Acrylic bone cement is widely used as a structural material in orthopaedics, dentistry, and orofacial surgery. Although bone cement celebrates four decades of success, it remains susceptible to fatigue fracture. This type of failure can directly lead to implant loosening, revision surgery, and increased healthcare expenditures. The mechanism of fatigue failure is divided into three stages: 1) fatigue crack initiation, 2) fatigue crack propagation, and 3) fast, brittle fracture. Adding reinforcing fibers and particles to bone cement is a proposed solution for improving fatigue performance. The mechanical performance of these reinforced bone cements is limited by fiber ductility, fibermatrix de-bonding, elevated viscosity, and mismatch of fiber size and scale of fatigue induced damage. In this dissertation, I report that adding small amounts (0% - 10% by weight) of multiwall carbon nanotubes (MWNTs) enhances the strength and fatigue performance of single phase bone cement. MWNTs (diameters of 10-9 10-8 m; lengths of 10-6 10-3 m) are a recently discovered nanomaterial with high surface area to volume ratios (conferring MWNT bone cement composites with large interfaces for stress transfer) that are capable of directly addressing sub-microscale, fatigue induced damage. MWNTs (2wt%) significantly increased the flexural strength of single phase bone cement by a modest 12%; whereas, similar additions of MWNTs dramatically enhanced fatigue performance by 340% and 592% in ambient and physiologically relevant conditions, respectively. Comparing the fatigue crack propagation behaviors of reinforced and unreinforced single phase bone cements revealed that the reinforcing mechanisms of MWNTs are strongly dependent on stress intensity factor, K, a numerical parameter that accounts for the combinatorial effect of the applied load and the crack size. As the crack grows the apparent stress at the crack tip intensified and the MWNTs lost their reinforcing capabilities. For that reason, it is likely that the predominant role of the MWNTs is to reinforce the bone cement matrix prior to crack initiation and during the early stages of crack propagation. Therefore, MWNTs are an excellent candidate for improving the clinical performance of bone cement, thereby improving implant longevity and reducing patient risk and healthcare costs.

No description available.

Wind turbines are classic examples of structures where their operating lifetime is controlled by the fatigue properties of the material. This is exacerbated by the 2D nature of the composite materials used in blade construction which are typically fabrics in a variety of formats (e.g. non crimp fabrics, uniweave, woven). The formation of internal detailed shapes within the blade, allowing features such as spars, shear webs and other connections, inevitably requires these 2D material configurations to be formed into 3D shapes. This introduces positions within the structure where load transfer occurs across regions with no fibre reinforcement. These weak areas become natural positions for the initiation of damage that can occur well before fatigue damage would be expected in the basic material subject to simple in-plane loading. The aim of this study is to modify and improve the blade structure in order to extend its working life and minimize geometry related fatigue issues. To achieve this goal T-sections have been manufactured as representative element of the blade's spar. T-sections have been made of carbon or glass fabric infused with epoxy resin using a vacuum-assisted resin-transfer moulding technique. The structure has been modified with different toughening techniques to increase its interlaminar fracture resistance (toughness) and hence delay or stop crack propagation. Methods such as the use of veil layers, tufting and 3D weaving techniques have been employed to improve the interlaminar fracture toughness of the T-joint. The changing parameters in samples are, the addition of the veil layer to the composite structure, veil material, tufting stitches and use of different 3D fibre weaving architectures in the fabrication of the composite T-joint. For T-joint testing, there was no standardised specimen shapes and no standard for specimen dimensions; as well as no test fixture designs or test procedures. Consequently, it was required to design a test rig and develop a test procedure for tensile and fatigue tests of T-joints. An additional investigation was performed to establish test specimen geometry suitable for testing in available Instron machines. Manufactured specimens were quasi-static and fatigue tested. Test results were compared and showed that 3D woven and polyester veil T-joints had the best performance among modified structures. However, it has been found that these structural modifications are performing differently in quasi-static and fatigue loading. The 3D woven four layer to layer inter wave sample that showed the best result in a quasi-static test was not the one with the best fatigue results but it was amongst the ones with the highest performance. SEM and optical microscopy were used to investigate fractured specimens in an attempt to establish the mechanisms involved in the fracture process of the T-joint. Finally, based on test and investigations results it has been concluded that the 3D weaving was the most effective modification to improve the static and fatigue properties of the T-joint. The T-joint modified with polyester veil showed the second best performance in both static and fatigue tests but the addition of the polyamide caused had negative effects on these properties.

