Ultrasonic holography in the inspection of rotor forgings

Ultrasonic holography in the inspection of rotor forgings E.E. Aldridge, A.B. Clare, D.A. Shepherd and C.G. Brown

Trials were carried out to demonstrate that ultrasonic holography can be successfully applied on the shop floor to the inspection of rotor forgings for estimating the position, size and shape of flaws situated at considerable depths (about 450 mm) within the steel. A helical scan, derived from the lathe in which the forging sat, was used and the various aspects of the resulting holography as they relate to the inspection are described. Details of the trials and the results obtained, illustrated with examples, are given.

Ultrasonic holography is now a well established technology 1'2 which is being applied to the field of non-destructive testing, albeit gradually. It is a means of enabling the ultrasonic image of a flaw to be seen optically and so make it possible to estimate the size and shape of the flaw accurately. In principle this accuracy is not affected by the depth at which the flaw lies and thus ultrasonic holography becomes very attractive for assessing flaws in thick section material. At Harwell, by 1969, a scanning system of ultrasonic holography had been developed on behalf of the NDT Centre there with the aim of evaluating its usefulness in nondestructive testing. This particular system, described elsewhere, 2,a was developed for its inherent flexibility and sensitivity in ultrasonic terms; the complexities of the holography are carried by the electronics. In the last four years the Central Electricity Generating Board (CEGB) has sponsored Harwell in developing ultrasonic holography towards the inspection of rotor forgings. One particular hope was that the inclusion clusters, which occur along the centres of rotors, would be able to be positioned and sized to the extent necessary for fracture mechanics analysis. The ability of conventional ultrasonics to perform this task is limited by the long testing distances and has not been demonstrated conclusively.4,s A preliminary feasibility study was successfully carried out to ascertain the extent to which ultrasonic holography was capable of producing useful images of inclusive clusters. 3,s This was followed finally with two workshop trials in which an Intermediate Pressure rotor forging (held back from scrap for these trials) was examined using a helical scan derived from the motion of the lathe in which the forging sat. A general review of this work relating it to the wider context of rotor inspection and conventional techniques has been given elsewhere.5 The various aspects of ultrasonic holography arising from the use of helical scanning, with particular reference to these trials, is described below. All the authors are with the Electronics and A.P. Division, AERE, Harwell, England.

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Helical scanning Cylindrical bodies which have to be ultrasonically inspected in some detail, are most conveniently examined by rotating the body about its axis of symmetry whilst moving the ultrasonic transducer parallel to this axis. The result is that relative to the surface of the body the transducer moves in a helix, the pitch of which depends upon the relative velocities of the two motions and which can be adjusted to suit the inspection. In ultrasonic holography, this represents a particularly simple type of scan in that using contact probes for example, the geometry will be decided by the surface of the body independent of any external reference, ie there is no lining up problem and it permits the use of lathes for scanning. This latter is important bearing in mind that rotor forgings in the rough state can weigh about 200 t and be up to 15 m long. Also, inspection can be carried out in the initial machining stages with the possibility of making it at the same time as the machining. The general case of ultrasonic holography in cylindrical geometry has been examined elsewhere2'6 and discussion here will be limited to the salient factors as they relate to the Harwell system 2'3 which was used for this work. In this system the hologram is recorded on a facsimile ('wet paper') recorder which is synchronised to the scan by means of stepping motors and digital control. Although the scan is in cylindrical geometry, the hologram is recorded as a fiat plane and the electronic reference is with respect to this plane. The basic format for image reconstruction is unchanged, although the resultant image is affected by the change in geometry between the scan and the recording. Thus, in the optical reconstruction, the optical bench arrangements up to the Fourier plane (where the spurious image and direct transmission are removed by an optical stop) are nominally identical to those for the cartesian scan hologram. The light allowed to pass through the Fourier plane is that concerned with the true image and, due to astigmatism, it is only from this point on that the optical arrangements become substantially different.

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Astigmastism

Fig. 1a shows a cylindrical object of radius ro being scanned in pulse echo by a transducer situated at a radius R. The corresponding hologram is sketched in Fig. lb and since it is fiat is referred to cartesian axes XYZ. Referring these axes in line to Fig. 1a, the X axis lies along the radius, the Y axis lies around the circumference and the Z axis is parallel to the axis of the cylinder. It is assumed that the hologram plane corresponds to the scan plane being cut at -+ 180 ° from the origin as shown. Thus the Y coordinate corresponds to the angular displacement of the scanning transducer from an angular datum chosen to correspond to the XZ plane.

