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Surface Crack Detection Using Rayleigh Waves - in immersion: A Novel Application of a Known Principle Dr. Sebastian Gripp Alzenau Contact

Inspection of forged compressor and turbine disks

Highest demands are established for the disks in stationary gas turbines for the power production with regard to their degree of purity and surface quality, lending to their extreme thermal and mechanical load under operation. Typical sizes of the disks roughly range from 800 mm to 2.000 mm in diameter at varying wall thicknesses of approx. 30 -400 mm. The weight of such a disk can go up to several tons.

Fig 1: Typical cross section of a forged disk in contoured state

Contoured forging

In view of this background it is tried now to forge the disks different from the conventional process, where the disk is forged flat, machined, full volume inspected (by UT), contoured by machining again, surface inspected (by MT), and delivered. In contrast, the final contour is in this advanced process nearly achieved by forging. The gain is multiple: one mechanical processing step can be omitted, material and processing time are saved. The production becomes leaner and faster, a key factor for increasing the efficiency in this highly competitive field.

Conventional and advanced forging process
flat: forging heat treatment flat machining vol. inspection (UT) contouring surface inspection (MT) delivery	contoured: forging heat treatment machining inspection (vol. / surface) delivery

Automated UT inspection system

The advantages of contoured forging outlined above only can be obtained if it is assured that the quality of inspection for material defects in the workpiece isn't reduced. For the VSG Energie- und Schmiedetechnik, Essen, Germany, a system has been developed, in which such contoured disks can be subjected to a mechanised ultrasonic volume inspection. At this the same detection sensitivities are achieved in immersion technique through concave, convex and complex curved surfaces, as are reached by the current, inspection techniques through plane surfaces.
The system works in immersion technique with a turn table. The inspection is carried out single sided from top, for the opposite side the disk is flipped over (see fig. below).

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.

Control software

For control of the system, data acquisition and evaluation an extensive software package has been developed which consists of the following main modules:
• Teach module
• Inspection module
• Re-evaluation module

Teach module
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.

Inspection module
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 3a:	Fig 3b:
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.

Considerations on surface crack detection

Since the disks must be inserted into an automated volume inspection system anyway, the idea suggested itself, if also the surface inspection could be carried out in parallel with the volume inspection. This would greatly improve the documentation and reliability of this testing step on top of the time and handling advantage.

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.

Surface crack inspection with Rayleigh waves

Two procedures making use of surface waves -also called Rayleigh waves-, were developed. In both cases the Rayleigh wave is generated by an incident longitudinal wave in immersion technique, which must meet the correct incidence angle very precisely.

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.

Through transmission of the surface

The first method uses a kind of Through transmission technique of the inspected surface employing separate transmitter and receiver transducers:

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.

Fig 5:

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.

Impulse echo inspection with surface waves

The second method of surface crack detection is based on the reflection of Rayleigh waves at surface cracks. So it is an impulse echo approach.

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.

Fig 7:

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.

Industrialisation

Unfortunately, the physical possibility of the detection of surface cracks with Rayleigh waves is only one half of the problem solution. Also important is the practical performance and the ease of use of the testing machine.

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.

Further development

Both methods will be further refined in future. Especially big potential appears to be in the through transmission method regarding the sensitivity. The pulse echo method against this is particularly impressive because of its applicablity on curved surfaces and its high spatial resolution, which may in future allow the imaging of surface defects and probably even character recognition of writings.

Literature:

1. Sebastian Gripp, Knut Escher: Automatisierte Prüfung von Kompressor- und Turbinenscheiben mit Endkontur. Proceedings Jahrestagung der DGZfP 1999.

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