Study On The Damage Of Cracked Metallic Material Under High Electrical-thermal And Mechanical Loads

The working current in electromagnetic launching is in order of mega-ampere and local electric current density can be in order of 10~9A/m~2.Heat accumulation due to extreme Joule heat causes material temperature increasing rapidly to melting point.Meanwhile,there is Lorentz force on the armature and complex cyclic electromagnetic force on the rail during multiple launching process.The electromagnetic launch is a transient electric-thermal-mechanical coupled process.When crack or defect exists in armature and rail material,high electric-thermal load and mechanical load can cause damage like crack propagation and local melting,which severely affecting launching efficiency and system longevity.This paper is aimed to study damage evolution of cracked material in electromagnetic launching using experimental and theoretical methods.The main research contents and results are as follows:Experiments with different crack lengths and current loads are conducted for aluminum alloy.Parallel theoretical model and finite element model are established and comparison between calculating and experiment shows that,temperature is the main reason affecting crack tip damage and the temperature dependence of electrical resistivity has great influence on the calculation result.Atom diffusion and dislocation motion around crack tip are accelerated by high electrical-thermal load.Crack tip microstructure changes due to temperature gradient in the material.Microscope,scanning electron microscope(SEM),energy dispersive X-ray spectroscopy(EDS),X ray diffraction(XRD)and electron back-scatter diffraction(EBSD)are used to study microstructure change around crack tip.The change of micro hardness and mechanical property is gained by Vickers hardness test and tensile test.Relationship between crack tip microstructure change and macro performance is understood.The physical field governing equations and boundary conditions are established for current-carrying plate with a single-edge crack.Based on the theory of mathematics,complex variable function and coordinate mapping method are used to solve the equations.Analytic expressions of current,temperature and stress are deduced.The experimental platform is built including loading system which is consist of high-voltage pulse power supply and mechanical testing machine,and testing system which is consist of Rogowski coil,high-speed infrared pyrometer and high speed photography.Experiments to study cracked material damage under transient electrical-thermal and mechanical load are conducted and damage evolution of material melting,crack initiation and propagation are recorded.A simulation method is presented to solve the damage problem of cracked material under electrical-thermal and mechanical load.Based on indirect decoupling concept,the complex coupled-physical-field simulation is divided into four steps:coupled electrical-thermal,coupled thermal-displacement,XFEM crack propagation and phase transition.Using this new method,local material melting and crack propagation are simulated and the effectiveness of this method have been validated by comparing simulation and experimental results.