At present, the dominant technology for transducers in the field of Ultrasonic Non-Destructive Testing is piezoelectric. However, some industrially important applications,
like the inspection of components operating at high temperature or while in
motion, are difficult tasks for standard piezoelectric probes since mechanical contact
is required. In these cases, contactless NDT techniques can be an attractive alternative.
Among the available options, Electromagnetic Acoustic Transducers (EMATs)
can generate and detect ultrasonic waves without the need for a physical contact
between the probe and the test object, as their operation relies on electromagnetic,
rather than mechanical coupling. Since EMATs do not require any coupling liquid,
the experimental procedures for inspection set-up are simplified and a source of uncertainty
is eliminated, yielding highly reproducible tests that make EMATs suitable
to be used as calibration probes for other ultrasonic tests. A further advantage of
EMATs is the possibility of exciting several wave-modes by appropriate design of
the transducer. Unfortunately, EMATs are also characterized by a relatively low
signal-to-noise ratio and by a complex operation relying on different transduction
mechanisms that make their performance dependent on the material properties of
the testpiece.
The present work aims to develop a numerical model including the main transduction
mechanisms, the Lorentz force and magnetostriction, that can be employed as
a prediction tool to improve the understanding of EMAT operation. Following an
overview on the historical development of EMATs and their models, the theory describing
EMAT operation is presented. The governing equations are implemented
into a commercial Finite Element package. The multi physics model includes the
simulation of the static and dynamic magnetic fields coupled to the elastic fields
through custom constitutive equations to include magnetostriction effects. The
model is used to quantitatively predict the performance of a magnetostrictive EMAT
configuration for guided waves without employing arbitrary parameters. The results
are compared to experimental data providing a validation of the model and insight
on the transduction process. The validated model, together with experimental tests,
is exploited to investigate the performance of different EMAT designs for Shear Horizontal
waves in plates. The sensitivities of each configuration are compared and
the effect of key design parameters is analyzed. Finally, the model is used in the
evaluation of the performance of bulk wave EMATs on a wide range of steel grades.
Experimental data interpreted via numerical simulations are employed to investigate
the relative weight of the transduction mechanisms, with implications on the
applicability of EMATs on the range of steels usually encountered in inspections.