Browsing by Subject "Finite element"

It is well understood that loss of motion following spinal fusion increases strain in the adjacent motion segments. However, it is unclear if to date, studies on cervical spine biomechanics can be affected by the role of coupled motions in the lumbar spine. Accordingly, we investigated the biomechanics of the cervical spine following cervical fusion and lumbar fusion during simulated whiplash.

A validated whole-human finite element model was used to investigate whiplash injury. The cervical spine before and after spinal fusion was subjected to simulated whiplash exposure in accordance with Euro NCAP testing guidelines, and the strains in the anterior longitudinal ligaments of the adjacent motion segments were computed.

In the models of cervical arthrodesis, peak ALL strains were higher in the motion segments adjacent to the level of fusion, and strains directly increased with longer fusions. The mean strain increase in the motion segment immediately adjacent to the site of fusion from C2-C3 through C5-C6 was 26.1% and 50.8% following single- and two-level cervical fusion (p=0.03). On average, peak strains experienced in a lumbar-fused spine were 1.0% less than those seen in a healthy spine (p=0.61). The C3-C4 motion segment had disproportionately high increases in strain following cervical fusion. The C6-C7 motion segment experienced high absolute strain under all tested conditions but the increase in strain following fusion was very small. This study provides support for both the hypothesis that adjacent segment disease is associated with post-arthrodesis biomechanical influences and the hypothesis that adjacent segment disease is a result of natural history, and inherent structures at risk.

The purpose of this thesis was to examine progressive composite damage models available within LS-DYNA and to correlate the results of these models with drop weight impact testing and with the non-destructive evaluation techniques of shearography, thermography, and ultrasonic testing. The secondary purpose of this study was to assess whether shearography and thermography provide an adequate less expensive replacement to ultrasonic testing. For this investigation, three models were chosen: Chang-Chang, Chang-Chang + Tsai-Wu, and a Faceted Failure Surface

Model. For the experimental impact testing, two sets of specimens were chosen: a 16-ply lay-up and a 32-ply lay-up of carbon fiber pre-preg material. The panel specimens were tested at various impact energies and the displacement and force history of the impactor were recorded. The models showed good correlation for the force history with the experiments. Furthermore, the 16-ply models correlated well with the displacement history. However, due the penalty method implementation, the 32-ply models did not show similar peak displacement output. The damage shown by the models was compared to non-destructive evaluation techniques. The shearography and thermography showed significantly less damage than the ultrasonic scans, and therefore do not provide an adequate replacement to ultrasonic scanning. In looking at correlation between the models and the non-destructive evaluation techniques, the faceted failure surface showed significantly more damage due to its elastic-plastic type formulation.

In the finite element simulation of problems with contact there arises

the need to change the mesh and continue the simulation on a new mesh.

This is encountered when the mesh has to be changed because the original mesh experiences severe distortion or the mesh is adapted to minimize errors in the solution. In such instances a crucial component is the transfer of data from the old mesh to the new one.

This work proposes a strategy by which such remeshing can be accomplished in the presence of mortar-discretized contact,

focusing in particular on the remapping of contact variables which must occur to make the method robust and efficient.

By splitting the contact stress into normal and tangential components and transferring the normal component as a scalar and the tangential component by parallel transporting on the contact surface an accurate and consistent transfer scheme is obtained. Penalty and augmented Lagrangian formulations are considered. The approach is demonstrated by a number of two and three dimensional numerical examples.

Due to the increasing cost of the enhancement techniques in current projection lithographic techniques and the required time in developing new technology's

as feasible manufacturing technology, EUVL is considered as the leading candidate for production of 45 nm node and less. In EUVL, mask is held electrostatically against chuck. This electrostatic chucking process affects the nonflatness of the mask due to contact interaction and the voltage force between the mask and chuck. A fundamental understanding of chucking phenomenon is required to realize the SEMI P37 and SEMI P40 stringent flatness requirements.

The primary challenge is to understand and characterize the ability of electrostatic chucking phenomenon to acheive consistent and reliable shapes of chucked masks.

The objective of this thesis is to study the effect of initial nonflatness of mask and chuck, chucking voltage, chuck and mask dimension and gravity on the final nonflatness of the mask A finite element model of the mask and chuck with initial nonflat surface is developed. To predict the final nonflatness of the mask frontside with nm accuracy, the contact interaction between mask and chuck is modeled using van der Waals forces. These results are compared with penalty method for the runtime and accuracy of results

Every day, 1,300 children in the U.S. and an additional 34,000 children worldwide are born prematurely. This study acts as a feasibility study for a proposed ultrasonic technique for the identification of preterm birth risk factors using an acoustic technique known as Acoustic Radiation Force Impulse (ARFI) imaging. A 3D finite element model was constructed to optimize transducer ARFI parameters in a layered cervix structure prior to clinical evaluation. The transducer model optimized in this study was the AcuNav^TM (Siemens Medical Solutions, Mountain View, CA). Cervix model structural geometry and material properties were varied according to anticipated pregnancy induced property fluctuation. Transmitted ARFI acoustic fields were generated by applying a Field II derived pulse to the 3D model[15]. Optimization procedures were performed in the following order: focal depth evaluation, transmit frequency optimization, effect of material property variation and the application of ARFI shear wave speed calculation algorithms to a layered cervical structure. Results indicated that ARFI evaluation of a layered cervix structure was most feasible using an 8MHz transmit frequency in the focal range of 5-10mm axial depth. It was observed that material property estimation errors were most likely when ARFI excitations were focused near a material boundary. A phenomenon was noted where shear waves initiated in stiffer media were slowed as a function of their relative proximity to a more compliant medium. Overall, these simulation studies demonstrate that ARFI shear wave imaging in the cervix is feasible; a model has been developed that can be used to evaluate the accuracy of shear stiffness estimates in the cervix to help address the important clinical problem of premature cervical ripening.