Plasma-facing components (PFCs) usually need to withstand extreme incident heat flux conditions in nuclear fusion engineering. In situ measurements of deformation to fatigue failure of PFCs under high heat flux (HHF) tests are significant and essential to understand their thermal-mechanical behaviors under servicing conditions; these measurements can provide first-hand and important information for evaluating and optimizing design performance and manufacturing techniques. However, unlike traditional contact measurements with strain gauges, the contactless optical measurement technique is rarely applied to measure or monitor the deformation to fatigue damage process of PFCs employed in HHF tests. In this work, a comprehensive HHF experimental platform was established by combining an electron gun scanning HHF heating system with a three-dimensional digital image correlation (3D-DIC) measurement system based on a vacuum chamber and specially designed optical windows and light sources. The 3D-DIC technology was utilized to measure the field deformation of a divertor mockup under cyclic HHF loads. Validation and qualification tests were conducted on the comprehensive experimental platform to ensure the performance of the platform and the accuracy of the 3D-DIC method. The thermal-induced field deformation of a flat-type divertor mockup under the conditions of 1000 HHF cycles at 10 MW m−2 and 300 HHF cycles at 20 MW m−2 using the 3D-DIC technique was then measured. The mechanical behavior of the accumulated plastic strain and fatigue debonding failure of the W/Cu interface due to periodic thermal stress were captured in situ by the measured strain curves and contours for the first time during HHF tests. The results demonstrate the feasibility and accuracy of the 3D-DIC technique for in situ fatigue deformation and damage strain measurements of PFCs during HHF tests. The proposed methods and technologies are expected to be applied to measure and monitor the servicing performance of PFCs under servicing conditions.
Plasma facing components (PFCs) are key to enduring high heat flux (HHF) loading from high-temperature plasma in nuclear fusion reactors. Understanding their thermal-mechanical behavior and cracking failure mechanisms related to structural designs and fabrication technologies during high heat flux loading is of great significance for improving their servicing performance and R&D (Research and Development) levels. In this study, a particular deep cracking failure process on the tungsten layer of a flat-type divertor mockup during 1800 cycles of 10 MW m-2 HHF loadings is completely monitored and measured with a special improved digital image correlation (DIC) technique. It is found that the DIC measurement under the HHF loading environment is improved successfully to capture fine deformation and strain fields with a spatial resolution less than 0.35 mm so that field strain on a 1 mm thick copper interlayer and deep crack initiation at several microns scale on the tungsten layer are measured out. Based on both full field and local strain and displacement measurements of the target divertor mockup, the thermal mechanical behaviors from deformation to crack initiation and propagation are successfully measured and traced. It is revealed that for the baseline copper interlayer design of a flat-type divertor mockup, the accumulation of plastic strain in the copper interlayer during ratcheting damage induces enough tensile stress on the tungsten layer during HHF cycles, leading to cracking and fracture failures even in its elastic state earlier than the copper LCF lifetime. Current SDC-IC rules fail to cover this kind of ratcheting cracking failure mode in the design stage. New design models or mechanical validation rules to resolve this design blind spot should be established in the future.
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