Study Of The Influence Of CT Specimen Connect Stiffness On The Resonant Fatigue Testing System Dynamic Characteristics

Electromagnetic resonance fatigue testing system is a kind of device which is used to specimen fatigue crack propagation test and it is based on resonance principle.The standard compact tension(CT)specimen is the most common specimen in fatigue crack propagation test.And its connect stiffness can affect dynamic load measurement accuracy,control accuracy and the dynamic characteristics.CT specimen connect with the fixture through a pin shaft which can be seen as mechanical joint.Without proper lubrication,the effects of joint on the dynamic characteristics mainly caused by normal contact stiffness.In this paper,the connect stiffness of CT specimen is obtained by theoretical calculation and experimental measurement,and its influence on the dynamic characteristics of system and measurement error of dynamic load is studied.The main research works of the paper are as follows:1.A mathematical model of electromagnetic resonance fatigue testing vibration system is established.Then,the connect stiffness model of CT specimen and fixture based on the pin shaft mechanical joint is established.According to Hertz contact theory,the normal contact stiffness between CT specimen and fixture is calculated and analyzed.These works lay theoretical foundation for the accurate measurement.And the CT specimen stiffness is calculated by FEM software ANSYS.2.A method of measuring the CT specimen connect stiffness is proposed,which is based on the characteristic equation of the resonant fatigue testing vibration system.The stiffness valve of series of CT specimens with different crack lengths and the corresponding natural frequencies of the resonant fatigue test system are used as auxiliary variables,counterbalance mass is regarded as the objective optimization variables.Using the least square method and Newton iteration method to solve over determined nonlinear equations,and then the CT specimen connect stiffness and other parameters of system can be calculated.The experiment was performed to verified the effectiveness of the CT specimen connect stiffness measurement method.Furthermore,the effects of pin shaft dimension,static load and specimen material on the CT specimen connect stiffness were analyzed.3.The electromagnetic resonance fatigue testing vibration system characteristics are analyzed.Base on the steady-state output response of the system,the mathematical model of dynamic load measurement error,system natural frequency,testing load resonance amplitude and testing load amplitude frequency characteristics is established.And it was studied through theory and experimentation.4.Influence of CT connect stiffness on the resonant fatigue test system characteristics is studied.And the influence of CT connect stiffness on testing load amplitude frequency characteristics,system natural frequency,testing load resonance amplitude and dynamic load measurement error are analyzed under different conditions.The results of this paper provide a theoretical basis for the design,analysis and control of the resonant fatigue testing system.