Research On Mechanical Property Of Zylon/epoxy Composite And Mechanical Behavior In Pulsed Magnet

With the rapid development of scientific research in pulsed high magnetic field,some important scientific experiments have put forward requirements for higher strength of pulsed magnetic field.At present,the highest strength of the pulsed magnetic field has been exceeded 100 T.However,due to the problem of the lifetime and mechanical stability of the magnet,the highest magnetic field level of pulsed magnet used in actual scientific experiments is usually only 70?80 T.Pulsed magnets with higher magnetic field strength has not been widely used in scientific experiments.The main reason for further improvement of the pulsed magnetic field strength and affecting the safe and stable operation of the high field pulsed magnets is the huge electromagnetic stress inside the magnet.The structural stress analysis of the pulsed magnet has become the most important content for the high field pulsed magnet design.Pulsed magnet undergoes complex processing technics,works in high stress,high voltage,high current,extremely low temperature and other extreme conditions,and withstands the cycle loading of multiple factors,such as electricity,magnetic,heat and force.As a result,pulsed magnet is a complex multi-physics coupled strong electromagnetic system.There are many factors that affect its mechanical properties and fatigue life,and the mechanical behavior and fatigue failure mechanism are extremely complicated.With the pursuit of higher magnetic field strength,design of pulsed magnet with higher fatigue life and higher reliability,the mechanical properties of the magnet material should be widely tested.It is necessary to deeply analyze the complex mechanical behavior of pulsed magnet,further understand the distribution and evolution of stress/strain of the magnet material,and study how to reduce the deformation of the pulsed magnet.Therefore,the Zylon/epoxy composite used for reinforcement of the pulsed magnet structure has been chosen as the main research object in this dissertation.Simulation analysis and experimental tests will be carried out to investigate the mechanical properties of the Zylon/epoxy composite and the mechanical behavior of the pulsed magnet.The main contents and achievements are summarized as follows:In the mechanical properties study of the Zylon/epoxy composite,a new method for testing the strength properties by electromagnetic expansion ring loading is proposed.This method truly reflects the stress state of the fiber material in the pulsed magnet.A multi-physics coupling finite element model for the electromagnetic expansion ring is established to analysis the expansion motion and failure process of the composite expansion ring under electromagnetic loading.Then,the strength of the Zylon/epoxy composite is tested and evaluated.Based on destructive experiments of small pulsed magnets,the strength of Zylon/epoxy composites is further verified.Fatigue tests of the unidirectional Zylon/epoxy composite specimen are carried out to study the ultimate tensile strength and fatigue life under different stress levels.The stiffness reduction and damage evolution law of the Zylon/epoxy composites under fatigue loading are analyzed.For better reflecting the mechanics characteristic of the pulsed magnet,a cyclic plastic constitutive model is developed for the conductor material to simulate the cyclic deformation behavior under the condition of repeated electromagnetic force loading.For the fiber composite,the calculation and modeling method for off-axis stiffness of the laminated structure in the finite element analysis is given out.Using the Tasi-Wu tensor criterion as the strength criterion,a progressive failure model for the pulsed magnet unit is developed.The failure mode of the composite element is separated by defining stress variables,and the stiffness reduction is implemented by using a stiffness degradation law.The analysis and calculation accuracy analysis of the pulsed magnet structure directly depends on the results of the circuit,magnetic field,heat transfer,and other multi-physical field coupling calculation.Therefore,to simulate the multi-physics coupling characteristics of the pulsed magnets,a multi-physics coupling finite element model for the single-stage pulsed magnet is established.The current,magnetic field waveform and current density,temperature distribution is calculated.Using it as a base,the method of multi-physics coupling modeling is extended to the simulation analysis of the 100 T triple-coil magnet system.In mechanical behavior analysis of the pulsed magnet,the influence of the material damage caused by the bending pre-strain of the conductor material on the stress distribution in the magnet is analyzed with the cyclic plastic constitutive model coupled the tensile damage.Taking the 100 T triple-coil magnet system as an example,the optimal magnetic field contribution ratio of the different coils is studied with the objective of stress balance for each coil.Based on the progressive failure model of composite material,the failure process and failure strength of fiber composite in the pulsed magnet unit is predicted and analyzed.The stress analysis model of laminated composite material under column coordinate system is established,and the performance of spiral wounded laminated structure for reinforcement of magnet windings is studied.Through simulation of the cyclic deformation of the magnet unit,the influences of the cyclic plastic behavior of the conductor on the allocation mechanism of electromagnetic force,and on the evolution of the stress/strain of the magnet material under cyclic loading are analyzed.It is pointed out that increasing the axial reinforcement performance of the pulsed magnet is the content that the later magnet design needs to pay attention to.