Experimental Study On Mechanical Behavior Of First-stage Twisted Structure

Based on the unique space helix characteristics,the cable structure has high elastic modulus and excellent tensile strength compared with single strand.It is widely used in daily life and industrial fields,especially in some smart structures and energy facilities.For example,artificial muscles made of multi-strands of high polymer cable have become one of the important directions of modern smart structure research since muscles have lightweight structure,no stress lag and high energy density.In addition,the International Thermonuclear Fusion Experimental Reactor(ITER),a new energy facility,is expected to generate a strong magnetic field through a superconducting magnet wound by a cable conductor inside the tube(CICC)to restrain the motion of high temperature plasma.And ITER can control thermonuclear fusion as a result.Therefore,the mechanical behavior of CICC conductor directly determines the safety and functional design of the large device.Generally,CICC conductors are made up of nearly 1,000 superconducting strands by 4-5 grade stranding.Two superconducting strands with the same diameter and one pure copper strand form a first-stage stranded cable.Then three first-stage stranded cable form a second-stage stranded cable.Similarly,a helical stranded cable with complex spatial structure is established with lots of high-stage stranded cables.And the mechanical behavior analysis of the first-stage twisted structure becomes the premise and foundation of the prediction of the mechanical behavior of complex structure.In addition,a large number of contact deformations will occur on the strands during the preparation or operation.However,the research about strands' mechanical behaviors under contact deformations haven't been reported up to now.This master's degree thesis focuses on the experimental study of the twisted structure's mechanical behavior.Meanwhile,the thesis mainly does research on the experimental and theoretical prediction of the equivalent Young's modulus of the first-stage twisted structure.what's more,the thesis studies untwisting behavior of the first-stage twisted structure under the tensile load,and analyses the impact of contact deformation to the mechanical properties of the strand element.Main works are as follow:Firstly,the equivalent Young's modulus of the first-stage twisted structure is studied experimentally and theoretically.After the preparation of the first-stage twisted structure,the equivalent Young's modulus of the first-stage twisted structure is obtained by eliminating interstitial voids through cyclic loading.The experimental results show that the equivalent Young's modulus decreases with the decreasing of the initial helix angle.Then,the experimental results are compared with the classical Costello theory.It is found that there is a big error between the theoretical and experimental values of Costello due to the existence of torsional residual stress.In this paper,a unified linear relationship between torsional residual strain and initial helix angle of primary cable structures with different diameters is given.And a new modified model is established based on Costello's theoretical model.The results are in well agreement with the experimental results qualitatively and quantitatively.Secondly,the experiment on untwisting behavior of the first-stage twisted structure under tension is carried out.An experimental platform was designed independently to test the torsional behavior of the first-stage twisted structure under tension.The six-winding-one-rigid strand structure and the first-stage copper twisted structure were measured by the platform.The relationship between the torsional angle per unit length and the axial tension was analyzed.At the same time,the Impact on torsional behavior of the first-stage twisted structure during tension with the same diameter and different initial helical angles is measured.Finally,the influence of prefabricated contact deformation(indentation)on the stress-strain curve of the strand element is studied.In order to avoid damage of clamp of electronic universal testing machine to strand sample,a matching fixture was designed and manufactured independently,which ensured the accuracy of stress-strain curve measurement of strand element.Subsequently,the stress-strain curves of Nb3 Sn superconducting strands and T2 copper wires with indentation were tested.And the effects of indentation's depth and number on the stress-strain curves were considered.The results show that as the indentation depth is increasing,the length of plastic zone of stress-strain curve decreases gradually.When the indentation depth reaches a critical value,the plastic zone disappears and the fracture stress decreases gradually.When the indentation depth is the same,with the increase of indentation number,the fracture stress almost remains unchanged,while the length of plastic zone increases.