High-temperature Digital Image Correlation Research Articles

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.

Experimental Investigation of Allotropic Transformation of Cobalt: Influence of Temperature Cycle, Mechanical Loading and Starting Microstructure

The allotropic phase transformation in polycrystalline high-purity cobalt is incompletely reversible and exhibits a temperature hysteresis. This leads to the presence of a FCC metastable phase at room temperature, which alters the mechanical properties. Moreover, this phase transformation seems to be induced by the plastic deformation. The influence of thermal cycling and initial microstructure on the phase transformation has been analyzed with different experimental approaches, namely in situ x-ray diffraction, differential scanning calorimetry and high-temperature digital image correlation analysis. A multiscale analysis, under an in situ tensile test, has been adopted to follow the phase transformation induced by the plastic deformation. The main result shows that the transformation is initiated by basal slip mechanisms, in competition with twinning mechanisms during the second work-hardening stage.

Macroscopic and microscopic characterizations of Portevin-LeChatelier effect in austenitic stainless steel using high-temperature digital image correlation analysis

Using digital image correlation (DIC) analysis in an Fe-19Cr-13Ni-0.2C austenitic stainless steel (mass%), the tensile deformation behavior accompanied with the Portevin-LeChatelier (PLC) effect was investigated at high temperatures of 723-823 K under various applied strain rates of 10−4 s−1–5 × 10−3 s−1. With the help of the DIC analysis, the global strain in the gauge part of a tensile specimen and the local strain in a PLC band were precisely measured regardless of the lowering machine stiffness at a high temperature. The high-temperature DIC analysis could identify the critical strain and strain rate conditions where stress-strain curve indicated serrated flow. The analytical results revealed that dynamic strain aging in the austenitic stainless steel was controlled by the pipe diffusion of Cr atoms. Further, it was also confirmed that the local strain gently increased in a gauge part through a PLC band propagation, which promoted a repeatable A-type PLC band propagation in the same direction. Furthermore, the local strain was discontinuously developed by the repeatable formation of PLC bands; consequently, the local strain rate became remarkably high within a PLC band. In addition, the local strain rate in the PLC band monotonically increased with the increase in the number of PLC bands. From this viewpoint, it was discovered that necking deformation occurs suddenly at the PLC band, when the local strain rate exceeds the critical value, where dynamic strain aging is ineffective.

Experimental methods for determination of mechanical behaviors of materials at high temperatures via the split Hopkinson bars

Full understanding of the thermomechanical behaviors of materials at high strain rates and high temperatures are of great importance from not only scientific meaning but also practical value in engineering structure design and safety assessment. Great efforts have been made for abilities of operation the split Hopkinson bars, the most popular technique for experimental determination of mechanical behaviors of materials over the strain rates from 102 to 104 s−1 over the past 70 years, at high temperatures since 1960s. A review of experiment work is presented in this paper to give an overview of the development of experimental techniques at high temperatures based on Hopkinson bar systems. The principles of the split Hopkinson bar requires the loading bars avoiding temperature gradient or keeping relatively low temperature when performing high temperature testing. Techniques such as performing temperature gradient corrections, rapid heating or using special designed automatically assembled systems were proposed by researchers to enable the operation of the split Hopkinson bars at temperature as high as possible. Moreover, to the application of high speed photographic technique for capturing the dynamic deformation process of the specimen in high temperature Hopkinson bar testing, some key issues of eliminating the strong thermal radiation induced lights oversaturation and de-blurring of images due to insufficient exposure at high temperature and high strain rate condition, as well as fabrication of high contrast speckle pattern for high temperature digital image correlation measurement were also proposed. The technique can now enable the split Hopkinson bar testing to be performed at high temperature up to 1873 K under the loading conditions of compression or tension with the in situ observation and full field measurement of deformation as well. The paper concludes with summaries of the most important achievements and highlighting of the prospects, trends and remaining challenges for future research.

