Current Issue : April - June Volume : 2012 Issue Number : 2 Articles : 6 Articles
Background: Pelvic incidence, sacral slope and slip percentage have been shown to be important predicting\r\nfactors for assessing the risk of progression of low- and high-grade spondylolisthesis. Biomechanical factors, which\r\naffect the stress distribution and the mechanisms involved in the vertebral slippage, may also influence the risk of\r\nprogression, but they are still not well known. The objective was to biomechanically evaluate how geometric sacral\r\nparameters influence shear and normal stress at the lumbosacral junction in spondylolisthesis.\r\nMethods: A finite element model of a low-grade L5-S1 spondylolisthesis was constructed, including the\r\nmorphology of the spine, pelvis and rib cage based on measurements from biplanar radiographs of a patient.\r\nVariations provided on this model aimed to study the effects on low grade spondylolisthesis as well as reproduce\r\nhigh grade spondylolisthesis. Normal and shear stresses at the lumbosacral junction were analyzed under various\r\npelvic incidences, sacral slopes and slip percentages. Their influence on progression risk was statistically analyzed\r\nusing a one-way analysis of variance.\r\nResults: Stresses were mainly concentrated on the growth plate of S1, on the intervertebral disc of L5-S1, and\r\nahead the sacral dome for low grade spondylolisthesis. For high grade spondylolisthesis, more important\r\ncompression and shear stresses were seen in the anterior part of the growth plate and disc as compared to the\r\nlateral and posterior areas. Stress magnitudes over this area increased with slip percentage, sacral slope and pelvic\r\nincidence. Strong correlations were found between pelvic incidence and the resulting compression and shear\r\nstresses in the growth plate and intervertebral disc at the L5-S1 junction.\r\nConclusions: Progression of the slippage is mostly affected by a movement and an increase of stresses at the\r\nlumbosacral junction in accordance with spino-pelvic parameters. The statistical results provide evidence that pelvic\r\nincidence is a predictive parameter to determine progression in isthmic spondylolisthesis...
Basic principles of the strapdown inertial navigation system (SINS) using the outputs of strapdown gyros and accelerometers are\r\nexplained, and the main equations are described. A mathematical model of SINS is established, and its Matlab implementation\r\nis developed. The theory is illustrated by six examples which are static status, straight line movement, circle movement, s-shape\r\nmovement, and two sets of real static data....
The paper considers a production facility that might deteriorate suddenly at some point during the production run time; after\r\ndeterioration, nonconforming items are produced in a greater rate compared to the rate before deterioration. Moreover, the\r\nproduction facility may ultimately break down; consequently, the production lot is aborted before completion. If breakdown\r\nhappens, corrective action is started immediately; otherwise, the production lot is completed and preventive repair is implemented\r\nat the end of the production cycle to enhance system reliability. The mathematical model is formulated under general distributions\r\nof failure, corrective, and repair times, while the numerical examples are solved under exponential failure and uniform repair\r\ntimes. The formulated model successfully determines the optimal lot size in addition to the optimal process parameters (mean and\r\nstandard deviation) simultaneously....
Cutting operations using blades can arise in a number of industries, for example, food processing industry, in which cheese, fruit\r\nand vegetable, even meat, are involved. Certain questions will rise during these works, such as ââ?¬Å?why pressing-and-slicing cuts use\r\nless force than pressing-only cutsââ?¬Â and ââ?¬Å?how is the influence of the blade cutting-edge on forceââ?¬Â. To answer these questions, this\r\nresearch developed a mathematical expression of the cutting stress tensor. Based on the analysis of the stress tensor on the contact\r\nsurface, the influence of the blade edge-shape and slicing angle on the resultant cutting force were formulated and discussed. These\r\nformulations were further verified using experimental results by robotic cutting of potatoes. Through studying the change of the\r\ncutting force, the optimal slicing angle can be obtained in terms of maximum feeding distance and minimum cutting force. Based\r\non the blade sharpness properties and the specific materials, the required cutting force can be predicted. These formulation and\r\nexperimental results explained the basic theory of blade cutting fracture and further provided the support to optimize the cutting\r\nmechanism design and to develop the force control algorithms for the automation of blade cutting operations....
Three versions of Bartlett Lewis rectangular pulse rainfall models, namely, the Original Bartlett Lewis (OBL), Modified Bartlett\r\nLewis (MBL), and 2N-cell-type Bartlett Lewis model (BL2n), are considered. These models are fitted to the hourly rainfall data\r\nfrom 1970 to 2008 obtained from Petaling Jaya rain gauge station, located in Peninsular Malaysia. The generalized method of\r\nmoments is used to estimate the model parameters. Under this method, minimization of two different objective functions which\r\ninvolve different weight functions, one weight is inversely proportional to the variance and another one is inversely proportional to\r\nthe mean squared, is carried out using Nelder-Mead optimization technique. For the purpose of comparison of the performance\r\nof the three different models, the results found for the months of July and November are used for illustration. This performance is\r\nassessed based on the goodness of fit of the models. In addition, the sensitivity of the parameter estimates to the choice of the objective\r\nfunction is also investigated. It is found that BL2n slightly outperforms OBL. However, the best model is theModified Bartlett\r\nLewis MBL, particularly when the objective function considered involves weight which is inversely proportional to the variance....
The motion equations of anisotropic media, coupled to the mass conservation and thermoequilibrium equations of fluid, are\nstudied here based on the standard space of physical presentation for thermoelastic dynamics of anisotropic saturated porous\nsolids. By introducing a new compressible thermo-elastic model, a set of uncoupled equations of elastic waves are deduced. The\nresults show that the elastic waves and speeds of elastic waves are affected by both anisotropic subspaces of solids and thermal and\ncompressive coupling coefficients between fluid and solid. Based on these laws, we discuss the propagation behaviour of elastic\nwaves for various anisotropic solids....
Loading....