/ / .

Cut

R--ro cos ~ -

y;

-

2

R rocosa

ks 1 XL . m2

R tan

I

//

X

//

In Fig. la is sketched a cross (lines aa and bb) placed on the surface of the cylinder. The cylindrical coordinates of the centre of the cross are ro, a, Zo as shown. The resultant image as reconstructed is shown in Fig. 1b where the two arms of the cross are now imaged at different distances so producing astigmatism. Here, using small angle theory and restricting the hologram aperture correspondingly, it can be shown ~ that line aa appears at:

x2

/k

\

,/ \

Scan plane radius R

(1)

(2)

m

a

b

Z0

z~

=

--

(3)

/

m

-/~

where Xs and XL are the wavelengths of ultrasound and light respectively and m (> 1) is the factor by which the hologram is reduced in the photography to make the transparency. The position of the image depends very much upon a, it is virtual for a in the first and fourth quadrants and real in the second and third. Similarly line bb appears at:

xl

-

R--rocosa

;ks XL

1 m2

(4)

and the Y and Z coordinates are as before.

/,~

/

Using this idea, Fig. 2a illustrates the situation for a flat plane scan. It is clear that little information about the object point can reach the transducer unless it sits within the transducer beam. Hence the geometry of this beam defines the useful aperture size and for this aperture or larger, the resolution of the object is that of the transducer. Fig. 2b illustrates the situation for circumferential resolution in a cylindrical scan, where the transducer is oriented always

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--~,-

/ ,~

X2

b Fig. 1 (a) Geometry of cylindrical object being scanned in pulse echo and (b) hologram of the object illustrating astigmatism

Scan

Resolution

In pulse echo the echo sent back by the object is weighted twice over by the transducer beam pattern. For practical purposes this effect can be represented as if the ultrasonic field of the transducer were uniform and distributed over a cone of half angle equal to the ratio of the wavelength to the transducer diameter.

/ //

Object ~ / point

"-

/

Useful

~'~aJDn

Ce

~

"Tronsdocor

• /~

I

\ /

Transducer

Fig. 2 Resolution with planar scan (a, left) and with a cylindrical scan (b, right)

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JUNE 1977

to look at the axis. Because the transducer twists as it scans, the object point stays within the transducer beam much longer than in the cartesian case. From the geometry it is clear that if the scan radius is large enough the object point will always be within the beam. For this condition the resolution is no longer dependent upon the transducer. Another feature is that all sides af the object point are seen and recorded in the hologram, ie the angular aperture is 360 ° . Unfortunately a linear optical system cannot cope with this situation. Consequently the aperture on the optical bench is deliberately limited to 60 ° by means of a slit which can be moved to expose any 60 ° sector in turn. Assuming a sub-aperture of 0 radians, then relative to the scan the subaperture size is RO. Thus on small angle theory the circumferential ultrasonic resolution is approximately:

I,is

~Kz

!

"""

=

Ra

1

Jr-" "":;"

"-

"opti~otstar,

Fig. 3 Space frequency distribution with planar scan (left) and with cylindrical scan (right)

slit

1 ( R - - r o coso0 k s 0 ro cos

tK,

Fourier

Cy~:drical

(S) I_aser.

where A is the angular resolution and a is measured with respect to the centre of the aperture. This resolution looks to be large as it is referred to the scan plane. If it is referred to the cylindrical surface it becomes

ro A =

1 ( 20 1

rocosc~) R

Xs cos a

(6)

and for ro ~ R and a = 0 or rr, the resolution can approach the limit ~1 ks. At a = ½zr or {Tr no resolution is possible. The longitudinal resloution is identical with that of the flat plane and is given by: R

--

r o cos

2Zm where Zm is the size of the aperture in the axial direction.

