LIFT-OFF PERFORMANCE OF FERRITE ENHANCED GENERATION EMATS

Abstract

Electromagnetic Acoustic Transducers (EMATs) are non‐contact ultrasonic transducers capable of generating wide‐band ultrasonic waves on electrically conductive and magnetostrictive samples. The lack of physical contact makes EMATs particularly suitable for online inspection applications, or situations where samples may be moving or hot. The generation efficiency of a given EMAT on a given sample is dependent on the “lift‐off”, which is the distance between the EMAT and the sample surface. Efficiency dramatically reducing with increased lift‐off. This requirement to be in close proximity to the sample imposes a practical limit of operation and changes in lift‐off due to phenomena such as sample vibration can have practical implications in certain NDE applications. This paper describes some results from experiments comparing the performance of a ferrite enhanced EMAT design to one of our ‘standard’ EMATs, where we have substituted the permanent magnet from the standard EMAT with a suitable ferrite material. When the EMAT coil is placed in proximity to the ferrite, but not wrapped around the ferrite, the increase in the generated eddy current amplitude is significant, whilst the inductance or bandwidth of the EMAT is not significantly affected. Using a ferrite material eliminated eddy current losses in the permanent magnet, and also enhances the self‐field generation mechanism, which generates a repulsive normal force on the sample surface. Direct experimental results show the ferrite enhanced EMAT generation efficiency can be higher at large stand‐offs and is also less sensitive to lift‐off variations. Although the example we describe here only applies to EMAT generation, there are situations where ferrite could be used to enhance detector efficiencies.