Environmental Durability of Multiscale Glass Fiber/Epoxy Composites: An Assessment On Mechanical Properties and Microstructural Evaluation

Abstract

Fiber reinforced polymer (FRP) composites are nowadays potentially used all around the globe as these are capable of substituting the conventional materials for applications starting from mini toys to large aerospace components. It is thus necessary and crucial to examine the durability, reliability and sustainability of these materials. The present exploration is based on the environmental durability of glass fiber/epoxy (GE) and nanofiller (nano-Al2O3, nano-TiO2) enhanced GE composites. The environmental durability of the composites was studied for their mechanical and thermal performance after exposure to high temperature, liquid nitrogen conditioning and thermal shock.
The tensile properties were studied at 50% and 70% volume fractions of reinforcement at different crosshead speeds viz. 1, 10, 100, 500, and 1000 mm/min at 25°C, 70°C, 90°C and 110°C temperatures. The effects of nano-filler (nano-Al2O3, nano-TiO2) incorporation on the mechanical and thermal behavior of GE composites at various loading rates were also carried out. The flexural tests of the nano-filler enhanced composites were studied at different elevated temperatures. The properties enhancement/alteration of nano-fillers embedded polymeric composites appears to be in-service temperature-sensitive phenomenon. The thermal properties of the composites were evaluated using dynamic mechanical thermal analyser (DMTA) and temperature modulated differential scanning calorimetry (TMDSC), chemical analyses by Fourier transformation infrared (FTIR) spectroscopy, fractography analyses were carried out using field emission scanning electron microscopy (FESEM) and morphology of the nano-fillers were studied by transmission electron microscopy (TEM).
The tensile strengths of the investigated GE composites increase with the increase in crosshead speed at all test temperatures. At higher crosshead speed the response of the composite is primarily governed from the fiber phase and increase in load-carrying capacity can be attributed to fiber dominated mechanical response. Liquid nitrogen (LN2) conditioning for 0.25 h and 1 h caused an improvement of tensile strength up to 3.33% and 7.3% respectively as compared to unconditioned GE composites. The percentage strength improvement was high (up to ~12%) when the specimens were tested at 1000 mm/min loading rate.
The thermal-shocked specimens also exhibited higher Ultimate Tensile Strength (UTS) as compared to the unconditioned specimens. It can be stated that matrix hardening and residual stress generation during the conditionings would generally govern to produce a higher load-carrying capacity of the composites.
Addition of 0.1 wt.% nano-Al2O3 particles exhibited an improvement in the strength of nano-Al2O3/GE composites at all crosshead speeds. Similarly, 0.3 wt.% nano-TiO2 enhanced composites showed maximum strength improvement. Exposure to elevated temperatures deteriorated the flexural properties of GE as well as nano-Al2O3 modified composites, as expected. However, the extent of degradation remained higher in case of nano-Al2O3 modified composites. Exposure to elevated temperatures improves the flexural properties in case of 0.3 wt.% nano-TiO2/GE modified composites as compared to the RT specimens. The increase in strength of the nano-filler enhanced composites was attributed to effective stress transfer from the matrix to fiber through well-dispersed and well-bonded fibre-matrix interfacial regions. The FESEM results on fracture surface morphology indicated crack bridging and good fibre-matrix interfacial bonding. On the other hand, agglomeration of nano-fillers took place at higher concentration of nanoparticles which showed poor strength. Exposure to high temperature would cause damage and degradation in the polymer phase of the composites revealed as matrix cracking, riverline markings, cusps, the flow of matrix, delamination of fiber from the matrix, fiber imprints etc. The DMTA results were correlated with the mechanical and thermo-mechanical behavior of the FRP composites. Addition of nano-fillers caused a reduction in the glass transition temperature. Finally, the Weibull design parameters were analyzed as a function of nano-Al2O3 content and different test temperatures. Weibull analyses responded a reasonable agreement with the experimental results.