Method of local measurement of thickness of thin metal products using ultrasound

During excitation of ultrasonic Lamb waves in thin metal plates and sheets using a thermoacoustic emitter with diameter of 3 mm and more, the shape of ultrasonic pulse changes. As the diameter increases the initial pulse starts dividing into two. It is believed that due to temperature gradient the opposite sides of the thermoacoustic emitter act as sources of the divided ultrasonic pulses. The time of retardation of these pulses from one another is defined by the diameter of the thermoacoustic emitter and the velocity of Lamb waves propagation. This fact may be used for local measurement of the wave velocity since the length of the section under measurement does not exceed the diameter of the thermoacoustic emitter. Based on the functional connection between the velocity of ultrasonic Lamb waves and the plate thickness and oscillation frequency a method was suggested on experimental measurement of plate thickness. Confidence boundaries of cumulative relative and absolute measurement errors were assessed. For comparison, a similar assessment was performed for a case of measuring a sample’s thickness using a micrometer. A conclusion was made that if error in measuring thickness of metal sheets and plates is accepted as equal to or exceeding 5,0 %, then the method of measurement using ultrasonic waves excited by optical nano-pulses may be successfully used at production enterprises. Moreover, this method is quite promising while monitoring thickness of large-size thin metal products.