No description available.

Osteoporosis is a disease characterized by bone loss affecting 10% of the US population over 50 years old. The spine is one critical area affected by the disease. The effectiveness of experimental treatments can be tested on an ovariectomized rat osteoporosis model. As a result, lumbar vertebral bodies are often mechanically tested in uniaxial compression in order to determine whether or not the mechanical properties of the bone in ovariectomized rats improve with treatment. The irregular shape of rat vertebral bodies requires some specimen preparation to create two parallel loading surfaces for uniaxial compression testing. Two specimen preparation methods are reported in the current body of literature. One cuts the cranial and caudal surfaces to make them parallel to each other. The other cuts the caudal surface and uses bone cement to create a flat loading surface at the cranial end. In this thesis a total of twenty rat vertebral bodies were tested. Ten were prepared with a cut specimen preparation method and ten with an embedding method. Each specimen was tested in uniaxial compression and was microCT scanned before and after testing. Eleven parameters were calculated from the mechanical testing data and compared between the two groups using Student’s t-tests. The specimens were also categorized into six failure modes and locations observed in the microCT images. The embedded specimens showed a lower stiffness (p = 0.026), greater yield displacement (p = 0.007) and apparent strain at failure (p = 0.050). These differences were largely attributed to the embedded specimens being 1 mm taller than the cut specimens. The shorter size of the cut specimens affected the mechanical parameters. The cut specimens were easier to prepare and were less sensitive to end effect failures. The embedded specimens kept the endplate, which distributes the load from the intervertebral disk through the vertebral body, intact. In addition, the embedded specimens exhibited two failure modes, endplate failure and failure at the center of the vertebral body, observed in ex vivo human lumbar vertebral body testing, which suggests the interaction of the vertebral body with the endplate is an important factor in vertebral body failure in uniaxial compression testing. In conclusion, neither preparation method showed an overwhelming advantage over the other, and experimental parameters should be considered when choosing a loading surface preparation method. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2013-09-30 11:58:32.794

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Thesis (Ph. D.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2008. / Committee Chair: Lyon, L. Andrew; Committee Member: Breedveld, Victor; Committee Member: Hernandez, Rigoberto; Committee Member: Srininvasarao, Mohan; Committee Member: Weeks, Eric R.

No description available.

This study is concerned with the mechanical behavior of open-cell aluminum foams. In particular the compressive response of aluminum foams is analyzed through careful experiments and analyses. The microstructure of foams of three different cell sizes was first analyzed using X-ray tomography. This included characterization of the polyhedral geometry of cells, establishment of the cell anisotropy and statistical distribution of ligament lengths, and measurement of the ligament cross sectional area distribution. Crushing experiments were performed on various specimen sizes in the principal directions of anisotropy. The compressive response of aluminum foams is similar to that of many other cellular materials. It starts with a linearly elastic regime that terminates into a limit load followed by an extensive stress plateau. During the plateau, the deformation localizes in the form of inclined but disorganized bands. The evolution of such localization patterns was monitored using X-ray tomography. At the end of the plateau, the response turns into a second stable branch as most cells collapse and the foam is densified. The crushing experiments are simulated numerically using several levels of modeling. The ligaments are modeled as shear-deformable beam elements and the cellular microstructure is mainly represented using the 14-sided Kelvin cell in periodic domains of various sizes. Other geometries considered include the perturbed Kelvin cell, and foams with random microstructures generated by the Surface Evolver software. All microstructures are assigned geometric characteristics that derive directly from the measurements. Unlike elastic foams, for elastic-plastic foams the prevalent instability is a limit load. The limit load can be captured using one fully periodic characteristic cell. The predicted limit stresses agree with the measured initiation stresses very well. This very good performance coupled with its simplicity make the characteristic cell model a powerful tool in metal foam mechanics. The subsequent crushing events, the stress plateau and desification were successfully reproduced using models with larger, finite size domains involving several characteristic cells. Results indicate that accurate representation of the ligament bending rigidity and the base material inelastic properties are essential whereas the randomness of the actual foam microstructure appears to play a secondary role. / text

The compressive behavior of continuous fiber composites is not as well understood as their tensile behavior because research and industrial applications have until recently focused on the latter. Furthermore, most theoretical and experimental studies on the compression of composites have examined the case of unidirectional specimens with fibers along the loading direction (0�� fibers). While this is a logical approach since it isolates the failure mode specific to this geometry (kinking), the study of multidirectional laminates is essential because these are used in all practical applications. Few theories model the compressive behavior of multidirectional laminates. None of the theories account for the stress field or the sequence and interaction of the various observed failure modes (kinking, delamination, matrix failure) specific to the multidirectional configuration. The principal objective of this investigation is to construct a realistic theory to model the compressive behavior of multidirectional composites. Compression experiments have repeatedly shown that the initial failure mode was in-plane kinking of 0�� fibers initiated at the edges of the specimens. We decided to base our compressive failure theory upon interlaminar stresses because in multidirectional laminates these are known to exist in a boundary layer along the edges. This required development of an analytical theory giving the amplitude of these stresses at the free edges. We then incorporated these stresses into a new general microbuckling equation for 0�� fibers. The global laminate failure strain was determined through several fiber and matrix failure criteria. Theoretical predictions were compared with experimental results obtained from compression testing of graphite/thermoplastic laminates with the same ply sequence but different off-axis ply angles. The theory correlated well with experiments and confirmed that in-plane kinking was the critical failure mode at low and medium angles, while revealing that out-of-plane buckling was responsible for failure at high angles. Furthermore, the theory correctly predicted the sequence of various fiber and matrix failure modes. / Graduation date: 1997

No description available.

M. Tech. Engineering: Civil. / Aims investigate the response of cement stabilised sub-base layers to dynamic load by evaluating the changes in stiffness at known strain level, and to compare the stiffness from dynamic loads with those derived from the Unconfined Compressive Stress (UCS) test.

The ability to predict failure of laminated composites in compression has been doggedly pursued by researchers for many years. Most have, to a limited extent, been able to predict failure for a narrow range of laminates. No means, as yet, exist for predicting the strength of generic laminates under various load conditions. Of primary concern has been the need to establish the mode at failure in compression. Even this has been known to vary for fiber and matrix dominated laminates. This study has been carried out to analyze the failure of specimens with a hole made of laminates with various quasi-isotropic stacking sequences. Different stacking sequences are achieved by rotating a [±45/90/0]s stacking sequence laminate as a whole with respect to the loading axis of the specimens. Two- and three-dimensional finite element models, using commercial packages, were generated to evaluate the stresses in the region of the hole. Two different compressive failure prediction techniques based on distinctly different failure modes have been used. The validity of these techniques was measured against experimental data of quasi-isotropic specimens tested. To investigate the applicability of the failure criteria for different laminated composite plates, analyses were repeated for specimens with different stacking sequences resulting from the rotation of the laminate. The study shows the need for the use of three-dimensional analysis of the stress state in the vicinity of the hole in order to be able to accurately predict failure. It also shows that no one mode of failure is responsible for limiting the strength for all laminate orientations but rather the mode changes with change in stacking sequence. The failure of the laminate with a hole was seen to be very sensitive to the stacking sequence. Experimental data presented also shows that the peak strength obtainable from the laminate analyzed, [±45/90/0]s, is going to be in the off-axis configuration rather than on-axis placement of the stacking sequence with respect to the loading direction. / Master of Science