REVIEW OF SCIENTIFIC INSTRUMENTS 85, 045121 (2014)

Ultraviolet digital image correlation (UV-DIC) for high temperature applications Ryan B. Berke and John Lambrosa) Department of Aerospace Engineering, University of Illinois, 306 Talbot Laboratory, 104 S. Wright St., Urbana, Illinois 61801, USA

(Received 14 March 2014; accepted 9 April 2014; published online 28 April 2014) A method is presented for extending two-dimensional digital image correlation (DIC) to a higher range of temperatures by using ultraviolet (UV) lights and UV optics to minimize the light emitted by specimens at those temperatures. The method, which we refer to as UV-DIC, is compared against DIC using unfiltered white light and DIC using filtered blue light which in the past have been used for high temperature applications. It is shown that at low temperatures for which sample glowing is not an issue all three methods produce the same results. At higher temperatures in our experiments, the unfiltered white light method showed significant glowing between 500 and 600 ◦ C and the blue light between 800 and 900 ◦ C, while the UV-DIC remained minimally affected until the material began nearing its melting point (about 1260 ◦ C). The three methods were then used to obtain the coefficient of thermal expansion as a function of temperature for the nickel superalloy Hastelloy-X. All three methods give similar coefficients at temperatures below which glowing becomes significant, with the values also being comparable to the manufacturers specifications. Similar results were also seen in uniaxial tension tests. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4871991]

r Any warping of the air between the camera and the

I. INTRODUCTION

The ability to measure deformation and strain during loading at high temperature is important for the design of engineering structures meant to withstand extreme thermomechanical environments such as spacecraft re-entry, hypersonic flight, and the operating conditions of certain gas turbines, nuclear reactors, and high-temperature fuel cells. In many such cases the strain field is likely to be non-uniform, and so high spatial resolution in the strain measurement is desirable. Traditional strain gauges offer limited spatial resolution, contact with the material, and operate in a limited temperature range. A popular and versatile alternative means of measuring surface strain and displacement on a deformed object is Digital Image Correlation (DIC).1 In brief, the technique works by applying a high-contrast speckle pattern to the object’s surface, then taking high-resolution digital images before and after deformation. A computer algorithm then breaks the images into small subsets and compares the grayscale values of the pixels within each subset in the undeformed image to subsets in the deformed image to compute the relative displacements of the speckled surface. The technique is popular because it requires little surface preparation, is essentially non-contacting (with the exception of a speckle pattern if applied externally), and is able to achieve full-field displacement and strain distributions with sub-pixel precision. In order to use DIC at elevated temperatures, there are a number of challenges which must first be addressed:2

r The speckle pattern itself must be able to withstand the temperatures at which the tests are conducted without either breaking free of the surface or changing color/texture. a) Author to whom correspondence should be addressed. Electronic mail:

[email protected] 0034-6748/2014/85(4)/045121/9/$30.00

specimen due to heat haze must be minimal such that the optical environment remains consistent. r Any light emitted by the specimen due to increased black-body radiation at higher temperature must be mitigated such that it does not significantly affect the grayscale values recorded by the cameras. To address the first challenge, some authors have used a number of commercial paints and coatings which can withstand high temperatures, including refractory coatings which are rated up to 1760 ◦ C,3 and cobalt oxide powder which remains black up to its melting temperature of 1900 ◦ C.4 Alternatively, pre-oxidizing the specimen surface by a limited excursion to a slightly higher temperature can also be used.5, 6 To address the second point, it is common to use an air knife to mix the air in the line of sight of the cameras, thereby reducing the apparent distortion caused by heat haze.3 To address the third, several authors have used monochromatic blue lights to illuminate the speckle surface and blue band-pass filters to reduce the amount of black-body radiation that enters the camera. The filtering method works because as the temperature of the specimen increases, the emitted light is known to have higher intensity at longer wavelengths than it does at shorter wavelengths. By filtering the cameras to only accept blue light, which has a shorter wavelength than most visible light, DIC can be performed up to higher temperatures before the emitted light becomes bright enough to significantly impact the camera images. The blue light technique has been demonstrated with both 2D7 and 3D8 DIC set-ups, and has been used to measure thermal expansion in nickel superalloys up to 1000 ◦ C,2 stainless steel up to 1200 ◦ C,9 and a C/SiC composite up to 1500 ◦ C.3 The objective of this work is to extend the temperature range of DIC even higher using ultraviolet (UV) lights and

