Strain field measurements over 3000 °C using 3D-Digital image correlation
Introduction
Understanding the mechanical behavior of metals under high temperature conditions is of great significance in many fields, for example, in the aerospace and nuclear industries [1], [2], [3], [4]. In fusion engineering, tungsten, the most promising material for the plasma-facing inner wall of future nuclear fusion devices, can work at very high temperatures, owing to ion beam irradiation heating. Full-field strain measurement of a heated tungsten specimen is essential for the determination of its thermal physical properties, and is also important for the proper selection and development of the materials applied in future nuclear industries. Conventionally, high-temperature strain is measured by contact strain gauges and an extensometer. However, for temperatures greater than 1000 °C, strain gauges cannot meet the measurement demands because of the limited operating temperature range. In addition, these two techniques can only provide the average strain of the local area and therefore cannot be applied for full-field thermal deformation measurement. In contrast, non-contact optical measurement techniques based on digital image procession can effectively overcome the shortcomings of contact techniques [5,6] and therefore have gradually become the most effective methods in the field of high temperature experimental mechanics.
Non-contact optical measurement techniques have been widely used in the fields of aerospace, materials, biology and so on because of their advantages, such as simplicity, high measurement accuracy and low requirements of vibration isolation. Digital image correlation (DIC) is a non-contact optical metrology originally proposed by Yamaguchi and Peters et al. in the 1980s [7,8]. In order to meet the urgent need for three-dimensional (3D) shape and deformation measurements, a 3D-DIC technology based on binocular vision was developed [9,10]. In 1996, Lyons et al. fabricated speckles on samples and obtained the thermal expansion coefficient and elastic modulus of chrome-nickel-iron super alloy materials from room temperature to 650 °C using DIC [11]. In 2009, Grant et al. measured the Young's modulus and coefficient of thermal expansion of a nickel-base alloy from ambient temperature to 1000 °C through blue illumination [12]. The accuracy of strain measurements can be enhanced by improving algorithms or using artificial speckle patterns. Gao et al. proposed a high-efficiency and high-accuracy algorithm for 3D-DIC, known as the IC-GN2 algorithm [13]. In 2012, Chen et al. studied the speckle performance of aluminum and zirconia ceramic materials with a monochromatic light source and proposed a way to cure the speckles in advance for the sake of clearly speckle identification [14]. Yang et al. studied the real-time deformation of near-interface regions and the surface of thermal barrier coatings using a micro-DIC method with fabrication of a speckle pattern by spraying a mixture solution of alcohol and high temperature resistant particles [15]. Chen et al. studied the residual stress evolution regularity in thermal barrier ceramic coatings by micro-DIC and micro-Raman spectroscopy [16]. Lin et al. performed thermal shock tests on SiO2 and Al2O3 based on a DIC method [17]. In order to solve the high temperature self-luminescent interference problem, Berke and Lambros proposed a technical route for 3D-DIC measurements with an ultraviolet (UV) light source and UV cameras [18]. Dong et al. measured the in-situ 3D ablation shapes of a blunt cone subjected to arc heating with a temperature range of 1000–1868 °C using a UV stereo-DIC technique [19]. Wang et al. measured full-field strain mappings and a stress–strain curve for a carbon fiber-reinforced carbon (C/C) composite uniaxial tensile specimen at 2000 °C and obtained the Young's modulus [20]. Guo et al. measured the stretching deformation of carbon fibers at 2600 °C by using plasma spray tungsten powder for speckle preparation and the filters for image acquisition [21]. The authors’ group completed a series of fundamental studies on the DIC technique and measurement precision analysis and developed a general 3D-DIC measurement system (PMLAB 3D-DIC), which has been applied in many fields [22], [23], [24], [25], [26], [27].
In summary, according to the addressed DIC technique application cases under high temperature, most of them are under a temperature of less than 2000 °C, with few cases having been extended to an ultra-high temperature range of 2000–3000 °C. In order to measure the strain fields at ultra-high temperature (>2000 °C), there are several problems that need to be solved as listed in the following aspects:
- (1)
Very few speckle materials can be maintained at ultra-high temperature without peeling off or being burned out.
- (2)
Thermal radiation at ultra-high temperature decreases the quality of the images, leading to the failure of strain field calculations. These effects are especially difficult to eliminate in the ultra-high temperature range.
- (3)
The strong airflow disturbance caused by normal environmental atmosphere at high temperature can easily affect the speckle characteristics of images.
Solving the above problems has become an important topic. The key links may lie in three aspects: first, searching for suitable speckle materials for the ultra-high temperature range and improving the fabrication process of speckles onto specimens; second, developing an advanced filtering method to reduce the effects of thermal radiation; third, conducting the experiment in vacuum conditions to remove thermally induced airflow disturbance at high temperature. In this study, a high heat flux comprehensive experimental platform with a vacuum chamber is established and a 3D-DIC measurement system based on blue light sources is used as a strain measurement device. Furthermore, a new kind of speckle material and fabrication technology are developed to adapt ultra-high temperature under vacuum conditions.
This study is organized as follows. Section 2 introduces the sample preparation, including speckle material selection, fabrication and curing of the speckle pattern. In addition, experimental facilities and measurement systems are elaborated, after which three schemes are proposed to suppress the disturbance caused by thermal radiation at ultra-high temperature and a detailed description of the experimental procedures is given. In Section 3, the strain fields of tungsten specimen over 3000 °C are obtained and the measurement results are compared with available database and literature. Moreover, the fluctuations of measurement results are also assessed through analyzing the residual variance. Section 4 draws the conclusions of this work.
Section snippets
Sample preparation
The specimen is made of pure tungsten with dimensions of 20 mm × 20 mm × 50 mm. To ensure reliable measurements using the 3D-DIC technique, randomly distributed artificial speckle patterns, serving as a carrier of deformation information, are generally fabricated on the specimen surface. In high temperature deformation measurements, the speckle pattern made by spraying common paints normally burns out or peels off at high temperature. In this work, a simple yet effective speckle pattern
Thermal strain of tungsten ranging from 25 to 1200°C
According to Scheme 1, external blue light sources were adopted to illuminate the speckle pattern for camera imaging. In order to obtain clear images, the exposure time of the cameras was set as 40 ms for 25–600 °C and 25 ms for 600–1200 °C. We captured 696 images and 741 images with an acquisition frequency of 10 Hz corresponding to 25–600 °C and 600–1200 °C, respectively. The relative full-field strain exx was calculated separately in each temperature interval, as presented in Fig. 6, where
Conclusions
In the present work, with the aim of developing a non-contact measurement technique of mechanical deformation under an ultra-high temperature range, a high heat flux (~300 MW) comprehensive experimental platform was established in combination with an electron beam heating system with a three dimensional-digital image correlation (3D-DIC) measurement system. Based on the vacuum condition on the experimental platform, thermally induced airflow disturbance can be removed. Tantalum carbide powder
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Magnetic Confinement Fusion Energy Research of China (grant no. 2015GB121007), the National Natural Science Foundation of China (grant nos. 11627803, 11872354), the Major/Innovative Program of Development Foundation of the Hefei Center for Physical Science and Technology (contract no. 2018ZYFX001), and the Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDB22040502).
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