Optimal Stress Research Articles

The effect of stress distribution and displacement of open subtalar dislocation in using titanium alloy and stainless steel mitkovic external fixator – a finite element analysis

An external fixator is normally used by medical surgeons in treating subtalar dislocation due to its biomechanical characteristics that can providing an adequate stability, preventing deformity (mal-union and non-union), reduce rate of infections, and promoting fast healing process as compared to coventional internal fixator. Apart from the configurations and fixation techniques, previous studies has mentioned that the stability of external fixator can be altered and manipulated by using different materials, e.g. stainless steel, titanium alloy and polymer. To be noted, the current available research works that have been investigated on different materials of external fixator are still lacking, therefore, the present study is aims to conduct related study. The main objective of the research work is to simulate finite element model of foot and ankle joint associated with open subtalar dislocation in which treated with Mitkovic external fixator by using two different material properties; titanium alloy (Model 1) and stainless steel (Model 2). The three-dimensional model of foot and ankle joint were reconstructed using images of CT dataset. For the soft tissues, cartilages at the ankle joint were developed by offsetting the bone surfaces with 1 mm thickness while ligaments were modelled with linear links. Homogeneous and isotropic properties were assigned to the bone, Mooney-Rivlin model for cartilage and specific stiffness value for ligaments. In order to simulate stance phase during walking condition, an axial load of 350 N was applied to the proximal tibia bone. The results of von Mises stress demonstrated that Model 2 has a low magnitude (127 MPa) at the pin-bone interface of tibia bone, compared with Model 1 (369 MPa). As for the local displacement at the bony segment of fibula, Model 2 (3.3 mm) indicated high stability of the external fixator than Model 1 (7.4 mm). In conclusion, the use of stainless steel material for Mitkovic external fixator can provide adequate stability and optimum stress distribution.

Cultivation and quantitative single-cell analysis of Saccharomyces cerevisiae on a multifunctional microfluidic device.

Here, we present a multifunctional microfluidic device whose integrative design enables to combine cell culture studies and quantitative single cell biomolecule analysis. The platform consists of 32 analysis units providing two key features; first, a micrometer-sized trap for hydrodynamic capture of a single Saccharomyces cerevisiae (S. cerevisiae) yeast cell; second, a convenient double-valve configuration surrounding the trap. Actuating of the outer valve with integrated opening results in a partial isolation in a volume of 11.8 nL, i.e. the cell surrounding fluid can be exchanged diffusion-based without causing shear stress or cell loss. Actuation of the inner ring-shaped valve isolates the trapped cell completely in a small analysis volume of 230 pL. The device was used to determine the growth rate of yeast cells (S. cerevisiae) under under optimum and oxidative stress conditions. In addition, we successfully quantified the cofactor beta-nicotinamide adenine dinucleotide phosphate (NAD(P)H) in single and few cells exposed to the different microenvironments. In conclusion, the microdevice enables to analyze the influence of an external stress factor on the cellular fitness in a fast and more comprehensive way as cell growth and intracellular biomolecule levels can be investigated.

Optimized back analysis method for stress determination based on identification of local stress measurements and its application

This paper presents an optimization method, including the identification of local stress measurements and a two-stage back analysis, to determine in situ stress under complex conditions. Firstly, the macro-regional distribution characteristics of in situ stress can be interpreted by analyzing regional geological conditions, topography, as well as rock mass failure phenomena. Then, a stereographic projection method (SPM) is used to determine the distribution features of stress tensor on an arbitrary plane. On this basis, some representative measured data, which can reflect the distribution characteristics of in situ stress in the study area can be identified. After that, a first-stage back analysis by finite difference method (FDM) is used to calculate the field stresses from the selected in situ measured data. The obtained boundary stresses are taken to consist of a constant term, a term that varies linearly with depth, and a hyperbolic term. Further, a second-stage back analysis by discrete element method (DEM) is carried out to determine the local field stresses which are significantly affected by discontinuities and excavations. The FDM back analysis results can serve as the initial input stress conditions for DEM analysis, and the optimum stress conditions for DEM analysis will be reasonably obtained by using the uniform design method through the second-stage back analysis. To show the feasibility of the optimized method, it is applied to the Wudongde Hydropower Station to determine in situ stress. Based on local stress measurements by borehole stress relief method using hollow inclusion strain gauges, some representative measured data are selected by SPM. The unknown state of stress in the station is estimated through the first-stage back analysis by FDM. For some local key areas, encountered discontinuities or excavations in the vicinity, the local field stresses are calculated through the second-stage back analysis by DEM. Finally, the results are compared with those elicited from the borehole stress relief method. It is shown that the calculated results by the first-stage back analysis agree well with the measured ones on the whole, but the discrepancy is large in the vicinity of discontinuities as well as excavation disturbance. However, the calculated results by the second-stage back analysis roughly coincide with the measured data with lower allowable error.

