High-temperature Digital Image Correlation Research Articles

Improved image preprocessing method applicable to high-temperature digital image correlation measurement

Improved image preprocessing method applicable to high-temperature digital image correlation measurement

High-temperature thermal disturbance elimination method based on multi-channel extraction and correction.

Digital image correlation (DIC) technology has been widely used in high-temperature measurement fields. However, due to the complexity of high-temperature environments, there are many interference factors that limit the development of high-temperature DIC technology, among which thermal disturbance is one of the most significant factors that severely affects the measurement accuracy of high-temperature DIC. In this paper, a multi-channel separation technique combined with a low-cost laser speckle device is proposed to eliminate thermal disturbance errors in high-temperature DIC measurements. First, a blue laser speckle generation system is independently designed to produce the most suitable speckle particles, and the best laser speckle is determined and projected onto the blue background white spot pattern. Then a green LED illuminates the sample to provide illumination for the sample's own grayscale characteristics. A color camera collects photos, and the obtained images are processed with channel separation to extract and calculate the displacement of different channels. Finally, the displacement fields of the green and blue channels are subtracted to separate the thermal disturbance error and correct the measurement values. In this paper, a laser speckle projection system is first assembled, followed by a comprehensive evaluation of the projected speckle and, finally, a DIC experimental system is constructed for verification experiments at both room temperature and high temperature, and the corrected values are compared with the true values. The results show that the corrected values are highly consistent with the true values, which verifies the reliability of the proposed method.

Fabrication of Ultra-Stable and Customized High-Temperature Speckle Patterns Using Air Plasma Spraying and Flexible Speckle Templates.

Reliable and accurate full-field deformation measurements at elevated temperatures using digital image correlation (DIC) require stable and high-contrast high-temperature speckle patterns to be prepared on the sample surface. However, conventional high-temperature speckle patterns fabricated by the existing methods possess several limitations, e.g., easily fail to preserve original pattern features due to the harsh environment and heavily dependent on the operator's experience. In this study, we propose a reliable and reproducible high-temperature speckle fabrication method based on air plasma spraying (APS) and flexible speckle templates. This method involves covering the sample surface with pre-designed speckle templates and then spraying the melted speckle powders onto the specimen surface using an air plasma spray technique to obtain customized speckle patterns. The validity of the proposed method was verified by the speckle fabrication on both planar and curved samples and heating tests with these samples. Experimental results demonstrate that the speckle patterns made by the proposed method adhere well to the sample surface, remain stable during the heating process, and exhibit excellent agreement with the reference values in terms of the thermal expansion coefficients. The proposed method provides a reliable and efficient way to create customized and stable speckle patterns for accurate high-temperature DIC measurements.

Neutralizing the impact of heat haze on digital image correlation measurements via deep learning

Digital image correlation (DIC) techniques have shown excellent capabilities in the deformation measurements of materials and structures at high temperatures. However, as a crucial challenge for the high-temperature DIC measurements, heat haze, caused by uneven distributions of temperatures and refractive indices, can lead to considerable distortions to the captured images and severely decrease the measurement accuracy. Inspired by a recently established atmospheric turbulence neutralization neural network called TSR-WGAN, we propose a deep learning-based approach to neutralize the effect of heat haze on high-temperature DIC measurements. Specifically, the original distorted speckle images obtained in the experiment are fed into the TSR-WGAN network twice to obtain distortion-corrected speckle images. By processing these corrected speckle images with a conventional DIC algorithm, displacement and strain fields with mitigated heat haze effects can be determined. Both simulation and real experiments have been conducted to assess the proposed method. Results clearly show the effectiveness and robustness of the proposed method in correcting heat haze effects. One of the experiments particularly shows that the displacement fluctuations caused by the heat haze can be reduced by approximately an order of magnitude in the u direction and by a factor of five in the v direction. The proposed technique provides a much-needed tool for neutralizing the impact of heat haze on the DIC measurements under a high-temperature environment.

