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Thesis (Ph. D.)--Materials Science and Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Thadhani, Naresh; Committee Member: Doyoyo, Mulalo; Committee Member: Kecskes, Laszlo; Committee Member: Li, Mo; Committee Member: Sanders, Thomas; Committee Member: Zhou, Min.

D.Ing. / Unbound granular material has and is still being used with great success in the construction of road pavements in South Africa and many other countries around the world. Often this material is used in the main structural layers of the pavement with very little protection provided against high traffic induced stresses by way of a surface treatment or thin asphalt concrete layer. The performance of unbound granular pavement layers depend mainly on the level of densification and degree of saturation of the material in addition to the stress levels to which the layers are subjected. The main form of distress of unbound granular layers is the permanent deformation of the layer, either through the gradual deformation or rapid shear failure of the layer. Design engineers need accurate and appropriate design procedures to safeguard the road against such rapid shear failure and to ensure that the road has sufficient structural capacity to support the traffic loading over the structural design period. The recent trend in pavement design has been to move away from empirical design methods towards rational mechanistic-empirical design methods that attempt to relate cause and effect. Although a mechanistic-empirical pavement design method has been available in South Africa since the midseventies, increasing criticism has been levelled against the method recently. The models for characterising the resilient response and shear strength and estimating the structural capacity of unbound material have been of particular concern. The purpose of the research reported in this thesis was therefore to develop an improved mechanistic-empirical design model, reflecting the characteristics and behaviour of unbound granular material. The new design model consists of three components namely a resilient modulus, yield strength and plastic deformation damage model with each model including the effects of the density and moisture content of the material unbound granular where appropriate. The models were calibrated for a range of unbound materials from fine-grained sand and calcrete mixture to commercial crushed stone products using the results from static and dynamic tri-axial tests. An approximation of the suction pressure of partially saturated unbound material was introduced in the yield strength model and was validated with independent matric suction measurements on the sand and calcrete mixture. The yield strength model which is a function of the density and moisture conditions as well as the confinement pressure was calibrated for the individual materials with a high accuracy. A single plastic strain damage model was calibrated for the combined plastic strain data from all the crushed stone materials but a single model could not be calibrated for the plastic strain data of the natural gravels as these materials vary too much in terms of particle size distribution and the properties of the fines found in these materials. The formulation of the plastic strain damage model includes the density and degree of saturation of the material. A single resilient modulus model was calibrated for the combined resilient modulus data from all the materials excluding the data from a limited number of tests during which large plastic strain occurred. The resilient modulus model again ii incorporates the density, degree of saturation and the stress dependency of unbound granular material and is on an effective stress formulation for the bulk stress. Finally, the yield strength, resilient modulus and plastic strain damage models are combined in a mechanistic-empirical design model for partially saturated unbound granular material. Results from the proposed design method seem more realistic than results from the current design model and the model is not as sensitive to variation in the design inputs as the current design model is. In addition to this, the effects of the density and moisture content of the partially saturated, unbound granular material on the resilient response and performance of the material is explicitly included in the formulation of the proposed design model.

This dissertation involves the experimental investigation of commercially pure titanium (CP Ti) which was subjected to laser forming and mechanical forming processes. Commercially pure titanium grade 2 was formed to a radius of curvature of approximately 120 mm using three forming procedures, i.e. i) laser forming; ii) mechanical forming (stretched forming) and iii) a combined forming process (laser-mechanical forming). Fatigue testing revealed, for all the forming processes, that samples produced by laser forming performed the best at high load settings. However, mechanically formed specimens performed the best at low load settings, whereas the laser-mechanical process resulted in midway performance between laser and mechanical processing. Considering microstructure vs fatigue; impact vs fatigue; and residual stress vs fatigue; at high load settings it is evident that the microstructure is the dominant contributor to crack initiation and growth. Crack morphology of fatigue samples revealed that secondary cracks (parallel to main crack front) followed the grain boundaries of the Widmanstätten microstructure, whereas irregular secondary cracks grew parallel and through the twinning planes and along the grain boundaries of the equiaxed microstructure. Laser forming resulted in microstructural changes from equiaxed grains to a Widmanstätten structure due to fast cooling rates. Excessive twinning is developed within the equiaxed microstructure after the mechanical forming procedure. This is due to cold working / strain hardening. The combined process shows a combination of equiaxed grains and Widmanstätten microstructure. Residual stress relieved for all forming processes revealed an increase in the magnitude of the residual stress compared to the parent plate and that the maximum values were obtained at the inner radius of curvature (i.e. 118.4 mm). Laser forming revealed the highest values in residual stress whereas the other two processes i.e. mechanical and laser-mechanical forming exhibited an increase midway between the parent plate and laser forming. The second most influential factor with regards to fatigue was the magnitude of the residual stress, especially at medium to low load settings. When considering theoretical models to predict fatigue life it was found that the Goodman model showed the closest relation to the actual fatigue data when considering the entire theoretical curve. Vickers microhardness profiling was applied to the thickness of the samples for the parent plate and all forming processes. No significant hardening occurred due to the forming processes and differences in hardness were considered negligible. Charpy impact testing revealed that the laser formed specimens exhibited the most brittle behaviour when compared to the parent plate results. Mechanical formed specimens showed a slight increase in brittleness compared to parent plate whereas the combined process yielded results midway between the laser and mechanically formed specimens. Mathematical equations are formulated and presented for predicting the fatigue life of CP Ti grade 2 for the parent plate and the three forming processes. This study proved that the laser forming process can be successfully used as a production stage in the forming of CP Ti grade 2.

