Investigation Of Subharmonic Resonance On Contact Acoustic Nonlinearity

The nondestructive assessment and residual life estimation of in-service components/structures play a key role for engineering projects.Ultrasound has been widely utilized for this.Conventional linear methods based on reflection and refraction,diffraction and scattering attenuation,etc.are not sensitive to micro-cracks.The nonlinear ultrasonic technique which uses nonlinear behavior such as harmonic generation,has been proved to overcome this limit.However the nonlinearities in the measurement process such as introduced by amplifiers and boundary conditions are prone to lead false alarm.Using subharmonic resonance is a more efficient choice to solve this problem.Subharmonic components can only be generated by the contact between solid interfaces and not affected by experimental measurements.In addition,the nonlinear effects of fatigue crack and boundary condition can be distinguished by the threshold of subharmonic resonance.The following work to investigate the threshold behavior of subharmonic on contact interfaces are as follow:First of all,the background and research status of SHM were introduced,two physical mechanisms of harmonic generation crack were summarized.Secondly,the bolted joint structure is simplified to a two degree of freedom model,and the multi-scale method is used to explain the phenomenon of the subharmonic resonance and conditions for the generation of the subharmonic.Numerical simulation predicts the generation of the subharmonics and conditions for the subharmonics can be found with respect to the excitation frequency and excitation amplitude.Experimental results demonstrate subharmonic resonance method can effectively detect bolt looseness.Thirdly,a rough interface model was studied to obtain the interface stiffness.A single degree of freedom model based on Hertz contact was introduced to explain the subharmonic resonance on contact interfaces.Fourthly,the clamping condition was qualitatively modeled as two elastic,frictionless half spaces where the contact obeys the basic Hertz contact law.The nonlinear ordinary differential equation drawn from the developed model was solved with the method of multiple scales.The threshold of subharmonic generation and the nonlinear modulation was studied.Different threshold behaviors between the nonlinear boundary condition and the fatigue crack were found that can be used to distinguish the source of nonlinear subharmonic features.Experiments using an aluminum plate with a fatigue crack were conducted to quantitatively verify the subharmonic resonance range.Finally,the full text is summarized and future research is prospected.