Viscoplastic constitutive model for Ni-based directionally solidified superalloy: Experimental validation on notched specimen

This paper aims to validate a viscoplastic constitutive model and investigate the local and the full-field strain of notched specimen. A high temperature digital image correlation (DIC)-based test system was constructed to measure the full-field surface deformation. Surface strain fields of notched DZ125 specimens were measured under monotonic tensile loadings at 20 °C, 650 °C, 760 °C, and 850 °C. Taking the effect of anisotropy, applied stress, and temperature into account, numerical simulations were conducted using a developed viscoplastic constitutive model. DIC-based measurements agreed well with the simulated strains, with the minimum correlation coefficient of 95.47%. Both the numerical and experimental results show that the area of notch plasticity increases with temperature and applied stress, and the high strain zone nearly remains constant under various tensile loadings. For a given notch geometry, the strain mutation in high strain zone nearly remains constant no matter how the temperature and the applied stress change.

Change of exposure time mid-test in high temperature DIC measurement

Performing digital image correlation (DIC) at extreme temperatures has been greatly challenging due to the radiation which saturates the camera sensor. At such high temperatures, the light intensity emitted from an object is occasionally so powerful that the acquired images are overwhelmingly saturated. This induces data loss, potentially ruining the test, thus requiring the user to restart the test. For this reason, selection of an appropriate camera sensitivity plays a crucial role prior to beginning the test. Exposure time is a factor contributing to camera sensitivity and it is the easiest setting to manipulate during the test since it introduces minimal errors when comparing to other factors, especially in quasi-static tests. This paper examines the influence of changing exposure time mid-test on DIC measurement uncertainty. The investigation was conducted by rigid body motion experiments at room temperature and 1600 °C, respectively. Thereby, some recommendations are given to help DIC users assess their images at room temperature to extrapolate the exposure at extreme temperatures along with accompanying solutions to salvage data at high temperature.

Method for conducting in situ high-temperature digital image correlation with simultaneous synchrotron measurements under thermomechanical conditions.

This work presents a novel method of obtaining in situ strain measurements at high temperature by simultaneous digital image correlation (DIC), which provides the total strain on the specimen surface, and synchrotron x-ray diffraction (XRD), which provides lattice strains of crystalline materials. DIC at high temperature requires specialized techniques to overcome the effects of increased blackbody radiation that would otherwise overexpose the images. The technique presented herein is unique in that it can be used with a sample enclosed in an infrared heater, remotely and simultaneously with synchrotron XRD measurements. The heater included a window for camera access, and the light of the heater lamps is used as illumination. High-temperature paint is used to apply a random speckle pattern to the sample to allow the tracking of displacements and the calculation of the DIC strains. An inexpensive blue theatrical gel filter is used to block interfering visible and infrared light at high temperatures. This technique successfully produces properly exposed images at 870 °C and is expected to perform similarly at higher temperatures. The average strains measured by DIC were validated by an analytical calculation of the theoretical strain. Simultaneous DIC and XRD strain measurements of Inconel 718 (IN718) tensile test specimens were performed under thermal and mechanical loads and evaluated. This approach uses the fact that with DIC, the total strain is measured, including plastic strain, while with XRD, only elastic strain is captured. The observed differences were discussed with respect to the effective deformation mechanisms.

Temperature dependent evolution of anisotropy and asymmetry of α-Ti in thermomechanical working: Characterization and modeling

