The Analysis On The Crack Arresting By Using Electromagnetic Heating Effect

The application of crack prevention techniques is badly needed in practical engineering, because it has many practical applications such as in military affairs, space flight, aviation, shipping, oil production, nuclear engineering and machine manufacturing. It is a novel and effective method for stopping internal and surface crack propagation in metal components by using electromagnetic heating effect. However, both domestically and abroad, there is a lack of systematic theoretical and experimental research on crack arresting using electromagnetic heating effect in metal components, including metal dies. The work shown here was based on the thoughts that to do some systematic research on crack prevention using electromagnetic heating effect. Theoretical analysis, numerical simulation and experimental surveys are the three aspects of this investigation.The main work is outlined as follow:(1) The complex function method is used as a basis for the theoretical model of crack prevention by using electromagnetic heating effect. The heat source power, temperature and thermal stress fields are calculated around the collinear crack tips in metal conductivity when the pulse current is switch on. The relationships among the temperature, thermal stress to the parameters of current density and crack dimensions, are given out. It is also described that the stress field near crack tips caused by mechanical load and arresting the crack propagation by electromagnetic heating effect.(2) The coupled field analysis of crack arresting has been done by finite element method when a super-density pulse current is switched on. Using the commercial finite element software ANSYS, the temperature fields around crack tips are calculated by a thermal-electricity coupled process for a plate with two collinear cracks. The thermal stress field is simulated by a thermal-mechanical coupled process while discharging. Meanwhile, the stress field caused by mechanical load and stopping crack propagation by usingdischarging is also simulated. It has a good agreement between theoretical analysis and the simulation.(3) The experimental investigation of arresting crack propagation for some different metal material, including die steel, are finished by a pulse current generator (model ZL-1 and ZL-2) developed by the work team. The process of crack arresting is recorded by a high speed electrical camera. The results of theoretical analysis and numerical simulation are verified by experiment. The results of structural microscopic and electrical microscopic observation show that: a white-bright layer is formed around the crack tip. The hardness, density and toughness of the layer are much higher than the surrounding matrix. The microscopic principle of crack prevention by a pulse current discharge is clarified. The mechanical performance of the structure around the crack tip is tested after crack prevention. Also, Scanning Probe Microscopy is used to do mechanical property testing for the nanometer scale.(4) Through the microstructure analysis near crack tip after crack prevention by using electromagnetic heating effect, it is found that the phase transformation has been occurred. Due to the phase transformation, the value of the material has enlarged. Further more, the strain and stress of the phase transformation are caused. The physical simulation experiment of phase transformation is finished by thermal simulation equipment, GLEEBLE3500 made in America, for getting the strain of free transformation. Also, the residual strain of phase transformation has gotten by X-ray diffusion method. It is the groundwork for determining the residual stress of phase transformation.(5) The stress field caused by phase transformation is analyzed and determined by the residual strain of phase transformation.(6) The theoretical calculation of the phase transformation stress is finished by Eshelby model of imaginary cutting method. The residual stress intensity is calculated. The affection of the residual stress to the crackpropagation is discussed. The integration effect of stress caused by mechanical load and the residual stress of phase transformation is analyzed. It is found that the residual stress will have a long term effect for crack prevention.The results obtained show that the technique of arresting crack propagation by using electric magnetic heat effect has both academic feasibility and practicability. The work can provide the theoretical basis for solving practical problems.