Characterization of local residual stress on curved blades by ultrasound

Residual stress caused by Foreign object damage(FOD) plays an important role in predicting the rate of crack initiation and propagation, which will lead to the failure of aero-engine blades. At present, many studies focus on the measurements of residual stress on flat surface by ultrasonic testing methods, which are not fit for curved surface. To evaluate the local residual stress on curved aero-engine blades, here a immersion ultrasonic testing method is established by using leaky Rayleigh wave combined with V(z) curve which is the relationship between the output voltage of ultrasonic signal and the lifting location of transducer. In this work, the acoustoelastic coefficients of Rayleigh wave propagation in Ti-6Al-4V are measured by tensile loading system. The velocity of Rayleigh wave can be calculated by the oscillation interval of V(z) curve. Depending on acoustoelastic coefficients and Rayleigh wave velocity, the local stress in damaged area can be determined. The biaxial stress field around a impact site on curved blades has been measured by proposed method. Result show that local stress in less than 1mm 2 local area can be precisely obtained by the stress measurement system and the error of calculated stress is only less than 10% of yielding stress.