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Rev. Sci. Instrum. 85, 045121 (2014)

FIG. 1. Schematic of 8 × 8 mm gauge region test specimen (left) and photo of specimens with 8 × 8 mm speckled gauge region and 4 × 4 mm unspeckled gauge region (right).

optics in place of blue light, in a method we call UV-DIC. Tensile specimens of a nickel superalloy, Hastelloy-X, are heated using an induction heater and observed under unfiltered white light, filtered blue light, and filtered UV light. Displacements and strains on the heated surface are then computed with DIC. Although in this work we have applied UV-DIC to temperatures up to 1125 ◦ C, this has been because of a limitation of the material used. In principle, the UV-DIC method should be able to go to much higher temperatures than blue light DIC using a material with a higher melting point.

II. EXPERIMENTAL METHODS

Experiments were performed using cylindrical tensile specimens with square gauge regions. Rods of Hastelloy-X were purchased from American Special Metals Inc. The rods were 130 mm long and 19.05 mm in diameter, but except for the grips at either end were lathed down to a diameter of 12.5 mm. The gauge region was then further reduced to a square cross-section using wire EDM. A schematic and photograph of the machined specimens is provided in Figure 1. Two gauge region cross-sections were considered: an 8 × 8 mm square cross-section and a 4 × 4 mm square one. The 8 × 8 mm specimens were used initially to perform uniaxial tension tests at elevated temperature, but due to their mass were unable to be heated above 725 ◦ C. The 4 × 4 mm specimens were then selected to measure thermal expansion at higher temperatures.

A. Uniaxial tensile testing

A speckle pattern was applied to one side of each 8 × 8 mm square gauge region. In order to help the paint adhere to the surface, the specimens were first pre-oxidized to reduce the likelihood of any oxidation during later testing resulting in paint flaking off. To pre-oxidize, the specimens were heated in a furnace chamber at a rate of approximately 20 ◦ C/min to a dwell temperature of 800 ◦ C. The specimens were held at dwell for approximately 20 min, then cooled at a rate of approximately 20 ◦ C/min back to room temperature. A black speckle pattern was applied directly onto the oxidized metal surface of the 8 × 8 mm specimens using VHT Flameproof high temperature spray paint. The specimen was supported between two water-cooled grips and heated via magnetic induction. Induction heating is advantageous compared to other methods of heating because direct heating occurs only in the sample and not in the surrounding air, so distortions in images due to heat haze are minimized.10 The induction coil was built from 3.175 mm diameter soft copper tubing and internally cooled with chilled water. The coil was helically wound around the sample, with a gap at the gauge region to allow room for the camera to record images. A photograph of the experimental setup is shown in Figure 2 with blue light illumination, with a zoomed in portion to show the placement of the specimen inside the coil. Heating was performed under feedback control from a K-type thermocouple, and temperature was further monitored by three additional thermocouples along the length of the

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FIG. 2. Photos of experimental setup.

