Journal of Scientific & Industrial Research Vol. 60, November 200 I, pp 890-895

Studies for Detection of Mkro-crack Defects in Cultural Artifacts Using Holographic Non-destructive Testing Methods

Sushi I K Kaura ·, D P Chhachhia and A K Aggarwal

Coherent Optics Division , Central Scientific Instruments Organisation, Sector 30, Chandigarh 160 020, Indi a Received :27 April 2001: accepted: 05 August 2001

Non-destructive testing methods based on different hologram interferometric techniques have achieved a great importance for different research and industrial applications . Hologram interferometric (HI) techniques can be used wherever the presence of a structural weakness results in localized deformation of the surface when the specimen is stressed, either by the application of a load or by a change in pressure or temperature. On account of the high measuring accuracy of these techniques, they offer a unique application for non-destructive diagnosis of incipient faults or critical damages in precious and ancient artifacts, paintings, sculptures, and monuments caused due to environment effects and aging. Non-destructive testing methods based on different HI techniques; such as double-exposure, real-time, and sandwich HI have been investigated for detection of micro-c rack defects in cultural artifacts . Experimental results achieved for micro-crack detection studies on different types of artifacts are discussed.

Introduction

During the past several years, there has been a worldwide growth of interest in the protection and restoration of precious and ancient cultural heritage. Due to the passage of time and environmental effects, various cultural artifacts have been, by and large, unable to retain their original colour, shine, texture, and form. Lot of efforts are being made by humanists, artists, museologists, and archeologists to sensitize public opinion about the urgency and the gravity of the problem. Several researchers are working in improving the methods of diagnosis, intervention, and conservation of artifacts. Non-destructive testing methods based on different hologram interferometric techniques 1

-3 have

received lot of attention, during the past few years, for defect diagnosis studies on various artifacts. The application of these techniques can help the caretakers to initiate action for their timely restoration without injuring the artifacts. HI is essentially a method of measuring change in surface shape of any object brought about by some external means such as , stress or heating . Holographic non-destructive testing is based on the fact that a defect on the surface or in interior of an object may cause a typical surface deformation, which can be recognised as a fault specific deformation. A weak, defective area generally deforms in different way than faultless zones . Even if the typical dimensions of these

faulty zones are in the micrometer range, they can be clearly identified on account of high measuring accuracy offered by HI. Non-destructive \testing methods0 , based on different HI techniques, such as double-exposurer' 7,

real-timex and sandwich9 HI techniques have been considered for identifying defects and anomalies in cultural relics, wooden artifacts, bronzware, and ancient panel paintings . Their application and usefulness is linked to the structural composition oft he artifacts, which may vary from time to time.

Double-exposure HI technique involves record ing of two holograms, one of the artifact in its undisturbed state and the other of the artifact in some disturbed state, on the same holographic recording plate . In the reconstruction process, the two-recorded wavefronts interfere with each other to pmduce a fringe pattern, wherein the contour and spacing describe the change that occurred between the two exposures. The doubly exposed hologram may be reconstructed with a separate arrangement without the need for an accurate regi stration of the plate in its original positi on. Real-time HI technique involves the recording of a hologram of the artifact and the real-time observatio of the formation and evolution of the fringes, which are formed due to the interference between the real artifact and its holographic virtual image . Though this technique is useful in estimation of the location and the extent of the

KAURA eta/.: DETECTIO OF MICRO-CRACK DEFECTS I CULTURAL ARTIFACTS R9 1

Figure (-Schemati c or experimental layout for recording oil-axis holograms: L, He-Cd laser; BS, vmiable beam splitter: SF, spatial filter assembl y; CM , concave-renecting mirror: M, mirror; BE, beam expander; 0 , object; l-IP, hologram recording

plate

defect in the artifact under test in real time but practical difficulties are encountered in its app lication, where it is ei ther required to carry out in situ photograp hi c processing of the exposed holographic plate or a severe conditi o n in te rm s of cor re ct placement of th e holographic plate after the photographic processing is required to be satisfied . In sandwich HI technique, in stead of making a double exposure on one holographic plate, the two exposures are made on different holographic plates, which are then brought together in correct registe r. In order to obtain this register, two holographic plates are required to be used for each exposure . By late r combining the two reconstructed images, it is then possible to study the interference fringes caused by those dimensional differences of the object that have arisen between the two exposures. The sandwich HI technique thus adds the advantages of real-time HI technique to the possibilities of double-exposure HI technique. Here, investigations are carried out for non-destructive testing studies for detection of micro-crack defects in various artifacts by using different HI techniques; such as double­ex posure, real-time, and sandwich HI.

