Large Safety Factors Research Articles

Design of a New XY Flexure Micropositioning Stage With a Large Hollow Platform

This paper presents the design of a new flexure-based xy micropositioning stage with a large hollow platform, which is suitable for practical use with a microscope. The designed mechanism has a parallel-kinematic structure and is actuated by two voice coil motors. By employing multistage compound parallelogram flexures, the stage is designed as a four-layer structure, which produces a motion output platform with a large hollow space and large range of motion. Analytical modeling was carried out for parametric design of the stage. Evaluation results show that the designed xy stage exhibits a large safety factor and high natural frequency. Moreover, the large hollow platform is well-suited for practical applications with a microscope.

Analysis of slope stability considering the mechanism of the bolt-beam-root pile composite retaining structure

This project focuses on the development of a new type of retaining structure, namely Bolt-Beam-Pile Composite Retaining Structure. The proposed structure is a technical and economically feasible method enhancing the slope stability. A parameter sensitivity analysis carried out to evaluate the behavior of the structure under different scenarios, and the variation of the soil parameters. Finite element method was utilized to stimulate the model and perform the analyses. The results demonstrated that, compared with other three typical retaining structures the new Composite Retaining Structure has larger safety factor and smaller displacement.

Weibull and Bootstrap-Based Data-Analytics Framework for Fatigue Life Prognosis of the Pressurized Water Nuclear Reactor Component Under Harsh Reactor Coolant Environment

AbstractIn general, the fatigue life of a safety critical pressure component is estimated using best-fit fatigue life curves (S-N curves). These curves are estimated based on underlying in-air condition fatigue test data. The best-fitting approach requires a large safety factor to accommodate the uncertainty associated with large scatter in fatigue test data. In addition to this safety factor, reactor component fatigue life prognostics requires an additional correction factor that in general is also estimated deterministically. This additional factor known as the environmental correction factor Fen is to cater the effect of the harsh coolant environment that severely reduces the life of these components. The deterministic Fen factor may also lead to further conservative estimation of fatigue life leading to unnecessary early retirement of costly reactor components. To address the above-mentioned issues, we propose a data-analytics framework which uses Weibull and Bootstrap probabilistic modeling techniques for explicitly quantifying the uncertainty/scatter associated with fatigue life rather than estimating the lives based on a best-fit based deterministic approach. We assume the proposed probabilistic approach would provide the first hand information for assessing the maximum and minimum effects of pressurized water reactor water on the reactor component. In the discussed approach, in addition to the probabilistic fatigue curves, we suggest using a probabilistic environment correction factor Fen. We assume the probabilistic fatigue curve and Fen would capture the S-N data scatter associated with the bulk effect of material grades, surface finish, strain rate, etc. on the material/component fatigue life.

Theoretical and numerical study on the block-flexure toppling failure of rock slopes

Block-flexure toppling failure is frequently encountered in rock slopes. In this paper, we first proposed two theoretical models to calculate the factor of safety against block-flexure toppling, namely, step by step analysis model (LEM-SSAM) and overall analysis model (LEM-OAM). Then, we developed two special MATLAB codes for a quick stability estimation based on the proposed methods, respectively. Next, the universal distinct element code (UDEC) was used to study the failure mechanisms of block-flexure toppling. In order to realistically simulate the excavation of slope and obtain the detailed damage information of joints, we developed two FISH functions in UDEC. Finally, we conducted a comparative analysis of the limit equilibrium method and the numerical simulations through twelve theoretical models. The results show that many aspects of block-flexure toppling failure have been captured using UDEC combined with the developed FISH functions. The LEM-OAM and UDEC agree well while a larger factor of safety is obtained using the LEM-SSAM. It is found that the dip angles and friction angles of the joints dipping steeply into face have a great influence on block-flexure toppling failure while the influence of the cross joint spacing is less significant.

Evaluation of the neuromuscular junction in a middle-aged mouse model of congenital myasthenic syndrome.