The Effects of Seed Treatment with Melatonin on Germination and Emergence Performance of Pepper Seeds under Chilling Stress

Melatonin was first isolated from bovine pineal gland more than half a century ago as an important animal hormone and since then it was proved to be present in almost all forms of life including eukaryotic unicells, prokaryotes, fungi, algae, animals and plants. In this study, the effects of pre-sowing seed treatment with melatonin on germination and emergence performance of pepper seeds under chilling conditions were investigated. Seeds were immersed in 0, 1, 5, 10 or 25 µM melatonin solutions for 24 hours after which they were dried for one day and subjected to germination and emergence tests at optimum and chilling stress conditions. Exogenous melatonin treatment promoted pepper seed germination and emergence under chilling conditions. Treatment of seeds with melatonin especially in 1 or 5 µM concentrations significantly improved germination and emergence percentage and rate whereas dry seeds and seeds treated with 0 µM melatonin exhibited the lowest germination and emergence performance. Melatonin application also reduced the MDA and H 2 O 2 content and elevated SOD and CAT enzyme activity. The improvement in germination and emergence performance of pepper under chilling stress conditions following melatonin treatment may therefore be due to reduced lipid peroxidation and elevated activities of antioxidant enzymes.

Controlled vertical stress in a modified amplitude sweep test (rheometry) for the determination of soil microstructure stability under transient stresses

Although there are evidences that most, if not all, soil physical and mechanical processes on larger scales have their origin in processes and properties at the microscale, the range of methods to investigate micro-scaled soil behavior directly is limited. One method to detect flow and deformation behavior under steady and transient stresses is rheometry. This method is well established by means of the amplitude sweep test (AST) to detect viscoelastic properties of soils, i.e. stress-strain relationships under transient stresses that occur due to wheeling or trampling. This test is based upon a soil sample positioned between two parallel plates, where the upper plate rotates in an oscillatory manner with increasing stress or strain amplitude, and the lower plate is fixed. Depending on the retardation of the sample's shear stress reaction, viscous and elastic strain proportions are defined with the help of storage and loss modulus (G′ and G″), and their ratio (tan δ). However, the AST is strongly susceptible to sample preparation and the vertical force of the upper plate. Either a low force (e.g. 1N) was applied or samples with forces beyond a critical limit (e.g. 12–15N) were excluded. We used controlled force of 1, 3 and 10N, equivalent to compressive stresses of 2.04, 6.11 and 20.37kPa, to find the optimum settings to run the AST. The selected forces/stresses are either low, intermediate or high compared to those observed under undefined conditions in two South Brazilian soils (A and B horizon of a Typic Hapludox – Oxisol, and an Oxyaquic Hapluderts – Vertisol). Furthermore, field density and a common (high) density were applied to the homogenized soil material. The variability of the results generally decreased with increasing compressive stress level, and at highest stress variability was lower than under variable force/stress. The microaggregation of the Oxisol based upon iron oxides and hydroxides (pseudosand) and high kaolinite content in the B horizon resulted in a smaller range of recoverable elasticity than in the Vertisol. However, at large strains the elasticity was higher in the Oxisol, while the Vertisol exhibited alignment of the soil particles. The Oxisol generally showed a brittle rupture by means of a maximum shear stress; while in the Vertisol yielding occurred due to expandable clay. High compressive force and density simultaneously reduced the share of elastic deformation and promoted plastic yielding. The results do not allow yet defining an optimum compressive stress. Thus, we suggest investigating further force or stress levels and to test more soils, e.g. of high soil organic matter content and/or high or low clay, silt or sand content.