Optical reconstruction The astigmatism inherent in the system can be corrected by adjusting the aspect ratio of the hologram. However this will only work for one cylinder radius and a much more satisfactory solution is to use a cylindrical lens on the optical bench. For example if the plane of the hologram is imaged onto a diverging cylindrical lens operating on the circumferential coordinate and its focal length,f, is given by:

f-

R 2m2

Xs XL

(8)

then the net result is to remove the astigmatism and bring x2 of Equation (1) to be identical with xl of Equation (4). The increased resolution in the circumferential direction reflects itself as an increased space frequency or Fourier transform 1'7 bandwidth and consequently a change in the optical stop arrangement on the optical bench. Fig. 3a shows the normal arrangement (for the Harwell system) in the back focal plane of the Fourier lens. The outer dotted circle represents the limit set by the recording media. The

NDT INTERNATIONAL. JUNE 1977

Fig. 4 Optical system for reconstruction of the hologram without astigmatism

two inner dotted circles represent the limit set by the scan transducer and correspond to the space frequency distributions of the true and spurious images. The optical stop is an iris, the aperture of which is shown by the full line, and which permits only those rays associated with the true image to pass. The carrier is situated 45 ° off the axes to enable the aperture image to be removed. Fig. 3b shows the arrangement adopted for cylindrical scanning. The space frequency distributions are ellipses which, along the major axes are usually limited only by the recording media. The carrier is situated such as to separate the ellipses along the minor axes, the size of these being set by the resolution of the transducer. The optical stop is an edge which removes the ellipse associated with the spurious image as well as most of the space frequencies associated with the aperture. An outline sketch of the optical system is shown in Fig. 4. The helium-neon laser shines on the hologram through a slit which permits only 60 ° equivalent of the hologram to be illuminated. The emergent wave front then passes through the Fourier lens and the spurious image is removed by an optical stop consisting of an edge placed in the back focal plane of this tens. The residual wave front, which makes up the true image, passes through a cylindrical lens which corrects its astigmatism and thence to an eye piece for viewing or photographing. An example of the results obtained using a simple cylindrical object made out of ptfe is shown in Fig. 5. Fig. 5a shows the object which consists principally of two windows: one is cut out of the main body of the cylinder and the other is flat and stuck symmetrically to the main body at the same radius and position. Fig. 5b shows the best uncorrected image and it is characteristically squeezed along the axis. Fig. 5c shows the corrected image. A feature here, which is

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Equipment 70

Rodius 21ram

The equipment used was essentially from the laboratory. Since the lathe was to be used for the scan, an optical shaft encoder was attached to the back of the main spindle and used to control all the recording. In the fLrSt trial the lathe lead screw driving the saddle, on which the transducer was mounted, provided the longitudinal scan. Unfortunately this was not driven by a gear train from the spindle but by a separate electric motor and smearing was produced in the final images. In the second trial the longitudinal scan was obtained from a separate frame placed beneath the rotor (cf Fig. 6) and this was synchronised to the lathe spindle through a stepping motor and the digital signal from the encoder. As the rotor turned rather slowly, the paper of the facsmilie recorder tended to dry out during recording and give a poor hologram. Since the recorder gives a very good, quick indication if something is going awry it was retained for this purpose alone. The hologram data proper were recorded on magnetic tape on the shop floor and only later at Harwell were they transferred to the facsimile recorder for photography. The size of the holograms prior to photography was approximately 450 mm x 100 mm. The aspect ratio was kept the same as that of the scan. The transducer (only a single transducer working in pulseecho was used) gave most trouble. It had been hoped that a contact transducer could be used, but a number of difficult practical problems had arisen during the feasibility studies. Consequently it was decided to use a water column of sufficient length to avoid interference from the interface echo. With the rotor above it, the transducer was held in the base plate of a rectangular paxolin tube with the other end separated from the rotor by a small gap. There was a closed circulation system in which water was fed into the bottom of the tube and spilled out at the top to be collected in a drip tray from whence it was pumped round again. The water losses were minimized by rubbing the surface of the rotor with an oily rag to render it hydrophobic and by placing a flexible strip of rubber in contact with the rotor

b

Fig. 5 (a) Test object, (b) image with astigmatism, and (c) image with astigmatism corrected (all dimensions shown are in mm)

implicit in the making of the hologram, is that the curved window is seen as fiat and the flat window is seen as curved concave: note the highlight on the latter in contrast to the uniform appearance of the former.