gauge region. The placement of the thermocouples on the back side of the specimen and a plot of the temperatures recorded by each thermocouple is shown in Figure 3. The figure shows that the temperatures agree to within about 2%, indicating that temperatures were relatively uniform along the length of the gauge region. The 8 × 8 mm square gauge region specimen was heated to a nominal temperature of 725 ◦ C. The 8 × 8 mm cross-section sample was subjected to uniaxial tension under load control in a servohydraulic load frame. During heating, the specimen was protected against loads caused by thermal expansion using a load protect feature that limited loads to between +/− 0.17 kN. Once the desired temperature was reached and given time to achieve steady-state, the specimen was then loaded at a rate of 0.1 kN/s for 170 s, resulting in a maximum load that was approximately 110% of the yield stress of the material at that temperature. The stress-strain response obtained from loading the sample is provided in Figure 4. The “noise” in the strain reading is a result of Hastelloy-X exhibiting a Portevin LeChatelier (PLC) effect at these temperatures.11–13 The strain shown in Figure 4 is an average of the measurements over the UV-DIC field of view, which is only a portion of the gauge

section. Therefore the local strain variations induced by PLC bands propagating through the sample would be measurable at this scale. Throughout loading, a pair of CM-140GE-UV cameras, manufactured by JAI, collected images of the speckled surface at a rate of 2 Hz. The cameras were capable of detecting both visible and ultraviolet light with a resolution of 1392 × 1040 pixels. Although only 2D DIC is used in this work, two cameras at a small angle were employed in order to record simultaneously UV and blue or white light results for comparison purposes. No correction for the angle of the cameras was made in either case. Both cameras were equipped with matching UV1054B lenses capable of transmitting both visible and ultraviolet light, produced by Universe Kogaku Inc. The UV lenses transmit both UV and visible light uniformly at about 85% efficiency, but the 8 mm width of the gauge section was only able to fill about 280 of the available 1392 pixels at the highest magnification that the UV lenses were able to provide. To eliminate the glow emanating from the heated specimens, one of the two cameras was then fitted with an XNite330C ultraviolet-range band-pass filter from LDP LLC, while the other camera was left unfiltered. The experiment

FIG. 3. Placement of thermocouples on back side of 8 mm sample (left), and temperatures recorded by thermocouples during heating (right).

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B. Thermal expansion

FIG. 4. Stresses and strains applied to the 8 × 8 mm gauge sample during loading at 725 ◦ C. The strain is computed as the average value over the DIC observable area. The apparent “noise” is a results of the Portevin Le-Chatelier (PLC) effect of Hastelloy-X at these temperatures.

was later repeated with a BP470 blue band-pass filter from Midwest Optical Systems applied to the unfiltered camera. A plot of the transmissivity of the two filters, as well as the lenses and cameras, is shown as a function of wavelength in Figure 5. The specimen was illuminated by both the ambient light in the room and by longwave ultraviolet (365 nm peak wavelength) light. During tests in which the blue filter was used, the specimen was further illuminated with a blue LED ring light. Displacement and strain fields were computed using VIC-2D, a commercially available DIC algorithm produced by Correlated Solutions Inc. Images taken at room temperature were used as reference images. For the 8 × 8 mm gauge region samples, the gauge region took up approximately 280 × 530 pixels out of the images taken, and the correlation was performed using 41 × 41 pixel subsets with a step size of 5 pixels.

The 4 × 4 mm square gauge region specimens were also pre-oxidized prior to speckling, using the same heating, cooling, and dwell times as described above for the 8 × 8 mm specimens. However, it was found that the VHT paint discolored at temperatures on the order of 750 ◦ C, so the 4 × 4 mm specimens were instead painted with a flat white background of Aremco PyroPaint 634-ZO and speckled with black Aremco HiE-Coat CM-840 using an airbrush to create the random pattern. The black paint is rated to 1371 ◦ C, and the white is rated to 1800 ◦ C. The 4 × 4 mm specimen was heated using the same induction coil and feedback controller as the 8 × 8 mm specimen although the specimen was supported by water-cooled grips at only one end to allow only for thermal expansion without any load being generated. The specimen was heated in increments of 100 ◦ C up to a temperature of 500 ◦ C, and in increments of 25 ◦ C beginning from 550 ◦ C onwards. After each heating increment, the temperature was given time to reach steady state, and three images of the speckled surface were collected using each camera. The specimen was heated to a maximum nominal temperature of 1125 ◦ C, above which the specimen broke under its own weight at a location outside of the gauge region where the specimen is believed to have neared its melting temperature of about 1260 ◦ C. To achieve higher pixel resolutions, the UV1054B lenses that were used with the 8 × 8 mm square gauge region specimens were replaced by a pair of Navitar 60135 zoom lenses. The zoom lenses were able to magnify much more, fitting the 4 mm width of the gauge region to almost the full width of the image. However, the zoom lenses were designed primarily for use with visible light, with longwave UV just slightly below the limit of the transmission data provided by the manufacturer. The transmissivity and amount of distortion introduced by the lenses at this wavelength, if any, are unknown. Under bright enough lighting conditions it was found that the zoom lenses were able to transmit enough filtered UV light to observe the speckled surface of the specimen. To achieve bright enough images the gain and exposure time settings of the cameras were set to their maximum values of 341 and 61 967 μs, respectively, making the cameras more sensitive to the glow emitted from the specimen at high temperatures. III. RESULTS AND DISCUSSION A. Effects of filtering

