Experimental Study And FEM Analysis On Single And Second-hierarchical Helix Strand Under Tension Deformation

The helix strand structures have been widely applied in various fields of engineering technology.There are a large number of multi-hierarchical helix strands from various types of engineering applications such as cables and twisted cables to superconducting composite CICC structures in high-tech and large-scale scientific installations.Due to the structural characteristics and complexity of helix strands,there are often complicated mutual contact between strands,which brings great difficulties in the analysis and estimation of the mechanical properties,including the geometric modeling and force from single-level and multi-level structures.And deformation analysis and basic experimental analysis have not yet developed well with effective analysis.For the structure of 1 × 3 single-hierarchical and 3 × 3 two-hierarchical helix strands,the basic mechanical properties were studied from tensile test to finite element numerical simulation.Firstly,we used the strand twister to prepare single and second helix strands.A set of feasible solutions for the elimination of pre-stress in the strand during preparation process has been developed,which greatly improved the uniformity of the strand structure and structural stability.In order to effectively reduce the mechanical loading and clamping error in the helical strand under tensile deformation test,three circular drill chucks were independently improved and developed,which provides a holding force at the non-ideal strand section for tests.Furthermore,for the NbTi/Cu superconducting composite wire,the strands were twisted and the preliminary experiments of the mechanical properties at low temperature were carried out.The experimental results show that with the decrease of temperature,the equivalent elastic modulus of the strand gradually becomes larger;the change of the intercept of the strand also has a significant effect on the overall mechanical properties,and the elastic modulus increases with the intercept of helix strands.Secondly,in order to perform the mechanical analysis of helix strand structure,we carried out finite element numerical simulations of single-hierarchical and 3 × 3two-hierarchical strands.A Python script program was made for a rapid geometric modeling for the helical strand structure,including the application of boundary conditions and periodic boundaries and exposing forces,which can effectively improve the numerical modeling of complex strand structure.It is the basis for subsequent further finite element analysis.The mechanical simulations of helical strand structure under tensile loading were further performed.The numerical results showed that with the reduction of the helix angle of the strands,the equivalent elastic modulus of the strand structure decreases,that is,the tensile stress corresponding to the same axial strain is small.In the case of a small helix angle,the contact deformation between the strands has little effect on the mechanical behavior of the strands;for the single-hierarchical strands with the same axial strain,the results of axial force with the friction between the strands is always greater than those without friction;for the two-hierarchical strand structure,the contact deformation between the strands has a great influence on the mechanical behavior of the strand.