The need for economical, sustainable, safe, and secure shelter is an inherent global problem and numerous challenges remain in order to produce environmentally friendly construction products which are structurally safe and durable. The use of sisal, a natural fiber with enhanced mechanical performance, as reinforcement in a cement based matrix has shown to be a promising opportunity. This work addresses the development and advances of strain hardening cement composites using sisal fiber as reinforcement. Sisal fibers were used as a fabric to reinforce a multi-layer cementitious composite with a low content of Portland cement. Monotonic direct tensile tests were performed in the composites. The crack spacing during tension was measured by image analysis and correlated to strain. Local and global deformation was addressed. To demonstrate the high performance of the developed composite in long term applications, its resistance to tensile fatigue cycles was investigated. The composites were subjected to tensile fatigue load with maximum stresses ranging from 4 to 9.6 MPa at a frequency of 2 Hz. The composites did not fatigue below a maximum fatigue level of 6 MPa up to 106 cycles. Monotonic tensile testing was performed for composites that survived 106 cycles to determine its residual strength.

Filament wound composite structures are becoming more and more attractive to designers in the aircraft and aerospace industries due to increasing strength- and stiffness-to-weight ratios and falling fabrication costs. However, the interaction of some of the manufacturing process variables such as mandrel stiffness and thickness, winding tension and pattern, and cure cycle characteristics can lead to common defects such as delamination, matrix cracking and fiber buckling. A model of the filament winding process was developed to better understand the behavior of wound structures during fabrication. Specifically, the residual stress state at the end of winding, heat-up and cool-down was determined. This information is important because adverse stress states are the mechanism through which the process variables cause fabrication defects. The process model utilized an incremental finite-element analysis to simulate the addition of material during winding. Also, the model was refined and extended to include changes that occur in the material behavior during the cure. A fabrication analysis was performed for an 18 in. (457 mm) graphite/epoxy filament wound bottle. Two different mandrel models were examined, a rigid steel and a soft sand/rubber mandrel. At the end of winding, the composite layers in the model retained all of their initial winding tension for the steel mandrel but did exhibit significant loss of tension for the sand/rubber mandrel. The composite layers experienced a large increase in tension during heating for the steel mandrel but showed no significant recovery of tension for the sand/rubber system. / M.S.

Within the past several years, researchers have detected the presence of a third “phase” between the bulk fiber phase and bulk matrix phase in a polymeric matrix composite. This finite-thickness region — termed the interphase — possesses mechanical, physical, and chemical properties that are distinct from the fiber and matrix constituents. Thus, the interphase embodies the characteristics of the fiber/matrix bond, including the strength and stiffness of the bond. In essence, the interphase represents the composite system, since it defines the level of synergistic interaction that occurs between the load-carrying fibers and the binding matrix material. Recent interest in the interphase has spawned international conferences and a technical journal devoted to its study. Despite this spate of research, some very fundamental questions about the interphase have remained unanswered. One such question is: “What is best for the performance of a composite, a strong or weak or intermediate-strength interphase?” It is surprising that this question is even asked, since, until recently, it had been assumed that the stronger the fiber/matrix bond, the better the composite behavior. It is now known that this adage is far from true. Two formidable challenges await those who wish to correlate the strength of the interphase to the mechanical performance of polymeric matrix composite materials. First, one seeks to systematically alter the interphase in order to exploit this variable. In this study, fourteen material systems representing permutations of four carbon fibers, three matrix systems, percentages of fiber surface treatment, and three sizing conditions have been examined. Secondly, one needs to quantitatively characterize the properties of the resultant interphase in order to correlate the bond condition to the composite’s mechanical behavior. This investigation utilizes two techniques, the Continuous Ball Indentation Test and transverse flexure testing, as a means of interrogating the strength of the interphase. The influence of the interphase on the tensile and compressive strength and modulus of crossplied laminates possessing a center hole is investigated. Unnotched angle-ply ([±45]_ns) laminates are also tested in order to assess the role of the interphase in the strength of a “matrix-dominated” laminate. Fully-reversed (R =-1), axial fatigue of notched cross-plied laminates from each of the fourteen material systems 1s performed. During fatigue testing several data are monitored, including cycles to failure, dynamic modulus, and notch temperature. The tension-tension (R= 0.1) fatigue response of the unnotched angle-ply laminates is also studied. Results from X-ray radiography of fatigue-damaged specimens help to explain the relationship between the interphase and the initiation and propagation of life-critical damage mechanisms. Having observed the formative role played by the interphase in the performance of these laminates, an attempt is made to introduce variables representing the interphase into micromechanical models of composite behavior. / Ph. D.