FIG. 5. Transmissivity of the optics used.

The surface of the 4 × 4 mm specimen is shown at various temperatures in unfiltered white light in Figure 6, in filtered blue light in Figure 7, and in filtered UV light in Figure 8. Each set of images is accompanied by a histogram showing the distribution of grayscale values observed in each picture. As temperature increases the glowing emitted by the sample also increases, as evidenced by the gradual shifting of the grayscale distribution towards higher (white) grayscale values. For the specific camera settings used in these experiments, the onset of glowing occurs in unfiltered white light around 500 ◦ C, and images become completely saturated by

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FIG. 6. Images of the speckled surface at the temperatures indicated using unfiltered white light, and histograms of the grayscale values throughout the images.

around 600 ◦ C. In blue light, glowing begins around 700 ◦ C with total saturation occurring around 900 ◦ C. The grayscale distributions obtained using UV light remained minimally affected throughout most of heating all the way up to the

point where the material failed under its own weight at above 1125 ◦ C. Although some glow was present in the final UV images, the effect of the glow was weak enough that the speckled surface remained visible.

FIG. 7. Images of the speckled surface at the temperatures indicated using filtered blue light, and histograms of the grayscale values throughout the images.

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FIG. 8. Images of the speckled surface at the temperatures indicated using filtered UV light, and histograms of the grayscale values throughout the images.

It is worth noting that while the blue images demonstrated a significant level of saturation beginning around 800 ◦ C, other authors report successfully using blue-filtered DIC at temperatures as high as 1500 ◦ C.3 One reason why the saturation effects are so pronounced in this study is the high sensitivity of the cameras. In order to get bright enough UV images through the zoom lenses the aperture, exposure time, and gain in the cameras all needed to be increased. The

increased sensitivity of the cameras resulted in higher levels of saturation at lower temperatures by the light emitted from the specimens. It is expected that if the sensitivity of the camera is reduced, both the blue and the UV filters will be able to achieve higher temperatures with lower levels of saturation, and that the UV images will still saturate at higher temperatures than the blue. In order to reach high enough temperatures to observe more glow through a UV filter, a

FIG. 9. Displacement fields computed with DIC from images taken at room temperature and 500 ◦ C prior to loading using (a) unfiltered white light, (b) filtered blue light, and (c) filtered UV light.

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FIG. 10. Contours of (a) vertical displacement (in pixels), (b) horizontal displacement (in pixels), vertical normal strain, and (d) horizontal normal strain computed from UV images taken at 725 ◦ C between unloaded state and peak load.

material with a higher melting point than Hastelloy-X must be used. B. Uniaxial tensile testing

