The behaviour of sands during loading has been studied in great detail. However, little work has been devoted to understanding the response of sands in unloading. Drained triaxial tests indicate that, contrary to the expected elastic behaviour, sand often exhibit contractive behaviour when unloaded. Undrained cyclic simple shear tests show that the increase in pore water pressure generated during the unloading cycle often exceeds that generated during loading. The tendency to contract upon unloading is important in engineering practice as an increase in pore water pressure during earthquake loading could result in liquefaction. This research contributes to filling the gap in our understanding of soil behaviour in unloading and subsequent reloading. The approach followed includes both theoretical investigation and numerical implementation of experimental observations of stress dilatancy in unload-reload loops. The theoretical investigation is done at the micromechanical level. The numerical approach is developed from observations from drained triaxial compression tests. The numerical implementation of yield in unloading uses NorSand — a hardening plasticity model based on the critical state theory, and extends upon previous understanding. The proposed model is calibrated to Erksak sand and then used to predict the load-unload-reload behaviour of Fraser River sand. The trends predicted from the theoretical and numerical approaches match the experimental observations closely. Shear strength is not highly affected by unload-reload loops. Conversely, volumetric changes as a result of unloading-reloading are dramatic. Volumetric strains in unloading depend on the last value of stress ratio (q/p’) in the previous loading. It appears that major changes in particles arrangement occur once peak stress ratio is exceeded. The developed unload-reload model requires three additional input parameters, which were correlated to the monotonic parameters, to represent hardening in unloading and reloading and the effect of induced fabric changes on stress dilatancy. The calibrated model gave accurate predictions for the results of triaxial tests with load-unload-reload cycles on Fraser River sand.

The panel zone region in columns of seismic resistant steel moment frames are subject to very high shear forces during earthquake loading. Doubler plates are often used to increase the stiffness and strength of the panel zone. The methods and details used to attach doubler plates to columns can affect seismic performance of the panel zone and can also affect cost. The research reported in this thesis was aimed at developing an improved understanding of the advantages and disadvantages of various approaches for detailing and welding doubler plates to columns and how various details perform under cyclic inelastic loading. An extensive series of finite element analyses were conducted to study doubler plate attachment details. Both a shallow W14x398 column and a deep W40x264 column were studied in this research. This thesis provides a detailed description of the finite element modeling techniques used for the research and presents the results of an extensive series of analyses examining a wide variety of issues related to doubler plate design and detailing. / text

The behaviour of sands during loading has been studied in great detail. However, little work has been devoted to understanding the response of sands in unloading. Drained triaxial tests indicate that, contrary to the expected elastic behaviour, sand often exhibit contractive behaviour when unloaded. Undrained cyclic simple shear tests show that the increase in pore water pressure generated during the unloading cycle often exceeds that generated during loading. The tendency to contract upon unloading is important in engineering practice as an increase in pore water pressure during earthquake loading could result in liquefaction. This research contributes to filling the gap in our understanding of soil behaviour in unloading and subsequent reloading. The approach followed includes both theoretical investigation and numerical implementation of experimental observations of stress dilatancy in unload-reload loops. The theoretical investigation is done at the micromechanical level. The numerical approach is developed from observations from drained triaxial compression tests. The numerical implementation of yield in unloading uses NorSand — a hardening plasticity model based on the critical state theory, and extends upon previous understanding. The proposed model is calibrated to Erksak sand and then used to predict the load-unload-reload behaviour of Fraser River sand. The trends predicted from the theoretical and numerical approaches match the experimental observations closely. Shear strength is not highly affected by unload-reload loops. Conversely, volumetric changes as a result of unloading-reloading are dramatic. Volumetric strains in unloading depend on the last value of stress ratio (q/p’) in the previous loading. It appears that major changes in particles arrangement occur once peak stress ratio is exceeded. The developed unload-reload model requires three additional input parameters, which were correlated to the monotonic parameters, to represent hardening in unloading and reloading and the effect of induced fabric changes on stress dilatancy. The calibrated model gave accurate predictions for the results of triaxial tests with load-unload-reload cycles on Fraser River sand.

The behaviour of sands during loading has been studied in great detail. However, little work has been devoted to understanding the response of sands in unloading. Drained triaxial tests indicate that, contrary to the expected elastic behaviour, sand often exhibit contractive behaviour when unloaded. Undrained cyclic simple shear tests show that the increase in pore water pressure generated during the unloading cycle often exceeds that generated during loading. The tendency to contract upon unloading is important in engineering practice as an increase in pore water pressure during earthquake loading could result in liquefaction. This research contributes to filling the gap in our understanding of soil behaviour in unloading and subsequent reloading. The approach followed includes both theoretical investigation and numerical implementation of experimental observations of stress dilatancy in unload-reload loops. The theoretical investigation is done at the micromechanical level. The numerical approach is developed from observations from drained triaxial compression tests. The numerical implementation of yield in unloading uses NorSand — a hardening plasticity model based on the critical state theory, and extends upon previous understanding. The proposed model is calibrated to Erksak sand and then used to predict the load-unload-reload behaviour of Fraser River sand. The trends predicted from the theoretical and numerical approaches match the experimental observations closely. Shear strength is not highly affected by unload-reload loops. Conversely, volumetric changes as a result of unloading-reloading are dramatic. Volumetric strains in unloading depend on the last value of stress ratio (q/p’) in the previous loading. It appears that major changes in particles arrangement occur once peak stress ratio is exceeded. The developed unload-reload model requires three additional input parameters, which were correlated to the monotonic parameters, to represent hardening in unloading and reloading and the effect of induced fabric changes on stress dilatancy. The calibrated model gave accurate predictions for the results of triaxial tests with load-unload-reload cycles on Fraser River sand. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