The strong anisotropy, asymmetry and distorted evolution of plasticity in thermomechanical processing of high performance metallic materials such as α-Ti make it difficult to identify the deformation mechanisms. In this research, a general characterization framework was established to determine the anisotropic and asymmetrical deformation behavior of α-Ti under different thermomechanical loading conditions. The temperature dependent discontinuous constitutive framework was developed for modeling of the distorted evolution of plasticity. The link between crystal plasticity simulation and continuum modeling considering the evolving yield surface of materials with temperature was then established. By taking the thermomechanical working of thin-walled α-Ti tube as a study case, the high temperature digital image correlation (HT-DIC) measurement approach, and the electron backscattered diffraction (EBSD) based physical measurement of arc and small-column samples were proposed to obtain the fundamental tension/compression properties and texture evolution of the alloy at different working temperatures. A viscoplastic self-consistent (VPSC) crystal plasticity based numerical simulation was conducted to determine the temperature dependent anisotropy and asymmetry in plastic deformation. A multi-objective optimization method was used to calibrate the VPSC model parameters and an interpolation approach was employed to smoothly identify the nonlinear evolution of yield loci with strain and temperature. By considering five main deformation mechanisms, viz., prismatic slip, basal slip, pyramidal <c+a> slip, {10–12}<10-1-1> tension twinning and {11–22}<11-2-3> compression twinning, the interactive relationships among flow stress, R-value, yield locus, deformation mechanism, texture evolution and working temperature were established and elaborated. The major findings include: 1) α-Ti tube behaves a significant anisotropy and asymmetry in terms of flow stress, R-value, yield locus, and deformed texture at room temperature, but the anisotropy and asymmetry are significantly reduced at elevated temperatures due to the decrease of twinning activity; 2) The temperature dependency of twinning activity in α-Ti depends on the decrease of the critical resolved shear stress (CRSS) value of pyramidal <c+a> slip with temperature; 3) For the materials with the c-axes of grains towards to normal direction (ND), the increasing activities of pyramidal <c+a> slip, tension twinning and compression twinning decrease the R-value; 4) The activation of twinning generates significant anisotropy and asymmetry and leads to an irregular evolution of yield loci. The developed theoretical framework of characterization and modeling is efficient for producing the above findings and provides a basis for generating a whole spectrum of knowledge of the anisotropy and asymmetry in plastic deformation of the high performance materials that are generally difficult to characterize.

A high magnification UV lens for high temperature optical strain measurements.

Digital Image Correlation (DIC) measures full-field strains by tracking displacements of a specimen using images taken before and after deformation. At high temperatures, materials emit light in the form of blackbody radiation, which can interfere with DIC images. To screen out that light, DIC has been recently adapted by using ultraviolet (UV) range cameras, lenses, and filters. Before now, UV-DIC had been demonstrated at the centimeter scale using commercially available UV lenses and filters. Commercial high-magnification lenses using visible light have also been used for DIC. However, there is currently no commercially available high-magnification lens that will allow images to be taken (a) in the UV range, (b) at a submillimeter scale, and (c) from a relatively long working distance separating a specimen inside a test chamber and a camera outside the chamber. In this work, a custom UV high-magnification lens is demonstrated to perform high-magnification, high-temperature DIC measurements. To demonstrate the capabilities of this lens, a series of thermo-mechanical tests was run on a stainless-steel ring specimen. Two UV cameras performed simultaneous measurements: one at lower magnification using a commercial UV lens, and one with the custom high-magnification UV lens. At room temperature, the custom lens produces sufficiently bright images to perform DIC, while at high temperature (demonstrated to 900 °C) the images retained sufficient contrast while avoiding oversaturation. The lens can detect submillimeter rigid motion and tensile strains from long working distances and high magnification. These tests show that the custom lens is suitable for use in high-magnification UV-DIC measurements.

Synchronous full-field measurement of temperature and deformation based on separated radiation and reflected light

Optical method has been playing an important role in measuring temperature and deformation at elevated temperatures. Here we propose a simple and effective iterative algorithm to realize accurate and simultaneous measurement of the full-field temperature and deformation of specimens subjected to flame heating at temperatures up to 1000 °C. The algorithm separates the optical images of specimen surface into radiant and reflected parts. The radiant part is used to calculate the full-field temperature by an improved two-color method, and the reflected part is used to obtain the deformation of the specimen surface based on high-temperature digital image correlation method. Experimental validation shows that the proposed method can effectively eliminate the mutual interference between the radiation and reflected light, displaying great potential for synchronous measurement of temperature and deformation with sufficient accuracy by using one camera.