Before applying tensile load the sample was heated to the desired temperature under no load. Throughout this time images were also recorded. Displacement fields of the 8 × 8 mm specimen undergoing such uniform thermal expansion, i.e., prior to any loading, at a nominal temperature of 500 ◦ C are given in Figure 9. These results are from a correlation between an image at the unloaded state at room temperature and an image once 500 ◦ C temperature has been reached. At this temperature, where there is little sample glowing, all three lighting conditions yield similar displacement fields— consistent with purely thermal expansion—indicating that the images obtained using the UV-DIC method are equally

suited for DIC compared to the unfiltered and blue-filtered methods. After the desired temperature was achieved, uniaxial tension was applied to yield the sample. Correlation was then performed between an image at 725 ◦ C prior to loading but after heating, and an image at peak load at the same temperature. Thus only the mechanical strains would be included in such a measurement. Such strain and displacement contours of the 8 × 8 mm specimen under peak load at 725 ◦ C are given in Figure 10. Figure 10(a) shows that the vertical displacement increases linearly with vertical position, which is consistent with uniaxial tension. Figure 10(b) shows that the horizontal displacement decreases linearly with horizontal position, which is consistent with Poisson contraction under uniaxial tension. Figures 10(c) and 10(d) show, respectively, that the vertical and horizontal normal strains are relatively homogenous except near the edges of the contour, which is

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FIG. 11. Mean CTE as a function of temperature, computed with DIC using each of the three lights.

also consistent with uniaxial loading. The average vertical strain is 0.0112 with a standard deviation of 0.0052. The average horizontal strain is −0.0042 with a standard deviation of 0.0028.

images become too saturated to compute strains from which to continue to obtain valid measurements, while UV-DIC remains capable of measuring CTE to higher temperatures. IV. CONCLUSIONS

C. Thermal expansion

Performing similar thermal expansion experiments to higher temperatures required use of the 4 × 4 mm samples. To limit cooling, the samples were supported at only one end by the water-cooled grips, so no tension load was applied. The strain contours from the 4 × 4 mm specimen were used to compute the mean coefficient of thermal expansion (CTE) as functions of temperature, as shown in Figure 11. The figure includes results for blue and UV from two separate experiments. The coefficients were obtained by computing the mean normal strain for each image and dividing by the difference between the current and initial temperature. The strains in both the longitudinal and transverse directions were computed separately from three images collected at each temperature, resulting in six different CTE values at each temperature. The values plotted in Figure 11 are the average of the six values at each temperature. The figure also includes values for CTE obtained from the manufacturers. It is unknown how the manufacturer measured the CTE, but if the measurements were made with point-based rather than full-field techniques a discrepancy is possible because of local strain inhomogeneities introduced by the PLC effect in the Hastelloy-X. The figure shows that at low temperatures the unfiltered, blue-filtered, and UV-filtered images result in nearly the same values of CTE. At higher temperatures, as the glowing in the unfiltered and blue-filtered images becomes significant, the

In summary, UV-DIC is a method that allows displacements and strains to be measured at temperatures much higher than in conventional DIC by reducing the effect of the light emitted by the specimen. The UV-DIC method agrees with unfiltered and blue-filtered methods at lower temperatures for which glowing is less disruptive, and results in displacement and strain distributions which are expected for uniform thermal expansion and uniaxial tension loading. At higher temperatures, UV-DIC is able to continue measuring displacements and strains at temperatures above which the unfiltered and blue-filtered DIC methods have saturated. Using the expanded temperature range of UV-DIC, the coefficient of thermal expansion of Hastelloy-X is measurable to temperatures well beyond the range capable with unfiltered or blue-filtered DIC, or existing manufacturer’s data. In fact, the effects of glowing in UV-DIC remain minimal even nearing the melting point of Hastelloy-X, and UV-DIC can potentially be used to even higher temperatures on a material with a higher melting point. ACKNOWLEDGMENTS

The authors gratefully acknowledge the support of AFOSR Award No. FA9550-12-1-0386, for which Dr. David Stargel is the program monitor. We also wish to thank Dr. Gavin Horn for his expertise in induction heating.

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Ultraviolet digital image correlation (UV-DIC) for high temperature applications.

A method is presented for extending two-dimensional digital image correlation (DIC) to a higher range of temperatures by using ultraviolet (UV) lights...
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