Experimental Procedure

In order to carryout the proposed in vestigati ons, an ex perimental layout for recording good quality off­axis holograms of an object, as shown schemati call y in Figure I , is used. Light from a He-Cd laser (Kimmon model JK 4171, 180mW output power) is split by a NRC variable beam splitter (BS) into a reference beam and an object beam. Collimated reference beam is generated throu gh a spatial filter assembl y (45X- microsco pe objective and a S11m pinhole) in conjunction with a

Figure 2- Typical results of mi cro-crack detection studic' in a ceramic mug

beam is further split into two parts by a NRC variable beam splitter (BS). Both the beams are expanded, us ing beam ex pand e rs (45 X - mi c roscope objective ) to illuminate the object (0). The scattered light from the object is made to interfere with the reference beam on the holographic recording plate (HP). Standard Kodak D- 19 developer and R-9 bleach bath soluti ons are used for Agfa-Gevaert I OE75-NAH plates (w ith exposure energy of the order of 751JJ/cm2) to g ive hi gh effic iency and low noi se off-axis ho log ram s. B y us in g the experimental setup, as shown schematically in Figure I , we have attempted the appli ca ti ons of different HI techniques ; such as double-exposure, real-time, and sandwich HI for carrying out studies for detection of micro-crack defec ts in various artifacts .

Results and Discussion

The application of double-exposure HI techni que was investigated for the detection of micro-crack defects in two different artifacts; such as a ceramic mug and a model of Konark wheel carved in stone, in two separate experiments, where two successive exposures were made on the same holographic record ing pl ate. Since the first artifact the ceramic mug (under test) was hollow, it was first filled up with sand before making the first exposure. In the first experiment the first exposure was g iven by keeping the object (ceramic mug) in an undi sturbed state, while the second exposure was g iven after applying a predetermined stress (by putting a load of 25g on the top

892 J SCI !NO RES VOL 60 NOVEMBER 2001

Figure 3- Typical results of micro-crack detection studies in a model of Konark wheel

the second experiment the first exposure was g iven by keeping the artifact (model of Konark wheel) in an undisturbed state while the second exposure was g iven after applying a predeterm ined stress (mechanical load) on the back side of the artifact. The typical results obtained with these studies are shown in Figure 2 and 3, respectively, where it may be observed that at the location of micro-cracks the orientation and spacing of interference fringes are different in comparison to the other portions of the object, which is obviously indicative of a defective zone. Double-exposure HI technique facilitates the identification of defects in the overall area of the object but is found to have limited usefulness in detection of localized minor incipie nt faults in the artifacts. It is to be pointed that in double-exposure HI technique the two images that are compared cannot_ be changed during reconstruction and the resulttn g interference fringes can also be called frozen fringes. Because of this limitat ion of double-exposure HI technique, it becomes extremely difficult to find information on the intermediate states of the object; and difficulty is also faced in the elimination of undesired fringes occurring due to unwanted rigid body motions.

The application of real-time HI tec hnique was investigated for the detection of micro-crack defects in ~- ~ ... -•L~- -~• v;-~ ... .. L ...... ~-11-L ... - _ ..... 1 •• -.-l ... - • ... ~ • '\ -· ·- -

