Dependence of tungsten crack behaviors on heat source parameters under transient heat flow

As a plasma facing material in TOKAMAK, tungsten may suffer plastic deformations, cracking, melting, even ablation induced by transient heat loads, such as Edge Localized Modes (ELMs). Focused on investigation of cracking behaviors of tungsten under transient heat flux and its dependence on pulse parameters, powder metallurgical pure tungsten samples have been tested with 1-100 pulses of intense pulse electron beams (IPEB) at 5 ms and 1 ms pulse duration. Three types of cracks were observed, including primary crack, secondary crack and blunt crack. The primary cracks can be assorted as branch, annular and network cracks according to their patterns. Branch cracks were formed in the irradiation area center, and annular cracks appeared surrounded the irradiation area. The width of annular cracks increases as the pulse number increasing. The initiation and evolution of annular cracks are mainly studies in this paper. Considering the ductile-to-brittle transition property, an ideal elasto-plastic mechanics model is adopted in the thermodynamic calculation model of tungsten under the IPEB irradiation. Irreversible plastic deformation plays an important role, which determined the tensile stress distribution during the temperature decreasing. The formation mechanism of annular crack and its dependence on the heat source parameters are discussed, based on the calculation results of temperature, stress distribution and evolution of tungsten under different pulse parameters with this model. It is found that the formation mechanism of annular crack depends on the high radial gradient of energy density E, and the radius of annular crack obviously depends on the energy density deposited per pulse and the heat source boundary gradient for large volume sample. The influence of pulse number on branch crack and annular crack is different. The widening of the annular crack with the increasing of pulse number is caused by the axial residual plastic strain after pulse irradiation.