Research On Fatigue Damage Detection Of Bolted Joints Based On Vibration Response

Bolted joints are widely used in many engineering fields such as construction machinery and aerospace because of their simple assembly and interchangeability.They are commonly exposed to harsh vibrational environment for a long time,and the vibration fatigue is very often identified as their main mode of failure.Therefore,to achieve early diagnosis of their fatigue damage state,it is of importance to carry out the investigation of vibration fatigue damage detection of bolted joints based on vibration response.The main research work of the present thesis is as follows:Firstly,vibration fatigue testing of bolted joints under different preload moments was carried out.The vibration fatigue specimen was designed according to the standard,the nonlinearity of the vibration fatigue test platform was quantified,the linear curve of strain and vibration acceleration was fitted,the acceleration amplitude was used to control the experimental load level.the excitation frequency was adjusted based on the change of the intrinsic frequency of the specimen to realize the resonance fatigue test of the bolted joint,and finally the fatigue test of the bolted joint under different preload torques was carried out.Secondly,a multi-parameter damage detection method is developed,which enables to simultaneously extracting sixteen damage parameters from the acquired vibration response.The extracted damage parameters are divided into two categories:the linear ones and the nonlinear ones.The former includes natural frequency,damage indicators based on transmissibility function.The latter covers total harmonic distortion,statistical properties of time domain response under sine and multisine excitations,and damage indexes based on the nonlinear distortion of the transmissibility function.For the last parameters,their quantification methods are hereby particularly studied.It is found that the nonlinear damage index called as TNL demonstrates high sensitivity to damage.Considering the fact that damage parameters of different kinds would contain redundant damage information,a comprehensive damage parameter known as the principal component analysis(PCA)is thereby used to eliminate such superfluous information.By assessing the evolutionary trend of this comprehensive parameter,it is revealed that the crack initiation phase dominates the fatigue life of bolted joints under various tightening torques.Finally,the fatigue life and fatigue damage investigations of bolted joints subjected to different tightening torques are carried out.For the former,it is shown that with the increase in tightening torque value the fatigue life first decreases and then increases.Through the surface crack analyses,described in terms of crack deflection range and branching degree,it is found that the branching degree is the main factor affecting the fatigue life.The fatigue damage detection study carried out in the present thesis discusses the applicable conditions and detection effects of numerous damage parameters,and the established multi-parameter damage detection method can provide a reference for related damage vibration detection studies.