Investigation Of The Micro-crack Damage By Two-way Collinear Wave Mixing Method

The safety and reliability of critical components in service have been paid seriously attention in the engineering field.Especially for the fracture failure caused by fatigue damage,numerous studies have revealed that the initiation and propagation of micro-cracks are the important factors of component failure.Traditional linear ultrasonic detection technology is not sensitive to the micro-crack damage below millimeter.Based on the nonlinear effect between ultrasonic wave and microstructure of damage,the nonlinear ultrasonic detection technology can be employed to detect and evaluate the damage whose size is much smaller than the wavelength.However,the investigation on nonlinear ultrasonic detection and localization of micro-crack damage is still insufficient.In this paper,the detection mechanism of two-way collinear wave mixing method and zero frequency method is studied for random micro-crack damage.Firstly,the detection mechanism of two-way collinear wave mixing method for micro-crack damage is investigated based on numerical simulation.The numerical results show that if the resonant condition ?_L/?_T=2?/(?-1) is satisfied,when a pair of transverse wave and longitudinal wave propagate in coaxial direction and mix in the micro-crack damaged region,a resonant wave can be generated since the interaction between two primary waves and micro-cracks.The ultrasonic nonlinear parameter based on the resonant wave has a linear relationship with the density of the micro-crack and the size of the micro-crack area,and monotonically increases with the increase of the frequency of the mixed wave,but it is not sensitive to the friction coefficient of the crack surface.Meanwhile,the damage region of micro-cracks can be accurately located by the resonant wave signal.Secondly,low temperature fatigue test is conducted on an aluminum alloy AL-5052 sample to prepare micro-crack damage,and ultrasonic scanning test is conducted on the fatigue sample based on two-way collinear wave mixing method.The experimental results show that the micro-crack damage is the main micro damage form in cryogenic fatigue test.And the micro-crack damage region can be detected and located effectively by using two-way collinear wave method.Moreover,it is found that the high frequency resonant wave signal has higher sensitivity and resolution in the detection of micro-crack damage.Finally,the mechanism and propagation law of the zero-frequency component of the bulk wave in micro-cracks damage are investigated based on numerical simulation.The numerical results show that the zero-frequency component can be generated when a longitudinal wave propagates in the micro-crack region.The acoustic nonlinear parameter based on the zero-frequency component is linearly related to micro-crack density,the size of the micro-crack region and frequency of the fundamental wave,but not sensitive to the friction coefficient of the crack surface.In addition,the nonlinear parameter based on the zero frequency component is more sensitive than that based on the second harmonic.The results of this study provide the theoretical and experimental foundation for developing the nondestructive evaluation technology.