Morphometric and mechanical characteristics of Equisetum hyemale stem enhance its vibration

Main conclusionThe order of the internodes, and their geometry and mechanical characteristics influence the capability of theEquisetumstem to vibrate, potentially stimulating spore liberation at the optimum stress setting along the stem.Equisetum hyemale L. plants represent a special example of cellular solid construction with mechanical stability achieved by a high second moment of area and relatively high resistance against local buckling. We proposed the hypothesis that the order of E. hyemale L. stem internodes, their geometry and mechanical characteristics influence the capability of the stem to vibrate, stimulating spore liberation at the minimum stress setting value along the stem. An analysis of apex vibration was done based on videos presenting the behavior of an Equisetum clump filmed in a wind tunnel and also as a result of excitation by bending the stem by 20°. We compared these data with the vibrations of stems of the same size but deprived of the three topmost internodes. Also, we created a finite element model (FEM), upon which we have based the ‘natural’ stem vibration as a copy of the real object, ‘random’ with reshuffled internodes and ‘uniform’, created as one tube with the characters averaged from all internodes. The natural internode arrangement influences the frequency and amplitude of the apex vibration, maintaining an equal stress distribution in the stem, which may influence the capability for efficient spore spreading.

Stress analysis of mandibular implant overdenture with locator and bar/clip attachment: Comparative study with differences in the denture base length.

PURPOSEThe design of the attachment must provide an optimum stress distribution around the implant. In this study, for implant overdentures with a bar/clip attachment or a locator attachment, the stress transmitted to the implant in accordance with the change in the denture base length and the vertical pressure was measured and analyzed.MATERIALS AND METHODSTest model was created with epoxy resin. The strain gauges made a tight contact with implant surfaces. A universal testing machine was used to exert a vertical pressure on the mandibular implant overdenture and the strain rate of the implants was measured.RESULTSMeans and standard deviations of the maximum micro-deformation rates were determined. 1) Locator attachment: The implants on the working side generally showed higher strain than those on the non-working side. Tensile force was observed on the mesial surface of the implant on the working side, and the compressive force was applied to the buccal surface and on the surfaces of the implant on the non-working side. 2) Bar/clip attachment: The implants on the both non-working and working sides showed high strain; all surfaces except the mesial surface of the implant on the non-working side showed a compressive force.CONCLUSIONTo minimize the strain on implants in mandibular implant overdentures, the attachment of the implant should be carefully selected and the denture base should be extended as much as possible.

Stress analysis in functionally graded rotating disks with non-uniform thickness and variable angular velocity

Stress field in functionally graded (FG) rotating disks with non-uniform thickness and variable angular velocity is studied numerically. The elastic modulus and mass density of the disks are assumed to be varying along the radius as a power-law function of the radial coordinate, while the Poisson's ratio is kept constant. The governing equations for the stress field is derived and numerically solved using the finite difference method for the case of fixed-free boundary conditions. Additionally, the effect of material gradient index (i.e., the level of material gradation) on the stress field is evaluated. Our results show that the optimum stress field is achieved by having a thickness profile in the form of a rational function of the radial coordinate. Moreover, a smaller stress field can be developed by having greater mass density and elastic modulus at the outer radius of the disk (i.e., ceramic-rich composites at the outer radius). The numerical results additionally reveal that deceleration results in shear-stress development within the disks where a greater deceleration leads to greater shear stress; however this has almost no effect on the radial and circumferential stresses. Furthermore, the shear stress can cause a shift in the location of the maximum Von Mises stress, where for small deceleration, maximum Von Mises stress is located somewhere between the inner and outer radii, while for large deceleration it is located at the inner radius.

Solid oxide fuel cell interconnect design optimization considering the thermal stresses

The mechanical failure of solid oxide fuel cell (SOFC) components may cause cracks with consequences such as gas leakage, structure instability and reduction of cell lifetime. A comprehensive 3D model of the thermal stresses of an anode-supported planar SOFC is presented in this work. The main objective of this paper is to get an interconnect optimized design by evaluating the thermal stresses of an anode-supported SOFC for different designs, which would be a new criterion for interconnect design. The model incorporates the momentum, mass, heat, ion and electron transport, as well as steady-state mechanics. Heat from methane steam reforming and water–gas shift reaction were considered in our model. The results examine the relationship between the interconnect structures and thermal stresses in SOFC at certain mechanical properties. A wider interconnect of the anode side lowers the stress obviously. The simulation results also indicate that thermal stress of coflow design is smaller than that of counterflow, corresponding to the temperature distribution. This study shows that it is possible to design interconnects for an optimum thermal stress performance of the cell.

