Mechanical behavior characterization and modeling of vinyl ester and its carbon nanofiber composite including strain rate and temperature effects

In this study mechanical behaviors and modeling of vinyl ester and its carbon nanofiber reinforced composites were studied. Pyrograf III produced by Applied Sciences as a low cost carbon nanofiber was used as the fiber material. When small amounts of these carbon nanofibers (0.5 wt%) are combined with thermoset vinyl ester with good fiber-matrix adhesion, stiffness of the resulting composite can improve. Modulus of elasticity can improve further up to 19% with surface treatment (functionalization) of carbon nanofibers.;Preliminary tensile tests on the vinyl ester polymer were conducted to analyze the effects of matrix composition and curing systems. The composition of 55%VE-45%St with a suitable viscosity for nanofiber addition was selected. Curing at room temperature and step post curing on the plaques with no secondary post curing on the specimens were also found the most suitable curing procedures for vinyl ester.;Tensile, flexure, creep, and fatigue tests were conducted on vinyl ester and its carbon nanofiber composite to analyze the effects of temperature, humidity, strain rate, loading condition, nanofiber surface treatment, and nanofiber content on their mechanical properties. Tensile strength and modulus of elasticity of these materials were found to increase while elongation (i.e. ductility) decreased linearly with increasing strain rate and decreasing temperature. Humidity absorption also decreased the mechanical properties, especially the strength of these materials at RT.;A physical model (Standard Linear Solid model) was used to represent the stress-strain behavior of vinyl ester over a wide range of strain rates and temperatures. This model was used to predict the stress-relaxation and short term creep responses of this material, as well. The model showed reasonable agreements with experimental data. Ramberg-Osgood and Menges models were also used to represent tensile stress-strain and deformation behaviors of the vinyl ester and nanocomposite. The Menges model provided better representation of the observed behavior than the Ramberg-Osgood model for a wide range of temperatures and strain rates.;An analytical power-law relationship used to predict the creep deformation behavior of both materials showed good prediction of creep strain versus time, especially at lower temperatures and for lower applied stresses. A three-parameter Findley-type creep law was also used to predict the creep compliance of these materials. For nanocomposite, good correlations were found between experimental data and predictions at all temperatures.;Fatigue and cyclic deformation behaviors of both vinyl ester and nanocomposite were obtained by performing tension-tension cyclic tests at different constant stress amplitude levels. Both materials showed steady-state cyclic deformation condition and strains were predominantly elastic even at the high strain amplitudes. Statistical analysis of the fatigue data was performed due to large scatter. At lower stresses the nanocomposite appeared to be more resistant to cyclic stresses than the vinyl ester. SEM was also used to analyze the fracture surfaces of the specimens after fatigue experiments.;The extensive experimental work and models presented in this work for vinyl ester polymer and its nanocomposite can be considered new contributions to the field of mechanical behavior of these materials, as such studies were not found in the literature.