HERE'S HOW

Moird Gratings for High Temperatures and Long Times

Authors have extended previous work on various types of grating materials and application techniques and have refined it to the point when gratings of 1000 lines/in, and over can be applied to the surface of the specimen using simple laboratory techniques

by G. Cloud, R. Radke and J. Peiffer

Introduction For many years there has been a need to perform

strain measurements on mechanical parts and structures which must function for a long time at high temperatures and in difficult environments. Problems of this type are common in the nuclear-reactor industry and in the manu- facture of aircraft engines, to name just two examples. Sharpe' has reviewed several methods of measuring strain at high temperatures. A general conclusion might be that the conventional methods of strain measurements are severely taxed. They are likely to give poor results in this difficult situation, especially when long-term stability is needed. Resistance strain gages are generally deficient in ability to withstand high-enough temperatures and in long-term stability. Available capacitance gages l-~ give decent results, but they are expensive, bulky, and they give only point-by-point measurements. Sharpe's interfero- metric strain gage' will work at elevated temperatures for extended periods, but it also gives only point-by-point information.

The moir~ technique seems promising for measurements of this type because of its remote noncontacting capability. In general, it is not subject to errors resulting from material incompatibility, simility or thermal effects. In its modern sophisticated form, the sensitivity of the moird technique can be varied over a broad range anytime after a test is completed and the data permanently recorded on high-resolution photographic plates. It is, of course, a whole-field method which gives a picture of displacement over the entire area studied. It is possible to obtain one, two or three displacement components and, thereby, characterize the complete strain tensor at the surface of the specimen.

Cloud ~ had developed sophisticated moir~ techniques which involve high-resolution-grating photography by a modified camera and subsequent optical data processing of the grating photos to obtain displacement and surface strain. Owing to the simplicity of the approach, the technique is suitable for in situ applications, and it is

G. Cloud (SESA Member) is Professor of Mechanics, R. Radke (SESA Student Member) and J. Peiffer are Research Assistants, Department of Metallurgy, Mechanics and Materials, Science, Michigan State University, East Lansing, MI 48824. Approved manuscript received: July 5, 1979.

being developed for geotechnical strain measurements, in biomechanics situations, for specimens while under cyclic load, and for high-temperature severe-environment conditions. Durelli and Buitrago, 6 Johnson, 7 and Rao and Sciammarella 8 have also developed and applied moir~ methods for high-temperature strain measurement. These researchers did not utilize the two-step high-resolution photography and optical processing approach which seems especially appropriate for these difficult applications. A fundamental requirement in high-temperature moir~ work is to have a specimen grating which will remain highly visible through a test cycle which may, for example, last for 1000 h at over 600~ Luxmoore and Hermann 9''0 and Holister and Luxmoore, ' ' have explored various types of grating materials and application techniques, some of which are known to be suitable for high-temperature studies. The authors have extended this work and refined it to the point where gratings of 1000 lines/in, and over, which have the necessary temperature and deformation resistance, can be applied to the surface of the specimen using simple laboratory techniques. Some of these techniques are outlined below. It must be recognized that variations and hybridizations of these methods are possible and are often quite appropriate in a given experi- mental situation. With these techniques in hand, one is always able to devise a moir~ grating application technique which will be suitable for a given set of test conditions and which utilizes devices and materials which might be at hand.

Photoresist All of the grating application techniques discussed

below begin with the application of a photoresist grating onto the specimen surface. While other possibilities are as good, the photoresist approach is probably the simplest and most consistent for creating a high-quality primary grating on the specimen. The authors generally use Shipley Photoresist 1350J or 1350B with the appropriate thinners. It is applied by a spinning technique as recom- mended, although swabbing, dipping and spraying methods also work well for specific problems. The photo- resist is dried according to specifications and exposed through a grating submaster with unfiltered radiation from a mercury lamp. Development and postbaking are quite ordinary. The authors have made grids and three- way gratings by multiple exposure of the photoresist.

Experimental Mechanics �9 19N

H E R E ' S H O W

Fig. 1--Photomicrograph of 40-1prom grating of gold vacuum deposited onto stainless steel

Nothing more will be said here except to comment that there is a good deal of art involved in handling photo- resist, and some practice to get the techniques well in hand i s worthwhile since they are used over and over again.

