Nondestructive testing of defects in adhesive joints

esistance [2]. For converting it to an acoustic transparent material for using in encapsulation, we
need to match the acoustic impedance (product of density and velocity of sound through the
material) with seawater [3]. The present study focuses on effects of filler loading on acoustic
parameters and its physico-mechanical properties.
Experimental
Materials
The materials used for this study are EPDM rubber-Nordel IP4520 procured from Dow
Chemicals. Compounding ingredients like TMTD, MBT from ICI and ZnO, Stearic acid, sulphur,
Silica, Precipitated CaCO3, Talc, TiO2, Naphthenic Oil etc are sourced from local market.
Compositions
To study the effect of filler loading on acoustic properties such as Insertion loss and Echo
reduction, a series of compositions were made from EPDM rubber as given in Table-1.
NORDEL IP 4520 Medium diene Hydrocarbon Rubber is an amorphous ethylene-propylenediene
terpolymer with Low molecular weight and Good low temperature properties [4]. This was
selected with view to cater encapsulation by injection or transfer molding technique.
All compositions were made by mill mixing in a lab model two-roll mill. Cure characteristics
were studied using rheometer MDR-2000 at 150 o C and 160 o C temperatures [5]. To study the
reversion characteristics specimens were run for 60 minutes, 120 minutes and 180 minutes. The
cure characteristics of optimized compositions were reported in Table-2.
Acoustic Measurements
Windows, reflectors, baffles and anechoic coatings constitute a complete set of materials
insofar their function is concerned. Ideal specimens of the four materials will, respectively
transmit 100%, reflect 100%, transmit 0% and reflect 0% of the sound incident on them [1]. Each
of these four materials can be evaluated by measuring the percentage of sound transmitted
through and reflected from the material when a sample is immersed in water. These two
measurements are called “insertion loss” and “echo reduction,” are defined by
Insertion loss=20 log (Incident sound pressure / Transmitted sound pressure)
Echo reduction=20 log (Incident sound pressure/ Reflected sound pressure)
For studying Insertion loss and Echo reduction cylindrical specimens of 50mm diameter
and 200mm diameter with a height of 20mm were made by compression moulding. Echo
reduction behavior for 3 kHz to 15 kHz was studied using 50mm diameter specimen in vertical
pulse tube with standard projectors and hydrophone at bottom. Echo reduction and Insertion loss
behavior for the frequency range 1 kHz to 4 kHz was studied in vertical low frequency pulse tube
using 200mm diameter specimen. For Insertion loss specimen was submerged in water in the
pulse tube with projector and hydrophone located opposite sides of specimen. Mechanical
properties were evaluated from standard specimens punched out from moulded test slabs of
150mm x 150mm x 2mm. For compression set study standard moulded cylinders were used. All
properties were evaluated according to the relevant ASTM standards [6-7].
Short-term water absorption characteristics was studied for all specimens and reported in
table-3. Long-term water absorption carried out for 260 days and the values obtained were plotted
in Fig-1.