On the Evaluation of the Effect of Microstructure on the SNR of Ultrasonic Images and on the Velocity of L-cr Waves

Stress measurement using ultrasound employs the acoustoelastic theory that relates the variation of the velocity of elastic waves to the variation of stress in the material. However, not only stress has influence on wave velocity, but also the microstructure of the specimen being evaluated. The aim of this work is to verify the feasibility of using ultrasonic images to evaluate microstructural variations that affect the velocity of longitudinal critically refracted (L-cr) waves used in stress measurements. The images were generated applying the total focusing method (TFM) in signals obtained with an ultrasonic array. The signals recorded contain grain noise related to the material's microstructure. In this work, the time-of-flight (TOF) of L-cr waves and the signal-to-noise ratio (SNR) of ultrasonic images are obtained for samples of ASTM A36 steel with different grain sizes and shapes. The TOF is related to the wave velocity and the SNR to the grain noise intensity. The influence of stresses on the TOF and SNR was also verified. A linear relation between TOF and SNR was established with a variation rate of -76 ns/dB. No significant variations were observed in the SNR caused by the applied stress. Because the noise intensity of the images are sensitive only to microstructural variations, the technique presented has potential to be used to correct values of ultrasonic wave velocity in stress measurements. (AU)