Shylaja Srihari

Hot-wet property characterisation of a high-temperature cured glass-epoxy composite in immersion environment

The hygrothermal diffusion and degradation behavior of a high-temperature cured glass–epoxy (Epoxy Novolac – EPN) composite system was studied under two environmental conditions (i.e. 323 K and 343 K immersion in distilled water). Samples immersed at 343 K showed higher diffusivity (Dc) value and lower saturation time (tm) than those of 323 K immersion while the maximum moisture content (Mm) remained the same, good Fickian correlation were observed for the composite system. As regards the degradative effects, the glass transition temperature (Tg) of the composite decreased with increased moisture content showing a maximum drop of 30 C at full saturation, while the mechanical properties (ILSS and IPS) of saturated specimens degraded upto 26 and 33% respectively. Further these mechanical properties obtained at 70 C/85%RH test condition showed good correlations with those predicted by a theoretical equation (Chamis et al. An Intergrated Theory for Predicting the Hydrothermo Mechanical Response of Advanced Composite Structural Components, Lewis Research Center, Cleveland, Ohio, NASA Technical Memorandum 73812).

Post-curing Effects on Hygrothermal Behavior of RT-cured Glass/Epoxy Composites

Studies were carried out on RT-cured glass/epoxy composite specimens, subjected to different post-cure schedules (50, 60, 70, and 85°C and durations) and immersed until saturation in distilled water at 50°C. The effects of different post-cure schedules, on the moisture diffusion characteristics of the composite were studied. Results showed that the saturation moisture levels decreased with increased post curing, a trend attributed to increased matrix cross-linking, as evidenced by the increase in Tg (glass transition temperature) values with extent of post cure. The diffusion parameter-like composite diffusion coefficient (Dc) and time of saturation (t m) showed either insignificant or marginal changes, within the range of post-cure schedule considered in these studies.

Investigations on tensile creep of CNT-epoxy shape memory polymer nanocomposites

In this work, epoxy-based shape memory polymer nanocomposites (SMPnCs) were prepared using 0.5 wt% and 1 wt% multi walled carbon nanotubes (MWCNTs) pre-dispersed in epoxy resin. This was cured with triethylene tetraamine (TETA) curing agent. The glass transition temperatures (Tg) were determined using Advanced Rheometric Expansion System (ARES). The creep behaviour of the 0 wt% CNT SMPnC as well as the 0.5 wt% and 1 wt% MWCNT SMPnCs were studied through short term tensile creep test at different temperatures. Master curves of creep compliance were derived using a time-temperature superposition principle (TTSP) based on Williams-Landel-Ferry (WLF) equation. A Findley power-law model was used to predict the creep deformation behaviour of 0, 0.5 and 1 wt% CNT SMPnCs. Good correlations between experimental data and the predictive model were obtained for both SMPs particularly at lower temperature and above Tg.

Moisture Distribution Profiles in RT-Cured Glass/Epoxy Laminates of Different Thicknesses

Investigations were carried out on the moisture absorption behavior of RT-cured glass/epoxy composite specimens subjected to 333 K/Immersion in distilled water. Different laminate thicknesses (i.e., 0.25-2.25 mm) were studied for comparative evaluation of their moisture diffusion and distribution profiles across the thickness of the composite as a function of the exposure period. Moisture absorption data showed that the equilibrium moisture levels (Mm) remained practically the same for different laminates of the same fiber weight fraction (i.e., 0.62). Further, the times of saturation (tm) increased with the laminate thickness, while the absorption rates (slopes of the absorption curves) showed a reverse trend. It was also found that the Diffusion Coefficient (Dc) values of different thicknesses specimens ranged between 0.28 and 7.81(10 7)mm2/sec, indicating a negligible thickness effect on this diffusion parameter. The small variations of Dc (all of the order 10 7 mm2/sec) are attributed mainly to the variations in the edge areas for different thickness laminates.