Cengiz Baykasoglu

Railroad passenger car collision analysis and modifications for improved crashworthiness

In this study, crashworthiness assessment and suggestions for the modification of a railroad passenger car are presented. To assess the crashworthiness, collision of the railroad passenger car onto a rigid wall is simulated by using finite element (FE) methods. A full-length, detailed passenger car model is used in FE analyses. In order to validate the FE model, simulation results obtained for different types of static loading conditions in compliance with various scenarios defined in UIC CODE OR 577 are compared with experimental measurements before running collision analyses of the railroad passenger car. The good agreement between static tests and FE analyses results indicates that the FE model accurately represents the real structure. Following the FE model validation, analysis of the collision behaviour of the railroad passenger car consists of two stages. In the first stage, the crashworthiness of the initial concept design of the railroad passenger car is analysed. It was observed that local buckling takes place at various points, which prevents the desired progressive damage behaviour in the railroad car body. Having revealed the structural weaknesses, the initial design was modified and simulated again under the same conditions. Using size optimisation, thickness of some sheet metal components is changed in order to obtain the intended progressive damage behaviour. As a result of the modifications, the passenger car design with better crashworthiness properties was obtained, in which large plastic deformations occur around the collision side of the car while mainly elastic deformations occur in the cars body away from the bumpers.

Energy absorption of circular aluminium tubes with functionally graded thickness under axial impact loading

The main objective of this study is to investigate the effects of thickness-gradient patterns on energy absorption characteristics of aluminium-based circular tubes under axial impact loading. Functionally graded thickness (FGT) enables to obtain variable stiffness throughout the length of a structure; thus, it provides more efficient control of the crashworthiness parameters when compared with traditionally designed uniform thickness (UT) counterparts. In order to investigate the crash behaviour of FGT tubes, different thickness-gradient patterns are introduced to the axial direction of the tubes and then impact with a fixed rigid wall is simulated by using the nonlinear explicit finite element (FE) method. To show the efficiency of FGT tubes, crashworthiness performance of the FGT tubes are compared with their UT counterparts at the same weight. The effects of thickness range and aspect ratio of the tubes on their crash behaviours are also investigated. The simulation results show that the FGT tubes have superior crashworthiness performance than that of their UT counterparts and the crashworthiness parameters of the FGT tubes can be controlled and improved with the appropriate selection of geometric parameters of the tubes.

Effect of attachment types and number of implants supporting mandibular overdentures on stress distribution: a computed tomography-based 3D finite element analysis.

The objective of this study was to calculate stresses in bone tissue surrounding uncoupled and splinted implants that are induced by a bite force applied to the mandible and to determine whether the number of mandibular overdenture supporting implants in mandibular bone influence the stress distribution. A human adult edentulous mandible retrieved from a formalin fixed cadaver was used to define the geometry of finite element (FE) model and the FE model was verified with experimental measurements. Following the FE model validation, three different biting situations were simulated for the 2-, 3- and 4-implant retentive anchor as well as bar attachment overdentures under vertical loading of 100 N. As a result of the analyses, it was concluded that an increment in implant number and the splinted attachment type tended to cause lower stresses and the use of two single attachments seems to be a safe and sufficient solution for the treatment of mandibular edentulism with overdentures.

Crash and structural analyses of an aluminium railroad passenger car

Crashworthiness, strength and vibrational features of a railroad passenger car, which is originally made of steel members and then converted to an aluminium design, are studied. The finite element (FE) method is utilised for the static analysis in compliance with various scenarios defined in UIC CODE OR 577, modal analysis and simulation of the crash into a rigid wall. Firstly, a full-length, detailed passenger car model made of steel is used in FE analyses and the model is verified for the steel car body by comparisons with strain measurements and experimental evaluation of natural frequencies. The agreement between test measurements and FE results indicates that the FE model of the railroad car accurately represents the original structure. Following, effects of material change on the structural behaviour can be accurately judged based on the outcomes of the analyses. It is observed that the stress values and natural frequencies of the aluminium structure are almost equal to those of the original steel structure. Moreover, the crash energy absorption characteristics are within the acceptable tolerances for both cases. The final aluminium design is found to be about one-third of the weight of the initial steel structure while it preserves stiffness values within acceptable limits. In addition, an equivalent spring-mass system is developed to model the crash of both steel and aluminium passenger cars, which can be used for occupant safety investigations in future.

Development of a design for a crash energy management system for use in a railway passenger car

A design approach for a crash energy management (CEM) system for a N13-type railway passenger car used by the Turkish State Railway Company is developed in this paper. The components of the CEM system are honeycomb-structured boxes, primary energy absorbers, shear bolts, a sliding sill mechanism and a fixed sill mechanism that are located in the passenger-free space at the end of the passenger car. In order to investigate the benefits provided by the CEM system, a full-scale railway passenger car collision with a rigid wall is simulated by using dynamic/explicit finite element (FE) methods. The crushing force, secondary impact velocity, acceleration and velocity curves, and deformation modes are computed to allow a comparison of the crashworthiness performance of a passenger car equipped with the proposed CEM system with that of a conventional passenger car. Comparisons of FE analysis results show that a passenger car incorporating the CEM system has a superior crashworthiness performance to that of the conventional passenger car.

The influence of the attachment type and implant number supporting mandibular overdentures on stress distribution: an in vitro study, part I.

