tubes used to transport water downstream from a dam to an upstream irrigation canal.
Fatigue Cracks in Discharge Tubes
The discharge tubes cited in this case study are large diameter steel or galvanized steel pipelines that transport water upward from the river downstream of a dam to a canal used for irrigation water. Several types of stiffeners are deployed to reinforce these discharge tubes including collar stiffeners, “T” type stiffeners and “H” type stiffeners. Each discharge pipe also uses periodically spaced supports which consist of two fillet welded collars connected via cross bracing which is also welded to the discharge tube.
Figure 1: Discharge tubes showing periodically spaced supports.
The potential zones for fatigue cracks to be detected with alternating current magnetic field measurement (ACFM) and phased array ultrasonic testing (PAUT) are shown in Figure 1 and 2: 1) the fatigue crack propagates inward through the fillet weld throat 2) the fatigue crack initiates at the fillet weld toe and propagates through thickness 3) the fatigue crack propagates along the fusion line of the fillet weld and discharge pipe / weld. An example fillet weld fatigue crack detected by ACFM is shown in Figure 2. The crack is approximately 3” long and was classified as a Type 2 fatigue crack per Figure 1 and follow-up testing with PAUT and magnetic particle (MT) was performed. The fatigue crack shown at the fillet weld toe was further confirmed with dry visual magnetic particle testing using an AC yoke and red powder.
Figure 2: Discharge pipe fillet weld fatigue crack locations
Figure 3: Fatigue crack detected by ACFM, MT, and PAUT.
Alternating Current Field Measurement (ACFM)
The ACFM technique was created to detect surface cracks, and very near surface cracks through paint coatings and with minimal surface preparation compared to the more routinely used, and less expensive, magnetic particle and liquid penetrant non-destructive testing techniques. The alternating current field measurement method is also less sensitive to lift-off variation compared to conventional eddy-current testing. Due to the benefits of 1) non-contact testing, 2) minimal cleaning, and 3) quantitative detection, the ACFM technique is now widely used for steel weld inspections.
An ACFM excitation coil introduces a uniform current field which in turn generates a magnetic field at the surface plane of the tested component. In the presence of a crack, the induced current is disturbed and affects the magnitude and direction of the resulting magnetic field at, or just above, the surface. An alternating current field measurement probe is designed to, at the very minimum, measure variations in the magnetic field Bx (parallel to weld) and Bz (through thickness) directions. While slightly counterintuitive, the magnitude of the magnetic field in the axial direction Bx quantifies depth while the magnitude and phase of the measured magnetic field in the z direction measure the weld defect length.
Example ACFM data is shown in Figure 3 for Bx (top left), Bz (bottom left), and the butterfly plot (right) generated using a standard alternating current field measurement 5 kHz weld probe. The ACFM Bx data shows a long trough that was used to estimate the depth of the crack. The ACFM Bz data show shows where the crack starts and ends and corresponds to the Bx trough data. Notice the opposite polarity of the Bz data at the start and end of the crack. On the butterfly plot, the Bx amplitude is plotted on the vertical axis and the Bz amplitude is plotted on the horizontal axis.
Figure 4: ACFM Bx, Bz, and Butterfly plot.
Fatigue Crack Detection with Phased Array Ultrasound
While phased array testing (PAUT) is generally applied to full penetration joints, the technique may be applied to fillet welds and fatigue cracks that may initiate and propagate from stress concentrations like the weld toe. In this case, PAUT was used to determine if the crack detected by ACFM was a Type 2 or Type 4 fatigue crack. The phased array data generated by the Olympus MX-2 32:128 platform is shown in Figure 5.
The PAUT A-scan and S-scan are shown respectively in the left-hand and right-hand side of the phased array data shown below. The area was inspected using a 45 to 70 degree sectoral scan. The discharge tube wall thickness was approximately 13 mm.
Figure 5: Phased array that from a through wall fatigue crack.
The fatigue crack depth and length were sized using the 3 dB technique. The following cursors were used on the Omni-scan MX-2 length sizing: U(m), U(r), U(m-r), I(m), I(r), I(m-r), I*U(m-r). The PAUT sizing approach for estimating fatigue crack length is the same as conventional ultrasonic testing when scanning manually. 1) The reflection from the fatigue crack was maximized and marked as the central position. The probe was then moved in opposite directions along the weld axis and 3 dB points, corresponding to the start and end of the fatigue crack were noted.
The height of the fatigue crack is estimated using the same 3 dB technique. This process is performed using the S-scan and manipulation of the focal law or beam. Once the maximum fatigue crack reflection is located, the phased array focal law is increased and decreased until the 3 dB points are identified. The U(m), U(r), and U(m-r) cursors are then used to determine the fatigue crack height.
Crack tip diffraction sizing may also be used to determine the depth of cracks. For surface breaking inside diameter cracks, a strong corner trap is typically observed. In this case, the reference depth cursor should be placed at the ID and the measurement cursor through the maximum intensity of the deepest ligament reflection.