Gellan gum/clay hydrogels for tissue engineering application: Mechanical, thermal behavior, cell viability, and antibacterial properties

In this study, sodium montmorillonite (Na-MMT) was successfully modified by using n-hexadecyl trimethyl ammonium bromide (CTAB) via cationic exchange to obtain an organophilic-montmorillonite (CTAB-MMT). The Na-MMT, CTAB-MMT, and a commercial montmorillonite, that is, Cloisite15A were incorporated into gellan gum (GG) hydrogel and their mechanical, physical, thermal properties, biocompatibility, and antibacterial activities were investigated. The mechanical performance results show that the GG hydrogels containing Cloisite15A required smallest volume to achieve optimum compression stress, modulus, and compression strain at 5% (w/w) compared to both Na-MMT and CTAB-MMT at 10% (w/w). Swelling ratio of GG hydrogels increased upon addition of MMT, and water vapor transmission rate (WVTR) values of all hydrogels were in the range of 1106–1890 g m−2 d−1, which were comparable to WVTR values of commercial wound dressings. Thermal behavior shows that the inclusion of Cloisite15A in GG hydrogel improved the thermal stability than its counterparts. Cell studies exhibit that the GG incorporated with Na-MMT is non-cytotoxic to human skin fibroblast cells (CRL2522), and in contrast, the GG hydrogels incorporated CTAB-MMT and Cloisite15A revealed that the cells were dying and the cell growth depleted after being cultured for 72 h. Qualitative antibacterial study revealed that GG hydrogel containing CTAB-MMT only in the sample exhibits inhibition against the Gram-positive bacteria, that is, Staphylococcus aureus and Bacillus cereus, while there was no inhibition exhibited against Gram-negative bacteria ( Escherichia coli and Klebsiella pneumoniae).

Resonance modeling and control via magnetorheological dampers

A method to model and minimize resonant structural oscillations using magnetorheological dampers is presented. The response of the magnetorheological fluid flowing in a circular tube under a pressure gradient to the applied variable magnetic field is tailored to determine the optimum stress field in the fluid to mitigate resonance effects.

Temperature Dependencies of Inter-Diffusion Coefficient and Mass-Transfer Coefficient of Chemical Strengthening

Chemical strengthening is a method of generating compressive stress on the glass surfaces by ion-exchange to improve their strengths since old ages. As its targets have been expanding to larger and thinner glasses recently, different strength profiles are required depending on the applications and selecting appropriate ion-exchange parameters becomes even more important. Therefore, we developed a simulation model of chemical strengthening which enables predicting the stress profiles by ion-exchange in order to help optimizing the ion-exchanging parameters. Parameter study using the simulation model with change of temperature results in precise stress prediction, the compressive stress on the surface with a margin of error of ± 3% and the depth of layer with that of ± 10%. Furthermore, the parameter study introduced following two technical findings; (1) both inter-diffusion coefficient and mass-transfer coefficient could obey the Arrhenius equation, (2) both actual temperature and fictive temperature could influence on the stress relaxation. These findings are of great importance in comprehending the phenomena associated with ion-exchange. It has possibilities for beneficial feedback to the composition development and optimum stress design in accordance with each application.

Management of Laryngotracheal Stenosis - Still Remains a Challenge for Successful Outcome

Abstract Management of laryngotracheal stenosis (LTS) is still an enigma and remains a challenge even for an expert surgical team. The treatment of LTS is quite difficult and often associated with significant risk and complications. Even with the availability of many surgical options, proper selection of surgical technique in each clinical situation is the key behind successful outcome. Selection of appropriate surgical repair depends on the site of stenotic segment, severity, duration, and cause of functional impairment. It requires meticulous preoperative evaluation and planning. No single approach is ideal and often several techniques may be needed before decannulation can be achieved. The purpose of this review article is to discuss the pathophysiology, pre-operative evaluation, surgical principle, surgical technique, graft materials in reconstruction and prevention of LTS and optimum stress given on different suitability of surgical techniques with application of suitable one in relation to site of stenosis and other stenosis parameters.

