| The Nb3Sn strands are widely used in engineering applications for their high critical magnetic field. The superconducting magnet within the tokamak of the ITER carrying large current provides high magnetic field to constrain the plasma. The superconducting magnet will undergo periodic electromagnetic forces and thermal stress in low temperature and high magnetic field. The stable operation of tokamak mainly depends on the basic characteristics, safety and validity of the superconducting magnet. CICCs are composed of one thousand of multi-stage helical strands which consist of Nb3Sn filaments and copper. The changes of strain, temperature, magnetic field will lead to the fracture and fatigue damage of filaments. Therefore, it is necessary to study of the mechanical properties of the strands which will contribute to the design optimization and improve their superconductivity.We firstly establish the two dimensional crack model, then consider the bridging crack under the tensile stress and shear stress. The stress intensity factor and relative opening displacement can be obtained with the method of Westergaard function. The relationship between applied strain and stress intensity factor can be obtained. At the same time, we compare two different damage modes that may exist during the extension of crack. The effects of twist pitch on the energy release rate and stress intensity factor are discussed. The influences of twist pitch, crack number and the width of filament and bronze on critical strain are analyzed.In the Nb3Sn strands, many micro voids or cracks distribute in different regions. Hence, the complex fracture behavior will happen when they interact with each other. For this reason, we consider the two collinear cracks problem. The results presents the relationship between crack number and strain. Then we compare the results of the applied strain with experiment.In reality, the superconducting magnet is composed of thousands of strands which are multistage cabling. The mechanical behavior becomes complicated because of the complex geometry. The extension of the crack is affected by more parameters because of periodic electromagnetic forces and thermal stress in the high magnetic field. Based on the thin rod theory, we get the geometric relationship of the stain, helix angle and rotation angel in every stage. We also analyse the changes of stress intensity factor and energy release rate with the helix angle, twist pitch and rotation angle. The stress on the crack’s interface c an be calculated in two ways which result in different values of stress intensity factor. The results are compared. At the same time we discuss the stress on the section of the multistage helical strands, and the results provide some reference for the optimal design of the superconducting strands. |
