Nondestructive testing of defects in adhesive joints

Transducer using Terfenol-D/epoxy composite
Reji John, Tusarkanta Sahoo, K A Thomas, Laly Joseph and K P B Moosad
Naval Physical and Oceanographic Laboratory
Defence Research and Development Organisation, Kochi-682021, India
Email: tsonpol@vsnl.com
Abstract:
Terfenol-D (alloy of iron, dysprosium and terbium rare-earth materials) has received considerable
attention as a material for magnetostrictive transducers and actuators due to its giant magnetostriction.
Nevertheless, device development using this material has always been hampered by problems related
to eddy currents and high brittleness of the material. Both of these problems can be effectively
addressed by developing composites of magnetostrictive materials in epoxy matrices. A study was
carried out on a Terfenol-D /epoxy composite to assess its suitability as a transduction material for the
application of low frequency underwater transducers. Composites with different percentages of
volume filling of Terfenol-D powder having 0- 300 micron sizes were prepared and their magnetic
and transduction properties were studied. The maximum attainable magnetostriction for a composite
of 60% volume filling of Terfenol-D powder was found to be 450 ppm. In order to assess the
suitability of the material for fabricating low frequency underwater transducer, the composite
material was cast into rods (8 mm diameter X 28 mm length) and were employed in a Flextensional
Transducer, as a transduction material . Underwater acoustic measurements were carried out and the
results are compared with those of a similar transducer using monolithic Terfenol-D material, in the
same geometries.
1.Introduction
Terfenol-D material has received considerable attention as magnetostrictive transducer and
actuator material due to its giant magnetostriction. Nevertheless, device development on these
materials has always been hampered by an eddy current induced bandwidth limitation to a few
kilohertz and brittleness imposed challenges to produce complex shapes and designing of devices. By
contrast, magnetostrictive polymer composite can be developed to alleviate the problems intrinsic in
bulk material by incorporating magnetostrictive Terfenol-D particles into a passive polymer matrix.
Recognizing the advantages of Terfenol-D composites, researchers began integrating these
composites in sonar transducers. The motivation is partially explained from a preliminary study
showing that monolithic Terfenol-D is superior to PZT in terms of power capability [1]. This idea
was reinforced in a subsequent study suggesting that ‘‘composite Terfenol-D should be superior to
both monolithic Terfenol-D and PZT in transducers for sonar arrays operating in the 20–30 kHz
range’’ [2]. Of particular importance in sonar transducer design is the strain output and the modulus
of the composite under different loading conditions. These properties dictate the work output
delivered by the transducer. However, the unavailability of a comprehensive characterization of strain
output, modulus, and coupling coefficient under different loading conditions and volume fractions
limits the use of magnetostrictive composites. The intention of the study is to develop a Terfenol-D/
Epoxy composite having optimum modulus and large Magnetostriction to enhance the stress output
for actuator applications. Also it is included in this paper, studies on the strain output of the
magnetostrictive composites as a function of particulate volume fraction of Terfenol-D powder.
2. Experiments
2.1 Fabrication of magnetostrictive composite material The 1–3 composites are fabricated using
a low viscosity epoxy (viscosity ~100 cps, Roto polymers, Chennai) as the matrix material and
Terfenol-D powder (0-300microns, Etrema, USA ) as the particulate filler. Low viscosity epoxy is