Field trials Both workshop trials were carried out at the River Don Works, Sheffield, of the British Steel Corporation under the aegis of the Non-Destructive Testing Applications Centre of the CEGB: the first during December 1974 and the second at the beginning of May 1975. The general situation and the main features of the forging, which was held back from being scrapped, are shown in Fig. 6 taken during the second trial. Two faces of the rotor, faces 4 and 5 shown in Fig. 6, were picked as being of special interest and they had previously been machined to about 3.2 pm CLA*. Emphasis was placed on the core region. The rotor was in an early stage (the 'black' stage) of its metallurgical forming process. The diameter of face 4 was approximately 1.12 m and that of face 5, 0.93 m. *Centre Line Average

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Fig. 6

Second workshop trial in progress

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along the water exit edge of the box. It was found necessary to put a full flow water fdter of 100/ma pore size in the circulation system. Without the filter the water as supplied on the shop floor was ultrasonically very dirty. It had been expected that a focussed transducer could be used, but eventually a small plane transducer was found to be more satisfactory.

Trial results

Four holograms were completed on the first trial of which three yielded constructions, two for face 4 and one for face 5. An example from face 4 is shown in Fig. 7a in which the longitudinal smearing due to the non-synchronous rotation of the lathe lead screw is apparent. This hologram covered a region of radius from 5 mm - 125 mm. The flaw is at a radius of 107 mm, ie at a depth within the steel of 453 mm and is approximately 30 x 40 mm in size and slightly over 20 mm in depth. It was the largest and brightest feature on this face. The experience gained on the first trial enabled the procedures on the shop floor for the second trial to run much more smoothly and ten holograms were completed. Of these, two were scrapped which left four for face 4 (covering regions of radius 10-144 mm, 102-236 mm, 194-328 mm, 2 8 6 420 ram) and four for face 5 (covering regions of radius 6 - 1 3 9 mm, 9 5 - 2 2 8 mm, 184-317 mm, 273-407 mm). The axial lengths on the rotor were about 700 mm. Of these holograms four showed no defects. Examples from the others are shown in Figs 7b, 8 and 9.

Fig. 8 Flaw from face 4 situated at a depth in steel of 488 mm (2nd trial)

Fig. 9 Flaw from face 5 situated at a depth in steel of 424 mm (2nd trial)

In Fig. 7b the image shown should be compared with that of Fig. 7a as it is the same defect. The former is much sharper, partly due to the synchronized longitudinal scan but partly due to an adjustment in the phase reference to enable optirnisation of the recording resolution and also to a wider range gate. This time the cut in the hologram was chosen so as to avoid any defect straddling it as happened with one or two defects in the first trial. Because of these changes it is unrealistic to expect precise point to point correspondence in the pictures where there are patches of light. However, where there are areas of darkness it is reasonable to expect some degree of correspondence since apart from changes in the gate position changes in the resolution should not produce light where there was no light before. In this case the two pictures correspond quite well, eg where the black surround intrudes into the defect pattern from the left tending to separate the upper part of the defect from the larger lower part. Some judgement is always needed when deciding the optimum focus on the optical bench for positioning the flaw and in Fig. 7b it appears at a radius of 101 mm which is 6 mm less than in Fig. 7a. This is within the estimated limits of error as discussed in the next section. Fig. 8 shows another defect from face 4. This occurs at a radius of 72 mm and is about 50 mm long by 10 mm wide. The vertical pattern on the right is the aperture diffraction from the edge of the hologram. This defect is only 90 mm from this edge.

Fig. 7 Top (a) flaw from face 4 situated at a depth in steel of 453 mm with non-synchronous longitudinal scan (1st trial), and bottom (b) the same flaw with synchronous longitudinal scan (2nd trial)

NDT I N T E R N A T I O N A L . JUNE 1977

Fig. 9 shows a defect from face 5. This occurs at a radius of 42 mm, ie at a depth within the steel of 424 mm, and is about 210 mm long by 10 mm wide. Until destructive tests are carried out the nature of these defects is not known. (Destructive examination has already

119

started under the auspices of the CEGB-Manufacturers Study Group in Holography. 9) In all, 30 defects were found and sized holographically in faces 4 and 5.