Reduced expression of the vesicular acetylcholine transporter (VAChT) leads to changes in the distribution and shape of synaptic vesicles (SVs) at neuromuscular junctions (NMJs), suggesting vesicular acetylcholine (ACh) as a key component of synaptic structure and function. It is poorly understood how long-term changes in cholinergic transmission contribute to age- and disease-related degeneration in the motor system. In this study we performed confocal imaging, electrophysiology, electron microscopy, and analyses of respiratory mechanics of the diaphragm NMJ components in 12-month-old wild-type (WT) and VAChTKDHOM mice. Diaphragms of NMJs of the VAChTKDHOM mice were similar to those in WT mice in number, colocalization, and fragmentation of pre-/postsynaptic components. However, they had increased spontaneous SV exocytosis, miniature endplate potential frequency, and diminished MEPP amplitude. No impairment in respiratory mechanics at rest was observed, probably due to the large neurotransmission safety factor of the diaphragm. The present findings help us to understand the consequences of reduced ACh release at the NMJs during aging.

Scantling assessment of large yacht rigs using carbon-fiber and aluminum masts

Weight saving is crucial in naval architecture to reduce costs and improve performances. In sailing ships design, rig's scantling is important also for ship stability, as rig's weight generates heeling actions on the vessel, due to heeled navigation. Therefore, rig's weight saving remarkably reduces ship's heeling, allowing to increase the wind exposed area, consequently improving the sailing performances. However, because of the complexity of the structure and uncertainties in loading conditions, the structural scantling of a rig implies the adoption of high safety factors. Furthermore, the increasing trend to use composite materials represents an additional source of complexity due to their anisotropic non-homogeneous nature and uncertainties on material properties, and generally implies even larger safety factors, besides implying high construction costs.The present work shows a comparison between an existing 76 m carbon mast and an aluminium alloy one. Suitable FEM models of both rigs were built up and used to assess the structural system strength according to applicable class rules. The cross-section stiffness of the aluminium mast was optimized in order to make masts’ performances comparable. The optimized aluminium mast, of comparable weight of the carbon one, results to be more cost-effective for such range of sailing vessels.

Structural Safety Evaluation of Precast, Prestressed Concrete Deck Slabs Cast Using 120-MPa High-Performance Concrete with a Reinforced Joint.

Prestressed concrete structures are used in various fields as they can reduce the cross-sectional area of members compared with reinforced concrete structures. In addition, the use of high-performance and strength concrete can help reduce weight and achieve excellent durability. Recently, structures have increasingly been constructed using high-performance and strength concrete, and therefore, structural verification is required. Thus, this study experimentally evaluated the structural performance of a long-span bridge deck slab joint, regarded as the weak point of structures. The specimens were designed with a 4 m span for application to cable-stayed bridges. To ensure the required load resistance and serviceability, the specimens comprised of 120 MPa high-performance fiber-reinforced concrete and were prestressed. The deck slabs satisfied all static and fatigue performance as well as serviceability requirements, although they were thinner than typical concrete bridge deck slabs. The study also verified whether the deck slabs were suitable to help implement precast segmental construction methods. Finally, the results confirmed that the structural performance of the developed prestressed concrete (PSC) deck slab was sufficient for the intended bridge application as it achieved a sufficiently large safety factor in the static and fatigue performance tests, relative to the design requirement.

Method identifying oversizing of mechanical ventilation systems in office buildings using airflow and electrical power measurements

Heating, Ventilation and Air-Conditioning engineers face high demands from their clients to deliver reliable, optimized solutions that perform acceptably in terms of energy use and provided comfort. However, time and resources are scarce to deliver an optimized solution. To ensure that the solution works under most operation scenarios, the design is typically based on a combination of conservative rules-of-thumb, general guidelines and a large safety factor. The consequences are building service systems designed for operating conditions that never or very rarely occur leading to oversized systems. The objective of this paper was to propose a method for identifying oversized air-handling units with variable speed drives. It was demonstrated on a case study including six air-handling units in an office building. The method was able to determine that the air-handling units were not grossly oversized or undersized by comparing the measured airflow and SFP from the part- and full-load operation to the design airflow and SFP. However, the method should be extended to include additional performance criterion such as indoor climate and thermal efficiency to be able to conclusively confirm the size of the units.