Serrated flow accompanied with dynamic type transition of the Portevin–Le Chatelier effect in austenitic stainless steel

• By introducing high-temperature DIC analysis, the PLC effect in an Fe-19Cr-13Ni-0.2C austenitic stainless steel was investigated. • Various types of serrated flow associated with the diffusion of interstitial C atoms appeared at temperatures from 473 K to 623 K, and the type dynamically changed even at an applied strain rate. • High-temperature DIC analysis revealed the dynamic type transition of serrated flow obeyed the variation in PLC band nucleation and propagation behavior. • Numerical modeling proves that the spatial-temporal coupling effect plays an important role in the microscopic nucleation manner of the PLC band. In order to further understand high-temperature deformation behavior and its-related mechanical properties, the Portevin–Le Chatelier (PLC) effect in an Fe–19Cr–13Ni–0.2C austenitic stainless steel was investigated using high-temperature digital image correlation (DIC) analysis. Under the tensile testing at temperatures from 473 K to 623 K, different types of serrated flow appeared even at a constant applied strain rate, and the type transition took place dynamically in a certain order with deformation time. DIC analysis revealed that the dynamic type transition of the serrated flow obeys the PLC band propagation behavior, and that the transition of the PLC band propagation behavior could be attributed to the PLC band nucleation manner. Numerical modeling also proved that the nucleation manner of PLC band nucleation is determined by the spatial-temporal coupling effect.

Correcting for digital image correlation speckle inversion at high temperature using color cameras.

Digital image correlation (DIC) is a popular, noncontacting technique to measure full-field deformation by using cameras to track the motion of an applied surface pattern. Because it is noncontacting, DIC can be performed for extreme temperature applications (e.g., hot-fire rocket testing of carbon composite rocket nozzles) under harsh conditions during which bonded gauges are damaged. Speckle pattern inversion is a phenomenon that sometimes occurs while performing high-temperature DIC. During speckle pattern inversion, portions of the surface pattern that were initially darker at room temperature (e.g., graphite) may emit more light due to blackbody radiation than the portions that were initially paler, thereby producing images in which the pattern appears inverted at high temperature relative to the initial pattern at room temperature. This phenomenon can prevent the correlation algorithm from being able to resolve the displacements between images. This work compares three methods to mitigate speckle pattern inversion: (A) the subtraction method, a recently-published technique in which two high-temperature images are subtracted to remove unwanted light; (B) the filtering method, a popular technique in which optical bandpass filters screen out unwanted light; and (C) the histogram rescaling method, a proposed new method that pairs a color camera with a blue light source and uses information from the green sensor of the camera to correct against inversion in the blue sensor through postprocessing. The histogram rescaling method is shown to successfully eliminate speckle pattern inversion and has the added advantages that it does not require quasi-static loading to be able to compensate for speckle pattern inversion, nor does it impose thick-glass distortions caused by the optical filter.

Remaining lifetime assessment of 141,000-hour service-aged forge 91-pipe 91 steel header welds with novel in-situ digital image correlation

Condition assessment of welded steam components is a crucial activity for safe power plant operations. In this work, remaining lifetime of a 141,000-hour service-aged large bore girth weld between an SA-182 forged 91 (F91) reducer and an SA-335 pipe 91 (P91) header was assessed with a specially designed in-situ high temperature digital image correlation (DIC) system. Nonuniform creep deformation/damages in this service-aged multi-pass F91-P91 welds were quantified and characterized. The in-situ DIC strain measurements reveal the nominal creep strain averaged over the gauge length was consistently below 3%, and the localized creep strain value at failure in the HAZ was ∼10%. Advanced metallurgical characterization was executed in the as-received and posttest conditions to determine the contributions of preexisting creep damage, the so-called “soft zone” in the HAZ, and microstructural variations in the HAZ to the observed Type IV failure. Creep failure occurred in the most predamaged region and this location is adjacent to, but not inside, the documented soft zone before the test. The higher fraction of inclusions in F91 steel contributed significantly to the lower creep resistance observed in both base metal and HAZ.