Positional accuracies In the case of actual defects, most of which tend to have irregular boundaries, instrument errors are swamped by the judgement as to which focussing pattern to use. However once a given pattern has been chosen, or if the defect happens to be a small isolated one, the radial position can be ascertained to within about 5 mm. Calibration errors on the optical bench were 1 mm thus the total error on the radial position is -+ 6 mm. Having chosen the circumferential datum on the surface of the rotor, it was necessary to bring it out to a more accessible point on the rim of the face plate of the chuck. It was then necessary to provide a bench mark on the bed of the lathe to enable an estimate to be made of the proficiency of the lathe operator in stopping the lathe with the rim mark opposite the bench mark. The next operation was to transfer the datum along the rotor to where the transducer was located and to centre-up the transducer beam. The total error from these operations was estimated to be about 6 ° . The Vernier errors on the optical bench contributed a further 2 ° making a total error of -+ 8 ° in determining the coordinate. The axial datum, referred to the centre of the transducer beam, was determined to within about 15 mm. The Vernier errors on the optical bench added a further 20 mm, making the total error -+ 35 mm.

Upon comparing these three criteria, it is seen that the resolution was set by the facsimile recorder but not to an extent which was greatly removed from the fundamental wavelength limit. It is probably not possible to extract any positive confirmation for the resolution from the defect pictures because there is no pre-knowledge about the defect shape but the deductions which can be made appear to be consistent. For example by measuring from centre to centre of the highlights of Fig. 7b, the axial resolution would appear to be better than 6 mm and the circumferential one, better than 3 mm.

Conclusions It has been demonstrated that ultrasonic holography can be successfully applied on the shop floor in typical working conditions although unfortunately, in its present state of development, only with highly skilled personnel. It has realised its technical promise for estimating the size and shape of flaws situated at considerable depths within steel. It provides a very convenient record of these flaws from which independent judgements can be made subsequently by later assessors. For example face 5 had 20 flaws in a cylinder of radius 317 nun and length 700 mm all recorded on three holograms each one of which was (ultimately) a small photographic transparency. Also, as a bonus, when applied to rotor forgings of this simple cylindrical geometry it has promise as a search tool, ie in this particular application it could conceivably replace conventional testing.

References 1

Sizing accuracy The ultrasonic frequency used was 2.5 MHz in compression. The wavelength in steel is 2.4 mm giving a resolution limit of about 1.2 mm. Beam plots 6 show that when a plane beam transducer is energised under essentially single frequency conditions there is a focussing action in the region between the Fresnel and Fraunhofer zones such that the beam width between the 3 dB points is about ¼ of the transducer diameter. Applying this result to the present case where the transducer diameter was 10 mm gives a beam width, and hence roughly a resolution, of 2.5 mm. The 0.46 m facsimile recorder has a resolution of 4 lines per mm and the surface of the rotor was scaled onto the recorder by a factor 0.14. Hence axial resolution was 3.7 ram.

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2 3 4 5 6 7 8 9

Hildebrand, B.P. and Brenden, B.B. 'An introduction to

acoustical holography' (Plenum Press, New York, 1972) Aldridge,E.E. 'Acoustical holography', (Merrow Publishers, 1971) Aldridge,E.E. 'Ultrasonic holography and non-destructive testing', Mater Res Stand 12 No 12 (December 1972) pp 13-22 Livevsidge,D.B., Fearn, G.A. and Dodgson, M.W. 'Ultrasonic assessment of unbored rotor forgings', Non-destructive Testing (1968) pp 385-400 Coffey, J.M. 'Ultrasonic holography', NDT '76 Conference, London 26-28 April 1976 Physics in Technology (July 1976) pp 146-153 Ceil,F.G. and Mott, G. 'Cylindrical scan acoustical holography', Proc 4th Int Symp Acoustical Holography (Plenum Press 1972) pp 335-350 Aldridge,E.E. and Sayers, J.L. 'Fourier transforms and acoustic diffraction', Ultrasonics 4 (July 1966) pp 131-135 Aldridge,E.E. and Liddington, B.H. 'Progress report on the evaluation of ultrasonic transducers', Ultrasonics for lndustry (lliffe Science and Technology Publications, 1970) pp 37-39 Coffey, J.M. Private communication

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