Due to the rapid expansion of applications of composite materials, investigations of their properties have greatly increased. Since theoretical and numerical methods have many limitations for anisotropic materials, experimental methods are sometimes the only way to answer the questions. It has been proved that moire interferometry is a powerful technique in the study of composite materials. The high sensitivity and resolution of a measurement technique is the key to determining the properties of a material which has a fine and complicated structure such as fiber reinforced composite laminates. In this paper, a carrier fringe method is introduced to increase the resolution of the fringe gradient in the moire technique. The ability of measurement is extended to the micromechanics region. High strain concentrations and the dramatic displacement variations can be determined by measuring the slopes of carrier fringes. Strain distributions across the plies (with the thickness of 125 μm) in graphite/epoxy composites and strain concentrations in the resin-rich zones (with the thickness of 10 μm) between neighboring plies are revealed by the carrier fringe technique. Three experiments are presented to show the effectiveness of the application of carrier fringes to resolve fringe gradients and obtain strains. The current moire technique is limited to the optical laboratory because it is extremely sensitive to the disturbance of the environment. A vibration with magnitude of 0.2 μm can completely wash out the contrast of a moire fringe pattern. The study has been done in moving moire interferometry off the optical table. Vibration insensitive moire systems are investigated to extend the moire technique to the tests of large structures and using testing machines for loading. Vibration problems are discussed and the new ideas for eliminating vibration effects are presented. Six representative schemes are analyzed and three of these systems are built to perform experiments in rough environments such as on a hydraulic testing machine. The results show the great success of these new systems. / Ph. D.

The objective of this study was to investigate the bending strength and failure characteristics of AS4/3501-6 and AS4/1806 graphite/epoxy angles sections as a function of processing induced defects and porosity. The angle sections were removed from 30-inch long angles fabricated at Lockheed Georgia Company with two quasi-isotropic stacking sequences, (± 45/90₂/ ∓ 45/0₂), and (± 45/90₂ ∓ 45/0₂)₃. Various degrees of porosity were introduced into the angles using four processing techniques: a standard lay-up, a solvent wipe during lay-up, moisture introduction between plies during lay-up, and a low pressure cure cycle. Two 2.5-inch wide angle sections, each with a 1.5-inch short leg and a 3.0-inch long leg, were bonded together along their long leg to form a T-shaped specimen. Bending of the T-specimen was introduced by pressing up on the underside of the flanges while holding the base of the specimen fixed. The experimental results have shown a significant effect of the processing induced defects on the failure load and bending stiffness for AS4/3501-6 specimens, but not for AS4/1806 specimens. An anisotropic analysis of the angle curved section was performed using Lekhnitskii's stress function approach. Stress and strain fields were studied and two failure criteria (Dual maximum stress and Tsai-Wu) were investigated in order to predict T-specimen failure load and failure mode. Reasonable correlation between prediction and experiments was found for the AS4/3501-6 (± 45/90₂/ ∓ 45/0₂)₃ T-specimens, but both failure criteria were found to be too conservative in predicting failure for the AS4/3501-6 (± 45/O₂/ ∓ 45/90₂)₃, T-specimens. The predicted failure modes were in good agreement with the experimental observations for both Iaminates. / Master of Science

Several damage modes, including fiber breakage, delamination, and transverse cracking, have been observed to contribute to the mechanical degradation of fiber-reinforced composite laminates. In this investigation, the effect of transverse cracking on laminate stiffness was studied. Four. glass-epoxy laminates ([0,90₃]_s, [90₃,0]_s, [0,90]_s, and [0,±45]_s) were evaluated. Two experimental test sequences were performed. In the first test sequence, longitudinal stiffness was measured at various stages of damage development. Damage development was monitored via edge replication. In the second test sequence, four laminate stiffnesses (E_xx, v_xy, G_xy, and D_yy) were measured in the undamaged and near-saturation damage states. Two analytical models were evaluated. A one dimensional shear lag model was used to predict longitudinal stiffness as a function of crack density for the [0,90₃]_s and 90₃,0]_s laminates. Correlation between theory and experiment was good. A modified laminate analysis was used to predict four laminate stiffnesses (E_xx, v_xy, G_xy, and D_yy). Except for the [0,±45]_s case, a laminate in which significant amounts of damage - s other than transverse cracking were observed, agreement between pre- · dieted and observed stiffness changes was good. / Master of Science

