Laser-ultrasonic absorption measurements in low carbon steels

We have refined the contactless laser-ultrasound reverberation technique to measure ultrasonic absorption on small metallic samples. In this technique , a sample is supported by a holder which is ultrasonically decoupled from the sample. A pulsed laser is used to generate an acoustic pulse. After the pulse has mode converted and scattered sufficiently to fully insonify the sample, the decrease in the noise-like ultrasonic signal is recorded as a f unction of time using a laser-interferometer. A joint time-frequency analys is technique is used to extract an absorption spectrum from the signal. In this paper, the technique is demonstrated in a frequency bandwidth ranging from 1 to 7 MHz, and in a dynamic range of 0.003 to 0.3 dB mu s(-1). Measur ements made on samples of three low-carbon steel grades, namely ultra low c arbon (ULC), low carbon (LC), and high strength, low-alloy steels (HSLA), c learly show that ultrasonic absorption varies with steel grade. The techniq ue was utilized to study the effect of a magnetic field on the ultrasonic a bsorption of an annealed ultra low carbon steel sample. It was found that m agnetoelastic effects are responsible for a large fraction of the total abs orption. (C) 2000 Published by Elsevier Science S.A.