An Assessment of Mechanical Behavior of Fibrous Polymeric Composites Under Different Loading Speeds at Above- And Sub-Ambient Temperatures

Abstract

Advanced fibrous polymeric composites are one of the most successful composite material systems due to its wide range of advantages such as high specific strength and stiffness, fatigue properties and corrosion resistance. Composite structures undergo different loading conditions i.e. from static to dynamic during their service life. During a cruise cycle an aircraft structure undergo different temperatures starting from ambient temperature on ground to during flight at 30,000 ft (-50˚C) and +50˚C during stays at the tropic and arid places. The polymer matrix is more susceptible to these changes than the fiber and thus dominates the mechanical behavior of FRP composites. Polymers are characterized as visco-elastic materials that their mechanical properties are strain rate dependent or they are called as sensitive to the rate at which loaded. The present experimental investigation uses flexural test to assess the effects of thermal conditioning at above- (50˚C) and below- (-50˚C) ambient temperature for multilayered laminates of 60 weight percentages of silane treated E-Glass fiber/epoxy composites and also with PAN based high strength epoxy compatible carbon fiber/epoxy composites. The state of interaction between the fiber and matrix was reflected in the ILSS values measured by 3 point-bend test with an Instron tensile testing machine with five increasing crosshead speed ranging from 1, 10,100,200 and 500 mm/min. Thermal conditioning at +50˚C is to induct further polymerization process in terms of epoxy embrittlement and along with the development of penetrating and/or semi penetrating network at the fiber/matrix interface. Whereas at −50˚C, the polymer chains get frozen due to which the deformation process is reduced results in less polymer relaxation i.e. it gets hardened. At higher crosshead speed due to shorter load assisted relaxation time, there is reduction in ILSS. The polymer gets more time for relaxation at lower crosshead speeds; as a result there is enhancement of ILSS values. The failure mechanisms are changing with changing in loading rate from static to dynamic. Fracture processes at the crack tip are controlled by thermal relaxation time and mechanical relaxation. At higher strain rates the heat generation was much faster than heat removal due to quasi-adiabatic heating which increases the fracture strain. In both the systems the locus of failure will shift from fiber polymer interface to the matrix itself that means instead of adhesion failure the predominating failure may be cohesive failure and that too shear cusp formation. FTIR analysis depicts that the band at 2609 cm-1 in the spectrum of carbon/epoxy composite can’t be seen properly in the spectra of the glass/epoxy systems. Carbon fiber may react with the OH groups which supports that the ILSS values are higher for CFRP than GFRP. DSC analysis shows an increase in glass transition temperature (Tg) after thermal conditioning for glass/epoxy composites. But for carbon /epoxy systems due to strong adhesion between the fiber and matrix Tg value is more as compared to glass/epoxy systems. But with increase in thermal conditioning time the Tg decreases due to the breakage of secondary bonds. AFM surface topography reveals that fiber/matrix height difference gradually increased with the increase of thermal treatment time at 50˚C for 5 hours suggested that residual stresses are developed due to this shrinkage. Implication of thermal conditioning most often lead an improved adhesion of the interface (at above ambient) and/ or increased crack density (at below ambient) temperature. These changes might lead further complications in accessing the loading rate sensitivity which itself as contradictory as on today.