Deposited Metal-film Gratings In this technique, the specimen with its photoresist

grating is placed in a vacuum deposition unit and a film of precious metal such as platinum, gold, or even aluminum, is deposited over the whole surface. The result is a relief grating in the metal coating. The grating may be used as is. Indeed, gratings of aluminum deposited o n titanium have been formed in this way and used without further treatment in fatigue studies which ran for several days at 300~ At high temperatures, the photo- resist may or may not burn away or vaporize. The only serious problem is that, if the photoresist disappears and allows the metal overcoating to sink to the specimen surface, the result will be essentially a uniform coating of metal on the Specimen, and the grating will be hardly visible.

Increased,grating visibility may be obtained by washing the specimer~: with acetone or some other solvent to remove the photoresist lines along with their overcover of metal. The intermediate lines of precious metal attached to the specimen are left. The result is a precious metal relief grating on the specimen surface. Visibility of the grating may be enhanced by appropriate choice of metal and by chemical treatment. A practical example is that a copper grating deposited on aluminum may be darkened by simple exposure to the air or by heating i t slightly. Overall, this technique is very simple if one,,has access to

a vacuum-deposition unit. An alternate approach which accomplishes about the same thing is to electroplate the metal film onto the specimen. The electroplating method is, however, somewhat more ticklish, although less exotic equipment is required.

Figure 1 shows a 40 lpmm (lines per millimeter) gold- film grating which has been deposited on a stainless-steel specimen for testing at 600~ for long times.

Etched Gratings This well-known technique has been explored and

described by Rao and Sciammarella, ~ by Durelli and Buitrago, 6 and others. The grating is produced in photo- resist and the specimen etched by electroetching or chemical milling. While simple in theory, this method is technically quite complex in that it is difficult to reproduce fine gratings without undercutting the photoresist and destroying the grating. All things considered, the authors find this etching technique to be considerably more ticklish and demanding of time and expertise than is the vacuum- deposition method.

Refractory Gratings It has been demonstrated that refractory paint such as

"Pyromark" , marketed by Tempil Division of Big Three Industries, South Plainfield, NJ will highlight etched 6.8 gratings. It appeared that this material could be "used to create a grating directly on the surface of the specimen. A successful simple technique has been developed which requires that the grating be fabricated first in photoresist. The refractory paint is then smeared or sprayed over the specimen. After it has cured, it is rubbed down to the surface of the photoresist. The resist can be removed or

Fig. 2--Photomicrograph of 40-1pmm refractory-paint grating

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HERE'S HOW

Fig. 3--Sample fringe pattern produced by optically processing images of refractory-paint gratings after 600~ for 1020 h. Specimen at 600 ~ C

left in place to vaporize during the high-temperature test. This approach has proved satisfying in that it produces a high-contrast relief grating which is very easy to photo- graph. The paint may be colored to improve contrast. Visibility of the grating actually improves if the substrate oxidizes during a test and a light-colored refractory is chosen. A possible serious problem is that the refractory paint might not adhere well enough to the surface during a mechanical test at elevated temperatures. Tests to explore the time, temperature and strain limitations of this appli- cation procedure are in progress. It is important to note that, with the grating photography and optical data- processing techniques, minor cracking of the refractory is not serious. Cracks are recorded as optical noise which is eliminated during the optical processing, Even minor flaking of the refractory would not be a problem,

Figure 2 shows a photomicrograph of a Pyromark refractory deposited directly on a stainless-steel specimen for testing at 600~ and over. This grating was produced by the method described above. It was rather carelessly prepared during a technique-development experiment, and a large number of scratches and inconsistencies are visible in the grating. Again, however, these scratches and dis- continuities are recorded as optical noise and filtered out during optical processing. The specimen pictured here has already been tested at 600~ for 1020 h and then at 750~ for 20 h. There is no sign of deterioration or poor adhesion. In fact, the moir6 results and grating photo- graphs have improved during the test as a result of oxidation of the substrate. A moir~ mismatch fringe pattern obtained by the most elementary optical-data processing of a photoplate of this grating appears in Fig. 3.

