Research On Ultrasonic Nonlinear Detection The Stress And Fatigue Damage Of Metallic Parts

Under the cyclic load in service,the key parts in sugar making equipments would appear service injuries,such as the early performance degradation,fatigue damage,cracks and residual stress.These damages are key factors affecting the safe operation of sugar making equipments.Therefore,it is of great practical significance to ensure the effective service of metal components for detection and evaluation the service injuries.In this paper,the ultrasonic nonlinear technique is applied to effectively detect and characterize the stress,fatigue damage and fatigue crack of parts in sugar-making equipments.Through the experimental research based on ultrasonic nonlinear technology,the ultrasonic nonlinear response of metallic parts with different service injuries is explored,the correlation among ultrasonic nonlinear parameters,material structure variation and service damage is analyzed,and the ultrasonic nonlinear parameters are used for characterization the service damage of mechanical parts.Then,the early fatigue damage of press gear,the key part of sugar making equipment,is detected by applying the ultrasonic nonlinear technique.In addition,according to the characteristics of ultrasonic nonlinear output signal,the chaos and fractal theory is used to analyze the signal,which provides a new analysis method for ultrasonic nonlinear detection technology.The main work of this article includes:(1)The stress characterization of metallic parts using the ultrasonic nonlinear technique.When the stress of specimen increases,the stress sensitivity coefficient of ultrasonic nonlinear coefficient is far greater than that of wave velocity;and the stress detection resolution based on ultrasonic nonlinear coefficient is about lOMPa,while the resolution based on wave velocity is about 30MPa.Therefore,the nonlinear property of critically refracted longitudinal wave can be applied to effectively characterize the stress state of metallic parts.When the propagation direction of critically refracted longitudinal wave is consistent with the stress direction,the stress prediction model based on ultrasonic nonlinear comprehensive parameter is established,which can effectively predict the stress state of specimens.While the propagation direction of ultrasonic and stress direction are not the same(along X1 and x2 direction,respectively),the stress prediction models at X1 and x2 direction are established,then the magnitude and direction of principal stress can be predicted according to the parallelogram law.This method can effectively predict the magnitude and direction of principle stress.(2)Fatigue life prediction and fatigue crack detection of metallic parts based on wave mixing technology.Specimens with different fatigue life are performed the collinear wave mixing experiments,the relationship between mixing nonlinear coefficient and fatigue life of specimen is analyzed,the variation of micro-structure of material is explored and the fundamental causes of mixing nonlinear coefficient variation are analyzed.The mixing nonlinear comprehensive parameter is proposed,and the fatigue life prediction models based on mixing nonlinear coefficient are established,which can effectively predict the fatigue life of specimens.In non-collinear wave mixing detection,the detection points near the fatigue crack are analyzed to obtain a nephogram distribution of mixing nonlinear coefficient,which can visualize the fatigue damage near the crack.By combining the collinear mixing and non-collinear mixing technique,the location and length of invisible fatigue cracks in metal specimens are measured.(3)Early fatigue damage detection of pressure gear,the key component of sugar making equipment.The ultrasonic nonlinear tests are performed on 45 steel specimens in the four stages of fatigue nucleation,micro-crack initiation and propagation,macro-crack formation and failure,the variation law of ultrasonic nonlinear coefficient is analyzed,the relation between ultrasonic nonlinear coefficient and microscopic structure of material is investigated.The intrinsic relationship between the material microstructure,ultrasonic nonlinear coefficient and fatigue damage is established,which lays the theoretical foundation for fatigue damage detection of gear.The ultrasonic nonlinear experiments are performed on the tooth root and tooth centre of gear respectively,the second and third order relative nonlinear coefficients of detection points are calculated to evaluate the early fatigue damage.The detection results are consistent with the actual fatigue damage condition of gear.(4)Chaotic characteristics analysis of ultrasonic nonlinear output signals and Duffing oscillator detection.The ultrasonic nonlinear output signal is a non-linear time series which reflects the internal damage of metallic parts and undergoes complex propagation.In order to effectively extract its nonlinear characteristics,the chaos and fractal theory is used to analyze the ultrasonic nonlinear output signal.Chaotic characteristic parameters are calculated,such as Lyapunov exponent,correlation dimension and K entropy.Chaotic characteristic parameters and ultrasonic nonlinear coefficient have the similar variation trend.Furthermore,the chaotic characteristic parameters can effectively characterize the evolution of fatigue cracks and reveal the nonlinear property of fatigue crack propagation.The improved Duffing oscillator is applied to detect the ultrasonic nonlinear output signal,which can not only detect the second harmonic signal under strong background noise,but also can estimate the amplitude of weak signal.