Objectives:The main goal of this study was to compare the stress distribution of mandibular overdentures (OVD) with different numbers of supporting implants and single versus splinted attachment types. Materials and Methods:Four different biting situations were simulated for the 2-, 3-, and 4-implant retentive anchor as well as bar attachment OVDs on a formalin-fixed cadaver mandible, and strains were recorded under vertical loading of 100 N. Results:The calculated von Mises values from measured strains in all measurement sites and loading conditions for nonsplinted attachments (retentive anchor) were higher than splinted (bar) attachments. Conclusions:It may be concluded that in cases with low quality and quantity of bone, the increase in number of implants and the use of a splinted attachment should be preferred to reduce forces emerging around the implants during function. The use of 2 single attachments in cases with good bone quality and ideal size implants still seems to be a safe and sufficient solution for the treatment of mandibular edentulism with OVDs.

Rollover crashworthiness analysis of a railroad passenger car

Rollover of railroad passenger cars is one of the most serious accidents having high rates of injuries and fatalities. Therefore, the railroad passenger car structures should satisfy the rollover crashworthiness requirements in order to protect the occupants. In this paper, rollover crashworthiness of a railroad passenger car called ‘N13-type’ used by Turkish State Railways is investigated. To assess the rollover crashworthiness, rollover simulations of the full scale and detailed passenger car models onto a rigid ground are completed by using a non-linear explicit finite element code, Abaqus. The numerical model is validated with static and modal experimental measurements before running rollover analyses of the railroad passenger car. Good agreement is observed between tests and finite element results, which indicate that the numerical model accurately represents the real structure. Simulations are performed in accordance with the regulation of ECE R66. Based on this standard, two different rollover scenarios are considered. In the first scenario, lateral rollover simulation is performed at the standard angular velocity due to ECE R66 regulation. The second scenario is performed at the double of this angular velocity to investigate the crashworthiness behaviour under exaggerated conditions. As a result of the analyses, it is observed that the residual space remains undamaged during and after the rollover, and passenger car is able to pass the required regulations.

VIBRATION AND ELASTIC BUCKLING ANALYSES OF SINGLE-WALLED CARBON NANOCONES

This paper reports the result on elastic buckling and vibration behaviors of singlewalled carbon nanocones (SWCNCs) having the potential usage in atomic force microscope and scanning tunneling microscope tips. The modeling work employs the molecular mechanics based finite element approach in which Euler-Bernoulli beam element formulations are used with consistent mass matrix. Free-free, free-clamped and clamped-clamped boundary conditions are considered in vibration analysis of SWCNCs; on the other hand, axial compression and bending loading conditions are taken into account in elastic buckling behavior of SWCNCs. The effects of cone height and disclination or apex angles on the buckling force and natural frequencies of SWCNCs are investigated. Vibration analysis results indicate that the natural frequency decreases with increasing cone height in all types of SWCNCs, whereas it increases as the disclination angle increases. Buckling analysis results indicate that as the disclination angle increases, the critical buckling load increases in axial compression loading and decreases in bending loading. In addition, it is observed that bending loading is more critical than axial compression loading for buckling behavior of SWCNCs if the disclination angle increases. Cengiz Baykasoglu, Alper T. Celebi Esra Icer and Ata Mugan

Thermal-stress analysis of ceramic laminate veneer restorations with different incisal preparations using micro-computed tomography-based 3D finite element models

Main objective of this study is to investigate the thermal behavior of ceramic laminate veneer restorations of the maxillary central incisor with different incisal preparations such as butt joint and palatinal chamfer using finite element method. In addition, it is also aimed to understand the effect of different thermal loads which simulates hot and cold liquid imbibing in the mouth. Three-dimensional solid models of the sound tooth and prepared veneer restorations were obtained using micro-computed tomography images. Each ceramic veneer restoration was made up of ceramic, luting resin cement and adhesive layer which were generated based on the scanned images using computer-aided design software. Our solid model also included the remaining dental tissues such as periodontal ligament and surrounding cortical and spongy bones. Time-dependent linear thermal analyses were carried out to compare temperature changes and stress distributions of the sound and restored tooth models. The liquid is firstly in contact with the crown area where the maximum stresses were obtained. For the restorations, stresses on palatinal surfaces were found larger than buccal surfaces. Through interior tissues, the effect of thermal load diminished and smaller stress distributions were obtained near pulp and root-dentin regions. We found that the palatinal chamfer restoration presents comparatively larger stresses than the butt joint preparation. In addition, cold thermal loading showed larger temperature changes and stress distributions than those of hot thermal loading independent from the restoration technique.

NONLINEAR FRACTURE ANALYSIS OF CARBON NANOTUBES WITH STONE-WALES DEFECTS

In this paper, a molecular mechanic based finite element model is employed to investigate the effects of Stone-Wales defects on mechanical properties of armchair and zigzag carbon nanotubes by considering large deformation and nonlinear geometric effects. Nonlinear characteristic of the covalent bonds are obtained by using the modified Morse potential and effects of the large deformation and geometric nonlinearities are considered by updating the atomistic coordinates of the original nanotube structure at each load step. The results show that the fractures of all types of carbon nanotubes are brittle, but armchair nanotubes are stiffer than zigzag nanotubes and these defects significantly affect the mechanical performance of nanotubes. Fracture initiation and crack propagation direction issues are also studied. It is shown that the direction of crack propagation in armchair nanotube is in the maximum shear directions having an angle of ±45° along its circumference. Comparisons are made with the failure stress and strain results reported in literature that show good agreement with our results.