An electromagnetic acoustic transducer (EMAT) is presented for excitation and detection of linear polarised shear waves at normal incidence with the use of a horizontal magnetisation of the specimen. The sound field is optimised for the measurement of the wall thickness of steel plates and pipe joints. Special focus is on determining the remaining wall thickness in case of metal loss (e.g. general corrosion, pitting corrosion). If metal loss is present at the transducer-near side, it is not possible to determine the remaining wall thickness because the lift-off of the EMAT sensor caused by the wall thickness reduction may lead to a complete loss of the ultrasonic signal. In order to ensure reliable measurement for this case, the EMAT technique is combined with the eddy current (EC) technique and the magnetic flux leakage (MFL). The EC technique, that uses the EMAT excitation signal as a pulsed EC excitation, is able to detect metal loss at the transducer-near side by measuring the transducer lift off, simultaneously to the electromagnetic ultrasonic inspection. Additionally, a MFL signal is derived by making use of the horizontal magnetisation and the EMAT receiver coil as flux leakage sensor. The MFL signal can be separated and analysed by appropriate filtering because it contains mainly low-frequency components as compared to the high frequency EC signal respectively ultrasonic signal. By combining the different inspection technologies the disadvantages of the individual techniques are eliminated. As a result, a sensor has been developed, that is able to measure the (remaining) wall thickness of a component as well as to determine the location in the wall of a detected metal loss. A further advantage of the new sensor conception is that essential hardware components can be used in parallel. Results obtained so far show that the new probe meets the requirements for high-resolution in-line inspection.
展开▼