STUDY OF THE STABILITY OF COASTAL RECLAMATION SLOPES BY USING GEOTEXTILES AS SOIL STRENGTH

This study presents a simulation of coastal reclamation deposits with geotextiles in areas with the weakest soil profile. Case studies related to deeds and improvement of slopes with woven geotextiles. Observations and investigations show that the land is sand soil which has a low carrying capacity. Weak soil parametric effects, geotextile webbing, external loads, and earthquake loads were analyzed by PLAXIS. The calculation results show reinforcement with woven geotextiles and against external loads and earthquake loads, having a large safety factor.

Critical time step for DEM simulations of dynamic systems using a Hertzian contact model

SummaryThe discrete element method (DEM) typically uses an explicit numerical integration scheme to solve the equations of motion. However, like all explicit schemes, the scheme is only conditionally stable, with the stability determined by the size of the time step. Currently, there are no comprehensive techniques for estimating appropriate DEM time steps when a nonlinear contact interaction is used. It is common practice to apply a large factor of safety to these estimates to ensure stability, which unnecessarily increases the computational cost of these simulations. This work introduces an alternative framework for selecting a stable time step for nonlinear contact laws, specifically for the Hertz‐Mindlin contact law. This approach uses the fact that the discretised equations of motion take the form of a nonlinear map and can be analysed as such. Using this framework, we analyse the effects of both system damping and the initial relative velocity of collision on the critical time step for a Hertz‐Mindlin contact event between spherical particles.

Assessment of the influence of an external dynamic force on the mixing of non-isothermal flows in the reactor plant

Today one of the main problems to be solved is increasing the safety of nuclear power plants (NPP). Possible shortcomings of the reactor plant (RP) can be identified by creating full-scale or large-scale prototype models for testing. There is an urgent need to create models for a more detailed study of some complex processes (flow mixing, heat and mass transfer and flow mixing under the action of dynamic forces). For example, the negative impact of these processes on the switchgear makes it difficult to ensure the natural circulation of the coolant (NCC) in all modes of operation, which could significantly increase the switchgear safety. At modern NPP this condition is not achieved either and the problem is solved by large safety factors. The purpose of this work is to evaluate the influence of external dynamic forces (EDF) on the processes of mixing of non-isothermal flows in the simulator of the core, applied to the research stand «Single-loop RP model». For the research stand we have designed a swinging platform simulating EDF. The main method for obtaining experimental data is direct layer-by-layer temperature sensing. We have also constructed a simple one-dimensional mathematical model describing these processes, based on Newton's Second Law. The adequacy of model selection has been confirmed experimentally. We have plotted graphs of temperature fronts for the experimental model and made a graphic representation of the coolant temperature field at the model entrance / exit for the dynamic mode. A mathematical equation has been obtained for describing the effect of EDF on the length of the Prandtl mixing path for the one-dimensional approximation. Based on the analysis of the results obtained, we can conclude that the external dynamic force has a negative effect on the mixing of non-isothermal flows. They change the length of the mixing path in comparison with the stationary mode by an average of 5–10 %, which, when transferred to a real NPP, can significantly affect the NCC and will need to be taken into account in the form of correction factors at the design stage.

