MULTI-SCALE CHARACTERIZATION AND MODELLING OF NANOSILICA REINFORCED CARBON/EPOXY COMPOSITES FOR FILAMENT WOUND STRUCTURES

Open Access
- Author:
- Vashisth, Aniruddh
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 04, 2017
- Committee Members:
- Dr. Charles E. Bakis, Dissertation Advisor/Co-Advisor
Dr. Charles E. Bakis, Committee Chair/Co-Chair
Dr. Clifford J. Lissenden, Committee Member
Dr. Kevin Koudela, Committee Member
Dr. Adri van Duin, Outside Member
Dr. Thomas Juska, Committee Member - Keywords:
- Ballistic impact
carbon epoxy composite
molecular dynamics
filament winding
nanosilica - Abstract:
- Fiber reinforced composite driveshaft are seen as an attractive replacement for metallic driveshaft in helicopters. Rigid matrix composite (RMC) are composites reinforced with a strong fiber such as carbon fibers in a rigid matrix such as an epoxy. Matrix plays an important role in the determining the properties of the fiber composites, for instance mechanical and thermal properties can be tailored by changing molecular structure of the polymer matrix or incorporating nanoparticles in the matrix. The objective of this research is to evaluate, experimentally and analytically, at multiple length scales, the effects of adding spherical nanosilica (NS) particles on the thermomechanical properties of carbon/epoxy composites. Starting from a molecular scale, a novel thermodynamically fundamental mechanism is developed that simulates the cross-linking of epoxide and an aromatic amine (DETDA) curing agent. Good agreement between the simulations and the experimental results were seen for tensile modulus, glass transition temperature, and mass density. The complete simulation procedure, including cross-linking, equilibration and testing of the polymer, was carried out using reactive force fields. Epoxies cured with three different curing agents (3,3’-diaminodiphenylsulfone (DDS), 4-4’-DDS, and DETDA) were included in all the experiments. Epoxies with variable concentrations of NS were mechanically characterized for modulus, fracture toughness, density, and glass transition temperature. The addition of NS improved the modulus and fracture toughness, although it also decreased the glass transition temperature slightly, possibly due to the diluent component of the master batch of NS-filled bis F. This investigation provides, for the first time, a full set of 2-D lamina properties of T700 carbon fiber composites with 0 wt % NS (0NS) and 37 wt % NS (37NS) cured with DETDA. Equations for predicting the 2D properties as functions of NS and carbon fiber contents were developed using micromechanics principals and backed-out properties of T700 fibers. At the lamina level, NS increased the longitudinal and transverse compression strengths and descreased the transverse coefficient of thermal expansion, which are beneficial for overall composite performance. Using these predictive equations, a simple design exercise was conducted for [±θ]5 carbon/epoxy driveshafts with and without NS, considering strength, buckling, torsional stiffness, and whirl. The use of NS improved the safety factors for strength, buckling, and torsional stiffness for all ply angles. However, using NS increased the weight of the shaft and consequently reduced the safety factor for whirl at ply angles less than approximately 60 degrees, which is the realm where NS does not significantly increase the bending stiffness of the shaft due to the dominating effect of the fibers. Tubular specimens with 0, 15, 25 and 34 wt % NS cured with 4,4’DDS and 0 and 37 wt % NS cured with DETDA were ballistically impacted at normal incidence and evaluated for damage area and residual torsional strength. This is the first study on NS-filled composites with ballistic impact where the projectile punctures through the composite. Incoporating NS into the composite decreased the projected damage area, increased the residual torsional strength, and increased the energy absorption per unit projected damage area. Specimens manufactured with 15NS/DDS and 25NS/DDS exhibited the highest residual strengths. An analytical model was developed to calculate the damage area and residual projectile velocity after normal-incidence impact. This model is an extension of a previous energy-based model for cross-ply laminates, with original modifications developed for angle plies and dual walls. Good agreement between the experimental and analytical residual velocities was obtained. Analytical and experimental damage areas display a similar trend with NS content, but the analytical damage areas were always less than the experimental areas. Tubular specimens with 0 and 15 wt % NS with DETDA curing agent were obliquely impacted with a ballistic projectile and subjected to residual torsional strength testing. No significant change in damage area or residual torsional strength was seen in these experiments, indicating a negligible effect of NS in an impact scenario where the oblique path of the projectile, rather than the toughness of the composite, is the main factor affecting the size of the damage zone.