Effect of Seawater and Temperature Upon Carbon Fiber/Vinyl Ester (CFVE) Composites and its Constituents

An experimental and computational investigation has been carried out to: (1) develop micromechanical characterization techniques for polymeric composites and (2) evaluate the effects of seawater and temperature upon the micro- and macro-response of the carbon fiber vinyl ester composites (CF/VE). Compression properties degradation due to short-term and long-term exposure to seawater is evaluated. A 12-16 % reduction in compression strength is observed due to long-term exposure to seawater, which is attributed to a decrease in interfacial shear strength (IFSS) due to long-term exposure to seawater. The developed Push-out methodology is used to characterize the IFSS. The presented approach consistently yields interfacial failure. However, the failure is found to be thickness dependent. Therefore, a Weibull-based methodology is adopted to quantify the degradation in IFSS for a given interfacial volume.

Subsequently, in Chapter 4, an energy-based parameter, R, is introduced to characterize the local crystal structure of carbon fiber, which is shown to be valid for an extensive range of standard to high-modulus PAN carbon fibers. The presented technique offers spatially resolved qualitative estimation of the crystal parameters. Chapter 5 presents a nanoindentation-based methodology to evaluate the Mode-I fracture toughness using the indentation-based load, the crack length, and the parameterobtained using finite element modeling. It is found that the fracture toughness of carbon fiber decreases as the modulus of the PAN-based carbon fibers increases.

Finally, in Chapter 6, the developed techniques are used for micromechanical characterization of CFVE exposed to extreme long-term sea water (15 years) and extreme temperature temperatures (±80 ºC) conditioning. It is found that high temperature (80 ºC) exposure causes cracking, an increase in glass transition (T_g), and an increase in IFSS. On the other hand, low temperature (-80 ºC) causes a decrease in IFSS while the T_g remains unaffected. For wet-aged coupons, the crack density tends to increase while the residual stresses tend to decrease. It is concluded that as the consequence of thermal aging and an increase in IFSS, short-term exposure at annealing temperatures can cause an improvement in the matrix-dominated fatigue response of CF/VE.