During the fabrication of a filament-wound composite case, which includes the winding and cure stages, the tension in the fiber can change significantly. If the level of fiber tension decreases excessively during fabrication, fiber slippage and clumping can occur. The resulting resin rich areas can significantly decrease the strength of the composite case. The objectives of the present investigation were 1) to measure the change in fiber tension during the winding and cure of a composite case wound with prepreg material, and 2) to calculate the change in tension during cure using a simulation computer program. Of particular interest was the loss of fiber tension due to resin flow (RFTL). A total of twenty-four tension loss experiments were performed using Amoco’s Thornel T40 fiber and T40/1908 prepreg materials. The parameters which were varied in the experiments were spool tension, oven heating rate, and the number of composite layers. Some of the experiments were designed to isolate and measure RFTL by comparing the changes in tension of winds of dry fiber and prepreg material. This method was not successful due to a similarity in prepreg and dry fiber tension loss characteristics. Low spool tensions were found to result in more tension loss due to resin flow (RFTL). RFTL was also greater for an increased number of layers, but was not affected by oven heating rate. During winding, significant tension loss occurred, probably due to deformation of the prepreg tow at room temperature. The change in fiber tension during cure was calculated using an existing cure simulation code (FWCURE) which was modified in this work to include the contribution to fiber tension made by the thermal expansion of the mandrel during cure. The revised code is called FWEXPAND. By adjusting the permeability model in FWEXPAND, the fiber tension during the cure of a single layer wind was accurately calculated. The predicted total RFTL of two multi-layer winds agreed reasonably well with the measured RFTL, but the rate of tension loss was overpredicted. Complete RFTL and full compaction occurred during the first ramp of the cure cycle in all of the experiments. / Master of Science

Thesis (M.Tech.: Mechanical Engineering)-Dept. of Mechanical Engineering, Durban University of Technology, 2007 xvi, 123 leaves / Interlaminar cracking or delamination is an inherent disadvantage of composite materials. In this study the fracture properties of nano and fibre-reinforced polypropylene and epoxy composite structures are examined. These structures were subjected to various tests including Single Edge Notched Bend (SENB) and Mixed Mode Bending (MMB) tests. Polypropylene nanocomposites infused with 0.5, 1, 2, 3 and 5 weight % nanoclays showed correspondingly increasing fracture properties. The 5 weight % specimen exhibited 161 % improvement in critical stress intensity factor (KIC) over virgin polypropylene. XRD and TEM studies show an increase in the intercalated morphology and the presence of agglomerated clay sites with an increase in clay loading. The improvement in KIC values may be attributed to the change in structure. Tests on the fibre-reinforced polypropylene composites reveal that the woven fibre structure carries 100 % greater load and exhibits 275 % lower crack propagation rate than the chopped fibre specimen. Under MMB conditions, the woven fibre structure exhibited a delamination propagation rate of 1.5 mm/min which suggests delamination growth propagates slower under Mode I dominant conditions. The woven fibre / epoxy structure shows 147 % greater tensile modulus, 63 % greater critical stress intensity factor (KIC), and 184 % lower crack propagation rate than the chopped fibre-reinforced epoxy composite. MMB tests reveal that the load carrying capability of the specimens increased as the mode-mix ratio decreased, corresponding to an increase in the Mode II component. Delamination was through fibre–matrix interface with no penetration of fibre layers. A failure envelope was developed and tested and may be used to determine the critical applied load for any mode-mix ratio. The 5 weight % nanocomposite specimen exhibited a greater load carrying capability and attained a critical stress intensity factor that was 10 % less than that of the fibre-reinforced polypropylene structure, which had three times the reinforcement weight. Further, the nanocomposite exhibited superior strain energy release rates to a material with ten times the reinforcement weight. The hybrid structure exhibited 27 % increase in tensile modulus over the conventional fibre-reinforced structure. Under MMB conditions, no significant increase in load carrying capability or strain energy release rate over the conventional composite was observed. However, the hybrid structure was able to resist delamination initiation for a longer period, and it also exhibited lower delamination propagation rates.

