Nondestructive testing of defects in adhesive joints

Studies on EPDM based compositions for underwater acoustic
applications
Shajahan. K, Jayesh. P, Krishnaveni.K.P, Vasudevan.S and Reji John
Naval Physical & Oceanographic Laboratory
Defence Research & Development Organisation,
Kochi-682021
Email: tsonpol@vsnl.com
Abstract
Polymeric compositions are widely employed in various underwater acoustic
applications. The major uses of these composites are for packaging underwater electronic
gadgets, which cannot function if directly exposed to seawater environments. These composites
have to function as acoustic windows, reflectors, absorbers or a combination of these, depending
on the frequency of interest. In this paper, the acoustic properties of EPDM rubber formulations
are studied for its window application. A series of compositions are made by incorporating
different ingredients at various loading levels and evaluated for its acoustic parameters. All
compositions are studied for its cure characteristics at various temperatures and time using
MDR2000 rheometer. The compounds developed are found to be reversion resistant beyond its
cure conditions. Compositions ranging from 150-250 pphr reinforcement loading are found to be
transmitting acoustic energy with negligible loss. Circular discs of diameters 50mm and 200mm
with 20mm thickness are used for acoustic studies. The insertion loss and echo reduction
behaviour are studied in a vertical water filled pulse tube using standard acoustic projector cum
receiver. Physical and mechanical properties are evaluated as per ASTM standards. Results
showed that the material could be a good candidate for acoustic window application. The
negligible water absorption and high underwater stability make this material suitable for sub-sea
application. Studies showed that insertion loss of the material is 0.2dB in the range of 2 kHz to 15
kHz. The water absorption of the material is 0.002% when immersed in both fresh and saline for
24 hours at room temperature. The weight gain due to water absorption, when submerged in
water for more than 260 days is negligible.
Keywords: encapsulants, EPDM, echo-reduction, insertion loss
Introduction
Polymeric compositions are widely employed in various underwater acoustic
applications. The major uses of these composites are for packaging underwater electronic
gadgets, which cannot function if directly exposed to seawater environments. These composites
have to function as acoustic windows, reflectors, absorbers or a combination of these, depending
on the frequency of interest [1]. In this paper, the acoustic properties of EPDM rubber
formulations were studied for its window application.
Traditionally a number of polymers are used for underwater applications. Transducer
encapsulation is the area where requires maximum acoustic energy transmission property , while
an acoustic baffle or mount requires maximum sound energy absorption or isolation. Poly
chloroprene, Halogenated Butyl, Nitrile, Styrene Butadiene rubbers and Thermoplastic elastomers
are some of the candidates currently used for these applications. Each one has their merit and
demerits but will operate moderately well in their functional parameters. One of the key concerns
of these materials is their higher percentage of water absorption. With continuous exposure to sea
environments, these materials can take considerable amount of water in and related changes in
electro-acoustic parameters. These effects are become cumulative coupled with ageing factors.
In the present study, the one of the environmentally stable rubber viz. EPDM was studied
for acoustic properties. EPDM has its inherent advantages of age resistance and superior water