While performing indentation test on neat epoxy specimens at ambient temperature (30ºC) and elevated temperatures until up to glass transition temperature of epoxy resin (50 ºC, 70 ºC, and 90ºC) I noticed two different type of damage modes. One among the damage mode was evident; it was matrix cracking at ambient temperature. But there a failure on the specimens while exposing to higher temp, but not matrix cracking. Please help me in this regard.
At glass transition temperature the storage modulus suddenly start dropping and there is large segmental mobility of matrix generating chain to chain friction. Under this condition the additional load might have caused failure
I also agree somewhat with Asit Baran that due to frictional heat, more thermal stress might have developed, due to which this may happen.
At or near the glass transition temperature, failure modes, matrix-matrix interactions, changes in storage modulus, chain-to-chain mobility, segmental mobility, etc. are essentially entwined with the glass transition process. I am unsure how they could be decoupled.
M Talha Junaid request is important to discuss this matter. As the temperature increases, the matrix might be soften due to the reason given by Olagoke Olabisi and Mr Asit Baran Samui. Therefore, you may not see the cracking (maybe only micro cracking at certain temp.) because the epoxy just elongate (what is the difference elongation between the ambient and the others, I presumed they are different between the sample and longer for the higher temp.). As for the ambient temp. the sample the movement of polymer chains are more rigid and this might cause the sudden breaking in chain (thus creating cracking) when subjected to higher loading
The indentation test is the worst experimental procedure to ascertain the failure mode of materials. It is a usefull test to evaluate the effects of heat and mechanical treatments and eventually correlate the resulting hardness with the strength of ordinary materials.
Concerning polymers (including thermosetting resins) it must be mentioned that if the indentation rate is fast enough , the brittleness of the resin will emerge even if the Test temperature is close to the Tg. At the same time at lower temperatures, if the the indentation rate is low enough a yielding-type failure will be observed (namely , a permanenent deformation). In all, the response of polymers depend on the timescale of the mechanical perturbation and the window of relaxation/retardation spectrum where the characteristic time of the perturbation falls in.
if you are still interested please look at my publication list. decades ago I did a lot in microhardness testing with polymers. May be you will find some useful answers to your question. much success.
Really, for the thorough analysis of your observations the comparison of usial static tests and contact indentation of epoxy specimens will be fruitful. Once again, the remarks noted by Asit Baran Samui, Alberto D'Amore and other colleagues are seemed very correct and following their answers I try to help you relating to my publication list in field of contact mechanics including the paper "Estimation of Properties of Elastomers by Indentation with a Constant Rate".
All the best