Sample Hybrid Technique One example of a hybrid technique for creating a

moir6 specimen grating is to combine the ideas outlined above. A metal relief grating is first made by vacuum deposition. The spaces between the grating lines are then filled with refractory material by spraying the refractory

and then sanding off the excess to the level of the metal dots or lines. Such a grating would cover the whole speci- men surface, and combinations of grating metal and refractory could be chosen to give good contrast for the test conditions. It is likely that a grating so prepared would exhibit good adhesion and maintain its integrity through rather severe strain cycles. Tests of gratings prepared in this way are in progress.

Additional Comments Specimen finish and grating photography lighting are

important factors to consider when trying to photograph gratings for subsequent optical-data analysis or when imaging gratings for in-camera superposition. For most applications, a lapped matte surface is satisfactory. Highly polished surfaces tend to create difficulties in photography or imaging because of specular reflection, but they are better for certain applications. Lapping works best on flat surfaces. Very fine sand blasting works well on curved shapes. For most experiments, lighting should be arranged so that the plane of the camera and the lamp is perpendicular to the specimen grating, and the lighting is off the optical axes of the camera by about 30 deg. Such an arrangement will tend to emphasize both contrast and relief aspects of the grating. In general, if the relief characteristic is most important, then the light should be directed from further-off axis. If contrast is strong, then the light may be brought closer to the optical axis. There are no set rules, however, and for any given experiment a major investment of time may need to be devoted to moving the light, whether continuous or strobe, around to obtain the best grating visibility.

References 1, Sharp, W.N., Jr., "'Strain Gages for Long-term High-tempera-

lures Strain Measurement, " EXPERIMENTAL MECHANICS, 15 (12), 482-488 (Dec. 1975).

2, Noltingk, B.E., Owen, C. and O'Neill, P., "'High-Stability Capacitance Strain Gauge for use at Extreme Temperatures, "" Proe. lnst. Elec. Eng., 119, 897-903, A72-37210 (July 1972).

3, Gillette, O. and Mullineaux, J., "'Development o f High-Temperature Capacitance Strain Gauges, "" 1SA Transactions, $ (1), 52-61, A69-37006 (1969).

4. Sharpe, W.N., Jr. and Martin, D.R., "'An Optical Gage for Strain~Displacement Measurement at High Temperature Near Fatigue Crack Tips, "" Air Force Materials Laboratory, Tech. Report AFML-TR- 77-153, Wright-Patterson AFB, OH (1977).

5. Cloud, G.L., "'Measurement o f Strain Fields Near Holes using High-resolution Grating Photography, Optical Data Processing, and Digital Data Reduction," EXPERIMENTAL MECHANICS (accepted for publication).

6. Durelli, A.J. and Buitrago, J., "'Residual Strains in a Stainless Steel Plate with a Circular Hole and Subjected to Reverse Loading at Elevated Temperature," From Pressure vessels and Piping: Verification and Qualification o f Inelastic Computer Programs, Edited by Corrum & Wright, A S M E (1975).

7. Johnson, L., "'Moir~ Techniques for Measuring Strains During Welding, "" EXPERIMENTAL MECHANICS, 14 (4), 145-151 (April 1974).

8. Rao, R.K. and Sciummarella, C.A., "'Fracture Study o f Stainless Steel Rings at llO0~ '" National Science Foundation Report 77-1 for C6ntract NSF Eng. 75-11160 (Dec. 1977).

9. Luxmoore, A. and Hermann, R., "An Investigation o f Photoresists for use in Optical Strain Analysis, "" J. Strain Anal., 5 (3), 162 (July 1970).

10. Luxmoore, A. and Hermann, R., "'The Rapid Deposition o f Moird Grids, '" EXPERIMENTAL MECHANICS, 11 (5), 375-377 (Aug. 1971).

11. Holister, G.S. and Luxmoore, A., "The Production o f High- density Moird Grids," EXPERIMENTAL MECHANICS, 8 (5), 210-216 (May 1968).

Expe r imen ta l Mechan i cs �9 21N