Nondestructive testing of defects in adhesive joints

Results and Discussion
The cure characteristics of the optimised compositions are given in table-2. As filler
loading increases scorch safety (ts2) decreases from 9 minutes to 2 minutes.
All the trends generally observed in the rubber compounds are also seen here. Energy dissipation
capacity of the compound can alter by filler loading, as it is seen in the table. However, as the
filler loading level increases the processability and mould flow characteristics seem to be
decreased.
The physico-mechanical properties evaluated are presented in table-3. As observed
generally with rubber compounds, higher the filler loading greater the density, hardness,
compression set and tear strength etc with corresponding decrease in elongation at break. The
maximum tensile strength in the talc alone series (E1-E5) was for E4 composition having 200phr
filler loading. This may be due to the reinforcing effect of talc at that particular loading. For E6
the increased tensile strength was due to silica reinforcement. For E5 and E6, the elongation at
break fall below 100% due excessive polymer matrix dilution. Water absorption for 24 hours was
lowest for E4 (0.0247%) due to the highest level of exfoliation occurring because of optimum talc
reinforcement.
Long-term water absorption characteristics of compositions are studied and plotted in
figure-1. Compounds E3, E4 and E5 show the lower percentage of water absorption even after
immersion for 259 days. The blank compound EB, shows a decreasing pattern due to the leaching
of organic compounding ingredients [8]. Compound E6 is having higher percentage of water
intake due to the presence of additional filler loading of precipitated CaCO3 and Silica.
Generally all the EPDM compounds are acoustically transparent however, from the
figure-2, it is seen that acoustic insertion loss of compound E4 and E5 are negligible to the extent
of 0.02dB in the frequency range of 0.5 kHz to 4 kHz. These compounds can be good candidates
for the developing acoustically transparent encapsulating material in the above frequency range.
The echo reduction behaviour of compounds in the frequency range of 3 kHz to 15 kHz
evaluated using 50mm dia pulse tube specimens are plotted in figure-2. It is evident from the plot
that all compounds except E3 and E4 are reflecting acoustic energy back to the source with
almost negligible loss. This means that, compounds E2, E5 and E6 are acoustically transparent in
this frequency range due to better acoustic impedance match. Compounds E3 and E4 can be used
for anechoic liner application due increased echo reduction.
Echo reduction pattern of 200mm dia pulse tube specimens in the frequency range of 0.5
kHz to 4 kHz are shown in fig-4. Here similar behaviour as observed in fig-3 is obtained.
Compounds E5 and E6 are less reflecting than E2, E3 and E4. Hence, E5 and E6 could be used
for application where acoustic transparency is required.
Conclusions
Generally, all EPDM compositions are found to be acoustically transparent. Acoustic
properties are found to be frequency dependent. However, for obtaining maximum underwater
acoustic transparency, compounding with reinforcing filler like talc at 200-phr level could be a
better choice with increased water resistance. The composition E4 and E5 can be used as an
underwater encapsulant where acoustic transparency is required. The adhesion of EPDM
compositions has to be studied and modified for better interfacing with metals and ceramics for
device fabrication [9].
Acknowledgements
Authors wish to acknowledge the constant inspiration of Shri. Vishnubhatla RMR,
Associate Director, NPOL. We gratefully acknowledge the Director, NPOL for granting
permission to publish this work.