Space simulated thermally induced deformations and stresses in continuous fiber-reinforced composites were investigated with a micromechanics analysis. The investigation focused on two primary areas. First, available explicit expressions for predicting the effective coefficients of thermal expansion (CTE's) for a composite were compared with each other and with a finite element (FE) analysis, developed specifically for this study. Analytical comparisons were made for a wide range of fiber/matrix systems, and predicted values were compared with experimental data. All of the analyses predicted nearly identical values of the axial CTE, α₁, for a given material system, and all of the predictions were in good agreement with the experimental data. Results from the FE analysis, and those from the solution of a generalized plane strain boundary value problem, were in excellent agreement with each other and with the experimental data for the transverse CTE, α₂. Less rigorous formulations were in poor agreement with the experimental data. The second area of investigation focused on the determination of thermally induced stress fields in the individual constituents. Stresses predicted from the FE analysis were compared to those predicted from a closed-from solution to the composite cylinder (CC) model, for two carbon fiber/epoxy composites. A global-local formulation, combining laminated plate theory and FE analysis, was used to determine the stresses in multidirectional laminates. Thermally-induced damage initiation predictions were also made. The type of analysis (i.e. CC or FE) was shown to significantly affect the distributions and magnitudes of the predicted stresses. Thermally-induced matrix stresses increased in absolute value with increasing fiber volume fraction but were not a strong function of fiber properties. Multidirectional [0₂/±θ]s laminates had larger predicted thermally induced matrix stresses than unidirectional ([0]) laminates, and these stresses increased with increasing lamination angle θ. Thermally-induced matrix failure predictions, using a maximum stress failure criterion based on the normal interfacial stress component and the measured transverse lamina strength, were in excellent agreement with experimental data. / Ph. D.

Natural fibers like kenaf, hemp, flax and sisal fiber are becoming alternatives to conventional petroleum fibers for many applications. One such applications is the use of Non-woven bio-fiber mats in the automobile and construction industries. Non-woven hemp fiber mats were used to manufacture plywood in order to optimize the plywood structure. Hemp fiber mats possess strong mechanical properties that comparable to synthetic fibers which include tensile strength and tensile modulus. This study focuses on the use of hemp fiber mat as a core layer in plywood sandwich composite. The optimization of fiber mat plywood was done by performing a three factor experiment. The three factors selected for this experiment were number of hemp mat layers in the core, mat treatment of the hemp mat, and the glue content in the core. From the analysis of all treatments it was determined that single hemp mat had the highest effect on improving the properties of the plywood structure.

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.

This study is designed to provide the composites industry as well as the fire engineering industry baseline data for pyrolysis modelling of common fiber reinforced polymer (FRP) systems. Four resin systems and three glass contents will be considered. This matrix of FRP systems has been carefully fabricated and documented so as to provide“transparency" as to the system compositions. An important and interesting aspect of these FRP systems is that all the resins used are listed by the manufacturers as Class 1 or Class A per ASTM E 84. The FRP systems are being evaluated in bench scale modern fire test apparatuses (FPA, ASTM E 2058, and Cone, ASTM E 1354); detailed information on the FPA is provided. These apparatuses provide a range of measurements such as heat release rate that can be used to calculate engineering“properties" of these FRP systems. The“properties", such as minimum heat flux for proper ignition (found to range from 20 to over 100 kW/m2) and the b flame spread parameter, can then be used to compare the fire performance (flashover potential) of these FRP systems according to resin type and glass content. Additional instrumentation has also been added to the specimens to allow surface and in-depth temperatures to be measured. The additional measurements are used to complete a set of data for pyrolysis modelling and for calculating thermal properties of the composites. The effect of environment oxygen concentration and flaming and non-flaming decomposition are investigated in terms of fundamental pyrolysis behavior of the FRP systems. A general conclusion is that the phenolic composite has better fire engineering“properties" than the polyester composite but the glass is the controlling component of the composite with regards to temperature profile and resulting thermal properties.

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.

It has been known that the nucleation and growth of cracks and defects dominate the fatigue damage process in brittle or quasi-brittle materials, such as woven fabric composites and concrete. The behaviors of these materials under multiaxial tensile or compression fatigue loading conditions are quite complex, necessitating a unified approach based on principles of mechanics and thermodynamics that offers good predictive capabilities while maintaining simplicity for robust engineering calculations. A unified approach has been proposed in this dissertation to simulate the change of mechanical properties of the woven fabric composite and steel fiber reinforced concrete under uniaxial and biaxial fatigue loading. The boundary surface theory is used to describe the effect of biaxial fatigue loading. A fourth-order response tensor is used to reflect the high directionality of the damage development, and a second-order response tensor is used to describe the evolution of inelastic deformation due to damage. A direction function is used to capture the strength anisotropic property of the woven fabric composite. The comparisons between model prediction results and experimental data show the good prediction capability of models proposed in this dissertation.

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering. Johannesburg, October 2017 / This research looks into the manufacturing process of complex geometries using continuous fibre reinforced thermoplastics (CFRTP). The purpose of this work was to develop methods that will enable the production of defect free complex components. This was achieved by investigating the key process parameters in the CFRTP manufacturing process, and optimizing them in order to improve the quality of components. The investi- gations were performed with the aid of software making use of the finite element method, and this was found to be instrumental in predicting the formability of geometries. The re- search showed that the formability of complex geometry is largely determined by the ability of the laminate to be draped into the required geometry. The forming mechanisms that take place during the draping process can be linked to the formation of defects where draping is unsuccessful. The study also showed that the quality of the drape can be influenced by blank and tool design factors. It was also shown that the blank can be manipulated using a restraint mechanism to improve the formability of geometries. The effect of processing parameters such as forming speed, forming pressure and tool temperature were also investigated. The research resulted in the formulation of guidelines to follow when manufacturing CFRTP components. The developments that were made were successfully implemented to improve the formability of a complex component that had previously been difficult to form without defects. / MT2018