Analysis of syntactic foam – GFRP sandwich composites for flexural loads
The use of glass microballoon (GMB) — epoxy syntactic foams as a sandwich core material is studied. The skins and foam core are fabricated and joined instantaneously unlike the procedures followed in the previous studies. Each successive layer of the sandwich is fabricated when the previous layer is in a semi-gelled state. These sandwich samples are characterized for their properties under flexural loading. The failure modes and mechanical properties are carefully investigated. The change in fabrication technique results in a significant increase in the load bearing pattern of the sandwich. In earlier studies, debonding was found to occur prematurely since the bonding between the skins and core is the weakest plane. Using the current technique, core cracking occurs first, followed by skin fiber breaking and debonding happens at the end. This ensures that the load carrying phase of the structure is extended considerably. The sandwich is also analytically studied using Reddy’s higher order shear deformation theory. A higher order theory is selected as the sandwich can no longer be considered as a thin beam and thus shear effects also need to be considered in addition to bending effects.
Hierarchical levels of organization of the Brazil nut mesocarp
Aiming to understand Nature´s strategies that inspire new composite materials, the hierarchical levels of organization of the Brazil nut ( Bertholletia excelsa ) mesocarp were investigated. Optical microscopy, scanning electron microscopy (SEM), microtomography (MicroCT) and small-angle X-ray scattering (SAXS) were used to deeply describe the cellular and fibrillary levels of organization. The mesocarp is the middle layer of the fruit which has developed several strategies to avoid its opening and protect its seed. Fibers have a different orientation in the three layers of the mesocarp, what reduces the anisotropy of the structure. Sclereids cells with thick cell walls fill the spaces between the fibers resembling a foam-filled structural composite. The mesocarp has several tubular channels and fractured surfaces which may work as sites for crack trapping and increase toughness. The thick and lignified cell wall of sclereids and fibers and the weak interface between cells can promote a longer and tortuous intercellular crack path. Additionally, fibers with high strength and stiffness due to microfibrils oriented along the main cell axis (µ = 0° to 17°) were identified in the innermost layer of the mesocarp. Such an understanding of each hierarchical level can inspire the development of new cellular composites with improved mechanical behavior
Simulation of porosity by microballoon dispersion in epoxy and urethane: mechanical measurements and models
Effective mechanical properties of microballoon-dispersed epoxy and urethane are studied under quasi-static and dynamic loading conditions. Elastic modulus measurements of these mixtures over a volume fraction range of 0–0.45 are in good agreement with Hashin-Shtrikman lower-bound predictions for two-phase mixtures comprising of randomly distributed spherical pores in an elastic matrix. The measurements have also been predicted accurately by a LEFM based pore-flaw model for a selected flaw size to pore size ratio. These imply that the microballoons offer negligible reinforcement due to extremely small wall thickness to diameter ratio. Accordingly, feasibility of using these materials to simulate controlled porosity for tensile strength and fracture toughness modeling is explored. Measured tensile strength and fracture toughness values decrease monotonically similar to the Young's modulus variation with volume fraction of microballoons. Guided by the measurements linear elastic models for porous materials that predict tensile strength and fracture toughness of these mixtures are proposed and validated. The tensile strength predictions are in very good agreement with measurements for both epoxy and urethane compositions. The quasi-static crack initiation toughness prediction captures the measurement trends rather well in both cases. The agreement between the measurements and predictions are modest for epoxy matrix while they are good for urethane compositions. Based on fracture surface micrography, an empirical corrective procedure is advanced to improve the agreement between the measurements and the model. The dynamic crack initiation toughness measurements for epoxy, on the other hand, are in excellent agreement with the predictions.