Cette thèse contribue à la compréhension du comportement en fatigue des thermoplastiques renforcés par des fibres de verre courtes. Deux différents matériaux sont étudiés : Un mélange de polybutylène téréphtalate et polyéthylène téréphtalate (PBT+PET GF30) et un polyamide 66 (PA66 GF35). Les enjeux scientifiques de la thèse concernent les chargements multiaxiaux, l'influence de la contrainte moyenne et l'orientation de fibres sur la tenue en fatigue. En outre, les mécanismes de rupture sont abordés au travers de techniques dédiées et ciblées ce qui a permis de proposer un scénario de rupture en fatigue pour le PBT+PET GF30. L'enjeu industriel est de développer un outil de dimensionnement en fatigue. Afin de répondre à ces objectifs, des essais de fatigue sont effectués dans le domaine de la durée de vie limitée (103-106 cycles) et à amplitude constante pour les rapports de charge de R=0,1 et R=-1. L'effet de l'orientation de fibres est étudié sur la base d'essais à différentes orientations en traction ainsi qu'en cisaillement sur des éprouvettes plates. Des chargements multiaxiaux sont appliqués à des éprouvettes tubulaires afin d'évaluer la tenue en fatigue multiaxiale. Dans le but de réduire le nombre d'essais d'identification tout en conservant les effets à décrire (triaxialité, rapport de charge et orientation), un nouveau critère est proposé. L'effet de l'orientation des fibres est simulé en utilisant l'approche de Mori Tanaka afin de calculer les contraintes moyennées. Le critère est implanté dans une chaîne de calcul allant de la mise en œuvre jusqu'à la durée de vie et il est validé sur deux matériaux et une large base de donnée expérimentale.

Thesis (M.Tech.: Mechanical Engineering)-Dept. of Mechanical Engineering, Durban University of Technology, 2007 xvi, 123 leaves / Interlaminar cracking or delamination is an inherent disadvantage of composite materials. In this study the fracture properties of nano and fibre-reinforced polypropylene and epoxy composite structures are examined. These structures were subjected to various tests including Single Edge Notched Bend (SENB) and Mixed Mode Bending (MMB) tests. Polypropylene nanocomposites infused with 0.5, 1, 2, 3 and 5 weight % nanoclays showed correspondingly increasing fracture properties. The 5 weight % specimen exhibited 161 % improvement in critical stress intensity factor (KIC) over virgin polypropylene. XRD and TEM studies show an increase in the intercalated morphology and the presence of agglomerated clay sites with an increase in clay loading. The improvement in KIC values may be attributed to the change in structure. Tests on the fibre-reinforced polypropylene composites reveal that the woven fibre structure carries 100 % greater load and exhibits 275 % lower crack propagation rate than the chopped fibre specimen. Under MMB conditions, the woven fibre structure exhibited a delamination propagation rate of 1.5 mm/min which suggests delamination growth propagates slower under Mode I dominant conditions. The woven fibre / epoxy structure shows 147 % greater tensile modulus, 63 % greater critical stress intensity factor (KIC), and 184 % lower crack propagation rate than the chopped fibre-reinforced epoxy composite. MMB tests reveal that the load carrying capability of the specimens increased as the mode-mix ratio decreased, corresponding to an increase in the Mode II component. Delamination was through fibre–matrix interface with no penetration of fibre layers. A failure envelope was developed and tested and may be used to determine the critical applied load for any mode-mix ratio. The 5 weight % nanocomposite specimen exhibited a greater load carrying capability and attained a critical stress intensity factor that was 10 % less than that of the fibre-reinforced polypropylene structure, which had three times the reinforcement weight. Further, the nanocomposite exhibited superior strain energy release rates to a material with ten times the reinforcement weight. The hybrid structure exhibited 27 % increase in tensile modulus over the conventional fibre-reinforced structure. Under MMB conditions, no significant increase in load carrying capability or strain energy release rate over the conventional composite was observed. However, the hybrid structure was able to resist delamination initiation for a longer period, and it also exhibited lower delamination propagation rates.