Optical Detection of Phase Transformations in Steels: An Innovative Method for Time-Efficient Material Characterization During Tailored Thermo-mechanical Processing of a Press Hardening Steel

With an innovative optical characterization method, using high-temperature digital image correlation in combination with thermal imaging, the local change in strain and change in temperature could be determined during thermo-mechanical treatment of flat steel specimens. With data obtained by this optical method, the transformation kinetics for every area of interest along the whole measuring length of a flat specimen could be analyzed by the generation of dilatation curves. The benefit of this innovative optical characterization method compared to a dilatometer test is that the experimental effort for the design of a tailored component could be strongly reduced to the investigation of only a few tailored thermo-mechanical processed specimens. Due to the implementation of a strain and/or temperature gradient within the flat specimen, less metallographic samples are prepared for hardness analysis and analysis of the microstructural composition by scanning electron microscopy to investigate the influence of different process parameters. Compared to performed dilatometer tests in this study, the optical method obtained comparable results for the transformation start and end temperatures. For the final design of a part with tailored properties, the optical method is suitable for a time-efficient material characterization.Graphical

The Effect of Bit Depth on High-Temperature Digital Image Correlation Measurements

Digital Image Correlation (DIC) is a camera-based method of measuring displacement and strain. High-temperature DIC is challenging due to light emitted from the sample which can saturate the image. This effect can be mitigated using optical bandpass filters, but the maximum sample temperature range of DIC remains dependent on the camera and camera settings. Among camera settings, bit depth, also referred to as color depth or number of bits, has received insufficient attention in high-temperature DIC literature. In this work, the effect of bit depth on DIC measurements is investigated both analytically and experimentally. It is shown that if image noise is sufficiently low, then increasing bit depth reduces DIC random error. A new metric, the effective number of bits, is presented to determine the appropriate number of bits for DIC images. Using increased bit depth, reduced exposure time, and low-noise images, the maximum sample temperature for DIC measurements was shown to increase without negatively impacting random error.

Time-space Domain Elimination Method of High-Temperature Thermal Disturbance Test Error

The digital image correlation (DIC) test method has been widely used in structural deformation measurement due to its non-contact and full-field. However, eliminating the effects of temperature-induced thermal disturbance is a primary challenge in high-temperature DIC measurements. In this paper, the measurement error is regarded as the superposition of zero-mean random error and non-zero mean system error, a time-space domain disturbance error elimination method is proposed to eliminate the adverse effects of thermal disturbance. Firstly, the noise images are averaged in the time domain to reduce the DIC matching error caused by zero-mean random noise. Then the noise displacement field is locally weighted quadratically smoothed in the space domain to eliminate the non-zero mean system error. The experimental results show that the proposed method can improve the high-temperature deformation measurement accuracy compared with the single-domain processing method. This paper will provide a new method for improving the measurement accuracy in DIC full-field deformation measurement under a high-temperature environment.

High-temperature digital image correlation techniques for full-field strain and crack length measurement on ceramics at 1200 °C: optimization of speckle pattern and uncertainty assessment

High-temperature digital image correlation techniques for full-field strain and crack length measurement on ceramics at 1200 °C: optimization of speckle pattern and uncertainty assessment

Evolution of short-term creep strain field near fatigue crack in single crystal Ni-based superalloy measured by digital image correlation

This study measured the variation over time of the creep strain fields around a fatigue crack tip in a single crystal Ni-based superalloy in an open environment at temperatures up to 900 °C using digital image correlation (DIC). A high-temperature DIC measurement system was developed to overcome three difficulties: the degradation of random patterns, thermal radiation, and heat haze. Tension hold was applied to the specimens, and the time evolution of the creep strain fields around the crack tip was measured at different temperatures and under different loading conditions. The proposed DIC system measured the short-term creep strain fields with high reproducibility and reliability, and the strain and strain rate increased as the temperature and hold load increased. To confirm the utility of the measurement results, the stress distribution around the crack tip was calculated using the finite element method (FEM), and the calculation results were validated by comparing the calculated strain to the measurement results. The effect of creep deformation on the fatigue crack propagation (FCP) is discussed.