Figure 4 - Typical results of micro-crack detection studi es in an earthen pot

hollow, it was first fill ed up with sand be fore making the exposure for holographic recording. A sing le holographic exposure of the test object (earthen pot) in an undisturbed state is recorded with the holographic set up, as shown in Figure I. The developed hologram is then required to be rep laced exactly in th e same position at which recorded so as to avoid the introdu ction of spuri ous frin ges at the time of reconstruction with the identical reference beam, as used in the recording process. However, practical difficulties are encountered to meet th e severe condition of correct repos itioning of the holographic plate after the photographic process ing. To overcome this difficulty, an in situ photographic processing, in a liquid-gate (Jodon model MPH-45W), of the exposed holographic plate was carried out in our experimental studies. The test object (earthen pot) was then deformed by applying a predetermined stress (by putting a load of 30 g on the top layer of sand inside the earthen pot) so as to carry out the studies for the detection of micro-crack defects in real tim e . Thi s a ll owed instantaneous observation of the frin ges created by the interference of the stressed object with its holographic virtual image. The typical results obtained with these studies are shown in Figure 4, where it may be observed that at the location of the micro-cracks the orientation and spacing of the interference fringes is different, as compared to the other portions of the object, which is obviously indicative of a defective zone. This method is thus useful in the estimation of the locat ion and the extent of defect in the artifact in real time. Real-time HI method is more time consuming than the double-exposure - ... 11-L ... ..J T- _ ....J..J : L : ___ L _ _

KA URA e1 a/.: DETECTION OF MICRO-CRACK DEFECTS IN CULTURAL ARTIFACTS X9J

_hi'""'"~~· 0 ~____] (a) I

B2 F2

--fit 0 '

\ (b) \

Bt F2

~ m • •

(c)

\1 I I I II

Figure 5 - Schematic of experimental steps involved in sandwich hologram interferometry: first ex posure with undeformed object 0. (b)second exposure with deformed obj ec t 0 ' , (c) reconstruct ion with interference frin ges

emul sion layer of the holographic plate the fringe contrast is generally lower for rea l-time method than for other types of HI methods. It may be mentioned that with rea l­time HI method, the test obj ec t mu st be st ressed sufficiently to cause observable differential changes in the reg ions of default or anomalies ; whi ch sometimes makes it difficult to distin gui sh the fringes caused by the faults in the object, from those occurring due to unwanted rigid motions.

Our experimental invest igati ons, for carrying out micro-detachment studies in paper paintings pasted and mounted on a wooden panel board, by using sandwich holography technique 10 involved a s imultaneo us exposure of two holographic plates, with their emul sion­coated surfaces facing the object under test, mounted together in a special plate holder. Jn the first exposure the two plates B

1 and F

1 are ex posed with the unstressed

object, as shown schematically in Figure 5 (a). The object is then deformed slightly by heating the painting surface with the use of a current of moderately warm air from a hair dryer. It is to be seen that if there is lack of adhesion between the painting layer and the wooden panel board the detached regions will di sperse heat at a lower rate than the non-detached regions . This, in turn , will lead to the enhancement of temperature to a higher value in the localized regions where detachments are present. It is thus expected that the detached regions of the painting will behave differently from the non-detached regions. A second exposure of the deformed object is then made on a new oair of olates B, and E mounted together in

Figure 6-Typical result s of mi cro-detachment swdies of a ra per painting obtained wit h 8

1 F~ sandwich hologram

the spec ial plate holder [Figure 5 (b)]. Similarly, a series of exposures are made on different pairs of plates B

1 F,;

B4

F4

, and B5

F5after sequentially deforming the object

by further heating the painting surface. Temperature of the painting surface was increased by an increment of I oc between the success ive ex posures. A di g ital thermometer (Vaiseshika model 7730EP) was used to monitor the temperature of the painting surface . After all the exposed plates are processed, the back plate B

1

from the first pair is combined with the front plate F2

from the second pair to form the sandwich hologram. This sandwich hologram is then mounted in the same plate holder and illuminated with the original reference beam [Figure 5 (c)]. The typical results obtained with these studies are shown in Figure 6. It may be noticed that in some regions the shape of the interference frin ges depart from the mean local behaviour, thus revealing the presence of detachments in those regions. Some of these faults are indicated by arrows in Figure 6. It is to be considered that five exposures (made on fi ve pairs of plates such as B

1 F

1; B

2 F

2; B

3 F

3; B

4 F

4, and B

5 F

5) of

sandwich holograms correspond to 20 exposures ( I 0 holograms) of double-exposure holograms 11

· 12

• It is possible to study the total deformation of the object at any stage by using the sandwich holograms made by combinations of plates such as B , F"; B, F, ; B, F, and

