Mechanical Behavior Studies of Interpenetrating Polymer Network Based Hierarchical Composites Incorporating Glass Fiber, Nano-Al2O3 and Al Alloy Reinforcements

Abstract

In recent years, advanced fibrous/metal reinforced polymer laminated composites have been widely accepted in various aerospace, marine, automotive, energy, chemical, and civil industrial applications. Moreover, laminated composite materials have become competitive in structural industries, where weight saving is one of the highest priorities. The laminated composite materials replace the traditional metallic structures due to their high strength-to-weight ratio, low density, good corrosion resistance and ease of handling, which is essential for modern structural applications. However, the laminated composite material suffers from poor out of plane properties, generally due to usage of low toughened polymer matrix. Also, the weak interfacial adhesion between constituents affects the interface/interphase dominant critical mechanical properties of laminated composite materials. The modification of the matrix and pre-treatment of thin metal (aluminium) sheets could address both of these issues. In this study, following a systematic approach for developing a hierarchical composite, i.e., initially replacing the neat polymer matrix by an interpenetrating polymer network (IPN), namely epoxy-vinyl ester IPN (EVIPN) and subsequently reinforcing the same with a nanofiller (nano-Al2O3), is a novel approach to enhance the interfacial adhesion between constituents, thereby improve the laminated composites' out-of-plane mechanical performance. The present investigation starts with evaluating the mechanical behavior of glass fiber/epoxy (GE), glass fiber/vinyl ester (GVE), and glass fiber/epoxy-vinyl ester interpenetrating polymer network (GEVIPN) composites. The role of cure temperatures (140, 170, 200, and 230 °C) on the flexural behavior of all fabricated composites was examined. The results revealed that amongst various post-curing temperatures, 200 °C cure temperature resulted in optimal flexural properties for all experimented composites. The GEVIPN composite led to ~14%, ~22%, ~23%, ~32%, and ~22% improvements in flexural strength, tensile strength, interlaminar shear strength (ILSS), critical strain energy release rate during mode-I interlaminar fracture test (GIC), and mode-II ILFT (GIIC), respectively, over GE composite at optimal post cure temperature. Dynamic mechanical thermal analysis (DMTA) was conducted in the temperature range of 30 to 200 ºC to correlate all the composites' mechanical and thermo-mechanical behavior. The chemical restructuring of the GEVIPN composite was analyzed by Fourier transform infrared spectroscopy (FTIR). Fractography analysis was also performed to understand the possible failure modes of experimented composites. Further, the emphasis was given on comparing the elevated temperature flexural properties and long term creep behavior of the different composite materials (i.e., GE, GVE, and GEVIPN) has been carried out using the time-temperature superposition (TTSP) principle. At 30 °C, the GEVIPN composite showed the highest flexural properties over GE and GVE composites. The long term creep analysis test results revealed that the GEVIPN composite showed positive reinforcement efficiency for ~385 days with respect to GE and ~46.27 years with respect to GVE at 30 °C under constant stress of 40 MPa. It was observed that GEVIPN composite showed the highest creep resistance as compared to the other two composites at a lower temperature, whereas the opposite trend was observed at elevated test temperature (90 °C). Fractography analysis was done to identify the failure mechanisms and draw a comparison between the tested composites. The next objective is aimed to fabricate the multiscale glass fiber reinforced polymer (GFRP) composites via simultaneous implementation of two hybridization routes, namely, IPN formation and nanofiller (nano-Al2O3) addition, and elucidate their mechanical behavior. The results showed considerable increments in the mechanical performance of modified GEVIPN composites. Improvements of ~18%, ~8%, ~14%, ~19 %, and ~28% in flexural, tensile, ILSS, GIC, and GIIC values of nano-Al2O3 modified composites, respectively, were observed over composite without nano-Al2O3. Composite with 0.1 wt.% of nano-Al2O3 showed the highest mechanical properties in all the modes of testing except for mode-II ILFT testing, where composite with 0.4 wt.% of nano-Al2O3 showed the highest GIIC value. The synergy between the constituents and the thermal behavior of the composites were analyzed via FTIR spectroscopy and differential scanning calorimetry (DSC), respectively. Fractography was performed to understand the composites' failure modes and toughening mechanisms. Following, the effect of in-situ test temperature variation (30 °C, 60 °C, and 90 °C) on the flexural behavior of nano-Al2O3/GEVIPN composites, and the role of nano-Al2O3 content at each test temperature was also analyzed. The test results revealed that nano-Al2O3/GEVIPN composites significantly improved the mechanical degradation resistance at elevated temperatures. DMTA analysis was carried out to study the viscoelastic nature of all fabricated composites in the temperature range of 30 to 200 °C. Fractography analysis was performed to understand the underlying phenomena which dictate the mechanical performance at each test temperature. The last objective in the present study is aimed to fabricate and evaluate the mechanical performance of advanced fiber metal laminated (FMLs). It was previously learnt that simultaneous implementation of two hybridization routes namely, IPN formation and nanofiller addition is a promising route to improve the mechanical properties of polymer based composites. As 0.1 wt.% nano-Al2O3 reinforced EVIPN matrix based composite showed best amongst the experimented composites, further work was carried out to develop FML with this modified polymer as the matrix. Subsequently, to facilitate stronger interfacial adhesion between aluminium and matrix, aluminium surface was mechanically and chemically pre-treated before laminate fabrication. FML prepared by surface modified Al and 0.1 wt.% nano-Al2O3 modified IPN showed the best performance, i.e., ~23%, ~17%, ~24%, ~28 %, and ~37% in flexural, tensile, ILSS, GIC, and GIIC values over FML without any modification (i.e., no surface pre-treatment and without nano-Al2O3). Fractography validated the effect of modification methods on the micro scale phenomena dictating FML failure under various testing modes.