Improvement method of high-temperature digital image correlation measurement accuracy based on image processing

With the increasing application of digital image correlation (DIC) technology in the field of high-temperature measurement, thermal radiation and disturbance in high-temperature environment restrict the development of high-temperature DIC technology. In this study, we propose a method based on image processing to improve the measuring accuracy of DIC technique of high temperature for solving the above mentioned problems. Firstly, the image dehazing enhancement processing and the gray scale range stretching processing are used to eliminate the influence of the thermal radiation background light to enhance the image contrast. Then, the inverse filtering image recovery processing is used to eliminate the effect of the thermal perturbation. Lastly, a series of high-temperature image acquisition and thermal disturbance elimination experiments at 1200 °C are conducted. The image collected by the camera is enhanced and restored according to the above mentioned image processing method. Experimental results show that this method can acquire high-contrast clear images under high-temperature conditions, effectively eliminate the interference of thermal disturbance noise in high-temperature measurement and improve the measurement accuracy.

Error correction of glass mediums in high-temperature digital image correlation deformation measurement

The predecessor of China Optics was China Optics and Applied Optics Digest, which was founded in 1985. At that time, it was the only retrieval journal in the field of optics in China. At the end of 2008, China Optics and Applied Optics Digest was renamed

High-temperature digital image correlation techniques for full-field strain and crack length measurement on ceramics at 1200°C: Optimization of speckle pattern and uncertainty assessment

High-temperature mechanical tests coupled with Digital Image Correlation (DIC) on ceramics which exhibit rather low level of strain require to overcome extreme experimental conditions that usually can reduce significantly measurement accuracy. Thermal resistance of speckle pattern, black body radiation and heat haze are three main concerns, which should thus be taken into account while designing a high-temperature image acquisition setup. In this aim, an experimental procedure has been specifically designed in order to minimize the three above-mentioned disturbances. The main objective of this study is to select a suitable high-temperature resistant speckle pattern for mechanical characterization at 1200 °C (or above) on refractory ceramics. Most of tested speckle patterns were performed with white alumina adhesive and dark ceramic grains (silicon carbide or brown fused alumina). Different grain sizes of silicon carbide were tested. At first, different speckle patterns are compared in terms of DIC strain measurement uncertainty by discussing speckle features, some main DIC parameters and two image pre-treatments (low pass filter, image size reduction). Then, these speckle patterns are tested to analyse fracture behaviour of refractories through a Brazilian test. An enhanced digital image correlation technique (2P-DIC), dedicated to monitor the fracture behaviour, is applied to study the evolution of crack length. The best representation of crack progression has been achieved for sample surface covered with a fine SiC powder ranging from 50 to 100 µm. It is then possible to compare the fracture behaviour between 1200 °C and 20 °C and to show that the refractory exhibits more crack branching at 1200 °C in comparison with behaviour at room temperature.

A High-Temperature Digital Image Correlation Method and its Application on Strain Measurement of Film Cooling Holes

In this paper, a digital image correlation (DIC) method is developed and applied on film cooling holes in the submillimeter scale in high temperature. Compared with the traditional DIC method, the speckle patterning method and the optical system are improved. In detail, a kind of high temperature-resistant black paint is selected as the basecoat, and the white ZrO2 particles are evenly distributed on the specimen using high-pressure splashing method. Besides, to eliminate the radiation effect of the high-temperature specimen, the blue light source is used to illuminate the specimen, and the optical bandpass filter is placed in front of the camera to allow the blue light passing. In order to verify the DIC method, the strain measurement on a specimen with single skew hole is performed. The relative error in high temperature of the maximum strain between the measurement results and the numerical simulation results given by the finite element method (FEM) is 12%. The strain concentration factor of the single skew hole is measured as 1.83. Finally, the developed method is applied to the strain measurement of the structure with multiple film cooling holes in 870°C. The X-shape strain distribution can be observed at the hole with maximum stress, which suggests that the strain field of multiple holes has coupling effect. In addition, the strain concentration factor of multiple film cooling holes increases to 2.34.

