Master of Science / Department of Architectural Engineering / Donald J. Phillippi / For this thesis, research was performed on CFRP bonded composite lap-joints with one and two continuous laminas through the lap. Composite wraps used to retrofit existing structures use lap joints to maintain their integrity. The use of composites for retrofitting structures has many advantages over traditional methods, such as steel jacketing, and is becoming more widely accepted in the structural engineering industry. While much literature exists documenting the performance of composite wraps as a whole when applied to concrete columns, less information is available on the behavior of the lap-joint of the wrap. Developing a better understanding of how the lap-joint behaves will help researchers further understand composite column wraps. This research sought to determine what affect continuous middle laminas may have on the stiffness of lap joints and whether or not stress concentrations exist in the lap-joint due to a change in stiffness.

The degree of cure and temperature are consistent variables used in models to describe the state of material behaviour development for a thermoset during cure. Therefore, the validity of a cure kinetics model is an underlying concern when combining several material models to describe a part forming process, as is the case for process modeling. The goals of this work are to identify sources of uncertainty in the decision-making process from cure measurement by differential scanning calorimeter (DSC) to cure kinetics modeling, and to recommend practices for reducing uncertainty. Variability of cure kinetics model predictions based on DSC measurements are investigated in this work by a study on the carbon-fiber-reinforced-plastic (CFRP) T800H/3900-2, an interlaboratory Round Robin comparison of cure studies on T800H/3900-2, and a literature review of cure models for Hexcel 8552. It is shown that variability between model predictions can be as large as 50% for some process conditions when uncertainty goes unchecked for decisions of instrument quality, material consistency, measurement quality, data reduction and modeling practices. The variability decreases to 10% when all of the above decisions are identical except for the data reduction and modeling practices. In this work, recommendations are offered for the following practices: baseline selection, balancing heats of reaction, comparing data over an extensive temperature range (300 K), choosing appropriate models to describe a wide range of behaviour, testing model reliability, and visualization techniques for cure cycle selection. Specific insight is offered to the data reduction and analysis of thermoplastic-toughened systems which undergo phase separation during cure, as is the case for T800H/3900-2. The evidence of phase separation is a history-dependent Tg-α relationship. In the absence of a concise outline of best practices for cure measurement by DSC and modeling of complex materials, a list of guidelines based on the literature and the studies herein is proposed. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

This research program investigates the performance of a steel truss bridge when subjected to both localized web damage and a subsequent post-tensioned strengthening approach. The investigation utilizes a combined approach involving an experimental scale model bridge and a numerical computer model generated using the commercial finite element software RISA 3-D. The numerical model is validated using test data and further extended to parametric studies in order to investigate the theoretical load rating, strain energy, load redistribution, mode shapes and frequency of the bridge for control, damaged and strengthened states. The presence and severity of damage are found to significantly influence the global safety and reliability of the bridge. Also, higher order modes are more susceptible to changes in shape and frequency in the presence of damage. A recovery of truss deflection and a reduction of member forces are achieved by the proposed strengthening method.

The purpose of this research was to quantify the strength of novel composite repair methods for thin-walled composite structures. Carbon/epoxy plates were manufactured and repairs were made at Gloyer-Taylor Laboratories. At BYU, specimens were damaged in a controlled and repeatable process. Three damage modes were implemented: impact, groove, and abrasive damage. Tensile strength tests were performed on control, damaged, and repaired specimens. Four 24 x 24 in (60 x 60 cm) carbon/epoxy plates were received. Each plate was made up of seven plies cured together with epoxy resin for a nominal total thickness of 0.04 in (1.02 mm). The thickness, however, was not uniform: each plate had a smooth side and a wavy side. This resulted in inconsistent damage depth. The plates were cut at BYU using a water-jet cutter into 1 in. (25.4 mm) wide by 8 in. (203.2 mm) long test specimens. Test specimens were grouped into four categories: control specimens, specimens inflicted with damage by machining a shallow groove ranging from 0.012 — 0.018 in. (0.30 — 0.46 mm) deep, specimens inflicted with an abrasive-type damage ranging from 0.006 — 0.012 in. (0.15 — 0.30 mm) deep, and specimens subjected to impact damage ranging from 1.47 — 3.23 J. Five specimens were placed in the control group. Ten specimens were placed in each of the remaining damage groups. All ten specimens were damaged, but only five of each were sent to be repaired. The randomization of the thickness variable was prevented by the desire to repair damaged specimens as a group with a single repair rather than performing repairs on every individual specimen. The stress-strain behavior confirm the control specimens generally exhibited the best overall behavior, as expected. Most damaged specimens, including the repaired specimens, exhibited lower ultimate stress than the undamaged control specimens. The repaired specimens exhibited a higher initial stiffness than either the control or damaged specimens, due to the stiffness of the composite patch. Although repaired specimens should exhibit higher strength than damaged specimens, but this however, was not always the case. In particular, repairs did not improve the ultimate strength of the specimens damaged by abrasion. Correlations between the different damage types were developed, relating damage intensity and strength was approximately. This suggests further investigation is needed.