An error elimination method for high-temperature digital image correlation using color speckle and camera

Digital image correlation method is a kind of displacement and strain measurement technique based on visible light illumination. It can realize non-contact and full-field measurement with lower requirements on the measurement environment, thus it's widely used in testing and research for mechanical properties of materials at high temperature. But there are many factors affecting the measurement accuracy due to the complexity of the high temperature environment, of which the influence of the thermal disturbance on digital image correlation method is the most obvious. In order to reduce the influence of it and improve the measurement accuracy, this paper proposes a measurement method for eliminating thermal disturbance error in high temperature deformation measurement based on color speckle and camera. Two kinds of grayscale field information of different colors were endued to the surface of specimen, and we collected images by color camera with channel separation for each frame of them. Then we processed the grayscale field of two channels separately combined with the digital image correlation method, analyzed the information carried by two different grayscale fields, and a simple mathematical model was performed to separate the thermal disturbance error from the measurement results. Finally, we set up the experimental system, built the mathematical model between the two channels through static measurement experiment, which was used to extract and correct the thermal disturbance error of measurement results of the rigid body motion experiment. The high consistency between corrected result and true displacement value proved the feasibility and accuracy of the proposed method.

High-temperature DIC based on aluminium dihydrogen phosphate speckle

Digital image correlation (DIC) method has been widely used for measurement in high temperature environment, where speckle pattern that can withstand high temperature and maintain good stability is required. In this work we develop a simple method to fabricate speckle pattern for high temperature application. The curing temperature of the speckle is controlled under 350 °C to avoid potential damage to the substrate material. The speckle can resist temperature up to 1050 °C for minimum 0.5 h. Three point bending experiment was carried out at 700 °C to test the speckle. Results show that this high temperature speckle exhibits excellent adhesion to the substrate. This non-invasive speckle shows great potential for improving the measurement reliability at high temperature in engineering application.

Refraction error correction for deformation measurement by digital image correlation at elevated temperature

An effective correction model is proposed to eliminate the refraction error effect caused by an optical window of a furnace in digital image correlation (DIC) deformation measurement under high-temperature environment. First, a theoretical correction model with the corresponding error correction factor is established to eliminate the refraction error induced by double-deck optical glass in DIC deformation measurement. Second, a high-temperature DIC experiment using a chromium–nickel austenite stainless steel specimen is performed to verify the effectiveness of the correction model by the correlation calculation results under two different conditions (with and without the optical glass). Finally, both the full-field and the divisional displacement results with refraction influence are corrected by the theoretical model and then compared to the displacement results extracted from the images without refraction influence. The experimental results demonstrate that the proposed theoretical correction model can effectively improve the measurement accuracy of DIC method by decreasing the refraction errors from measured full-field displacements under high-temperature environment.

High-temperature delamination mechanisms of thermal barrier coatings: In-situ digital image correlation and finite element analyses

Through in-situ high-temperature digital image correlation (DIC) and finite element simulation, we unveil new, critical delamination and fracture mechanisms of dense vertically cracked thermal barrier coatings (DVC TBCs). Full-field DIC strain maps of cross-sectioned TBCs during thermal cycling up to 1200 °C reveal the coupled formation mechanisms of vertical and horizontal cracks, which showed that horizontal cracks were formed by vertical crack deflection/branching. Horizontal crack growth was characterized by crack bridging, crack shielding, and crack deflection. Internal defects and weak interfaces such as pre-existing voids and inter-splat boundaries promoted crack formation. Finite-element simulations revealed that the interaction between the vertical and horizontal cracks produced favorable strain energy release rates (ERR) for horizontal crack initiation and propagation on the interior of the coating compared to along the bond coat interface, which contrasts with conventional delamination mechanisms. These results indicate that high-temperature DIC is a powerful tool for in-situ measurement of microstructure-processing-deformation-failure relationships that are otherwise unobtainable.