894 J SCIIND RES VOL 60 NOVEMBER 200 1

B1 F

5• In addi ti on, incremental deformat ions of the object

can be analyzed by using the sandwich holograms made by combinat ions of plates such as, B 1 F2

; B2

F3

; B3 F~; B ~

F,. Thu s, sandwich ho lograms repre enti ng ma ny d~ffe re nt deformati on states of the objec t can be reconstructed in any combinat ion. The sandw ich HI technique 13 thereby offers more versati lity and overcomes many limitat ions of doub le-exposure HI technique. It is to be seen that though the image, reconstructed from a sandwich hologram wi th the same reference beam as used for recording, gives the fri nges very similar to that given by a conventional double-exposure ho logram but, by havin<Y the ima<Yes on two di fferent plates, it is possib le 0 0

to mani pul ate them and hence the fr inge pattern 14 1s A series of sandwich holograms B

1 F

3; B

1 F~. and B

1 F, are

also made by combin ing the back plate B1 from the fi rst

pair with the front plates F3

, F4

, and F5

from the third , fou rth and fift h pairs, respectively. These sand wich holograms are then mounted sequent ia lly in the same plate holder and illuminated with the ori ginal reference beam. In our studies, the sandwich holograms B

1 F

2; B

1

F · B F and B F represented th e di fferences in 1' I 4 ' I 5

t~mperature of + I, +2, +3, and +4 o C, respecti vely. Here, plus sign stands for enhancement of the temperature of the painting surface, as compared to its temperature at normal room temperature conditi on. The res ulting interfe rence patterns obtained from sandw ich holograms B F · B F · B F and B F can be used to study the

I 2 ' I 3' I 4' I 5 . surface defects occurring at any stage of defom1at10n caused due to heating. The typical results obtained with the studies of the sand wich hologram B1 F~ are shown in Figure 7. It may be observed from Figure 7 that in some reg ions of the painting the regul arity of the local mean behav iour of the interfe rence fringes is getting altered more predominantly th an in other reg ions, th ereby indicating the presence of detachments in those regions. It is thus seen that the shape of the interfe rence patterns on undetached portions of the painting are quite regul ar, whe reas in the areas of detac hment , strong loca l departures of the interfe rence fringes from the mean behav iour are exhibited. Some of th ese detac hment defects are indicated by arrows in Figure 7. It is further observed that the mean temperature rise required to make the method work is still low enough so as not to endanger the painting under study. This method may also be used for the detec ti on of micro-detachment de fects in the regions of interest in the paintings. A compensation of unwanted ri gid body motion, occurring between the two PYnrlC II rP'-.' rl 11 r in o r Pr nt·rlfn o ro •_"'l n h &:a " ht .-. ;n orl h "

Figure 7 -Typical results or micro-detachment studies or a paper pain ting obtai ned with 8

1 F~ sandwich hologram

manipulating the pos ition of one the sandw ich holograms during reconstruction.

Conclusions

It has been investigated that non-destructi ve testing methods based on different HI techn iques; such as double-exposure, real-time, and sandw ich HI hold a great prom ise for the de tec ti on of micro-crack defects in various artifacts viz., a cerami c mug, a model of Konark whee l carved in stone, an earthen pot and fo r th e diagnosis of micro-detachment defects in paper pa intings pasted and mounted on wooden panel boards. The re lat ive advantages and drawbacks of di fferent HI techniques have also been brought out. The applicati on of different HI techniques can also be considered fo r non-destructi ve di agnosis of incipient fau lts or critica l damages in various types of arti fac ts, paintings on other support bases, sculptures, and monu ments. The present study also hi ghlights th at depend ing on the nature (structural composition) and the conditi on of the arti facts, the applicati ons of different HI techniques can be considered fo r its optimum exploitati on with different va ri ati ons; such as doubl e-exposure, rea l-ti me, and sandwich HI fo r defect diagnos is studies.

Acknowledgement

The authors are grateful to Department of Sc ience and Technology, Government of India, New Delhi fo r ... L _ c: ·- _ ___ : - 1 -

KAURA eta/.: DETECTION OF MICRO-CRACK DEFECTS IN CULTURAL ARTIFACTS R95

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