Determination of relationship between Rayleigh waves velocity and stress with laser Doppler velocimeter

【Author】

Shoichi Kobayashi;

【Abstract】

<正> The velocity of Rayleigh wave propagating in an elastic body depends on the stress field of the body. Since the stress dependence varies with different materials and types of ultrasonic waves, it is necessary to obtain the quantitative relationship between stress and ultrasonic waves velocity for each interested material. This paper focuses on the relationship between Rayleigh waves velocity and stress for materials of aluminum alloy 5052 (A15052) and steel SS400 by use of a laser Doppler velocimeter (LDV).In the measurement system, a wedge piezoelectric transducer is adopted for generating Rayleigh waves, which converts longitudinal waves into Rayleigh waves. Rayleigh waves are detected with the LV-1300 laser Doppler velocimeter, which works on the optical heterodyne interferometer. In the LDV, an interferometer is installed in the sensor head. In order to measure wave velocity at two points, the LDV was specially designed for two-point detection. Two laser beams from the LDV are interchangeably selected as receiving sensors. The distance between the two beams is fixed at 4 cm. The time-of-flight (TOF) for Rayleigh waves propagating the fixed distance is measured by the sing-around unit, which works on the sing-around principle. The accuracy of the sing-around unit is about 0.1ns for 10,000 times accumulated number of trigger signals. In order to investigate the quantitative relationship between Rayleigh waves velocity and the stress field, tensile test is implemented. The stress caused by the loading in the specimen is measured by a gauge-meter, and at each load level, the TOF is measured with the sing-around unit, thus the velocity can be obtained.The stress-acoustic coefficients of Rayleigh waves in A15052 and in SS400 are-1.19×10-5(MPa)-1and -0.21×10-5(MPa)-1, respectively, which are in good agreement with that obtained by the "sharp edge" transducers. This shows that LDV is applicable to determining the stress-acoustoelastic coefficient and it may be also suitable for determining residual stress.

【Keywords】

Rayleigh waves velocity, laser doppler velocimeter, stress-acoustic coefficient, sing-around, stress

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