Speckle Pattern Inversion in High Temperature DIC Measurement

During hot fire rocket engine testing, non-contacting measurements are superior to bonded gauges because they are immune to burning, shaking loose, or damage due to the harsh testing conditions. Additionally, when compared to instruments which register at single points, Digital Image Correlation (DIC) has the added benefit in that it collects full-field displacement and strain maps over the duration of the test. However, for certain materials and paints under some circumstances of temperature and camera sensitivity, portions of the speckle pattern which were darker at room temperature may emit more light compared to the initially lighter portions of the pattern, resulting in a high temperature pattern which is inverted in comparison with that at room temperature. To address this inversion, a post-processing method is introduced wherein an inverted image containing only emitted light is subtracted from an image containing both emitted and reflected light, thereby generating an un-inverted image. The artificial high temperature image is subsequently correlated against the room temperature image to obtain full-field strains. The subtraction technique is then validated using optical bandpass filters to prevent significant amounts of emitted light from reaching the camera sensor. The two methods are mapped onto common coordinates and shown to produce comparable results. The subtraction method sufficiently mitigates speckle pattern inversion, but its key drawback is that it only works when there is negligible displacement between the subtracted images (i.e. quasi-static loading). It is therefore preferable to eliminate inversion from reaching the camera in the first place by using optical bandpass filters.

Heatwave distortion correction using an improved reference sample compensation method and multispectral digital image correlation.

Heatwave distortions, caused by unevenly distributed temperature and refractive index in the optical path, unavoidably occur in high-temperature digital image correlation (DIC) measurement. To eliminate these distortions, a multispectral DIC-aided reference sample compensation method is proposed. The proposed method first adheres a correcting transparent glass (decorated with fluorescent speckle patterns) onto the test specimen (sprayed with red speckle patterns). Then, by illuminating the specimen with ultraviolet- and red-light sources, the blue light excited from the correcting glass and the red light reflected from the specimen surface can be captured by a 3CCD camera, forming a color image. After separating the recorded color images into red and blue subimages, the original and the correcting displacement fields can be calculated from these two sets of subimages using the subset-based local 2D-DIC algorithm. By point-to-point subtracting the correcting displacement fields from the original ones, the heatwave distortions can be eliminated, and the corrected real displacement fields can be obtained. For validation, static heatwave experiments show the feasibility and accuracy of the proposed method in correcting heatwave distortions. A uniaxial tensile test of an aluminum specimen with a central hole was also performed, further confirming the practicality of the proposed method in correcting heatwave distortions and revealing heatwave-hidden deformation.

Overview of High-temperature Deformation Measurement Using Digital Image Correlation

Developments in digital image correlation (DIC) in the last decade have made it a practical and effective optical technique for displacement and strain measurement at high temperatures. This overview aims to review the research progress, summarize the experience and provide valuable references for the high-temperature deformation measurement using DIC. We comprehensively summarize challenges and recent advances in high-temperature DIC techniques. Fundamental principles of high-temperature DIC and various approaches to generate thermal environment or apply thermal loading are briefly introduced first. Then, the three primary challenges presented in performing high-temperature DIC measurements, i.e., 1). image saturation caused by intensified thermal radiation of heated sample and surrounding heating elements, 2) image contrast reduction due to surface oxidation of the heated sample and speckle pattern debonding, and 3) image distortion due to heat haze between the sample and the heating source, and corresponding countermeasures (i.e., the suppression of thermal radiation, fabrication of high-temperature speckle pattern and mitigation of heat haze) are discussed in detail. Next, typical applications of high-temperature DIC at various spatial scales are briefly described. Finally, remaining unsolved problems and future goals in high-temperature deformation measurements using DIC are also provided. We expect this review can guide to build a suitable DIC system for kinematic field measurements at high temperatures and solve the challenging problems that may be encountered during real tests.

Thermal radiation elimination method for high-temperature digital image correlation using polarization camera

Digital image correlation (DIC) is a material displacement and strain measurement technology based on visible light illumination. At high temperatures, the problem of thermal radiation seriously affects the quality of acquired images and restricts the development of high-temperature DIC technology which is increasingly applied in the field of high-temperature measurement due to stringent measurement temperature requirements. A thermal radiation elimination method based on the use of a polarization camera for high-temperature DIC measurements is proposed in this study. This method uses a polarization camera combined with a filter set to achieve clear image acquisition at 1200 °C and effectively eliminates the effects of thermal radiation on image acquisition. The gray average method and an image inverse filtering algorithm are adopted in this study to eliminate high-temperature thermal disturbances. Finally, a high-temperature DIC measurement system is independently designed, and a rigid-body displacement experiment is carried out on an FV566 steel specimen to obtain time–displacement curves. A set of uniaxial tensile tests is also performed on FV566 steel material to explore its strain field at 1200 °C.