Characterization and Modeling of Strength Enhancement Mechanisms in Composite Materials at the Nanoscale

摘要

The compressive failure of continuous fiber reinforced composites has received considerable attention in the recent years because it typically occurs at a stress level that is 40-50 % below the tensile strength of the composite. This paper is primarily focused on enhancing the compressive strength and stiffness of unidirectional polymer matrix composites (PMC) through a series of modifications in matrix composition using nanoparticle reinforcement, and by optimizing thermoplastic pultrusion manufacturing process variables. Low-cost commodity resins such as polypropylene (PP), suffer primarily due to low compressive strength. Enhancement of the compressive strength of pultruded thermoplastic composites can be achieved by improving the yield strength of the surrounding matrix in shear and reducing fiber misalignment in the composite through optimization of manufacturing process variables. Increase in matrix yield strength is obtained through the use of uniformly dispersed surfactant-modified nanoclay platelets, and decreasing fiber misalignment concurrently, thereby making use of positive synergies that may exist between these effects. A single-screw extruder was used to mechanically facilitate the dispersion. This new family of materials exhibits enhanced compressive stiffness and strength of the matrix material, by including exfoliated nano-scale montmorillonite particles in the fabrication of resin pre-impregnated (prepreg) glass fiber filaments. Preliminary test results on E-Glass/PP/MMT pultruded samples indicate approximately two-fold increase in modulus and strength with only 5% nanoclay loading, without adversely affecting PP resin viscosity. Scanning Electron Micrographs (SEM) was taken to examine the failure surfaces. Transmission Electron Microscopy (TEM) revealed all platelet morphologies, namely exfoliated, intercalated and stacked structures within the samples. Dramatic improvement in compressive strength (124%) and compressive modulus (110%) were observed with relatively low nanoclay loadings.Compression tests were also performed on E-Glass/PP/MMT specimens that have been aged for 15 months at room temperature. The effect of aging was found to be benign for low loadings of nanoparticle. However, at higher nanoparticle loadings, a significant decline in compression strength was observed in the aged specimens. Effect of nanoparticle loading on fracture toughness of neat polypropylene was studied using three-point bending of notched beam specimens. The results indicate an optimum in fracture toughness of PP occurring for a clay loading of 4 weight %, followed by a sharp decline with further increase in clay loading.