Optimum quadratic step-stress accelerated life test plan for Weibull distribution under type-I censoring

This paper presents the optimum stress changing times for 3-step, step stress accelerated life testing under the cumulative exposure model with type-I censoring. The lifetimes of test units are assumed to follow Weibull distribution. The scale parameter of the Weibull failure time at constant stress level is assumed to be a log-quadratic function of the stress level. We derive an optimum test plans to minimize the asymptotic variance of maximum likelihood estimator of given pth percentile of the distribution at a design stress. The optimum test plan based on simulated observations is illustrated through a numerical example. The maximum likelihood estimates and asymptotic interval estimates are obtained using R software.

Numerical study of magnetic circuit response in magneto-rheological damper

Purpose – The purpose of this paper is to evaluate the performance of the semi-active fluid damper. It is recognized that the performance of such a damper depends upon the magnetic and hydraulic circuit design. These dampers are generally used to control the vibrations in various applications in machine tools and robots. The present paper deals with the design of magneto-rheological (MR) damper. A finite element model is built to analyze and understand the performance of a 2D axi-symmetric MR damper. Various configurations of damper with modified piston ends are investigated. The input current to the coil and the piston velocity are varied to evaluate the resulting change in magnetic flux density (B), magnetic field (H), field dependent yield stress and magnetic force vectors. The simulation results of the various configurations of damper show that higher magnetic force is associated with plain piston ends. The performance of filleted piston ends is superior to that of other configurations for the same magnitude of coil current and piston velocity. Design/methodology/approach – The damper design is done based on the fact that mechanical energy required for yielding of MR fluid increases with increase in applied magnetic field intensity. In the presence of magnetic field, the MR fluid follows Bingham’s plastic flow model, given by the equation τ = η γ•+τ y (H) τ > τ y . The above equation is used to design a device which works on the basis of MR fluid. The total pressure drop in the damper is evaluated by summing the viscous component and yield stress component which is approximated as ΔP = 12ηQL/g3W + CτyL/g, where the value of the parameter, C ranges from a minimum of 2 (for ΔPτ ΔPη less than approximately 1) to a maximum of 3 (for ΔPτ/ΔPη greater than approximately 100). To calculate the change in pressure on either side of the piston within the cylinder, yield stress is required which is obtained from the graph of yield stress vs magnetic field intensity provided by Lord Corporation for MR fluid −132 DG. Findings – In this work, three different finite element models of MR damper piston are analyzed. The regression equations, contour plots and surface plots are obtained for different parameters. This study can be used as a reference for selecting the parameters for meeting different requirements. It is observed from the simulation of these models that the plain ends model gave optimum magnetic force and 2D flux lines with respect to damper input current. This is due to the fact that the plain ends model has more area when compared with that of other models. It is also observed that filleted ends model gave optimum magnetic flux density and yield stress. As there is reduced pole length in the filleted ends model, the MR fluid occupies vacant area, and hence results in increased flux density and yield shear stress. The filleted ends assist the formation of dense magnetic flux lines thereby increasing the flux density and yield stress. This implies that higher load can be carried by the filleted ends damper even with a smaller size. Originality/value – This work is carried out to manufacture different capacities of the dampers. This can be applied as vibration controls.

Compression Strength of Gellan Gum Hydrogel Incorporated with Organo-Montmorillonite and Cloisite 15A

The uniformly cross-linked gellan gum hydrogel with sodium montmorillonite (Na-MMT), organo-montmorillonite (CTAB-MMT) and Cloisite 15A were successfully prepared. The compression performances of the hydrogels were investigated. The results show that the GG hydrogels containing Cloisite 15A required smallest volume to achieve optimum compression stress, modulus and compression strain at 5% (w/w) compared to both Na-MMT and CTAB-MMT at 10% (w/w), respectively. The decrease in compression performances of gellan gum hydrogel at higher concentration containing those clays could be due to agglomeration process which created the entangled structure and bring up the brittleness of hydrogel properties. Overall, the presence of the clays significantly improved the mechanical performances of gellan gum hydrogels which beneficial to be used in tissue engineering.

Evaluation of the Stress Induced in Tooth, Periodontal Ligament & Alveolar Bone with Varying Degrees of Bone Loss During Various Types of Orthodontic Tooth Movements.

