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The main goal of this study is to develop a novel multi-body dynamic model for seat-occupant system of a manned spacecraft. The seat of a spacecraft and its components has significant effects on an astronaut’s body during flight mission, where sustained accelerations and impact loads are inevitable. Besides, conducting sustained acceleration and shock loads on the body of astronauts during flight missions is the key factor for health conservation. This study develops a new multi-body dynamic model for seat-occupant system of manned spacecraft to investigate the effects of spacecraft seat design. The astronaut is modelled as a four-jointed rigid link placed on a viscoelastic cushion. The cushion is modelled to present polyurethane foam, which is using in almost all automotive and aerospace applications. The equations of motion are derived and the effects of different parameters are investigated. To verify the developed model, a finite element model is presented to implement the same exact multi-body model. The results show that the presented analytical multi-body model is in good accordance with the finite element model. The effects of different parameters such as foam discretisation, foam thickness and geometry of seat frame on the motion of the system are illustrated. This novel model can be used in designing spacecraft seat to preserve an astronaut’s health. It also can be used in seat control applications and where computational efficiency is necessary. [ABSTRACT FROM AUTHOR]

Background and Aims: Hydroxyapatite coating has allocated a special place in dentistry due to its biocompatibility and bioactivity. The purpose of this study was to evaluate the relation between the hydroxyapatite thickness and stress distribution by using finite element method. Materials and Methods: In this paper, the effect of hydroxyapatite coating thickness on dental implants was studied using finite element method in the range between 0 to 200 microns. A 3D model including one section of mandible bone was modeled by a thick layer of cortical surrounding dense cancellous and a Nobel Biocare commercial brand dental implant was simulated and analyzed under static load in the Abaqus software. Results The diagram of maximum von Mises stress versus coating thickness was plotted for the cancellous and cortical bones in the range between 0 to 200 microns. The obtained results showed that the magnitude of maximum von Mises stress of bone decreased as the hydroxyapatite coating thickness increased. Also, the thickness of coating exhibited smoother stress distribution and milder variations of maximum von Mises stress in a range between 60 to 120 microns. Conclusion: In present study, the stress was decreased in the mandible bone where hydroxyapatite coating was used. This stress reduction leads to a faster stabilization and fixation of implant in the mandible bone. Using hydroxyapatite coating as a biocompatible and bioactive material could play an important role in bone formation of implant-bone interface. [ABSTRACT FROM AUTHOR]

Background and Aims: Different mechanisms have been developed for connecting abutment to implant. One of the most popular mechanisms is Tapered Integrated Screw (TIS), which is a Tapered Interference Fit (TIF) with a screw integrated at the bottom of that. The aim of this study was to investigate the mechanism of TIS and effective factors in employing TIS during design and implementation processes using an analytic method. Materials and Methods: Relevant equations were developed to predict tightening and loosening torques, contact pressure and preloads with and without bone tissue in this analysis. The efficiency is defined as the ratio of the loosening torque to the tightening torque. The effects of the change in elastic modulus and thickness of the bone on operation of this mechanism were investigated. Results: In this study, 14 independent parameters such as taper angle, friction coefficient, abutment and implant geometry that are effective on performance of TIS mechanism were presented. The role of some factors was shown in the performance of ITI implant using sensitivity analysis. Conclusion: It was shown that friction coefficient, contact length, and implant radius play major roles on tightening and loosening torques and efficiency of the mechanism. Furthermore, the results revealed that the change in the elastic modulus and thickness of the bone influenced the efficiency of the mechanism less than 15%. [ABSTRACT FROM AUTHOR]