Estimation of thin metal sheets thickness using piezoelectric generated ultrasound

In the industry, many metal structures are constituted by thin plates which undergo, during their use, fatigue, corrosion, defects etc., these effects often result in a thickness variation. It is so, important to be able to measure, accurately, the thickness of these plates. Furthermore, only one side of the plate is often available, this disadvantage increases the difficulty of measuring the thickness variations. Generally, ultrasonic waves are used to test these structures, without dismantling or modifying them. This paper presents a method to measure the thickness of thin metal sheet by using the propagation of the first antisymmetric A0 Lamb mode generated by a piezoelectric transducer. Principle is based on the proportionality relation which exists between the group velocity and the thickness of the thin sheet, at low frequencies. We put in evidence this relation which we test on thin metal sheets (150 mm long and 70 mm (58 mm for the sheet of 25 μm thickness) width) of which we know the thicknesses (25; 75; 100; 160 and 200 μm), measured with a palmer (micrometer). This work is a feasibility study which shows that it is possible to use piezoelectric transducers to measure small thicknesses with the A0 Lamb mode method.