Traditionally it was thought that the unidirectional strength in the fiber direction of fiber dominated composites was not influenced by the matrix material. As long as the fiber was not affected then the strength would remain. However this thesis will challange that belief. The unidirectional strength in the fiber direction of fiber dominated composites is influenced by the matrix material. The object of this study was to examine the quasi-static tensile strength of unidirectional polymer composites, and then use current analytic models to predict the experimental results. The different matrix materials were polyphenylene Sulfide (PPS), vinyl ester with two different fiber-matrix interface materials, and polyether ether ketone (PEEK). / Master of Science

Ultramicroelectrodes (UMEs) have gained considerable attention over the few past decades due to the important roles they play in electrochemical studies. Electrodes with dimension less than 25 mm can generally be classified as UMEs. These electrodes exhibit enhanced electrochemical properties as their dimensions get smaller hence making nanoelectrode (production of electrodes with limiting dimensions less than 100 nm) a continuing area of interest in research. Nanometer size electrodes have advantages of high sensitivity which enables them to be used in fields such as single particle characterization and single cell analysis, and fast electron and mass transport which permits use for studying short-lived and transient electrochemical reactions such as those involved in neurochemistry. Nanoelectrodes can be fabricated via a few different strategies which include but are not limited to electrochemically etching a thin metal wire down to a cone shape or flame-etching a carbon fiber, and chemical vapor deposition of carbon in nanopipette. This work seeks to employ the use of the laser-assisted pulling method to fabricate carbon fiber electrodes sealed in glass capillary tubes. Effects of various pre- and post- treatment techniques on electrode size and stability are explored. Key words: Electrodes, Electrochemical, carbon fiber.

Carbon Fiber Reinforced Polymer (CFRP) composites exhibit exceptional specific stiffness and strength properties. However, their use in structural applications is often constrained with high safety margins out of concern for their brittle and sudden failures. This study proposes manipulating the tensile failure mechanism by utilizing a discontinuous overlapped architecture, which has been demonstrated in the literature to non-linearize the tensile stress-strain response of CFRP laminates. Continuous Carbon fiber 3D-printing provides freedom in building complex morphologies and adjusting the resin content, enabling intricate discontinuous patterns for further tuning the stress-strain response. This study characterizes the constituents and tensile properties of 3D-printed continuous UD laminates. Then, an investigation is conducted on the mechanical tensile response of a 3D-printed discontinuous laminates design and the effect of discontinuity pattern length, and post-processing.

No description available.

A lightweight carbon fiber reinforced polymer (CFRP) c-channel beam was previously designed using analytical theory and finite element analysis and subsequently manufactured through a pultrusion process. Physical testing revealed the prototype did not meet the bending and torsional stiffness of the beam model. An investigation revealed that the manufactured prototype had lower fiber content than designed, compacted geometry, an altered ply layup, missing plies, and ply folds. Incorporating these changes into the beam model significantly improved model-experiment agreement. Using what was learned from the initial prototype, several new beam designs were modeled that compare the cost per weight-savings of different composite materials. The results of these models show that fiberglass is not a viable alternative to CFRP when designing for equivalent stiffness. Standard modulus carbon was shown to have slightly lower cost per-weight savings than intermediate modulus carbon, although intermediate modulus carbon saves more weight overall. Core materials, despite potential weight savings, were ruled out as they do not have the crush resistance to handle the likely clamp loads of any attaching bolts. Despite determining the ideal materials, the manufactured cost per weight-savings of the best CFRP beam design was about double the desired target. / Graduation date: 2013 / Access restricted to the OSU Community at author's request from Oct. 5, 2012 - Oct. 5, 2014

Submitted in fulfillment of the requirements of the Degree of M.Tech.: Mechanical Engineering, Durban University of Technology, 2017. / Fatigue crack initiation and propagation behaviour of CFRP have been of great importance because such composites are often used in engineering components that are subjected to continuous cyclic loading. The objective of this thesis work was to investigate the damage characteristics of the fatigue properties of CFRP composites by the modification of the polymer matrix with nanoclay addition. Carbon fibre reinforced epoxy was produced via vacuum assisted resin infusion moulding method (VARIM) with nanoclay concentrations of 0wt%, 1wt%, 3wt% and 5wt%. Tension-tension fatigue tests were conducted at loading levels of 90%, 75% and 60%. The frequency that was used was 3Hz with R value of 0.1. The results showed that at nanoclay percentages of 0wt%, 1wt% and 3wt% there was a consistent trend, where the number of cycles increased in fatigue loading percentages of 90%, 75% and 60%. At 5wt% nanoclay percentage the number of fatigue cycles dropped significantly at the 90% fatigue loading. The brittle nature of the 5wt% laminate became dominate and the sample fractured early at low fatigue cycle numbers. At the 75% fatigue loading, the number of cycles increased and at 60% fatigue loading the 5wt% nanoclay sample exceeded the number of cycles of all the nanoclay percentages by 194%. This was due to the intercalated arrangement of the nanoclays favouring the slow rate of surface temperature increase, during fatigue testing, at low fatigue cycle loading. The Crack Density analysis was performed and showed that at the same time in the fatigue cycle life, the 1wt% had 55 cracks, 3wt% had 52 cracks and the 5wt% had 50 cracks, for the 60% fatigue loading. This proved that it took longer for the cracks to initiate and propagate through the sample as the nanoclay percentage increased. Impact and hardness testing showed that the 5wt% exhibited brittle behaviour, which contributed to the results above. Scanning electron microscopy examination highlighted that the agglomeration of nanoclays delayed the crack initiation and propagation through the specimen and that the extent of fatigue damage decreased as the nanoclay percentage increased. A fatigue failure matrix was developed and showed that delamination, fibre breakage and matrix failure were the predominate causes for the fatigue failure. / M