·Table of Contents
·Materials Characterization and testing
Surface Crack Detection Using Rayleigh Waves - in immersion: A Novel Application of a Known PrincipleDr. Sebastian Gripp
|Fig 1: Typical cross section of a forged disk in contoured state|
|Conventional and advanced forging process|
|Fig 2: Overview of the mechanics|
An inspection bridge which carries three inspection systems can be moved over the inspection tank for the test. Three inspection systems (straight / oblique incidence and surface inspection) are located radially to the turn table.
This module must be entered at least once for each type of disk, as here the disk is made known to the system. In it, all disk type specific parameters, such as type of probe, inspection characteristics, evaluation sensitivity, geometry of the part, and so on are set. From this data the full surface is constructed automatically, and the complete mechanical and ultrasonic inspection setup for the scan derived. After this, the operator reviews and confirms the scan plan. Several utilities facilitate this procedure, such as smart probe positioning commands, sophisticated motion control, computer aided gate adjustment, etc. When this teaching procedure has been finished, the teach module needs only to be entered again in case of modifications to the scan plan.
The inspection module is the part of the software which is regularly used by the operator during normal operation. It is therefore equipped with only few options. Simply the type of the inserted disk is selected and the scan started. During the inspection the current positions of all inspection systems are displayed. A top view of the component with recordable indications is displayed in real time. Automatic geometry echo detection and discrimination greatly facilitates the automated inspection.
Data evaluation and reporting
The concept of projected amplitude storage is followed: While scanning the amplitude values inside the flaw gate are recorded together with their time of flight, and projected into the inspection volume taking the current sound path and velocities into account. Hence all indications from a flaw, which has been insonified from different directions, are mapped into the same volume and can easily correlated. Faulty areas in the specimen result in volumes of elevated amplitudes in the ultrasonic three dimensional representation. After the inspection is finished, a report is made containing a sketch (top and side view) and a table with all recordable indications.
|Fig 3: Reference disk with various flat bottom holes as reference reflectors|
Off line Re-assessment
It is possible to re-evaluate the previously scanned data selecting different evaluation thresholds. This allows the registration and analysis of all possible indications down to noise level. Reports can be printed, as well as 3 D visualisation or sections though the inspection volume.
It is possible in the USIS-CF architecture to have more than one system scan at the same time, in order to increase defect detection or scan speed.
Therefore NUKEM Nutronik GmbH, Alzenau, Germany, was requested to develop a procedure, which allows in parallel to the UT to inspect the close surface for surface breaking and near surface defects, i.e. cracks. It was one indispensable demand that the same test sensitivities as for the manual MT must be achieved. Notches of 1.6 mm length and 0.5 mm depth were defined as smallest detectable reference defects.
Inspection for far surface breaking defects is only insufficiently possible for wall thicknesses and geometries as present. Therefore a method had to be developed, which allows near surface inspection with high sensitivity and reproducibility.
Fig 4: Rayleigh wave Through transmission:|
(+) High sensitivity
(-) difficult to apply on curved surfaces
(-) Sensitivity dependent on flaw orientation
The inspection sensitivity with respect to defect length and penetration depth can be controlled by the size and frequency of the transducers as well as suitable electrical and acoustic matching.
The transmitter sends a longitudinal wave under a suitable angle of incidence on the surface to be tested. This excites a surface wave which spreads out along the material surface in the plane of incidence, and which itself again generates a longitudinal wave under the same angle, propagating in forward direction. If the surface wave hits a material separation, its spreading is handicapped and the through transmission signal reduced. It is a shadow technology, in which a defect shows up by a signal reduction. Problematic is the fact that to allow discrimination against the specular reflection echo, a considerable distance of the surface must be passed through by the surface wave, typically at least several tenths of an inch.
The following picture shows the inspection results of a test piece with electric discharge machined notches with dimensions (width/depth) (: 1 10/1 mm; 2 3/0.5 mm; (3) 1.6/0.5 mm). The rectangular form of the indications stems from the horizontal propagation of the surface waves. The horizontal width of the indications correspondingly reflects the length of the insonified surface.
In order to reduce the dependence of the inspection sensitivity with respect to flaw orientation, several such transducer arrangements must included in one probe, e.g. over cross or in hexagonal position.
Although the inspection sensitivity on flat surfaces is very high, due to the high critikality of the angle of incidence this concept isn't suitable for surfaces showing strong curvature, though.
Fig 6: Pulse echo inspection of Rayleigh waves
(+) applicable to curved surfaces
(+) low dependence of flaw orientation
(-) lower sensitivity
A transducer, which works both as transmitter and receiver, is shaped as a conical or spherical ring such that the longitudinal waves are concentrically emitted such that upon incidence onto the surface, they will generate radially emanating Rayleigh waves. If one of those Rayleigh waves hits a surface braking crack across its propagation direction, it is reflected back into the reversed sound path to the probe, including mode conversion.
Hence this is an pulse echo method, i.e. a defect is assigned an increased signal amplitude. Due to the axial symmetry of the problem the method is independent of the flaw orientation, the inspection sensitivity however is lower for little flaws, due to the equal radiation into the whole surface. Because of the point like approach, an inspection of even highly and multiply curved surfaces is feasible.
As example impulse echo results of the same test piece are shown as used for the through transmission.
In the system, three probe movement units work completely independent of each other in parallel: Two systems carry out volume inspection with different probes (one for near surface and one in depth inspection) while the third system goes for surface crack. The test results of all three systems are projected in their true location into the three-dimensional inspection volume. Grouping of the indications can be performed on request.
Great effort was dedicated to a simple use of the system and the interpretation of the inspection results. The operator gets a one page report which certifies the inspection and the results at flawlessness. At appearance of flaws the specimen is displayed in top and side view together with the flaw locations. For more precise evaluation, positions and amplitudes of all found defects are listed in a table, either grouped or individually. An example is shown below, which shows the inspection result of a test disk with artificial volumetric and surfaces defects.
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