The feasibility and application of gray scale adjustment method in high temperature digital image correlation

In this paper, the basic principle and application of linear gray scale adjustment method are investigated in high temperature digital image correlation (DIC) technology. First, the simple linear gray scale adjustment method is proposed, which can adjust the gray scale value of the saturated pixels and diminish the correlation error caused by the saturated pixels. Then, both the simulated high temperature images and DIC correlation results before and after the gray scale adjustment are provided and analyzed to verify its effectiveness, in which the displacement error decreased from 0.1 pixels to 0.04 pixels after the linear gray scale adjustment for high temperature images. Finally, the linear gray scale adjustment method is used to extract the displacement with high accuracy in high temperature experiment of SiC specimen, and the displacement error decreased from 0.5 pixels to 0.1 pixels after the linear gray scale adjustment.

Displacement field denoising for high-temperature digital image correlation using principal component analysis

ABSTRACTPrincipal component analysis (PCA) was extended to minimize the noise effect in digital image correlation (DIC) measurement under a high-temperature atmosphere environment. First, the principle of PCA was introduced, and the singular vectors and singular values for each component of the displacement fields from DIC were obtained. Then, the simulated high-temperature speckle images were developed to investigate the influences of noise on the DIC method under a high-temperature environment. Finally, the displacement fields of several special conditions were extracted from the simulated speckle images and experimental images; the effects of noise on the PCA were also analyzed.

High temperature digital image correlation evaluation of in-situ failure mechanism: An experimental framework with application to C/SiC composites

A high temperature digital image correlation (DIC) technique was developed, which was applied to study the in-situ fracture behavior of a carbon fibre reinforced silicon carbide matrix (C/SiC) composite. The displacement distribution and cracking information of the C/SiC single edge notched beam specimen can be monitored real-time, thanks to the improved DIC technique with special speckle patterns that can reach up to 1600°C. The results showed that the brittle to ductile transition temperature of C/SiC composites is about 1300°C. The new failure mechanisms of C/SiC composites at different experimental temperatures were further verified with the aid of X-ray diffraction and scanning electron microscope (SEM) techniques. In addition, the relationships between the fracture toughness, first-crack strength of C/SiC composites and environmental temperature were deduced. The proposed experimental method and testing results may shed some light on assessing the reliability and durability of C/SiC composites at high temperatures.

Improvement on measurement accuracy of high-temperature DIC by grayscale-average technique

In this paper, an effective grayscale-average technique is proposed to minimize the thermal disturbance in digital image correlation (DIC) measurement method under high-temperature atmosphere environment. First, both the grayscale-average technique and the simulated high-temperature speckle image are developed to investigate the influences of thermal disturbance on DIC method under high-temperature environment. Second, a high-temperature DIC experiment has been performed to verify the validity of the proposed grayscale-average technique. Finally, the displacement fields and average thermal strains of several temperature conditions are extracted from the simulated speckle images and the experimental images, also the effects of the image number and the temperature in the grayscale-average technique are analyzed. The results will provide a new method for improving the measurement accuracy in DIC full-field deformation measurement under high-temperature atmosphere environment.

High-temperature digital image correlation method for full-field deformation measurement captured with filters at 2600°C using spraying to form speckle patterns

A method is presented for obtaining good images of a sprayed speckle pattern on specimen surfaces at high temperatures, suitable for strain measurement, by digital image correlation (DIC) using plasma spray for speckle preparation in which a bandpass filter, neutral density filters, and a linear polarizing filter are used to reduce intensity and noise in images. This is accomplished by speckle preparation through the use of plasma spray and suppression of black-body radiation through the use of filters. By using plasma spray for speckle preparation and the filters for image acquisition, the method was demonstrated to be capable of providing accurate DIC measurements up to 2600°C. The full-field stretching deformation of the specimen was determined using the DIC technique. Experimental results indicate that the proposed high-temperature DIC method is easy to implement and can be applied to practical, full-field, high-temperature deformation measurements with high accuracy.