| Περίληψη: | In the present thesis, the mechanical behavior of multifunctional materials containing MWCNTs and flame retardants POSS compounds as well as of self-healing materials containing microcapsules has been investigated. Regarding the multifunctional materials containing MWCNTs and flame retardants POSS compounds, numerous mechanical tests have been performed, namely several types of quasi-static test and specifically tension, compression, three-point bending, GIC fracture toughness as well as dynamic tests of fatigue and CAI. Furthermore, a multi-scale model simulating the effect of the dispersion, the waviness, the interphase as well as the agglomerations of MWCNTs on the Young’s modulus of a polymer filled with 0.4% MWCNTs (v/v) has been developed. Representative Unit Cells (RUCs) have been utilized to determine the homogenized elastic properties of the hybrid material, which have been assigned to the Finite Element (FE) model of a tension specimen in order to predict the Young’s modulus of the filled material. Regarding the self-healing materials, the effect of embedded self-healing microcapsules on the ILSS behavior of carbon fiber reinforced composite materials has been studied and the self-healing efficiency has been assessed.
The materials under consideration are listed below:
Multifunctional materials enhanced with MWCNTs and flame retardants
Polymers enhanced with MWCNTs
Polymers enhanced with MWCNTs and GPOSS/DPHPOSS
Composite enhanced with MWCNTs and GPOSS
Multifunctional materials enhanced with self-healing microcapsules
Composites enhanced with microcapsules and catalyst
Regarding the multifunctional materials enhanced with MWCNTs, the results showed a significant increase in the tensile strength. On the other hand, all other properties investigated, namely compression, flexural as well as GIC fracture toughness properties were degraded. This may be attributed to the main reinforcement mechanism of CNTs, namely the pull-out mechanism which takes place during tension. As far as the fatigue testing is concerned, a slight decrease in the fatigue life has been observed in the range of the low stress levels; however, it gradually tends to coincide with the reference material resulting in a nearly same fatigue limit. The incorporation of the flame retardants GPOSS/DPHPOSS into the polymer further deteriorates the mechanical behavior of the filled material. However, GPOSS seems to fulfil the criteria as an effective flame retardant for polymers as compared to DPHPOSS.
Concerning the CAI testing, the results obtained from C-Scan analysis have shown a significant increase of the damaged area after the impact tests for the composite materials enhanced with MWCNTs as well as flame retardant, GPOSS. As a consequence, a reduced compression after impact strength has been found.SEM analysis has revealed MWCNTs agglomerations, while EDS analysis has revealed some areas of incomplete dissolution of the flame retardants GPOSS/DPHPOSS into the resin. The results underline the sensitivity of the mechanical behavior of the multifunctional polymers on the dispersion features of the additives and the significance of both, CNT agglomerates and GPOSS aggregates for the observed mechanical behavior.
Furthermore, a multi-scale model simulating the effect of the dispersion, the waviness, the interphase as well as the agglomerations of MWCNTs on the Young’s modulus of a polymer enhanced with 0.4% MWCNTs (v/v) has been developed. Representative Unit Cells (RUCs) have been employed for the determination of the homogenized elastic properties of the MWCNT/polymer. A comparison with experimental results obtained by tensile testing has been made. As the CNT agglomerates increase, the results showed a significant decrease of the Young’s modulus regarding the polymer filled with aligned MWCNTs while only slight differences on the Young’s modulus have been found in the case of randomly-oriented MWCNTs. This might be attributed to the low concentration of the MWCNTs (0.4% (v/v)) into the polymer. For low MWCNTs concentrations, the interphase seems to have negligible effect on the Young’s modulus. Furthermore, as the MWCNTs waviness increases, a remarkable decrease of the Young’s modulus of the polymer filled with aligned MWCNTs is observed. The experimental results in terms of the Young’s modulus are predicted well by assuming a random dispersion of MWCNTs into the polymer.
Regarding the multifunctional materials enhanced with self-healing microcapsules, the results indicate a general trend of a degraded mechanical behavior of the enhanced materials, as the microcapsules exhibit a non-uniform dispersion and form agglomerations which act as internal defects. A remarkable value of the self-healing efficiency has been found for materials with limited damage, e.g. matrix micro-cracks. However, for significant damage, in terms of large matrix cracks and delaminations as well as fiber breakages, the self-healing efficiency is limited. However, further investigation should be conducted in order to provide definite evidence.
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