Keywords: corrosion detection, Lamb waves, nondestructive testing, ultrasonics
Table of Contents
In the conventional configuration, straight beam probes scan the cylindrical wall to measure wall thickness and detect lamination and (planar) defects in the plane of the wall. Angle-beam probes are used to detect the defects oriented normal to the plane of the wall.
Conventional ultrasonic C-scan can inspect the straight sectors of the cylinder but is not suited for inspection of bends and hidden parts of a cylindrical structure. Moreover, inspection of the cylinder with C-scan is very time consuming since a point-by-point measurement is needed for a complete inspection of the structure and difficulties in inspecting around bends, are disadvantages of conventional ultrasonics. However, ultrasonic measurements are more appropriate and precise for small defects.
This work evaluates the ability of ultrasonic guided waves to detect several types of defects in cylindrical structures with different pre-selected wave modes. We also demonstrate ultrasonic guided wave method as an alternative to standard ultrasonic techniques and how to address their deficiencies. This approach is based on the use of ultrasonic guided waves or, generally, the resonant modes of propagation in the cylindrical geometry.
The problem of a hollow circular cylinder has been investigated in an exact formulation without any of the approximation usually associated with thin cylindrical shell theories [4,5]. Dispersion curves for all cylindrical shell modes also were derived; it was demonstrated that a subset of these modes is reducible to Lamb waves in the limit of small cylinder curvature (large radius) .
Generally, the resonant wave modes of cylindrical structure belong to two groups. The first group represents resonant wave modes propagating in the axial plane (axially symmetric modes) and the other group are modes propagating in the transverse plane of the cylindrical coordinate system (non-axially symmetric modes). Each group by itself can be divided further into two types of resonant modes. One type is unique to cylindrical geometry and the other type is equivalent to Lamb wave modes of the flat plate .
The two sets of resonant wave modes which are equivalent to Lamb waves are called the planar and axial modes. The planar modes propagate circumferentially and the axial ones along the cylinder. In this work, the axially symmetric modes equivalent to Lamb waves are used L(0,m)(m=1,2,3....) where
Figure 2 Guided Wave propagation in a cylindrical structure
Figure 4 Group Velocity Dispersion Curves
In the following section results show a strong detectability of defects in the straight cylindrical section as well as the end sections; a reproducible response to different cylinders inspected with different transducer types and high sensitivity to the chosen wave modes. From these results, the proposed ultrasonic guided waves method can be considered feasible for efficient detection and location of variety of defects in cylindrical geometry.
Figure 5 a) S0 transmission mode b) A1 transmission mode
Figure 6 shows the time response obtained from the So wave mode using the pulse-echo arrangement. We observe significant differences between the inspected area on the steel cylinder without defect (Figure 6a) and the inspected one with a 25x1.5x1.5 mm crack (Figure 6b). The time response shown in Figure 6a for a non-defected area of the cylinder shows only the So mode reflected approximately at 1200ms (micro seconds) from the end of the specimen. Waves arriving before the So wave mode represent the excitation (main bang) and the reverberation produced in the interface between the transducer element and the Plexiglas wedge. In Figure 6b, an additional wave packet corresponds to the reflection of the So mode from the crack is observed. This reflection from the defect was confirmed by comparing the actual and calculated distance between the transducer and the defect. This distance is calculated from the product of time-of-flight measurement and the group velocity of the excited So mode. An immediate conclusion from the time response for the pulse-echo arrangement test is that we can directly see any changes in the time response waveform. Existence of an additional packet of waveform is a strong evidence of the existence of defect. Therefore, a guided wave pulse-echo propagates over long distances of these cylinders and can detect flaws along its path.
Figure 7a shows the results obtained from inspection of the same cylinder with previously detected defect using an EMAT probe to excite So wave mode in pulse-echo arrangement . Slight differences in the position of inspection probe and interpretation of the results are occurred because EMATs transducer transmits in both directions. In Figure 7, EMAT transducer is placed faced to the defect resulting in an extra reflection of So time response. The first wave packet is the main bang exercised to excite the EMAT, the second wave packet represents a reflection produced from defect while third one is a reflection from the near end of the cylinder. For a longer time response (Figure 7b) we would be able to see another two reflections one after another. One is produced from the defect due to the reflection of the previously reflected wave from the near edge and the second is produced from the reflection of the secondly transmitted pulse from EMATs at the far edge of the cylinder.
Tests were continued on a half section cylinder whose ends were cut, giving a straight section 1000 mm long and sliced in the middle so that internal defects are easily introduced. The results in the following figures illustrate the detectability as well as the long range inspection capability of the guided wave procedure. Figure 9 shows results from crack-like defect detected at 190 mm away of EMAT transducer. Clear reflection from the defect is obtained as well as the near edge of the cylinder. In the second figure the transducer was placed on the other side of the cylinder so no reflection from the defect is observed. However, Figure 8 shows the reflection of the second far end of the cylinder.
Figure 8 PE response for a steel cylinder a) with defect b) without defect
c) two edge without defect d) two edge with defect
Figure 9 Circumferential pulse-echo response
Under different conditions, several experimental tests utilizing So and A1 modes with different frequency-thickness product were performed. Experiments showed that results can be repeated on different cylinders which is encouraging for adapting this technique to real time inspection. Yet our experimental conclusion on the sensitivity and applicability of using guided Lamb waves confirms that they can be used to inspect large areas in the axial and circumferential plane. Various modes can be induced and selected appropriately to optimize their propagation as a function of their selected characteristics. Sensitivity can be demonstrated from ability to detect, locate and validate structural damage in different type of cylindrical structures, also by demonstrating how ultrasonic guided wave method assesses different types of defects like fine surface-breaking flaws, crack-like defects, holes as well as wall thinning.
The Paper was presented on the UTonline Application Workshop in May '97
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