Experimental Study On Damage Failure Behavior Of Divertor Target Based On Digital Image Correlation

With the development of nuclear fusion engineering and science,the operation parameters of Tokamak devices in various countries are getting higher and higher,but there are still many difficulties to achieve the operation conditions of commercial fusion reactor.For future fusion reactors,long pulse and high confinement plasma operation mode is considered to be the first choice to obtain high fusion energy gain.In this mode,as one of the most important parts in Tokamak device,the divertor target will withstand extremely high heat flux(HHF)loading,which is prone to damage and failure behavior.Therefore,it is of great significance to study the damage and failure of the divertor target under HHF loading.The results can help us clarify the damage and failure mechanisms of the divertor target,and further provide guidance for the design and preparation process optimization of the divertor target.Under the steady-state HHF(10-20 MW m-2)loading,the divertor target material and its interface are prone to fatigue,fracture,debonding,leakage and other failure behaviors.At present,post-analysis and finite element simulation are the main methods used to study the failure behavior of divertor target under HHF loading.Due to the harsh service environment of the divertor target,experimental research on the high temperature mechanical behavior of the divertor target is rarely involved.Therefore,this paper focuses on the deformation characteristics and damage failure evolution process of the divertor target under HHF loading by means of experiments.In this paper,based on the improved digital image correlation(DIC)method and the comprehensive thermal fatigue test platform independently designed and constructed by our research group,the mechanical behavior and damage failure mechanism of divertor targets under HHF loading were systematically investigated experimentally.Firstly,the high temperature speckle preparation and high temperature thermal radiation suppression strategies for DIC method were studied.Secondly,the tungsten/copper interface debonding and tungsten cracking failure behavior of flat-type divertors were studied,respectively.Thirdly,the deformation response and the macrocrack growth rate of a full-tungsten diverter under cyclic thermal load were in-situ measured,and the formation and propagation mechanism of the macro-crack were analyzed.Finally,based on laser speckle DIC technique,a new method for identifying the crack on the top surface of the full-tungsten divertor plate is proposed.The main research contents of this paper are as follows:1.In order to develop a high temperature DIC technique for in-situ measurement of mechanical behavior of divertor target under HHF loading,a high temperature speckle resistant preparation process based on tantalum carbide powder was developed.The physical and chemical stability and robustness of tantalum carbide speckle in high temperature environment were evaluated by measuring the ultrahigh temperature deformation of tungsten target.Aiming at the large-span variable temperature condition of divertor target in HHF test,an ultraviolet-DIC measurement system based on adaptive exposure algorithm was proposed.By measuring the thermal strain and thermal expansion coefficient of a tungsten target at 1600-3200℃,the feasibility and accuracy of the measurement system were illustrated.2.Based on 3D-DIC technology and specially designed optical path,deformation characteristics and interface debonding failure behavior of a flat-type divertor under high heat flux were in-situ measured for the first time.Based on the measured strain distribution and strain evolution curve,it is found that the causes of tungsten/copper interface debonding can be attributed to the huge temperature gradient formed between the divertor target from the heat flux loading surface to the cooling channel and the mechanical properties differences in different materials.3.Using the improved speckle preparation technology,the macro crack initiation and propagation of a flat-type divertor tungsten armor in HHF fatigue test was investigated for the first time.The high-temperature mechanical behavior of different positions of the diverter plate in cyclic HHF loading is investigated.The influence mechanism of copper plastic accumulation on crack initiation and propagation in tungsten layer is expounded.Thermal fatigue results in a ratchet effect in the copper layer.The accumulation of plastic strain of ductile copper sandwich imposes a large tensile stress on the tungsten layer during the cyclic HHF loading,leading to its cracking failure,and accelerating crack propagation.4.Deformation characteristics and damage behavior of a full-tungsten divertor under HHF loading were measured based on adaptive exposure DIC and laser speckle DIC technique.The initiation and propagation mechanism of macroscopic crack in tungsten armor were analyzed,and the relationship between crack propagation and plastic accumulation was investigated.In addition,the feasibility and accuracy of measuring the deformation of the top surface of full-tungsten divertor with laser speckle DIC technique are preliminatively verified,and a new method for identifying the crack on the top surface of divertor tungsten target based on laser speckle DIC technique is proposed.