<p> A theoretical solution for the progress of consolidation of a saturated soil layer subjected to cyclic loading is obtained. A comparison between the theoretical solution and the experimental results of Kaolin samples consolidating under cyclic loadings is presented. </p> / Thesis / Master of Engineering (MEngr)

The first instability of circular tubes compressed into the plastic range is axisymmetric wrinkling, which is stable. Compressed further the wrinkle amplitude grows, leading to a limit load instability followed by collapse. The two instabilities can be separated by strain levels of a few percent. This work investigates whether a tube that develops small amplitude wrinkles can be subsequently collapsed by persistent cycling. The problem was first investigated experimentally using SAF 2507 super-duplex steel tubes with D/t of 28.5. The tubes are first compressed to strain levels high enough for mild wrinkles to form and then cycled axially under stress control about a compressive mean stress. This type of cycling usually results in accumulation of compressive strain; here it is accompanied by growth of the amplitude of the initial wrinkles. The tube average strain initially grows nearly linearly with the number of cycles, but as a critical value of wrinkle amplitude is approached, wrinkling localizes, the rate of ratcheting grows exponentially and the tube collapses. Similar experiments were then performed for tubes involving axial cycling under internal pressure and the combined loads cause simultaneous ratcheting in the hoop and axial directions as well as a gradual growth of the wrinkles. The rate of ratcheting and the number of cycles to collapse depend on the initial compressive pre-strain, the internal pressure, and the stress cycle parameters all of which were varied sufficiently to generate vii a sufficient data base. Interestingly, in both the pressurized and unpressurized cases collapse was found to occur when the accumulated average strain reaches the value at which the tube localizes under monotonic compression. A custom shell model of the tube with initial axisymmetric imperfections, coupled to the Dafalias-Popov two-surface nonlinear kinematic hardening model, are presented and used to simulate the experiments performed. It is demonstrated that when suitably calibrated this modeling framework reproduces the prevalent ratcheting deformations and the evolution of wrinkling including the conditions at collapse accurately for all experiments. The calibrated model is then used to evaluate the ratcheting behavior of pipes under thermal-pressure cyclic loading histories experienced by axially restrained pipelines. / text

Thesis (Ph.D.)--University of Wollongong, 2004. / Typescript. Includes bibliographical references: leaf 215-219.

Investment into offshore wind farms has been growing to address the growing threat of climate change. The majority of offshore wind turbines (both current and planned) are founded on monopiles, large circular steel pipe piles ranging from 4.0 m – 7.5 m in diameter. Based on available borehole records, most planned wind turbines in the UK will be founded in overconsolidated clay deposits. Monopile design is done via usage of the well established p-y curves. However, there are issues with the usage of the p-y curves. Firstly, the curves may be unsuitable to model the monopile’s behaviour as it is expected to behave similarly to a rigid pile rather than flexibly. Secondly, the curves may not accurately estimate the initial pile-soil stiffness. Thirdly, the curves are not comprehensive enough to account for the accumulated strain and stiffness changes resulting from cyclic loading. Considering these issues, research was carried out to improve the current design of monopiles in clay by carrying out displacement controlled monotonic and load controlled cyclic load tests in a centrifuge. Results from monotonic tests suggest that the DNV (2014) design methodology to construct p-y curves in clay based on Matlock’s (1970) soft clay criterion significantly underestimate stiffness. Findings suggested that the experimental p-y curves could be characterised through modification of the criterion. Modification of the criterion produced estimates that matched the 3.83 m monopile experimental curves. Pile toe shear force was observed to contribute little to ultimate lateral resistance and stiffness. Despite the marginal contribution, an effort was made to characterise the pile toe shear force. Estimates of the modified criterion on the 7.62 m monopile did not match the observations, indicating that further research should be carried out to improve the modified criterion. The cyclic tests displayed two distinct regimes; the stiffening regime and the softening regime. Results suggests that cyclic loads of different characteristics influence the locked in stress conditions of the soil which in turn influence the excess pore pressure behaviour, hence dictating whether the stiffening or softening regime takes place. Suggestions were made regarding the conditions that dictated whether the stiffening or softening regime would take place. In the stiffening regime, the stiffening rate decreased with increasing strain while as the accumulated rotation rate increased with vertical load for the same cyclic load magnitude. The softening regime was determined to be extremely detrimental as the high rates of softening and accumulated rotations could cause failure of the system in the short-term. Recommendations were made to estimate the cyclic stiffness and accumulated rotations resulting from both stiffening and softening regime.

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Previous research has provided evidence in support of apoptotic osteocytes playing a role in the bone remodeling process. In this study, we examined the regional and quadrantal variations of apoptotic and viable osteocytes in cyclically loaded and unloaded samples. Left tibias of C57 Black 6 Taconic mice (C57Bl/6) were cyclically loaded for either 2 weeks or 5 weeks, with the right tibias being used as controls. After loading, tibias were resected, processed, and then stained using either a TUNEL stain, to show apoptotic osteocytes, or a 2.0% methyl green solution, to reveal viable cells. Cross-sectional images from each tibia were then captured and analyzed in each region (distal, midshaft and proximal) and quadrant (cranial, lateral, caudal, and medial) by counting the number of osteocytes, both apoptotic and viable, and subsequently calculating the percentages and densities of those osteocytes. Individual analysis of each sample group showed that the 5 week loaded bones, with the most statistically significant p-values, had the most regional variations within the samples, specifically showing decreased apoptotic and viable osteocytes in the lateral quadrants. Comparative analysis revealed a statistically significant higher percentage and density of apoptotic osteocytes in 5 week loaded samples compared to all other samples. This provides further quantitative evidence in support of apoptotic osteocytes playing a role in bone remodeling.