This experiment investigated the effect of infill pattern shape on structural stiffness for 3D printed components made out of carbon fiber reinforced nylon. In order to determine the natural frequency of each specimen, nondestructive vibrational testing was conducted and processed using data acquisition software. After obtaining the acceleration information of each component, in response to ambient vibrational conditions and excitation, frequency response functions were generated. These functions provided the natural frequency of each component, making it possible to calculate their respective stiffness values. The four infill patterns investigated in this experiment were: Zig Zag, Tri-Hex, Triangle, and Concentric. Results of the experiment showed that changing the infill pattern of a 3D printed component, while maintaining a constant geometry and density, could increase mechanical stiffness properties by a factor of two. Comprehensively, the experiment showed that infill pattern geometry directly attributes to the mechanical stiffness of 3D printed components. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2020. / FAU Electronic Theses and Dissertations Collection

The effect of specimen size on the uniaxial tensile stress/strain response of sublaminatelevel scaled composite specimens, and the four point flexure load/deflection response of ply- and sublaminate-level scaled composite specimens was investigated. Three laminates were studied in the tensile program, namely [+30/-30/90₂]_ns, [+45/-45/0/90}_ns and [90/0/90/0|_ns, where n = 1, 2, 3, 4. Two material systems were used, namely AS4/3502 graphite/epoxy and APC-2 graphite/PEEK, to investigate the relative effect of resin toughness. Three laminates were also studied in the flexure program, The baseline lay-ups were (±45/0/90}_2ns, [0/90/0/90J_2ns and [±45/±45J_2ns, where n = 1, 2, 4. Ply- and sublaminate-level scaling were used to increase specimen thickness. All flexure specimens were of AS4/3502 graphite/epoxy. Enhanced X-ray radiography and edge photomicroscopy were used to examine damage development in specimens loaded to various fractions of their ultimate stress. This nondestructive examination was coupled with observations of critical events in the stress/strain response to try to correlate scaling effects with the damage development in the specimens. Analytical and numerical methods were employed in order to understand the stresses driving certain damage modes observed. 2-D and 3-D finite element models were used to find delamination stresses in an undamaged laminate, and an approximate clasticity approach was used to find stresses duc to cracks in the 90° plies. It was found that the tensile strength of the [+30/-30/90₂]_ns and [+45/-45/0/90}_ns laminates gencrally increased as n increased. This effect was more pronounced for the matrixdominated [+30/-30/90₂]_ns. Both the [+30/-30/90₂]_ns and the quasi-isotropic [+45/-45/0/90}_ns laminates seemed to be approaching a maximum strength, beyond which the strength scaling either stops, or is reversed. As # increased from 1 to 4, these two laminates exhibited a delay in the onset of certain damage mechanisms, such as delamination and transverse matrix cracking. The [90/0/90/0|_ns laminates showed no tensile strcss/strain response scaling effects, although the stress at which first ply failure occurred was found to increase as 7 increased. (±45/0/90}_2ns and [±45/±45J_2ns flexure specimens showed no strength scaling cffects when sublaminate-level scaling was uscd, but significant decreases in s{rength were found when specimen size was increased using ply-level scaling. [0/90/0/90J_2ns specimens showed no global load/deflection scaling effects. / Ph. D.

The effect of particle size matching and mismatching on the processability, and the mechanical and physical properties of particulate reinforced composites is investigated in this study. These composites were made from dry powder-powder blends. Polymer and reinforcement materials were chosen, characterized and molded into composite plaques. For the same particle volume fraction (400/0), stiffness was found to increase, in general, as particle size decreased. The intimacy of mixing, stiffness and strength improvements depended upon the reinforcement type. These results were compared with predictions from simple micromechanics models to gain a better understanding of their physical behavior. / Master of Science