Composite materials

A composite 3 meter ocean current turbine blade has been designed and analyzed using Blade Element Theory (BET) and commercial Finite Element Modeling (FEM) code, ANSYS. It has been observed that using the numerical BET tool created, power production up to 141 kW is possible from a 3 bladed rotor in an ocean current of 2.5 m/s with the proposed blade design. The blade is of sandwich construction with carbon fiber skin and high density foam core. It also contains two webs made of S2-glass for added shear rigidity. Four design cases were analyzed, involving differences in hydrodynamic shape, material properties, and internal structure. Results from the linear static structural analysis revealed that the best design provides adequate stiffness and strength to produce the proposed power without any structural failure. An Eigenvalue Buckling analysis confirmed that the blade would not fail from buckling prior to overstressed laminate failure if the loading was to exceed the Safety Factor.

In this research, the degradation of carbon fiber/vinylester composites in marine environments was experimentally investigated. Additionally, two types of carbon fiber surface treatments, namely Polyhedral Oligomeric Silsesquioxane (POSS) and the industrial surface treatment F0E, were evaluated to determine their effectiveness in creating a fiber/matrix (F/M) interface for use in the marine environment. Electrochemical Impedance Spectroscopy (EIS) was explored as a new application of an existing technique for use in measuring the amount of water at the F/M interface in carbon fiber/vinylester composites. EIS spectra were used to determine equivalent electric circuit models that allow for the prediction of water at the interface. The location of water within the composite was determined through Positron Annihilation Lifetime Spectroscopy (PALS). Interlaminar shear strength and transverse tensile tests were carried out for dry conditions and after hygrothermal exposure of the composites to study the influence of the integrity of the F/M interface on the macroscopic response of the composite.

In an effort to obtain an improved mode III fracture toughness test suitable for a testing standard, mechanics analysis, experimental testing, and finite element analysis (FEA) have been conducted. Of particular concern are the merits of one-point and two-point edge crack torsion (ECT) test methods, the influence of specimen geometry that overhangs beyond load/support points, and the influence of crack length on the compliance and energy release rate. Shear stress distributions at the crack front are determined to examine the uniformity of mode III loading and mode II influence. The shear stress distributions in the one-point and two-point tests are virtually identical, indicating that either of the two tests could be used interchangeably. Based on the uniformity of the mode III shear stress distribution along the crack front, it was found that the ECT specimen should have minimum overhang. Longer crack lengths tend to produce nonuniform shear stress distributions. A modified two-point ECT test fixture was developed to allow testing of specimens with a range of dimensions. This development enabled experimental verification of the results from the FEA overhang series. The specimens with a minimum overhang produced consistant mode III toughness data. The most reliable way to reduce data is through the original compliance calibration method. A modified ECT specimen was developed with a staggered crack front to produce uniform mode III crack growth. Finite element analysis of the modified ECT specimen shows a uniform mode III stress distribution along the crack front with little mode II interaction.

A novel approach has been introduced in making flexible armor composites. Armor composites are usually made by reinforcing Kevlar fabric into the mixture of a polymer and nanoscale particles. The current procedure deviates from the traditional shear thickening fluid (STF) route and instead uses silane (amino-propyl-trimethoxy silane) as the base polymer. In addition, a cross-linking fixative such as Glutaraldehyde (Gluta) is added to the polymer to create bridges between distant pairs of amine groups present in Kevlar and silated nanoparticles. Water, silane, nanoparticles and Gluta are mixed using a homogenizer and an ultra-sonochemical technique. Subsequently, the admixture is impregnated with Kevlar - bypassing the heating and evaporating processes involved with STF. The resulting composites have shown remarkable improvement in spike resistance; at least one order higher than that of STF/Kevlar composites. The source of improvement has been traced to the formation of secondary amine C-N stretch due to the presence of Gluta.

This research focuses on carbon fiber treatment by nitric acid and 3- (trimethoxysilyl)propyl methacrylate silane, and how this affects carbon/vinyl ester composites. These composites offer great benefits, but it is difficult to bond the fiber and matrix together, and without a strong interfacial bond, composites fall short of their potential. Silanes work well with glass fiber, but do not bond directly to carbon fiber because its surface is not reactive to liquid silanes. Oxidizing surface treatments are often prescribed for improved wetting and bonding to carbon, but good results are not always achieved. Furthermore, there is the unanswered question of environmental durability. This research aimed to form a better understanding of oxidizing carbon fiber treatments, determine if silanes can be bonded to oxidized surfaces, and how these treatments affect composite strength and durability before and after seawater exposure. Nitric acid treatments on carbon fibers were found to improve their tensile strength to a constant level by smoothing surface defects and chemically modifying their surfaces by increasing carbonyl and carboxylic acid concentrations. Increasing these surface group concentrations raises fiber polar energy and causes them to cohere. This impedes wetting, resulting in poor quality, high void content composites, even though there appeared to be improved adhesion between the fibers and matrix. Silane was found to bond to the oxidized carbon fiber surfaces, as evidenced by changes in both fiber and composite properties. The fibers exhibited low polarity and cohesion, while the composites displayed excellent resin wetting, low void content, and low seawater weight gain and swelling. On the contrary, the oxidized fibers that were not treated with silane exhibited high polarity and fiber cohesion.