Nondestructive testing of defects in adhesive joints

selected as matrix material due to the reason of easy removal of trapped air and wetting of the
powder and further maximizing the filler loading . Composites are manufactured in an aluminum
mold fabricated with a cavity of 8mm dia and 28 mm length. Appropriate volume fractions of
Terfenol-D particles are mixed with epoxy resin in a beaker. The mixed slurry is placed in a vacuum
for degassing. The degassed slurry is further poured into the mould just before the gelation time. The
mold is then sealed and placed between two permanent magnets (field strength ~ 1000 gauss). The
mould is further kept on rotation at room temperature during curing process to prevent the settling of
dense Terfenol-D particles.
The magnets placed at the two ends of the mould, produce uniform horizontal magnetic field through
the length of the cavity. This aligns the particulate into chains or pseudo fibers fashion, which when
cured get locked in the epoxy matrix results in a 1–3 composite. A 1–3 composite produces a higher
strain output than the particles are randomly aligned [3]. In addition to aligning the particles the
magnetic field also further aligns the domains inside the particles, however, since Terfenol-D is a soft
magnetic material the domains redistribute once the magnetic field is removed.
2.2. Magneto mechanical Characterisation
2.2 (a) compressive modulus.A universal testing machine (Zwick 1476) is employed to determine
the compressive modulus of the composite.
2.2 (b ) Magnetostriction The strain developed in the material is determined using a strain gauge
and whetstone bridge setup.
2.2 (c ) Magnetisation The saturation magnetization, remnant magnetic field and magnetic coercivity
are determined using walker scientific magnetometer
2.3. Transducer Fabrication
2.3 (a) Approach: NPOL has designed and developed Flextensional Transducers (FT) using
peizoceramic transduction material. The effort was to fabricate flextensional transducer using the
fabricated magnetostricitve composite material. FT consists of an elliptical aluminium shell,
encapsulated with neoprene rubber.
The main consideration for designing the driver using the GM material was the the space available in
the shell. Composite rods of 8 mm diameter and 28 mm length were fabricated as described above.
The conceptual sketch of the arrangement of the rods is shown in Fig. 1. The numbered items are (1)
magentostricitve composite rods 8 mm dia, 28 mm long [6 Nos.] (2) Tablet-type 20 mm dia, 10 mm
thick Samarium-Cobalt permanent magnets [12 Nos.] (3) 375 turns of 22 SWG Solenoids [6 Nos.]
(4) 30 mm dia, 27.5 mm long aluminum bobbins [6 Nos.] (5) 8 mm thick brass magnetic de-coupler
(6) I mm MS outer frame for magnetic flex return path (7) Aluminum shell [Elliptical cylinder]. The
solenoids are connected in parallel and designed to be powered by AC current with a current
limitation of 5 amp.). A compressive mechanical pre-stressing is given to the drive by elastic spring
of the flextensional shell. For this the total length of the drive module taken is 0.5 mm more than the
major axis of the shell. The drive module is inserted in the shell by mechanically pressing the shell in
the minor axis. Once the drive module is inserted the pressing is removed and the shell is loaded
against the drive and this will give sufficient pre-stress.
2.3(b) Design of magnetic circuit
Magnetic circuit was designed and analysed using a FE modeling software Vizimag R . The input
parameters are enlisted in table 1.On the basis of modeling a most suitable schemes is arrived at,
and the plots of which is shown in Fig. 2 (a) and (b). Magnetostricitive composite material drive is
shown in Fig. 3 and that of fabricated FT with is shown in Fig. 4.
2.3 (c) Transducer measurements
Measurements are made in the open water tank, keeping the transducer at a depth of 10 m. The sound
projected by the transducer is picked up by a standard B& K hydrophone. A similar FT, with
monolithic Terfenol-d rod drive was also subjected to measurements using the same set-up and the