FEM Investigation of Strength and Toughness in Reinforced Polymer Composites at the Micro and Nano Scales

of a dissertation at the University of Miami. Dissertation supervised by Assistant Professor Ryan Karkkainen. No. of pages in text. (144) The finite element method was used to evaluate microstructural strengthening and toughening effects in nanoparticulate reinforced polymer composites (nanocomposites) and in short aligned discontinuous fiber reinforced polymer composites. Nanoparticulate reinforcement is a well-known method of polymer toughening which can greatly expand the range of engineering applications for polymers. However, the mechanisms of nanoparticulate toughening, as well as complementary sub-micron fracture processes, are not well understood. Short, aligned, discontinuous carbon fiber reinforced thermoplastics show promise as a versatile, inexpensive material system with favorable manufacturability, but failure of the associated morphologies is also not yet well explored. In nanocomposites, two microstructural effects were considered and their relative importance to toughening investigated: the presence of a material interphase between the matrix and the nanoparticles, and sub-micron fracture process zone mechanisms which included polymer chain disentanglement, directional chain re-alignment, and consequent anisotropy. In the short aligned fiber composites, the effects of the fiber-matrix interface and of occasional fiber misalignment on strength and stiffness were quantified. To investigate the effect of a material interphase in nanocomposites on toughness, crack growth in a silica nanoparticle reinforced cyanate ester was simulated and a parametric study of interphase material properties was developed. It was found that altering the strength and stiffness of the interphase material caused changes in crack morphology, but did not completely explain the experimentally observed toughening effects of nanoparticulate reinforcement. To investigate sub-micron fracture process zone mechanisms, polymer chain disentanglement, directional chain re-alignment, and consequent anisotropy were considered in a dedicated user-defined material law for a structural epoxy. Crack growth through a domain of neat epoxy resin, as well as in the presence of nanoparticulate reinforcement, was modeled using the new user material. A parametric study was developed to investigate the relative importance of polymer chain disentanglement stress, energy absorbed by the highly localized inelastic deformation of chain realignment, and ultimate failure stress of loading-aligned polymer covalent bonds. To ensure the accurate representation of complex stress fields, methods herein were validated with known analytical predictions. It was shown that several of the nanoscale effects described in the new user material could be exploited to improve toughness and highlight the need to consider additional mechanisms in nanoscale fracture. The short aligned discontinuous composite material system, carbon fiber reinforced polymethyl methacrylate, was investigated using a finite element based micromechanical approach. Virtual tests were performed on highly detailed microstructural representations which included: fiber-matrix interface effects, fiber diameter, chopped fiber length, uniform fiber alignment / misalignment, fiber spacing, and the constituent properties of both fiber and matrix. Stiffness, strength, and failure mechanisms in this material system were observed and analyzed. It was shown that, without accounting for fiber misalignment, this material maintains a high modulus that is comparable to an equivalent continuous fiber reinforced polymer (139 GPa, 95% of a CFRP), and relatively high strength (2250 MPa, 80% of a CFRP). It is shown that there are two failure modes which depend on the toughness of the interface: fiber rupture or fiber pullout, and that the fiber pullout mode is generally weaker. Preliminary analyses indicated that small fractions of misaligned fibers could sharply reduce material properties. Explanations and predictions for this reduction