The force applied on to a tooth with periodontal bone loss may generate different magnitude and pattern of stresses in the periodontium when compared to a tooth with no bone loss & under the same force system. The intensity of the forces and moment to force ratios needed to be applied during an Orthodontic treatment must be adapted to obtain the same movement as in a tooth with a healthy periodontal support. Evaluation and assessment of the stress distribution during various types of Orthodontic tooth movement on application of Orthodontic force, at various levels of alveolar bone loss; & determination of the most ideal force system producing the Optimum Stress (i.e., stress within optimum range), uniformly (conducive to bodily movement of maxillary canine with varying degrees of bone loss). A human maxillary canine tooth of right side was simulated by means of Finite Element Method (FEM). Five different models were constructed with bone loss ranging from 0mm in model 1, to 8mm in model 5 (progressing at 2mm per model). Ten different loading conditions were applied on these models and the stress generated was charted at various occluso-gingival levels and surfaces around the tooth. The evaluation and assessment of the stress distribution during various types of Orthodontic tooth movement on application of Orthodontic force, at various levels of alveolar bone loss was done. The results showed that there was a high positive correlation between the increase in bone loss & the stress generated, suggesting an elevation in the stress with advancing bone loss. Additionally, the type of tooth movement was found to be changed with bone loss. During the determination of ideal force system it was found that the centre of resistance of the canine migrated apically with bone loss and an increase in the moment to force ratio (Mc:F) was required to control the root position in these cases. A high positive correlation exists between the increase in bone loss and the stress generated. Suitable modification should be done in the force system under bone loss conditions.

Long-range stresses generated by misfit dislocations in epitaxial films

For the first time, an equation is derived that relates the misfit parameter f, the number of misfit dislocation (MD) families, and the distances between MDs of the same family, and the projection value of edge Burger’s vector component on the interface. The equation is valid for various interface boundaries (hkl). To derive this equation, the long-range normal and shear stresses associated with MD distribution are considered. The optimum and nonoptimum stress releasing processes are discussed. The problem of threading dislocation density diminution with generation of the intersecting MDs having the same Burger’s vector (L-shaped MDs) is considered for (001) and (111) interfaces. It is shown that such MDs can be effectively generated only at the initial stage of the releasing process, since generating is accompanied by the increase in the level of long-range shear stresses.

Ceramics and defects

The feasibility of square-pulsed thermography nondestructive testing for the detection of defects in one ceramic material sample has been carried out by finite element (FE) analysis. In particular, a ceramic plate containing defects of different diameters, depths, locations, nature, and shapes has been numerically investigated by means of Comsol® Multiphysics computer program, taking into account the results coming from both a MATLAB™ script and the infrared thermography (IRT) technique. Indeed, the FE method simulates through a 3D model the heat transfer process induced into the ceramic material by two halogen lamps that have been applied in order to provoke an optimum thermal stress. Moreover, further defects like cracks arose beneath the surface of the plate due to the shrinkage process, have been discovered, and contrasted using a non-usual segmentation algorithm that when correlated in the time to IRT data simulates the thermo-elastic effect. Following the non-direct procedure proposed, both the depth of each defect and its main dimensions have been retrieved into a satisfactory accuracy.

Optimization of fillet stress to enhance the bending strength through non-standard high contact ratio spur gears

The present study aims to determine the improvement in the bending strength of the non-standard high contact ratio spur gears based on the balanced (optimum) fillet stress of the pinion and gear. The average number teeth in contact is more than two for high contact ratio gear drives. In the non-standard high contact ratio spur gears, the rack cutter tooth thickness factor is more than 0.5, whereas the standard rack cutter tooth thickness factor is 0.5. The maximum fillet stresses of the pinion and gear is not equal for non-standard high contact ratio spur gear drives when the gear ratio increases. In order to avoid the fatigue failure of the gear, the fillet stresses of the pinion and gear should be balanced. This balanced stress is predicted as the optimum fillet stress. Hence, the present study focuses to optimize the fillet stress with respect to the rack cutter tooth thickness factor of the pinion and gear through finite element analysis. Also, a parametric study is carried out to obtain the influence of some gear parameters, such as gear ratio, teeth number in the pinion, pressure angle, addendum height and corrected gear drives (S+, S− and So) on the optimum fillet stress with respect to the rack cutter tooth thickness factor of the pinion and gear.