As per my result after 30 days of sea water immersion,The Kevlar/Epoxy laminate shows the better performance during impact.Ans also impact strength increases with aging time.I want to know weather my result is right or wrong? What is the reason behind?
You can expect that your composite my have absorbed water, and maybe a significant amount of water. Water acts as a plasticizer, making the polymer (and hence the composite) become less hard: both modulus and strength decrease, whereas elongation at break increases much. Therefore, as the material becomes less fragile (i.e. softer), I would not be surprised that it can absorb higher shocks without breaking, in other words its impact performance can be improved. But these should be related to the decrease of other properties (see above).
Alain
I agree with Alain. Water does act as a plasticizer. You should also expect a decrease in shear strength which is typically better for impact resistance. I would weigh the composite before and after sea water immersion.
Alain and Claire are right. Over time under sea water, the laminate gets better conditioned. That is moisture becomes more uniform, avoiding stresses that would otherwise cause splitting, cracking and cavitation.
As a general rule, water is detrimental to mechanical properties, in part due to moisture swellling resulting in internal stresses and in part due to the plasticisation of polymers. However, exceptions occur.
As opposed to composites with inorganic reinforcements, in Nylon (kevlar) water is absorbed in the reinforcements as well as is the resin matrix. Moreover, if I recall correctly, each Kevlar fibre is itself constituted of multiple filaments and the friction between these is in part responsible for the good impact properties of the composite. It is therefore possible that the plasticized fibers contribute to dispersing more energy by being splitted. Just an assumption...
However, if charges are not added to prevent this, it is good to remember that Nylon can rot if left wetted for long periods and this might further degrade the material engineering properties.
best regards,
Laurent
I am assuming that the Kevlar reinforcement had a soured finish to increase the adhesion between the matrix and reinforcement. If that was the case then the bond between the Kevlar and epoxy was to strong, and resulted in a degradation of the impact strength by causing the Kevlar to prematurely shear . Most ballistic Kevlar composites use a starch-oil finish to increase the abhesion (not adhesion) of the composite. This allows the reinforcement to stretch and fall in tension dissipating more energy. Your sea water may be reducing the bond strength between matrix and reinforcement allowing for a more optimum bond strength for impact resistance.
Are you evaluating a structural composite (resin rich) or impact resistant composite (resin starved)? This will effect the amount of variation in impact resistance you may incur.
If you have a American security clearance and can justify a need to know check out the following publications deal with Kevlar based armors. These may help you to explain your results:
N. Rupert, E. E. Rowe, Jr., and C. J. Hall. "DDG 993 Material Characterization Program: Ballistic Evaluations Conducted at NSWC," NSWC-TR-81-286, NSWC, July 1981.
N. Rupert and B. D. Smith. "Evaluation of Fire and Ballistic Resistance Reinforced Plastic Composite," NSWC-TR-80-184, NSWC, May 1980.
B. D. Smith, J. M. Garrison, K. A. Mussilman, and N. Rupert. "Resin Selection and Characterization of Kevlar-Reinforced Plastic (KRP) Armors," NSWC-TR-89-2, NSWC, January 1979.
B. D. Smith and N. Rupert. "FFG 7 Survivability Program Quarter-Scale Fire Test for the Torpedo Magazine," NSWC/DL-TR-3393, NSWC, December 1978.
N. Rupert. "Ballistic Characterization of Kevlar Composite Armor Systems Using Fragment Simulators," NSWC-TR-79-92, NSWC, December 1978.
Resin rich means it has a high proportion of resin as an adhesive. Resin starved means is has little or no resin as an adhesive. Resin is phenolic just as lignin, and has replaceable OH groups for moisture exchange.
Preprint On thermoelastic impact modelling of frozen composite target...