The experiment of four-point bending behaviours of helicoidally laminated CFRPs

The purpose of this study is to mitigate the brittle behaviour of CFRP. CFRP is a composite material of carbon fibre and a thermosetting resin. As a feature, it has a low density than metallic materials such as steel and aluminium, and it is excellent in specific strength, specific stiffness, and is corrosion resistant. In the civil engineering field, CFRP is expected to be utilized for durability improvement and long life achievement. However, it is necessary to be careful when using it as a primary member, because the behaviour at the time of fracture is very brittle, thereby it is necessary to set a large safety factor at the time of design. One way to achieve the purpose of the current study is to follow a biological structure, so-called biomimetics. In this study, mantis shrimp is selected. The characteristic structure of its forefoot is that chitin fibres are stacked while changing helicoidally their angles. It is said that the forefoot can withstand a thousand times the force of its own weight because of its structure. Helicoidally laminated CFRP is fabricated by processing prepreg sheets of unidirectional carbon fibres. Four laminated structures were examined in which four-point bending experiments were carried out under displacement control of 2 mm/min, and load and displacement were recorded. During the experiments, the specimens were observed until fracture with a video camera. As a result, the brittle behaviour was improved significantly as the orientation angle difference of the fibre sheets decreased.

Grey theory study on the influence of karst-pile-soil coupling on the ultimate bearing capacity of pile foundation in karst development area

There are many factors influencing the ultimate bearing capacity of pile foundation in karst development area. The current design method usually adopts a large safety factor, which inevitably leads to an increase in engineering cost and a prolonged period. Grey theory is a kind of mathematical theory based on known information. It can describe the change law of unknown information by generating and developing some known information. Based on the above characteristics, combined with numerical simulation results, the effects of aspect ratio and karst scale on the ultimate bearing capacity of bridge piles are studied by using grey theory. The results show that the ultimate bearing capacity of piles increases with the increase of aspect ratio, and decreases with the increase of karst scale. The degree of influence of karst scale is larger than that of aspect ratio. Grey theory can better reflect the variation of ultimate bearing capacity of pile foundation under aspect ratio and karst scale, and the error between calculation result and numerical simulation result is about 9%. The grey theory can be applied to the estimation of the ultimate bearing capacity of bridge piles.

Stratified slopes, numerical and empirical stability analysis

Urbanisation means that many natural slopes in and around cities are often subjected to cuts resulting in dramatic changes in the geometry of slope faces mostly by increasing slope angle, which could lead to failures with catastrophic consequences. As most natural slopes are of non-homogeneous layered nature, understanding the stability behaviour of such slopes will be of utmost importance. The current practice in analysing slopes of complicated nature, geometrically and materially, is mostly to apply simplifications sacrificing accuracy leading to use of large factors of safety (FoS), which could undermine analytical and economic feasibility of projects. In this research study, limit-equilibrium and finite-element methods are used by the Oasys slope and Plaxis 2D software programs, respectively, empirically and numerically to model and analyse geometrically non-homogeneous stratified slopes with the aim of understanding the effects of non-homogeneity of geometry and materials on stability under various inclination angles of slope face. The analysis included determination of FoS as well as a sensitivity analysis looking into the combined effects of contributing parameters.

Behaviour and Modeling of the Bearing Capacity of Shear Stud Connectors

The bearing capacity of shear stud connectors from the simplified design equations in different building codes tends to be underestimated because of a relatively large safety factor. Finite element analysis using ABAQUS was carried out to simulate the force–deformation response of shear stud. The finite element model was verified by using push-out tests, and detailed parametric numerical studies were conducted to investigate the force–deformation response of different shear stud configurations. A new design equation was derived from linear regression based on the results of the parametric studies. The presented equation shows better agreement with the experimental data. Detailed comparison between the proposed equation and existing design equation in China code for shear stud strength was carried out. The results show significant reduction in the construction cost when the new equation is used.

LCF behaviour and a comprehensive life prediction model for AlSi10Mg obtained by SLM

The full potential of additive manufacturing (AM) components is today yet to be reached. Space and aerospace industries are still conservative in the use of AM parts for safety–critical applications, mostly because of the uncertainties and low reproducibility that are often associated with the process. One of the most challenging issues is the fatigue resistance. The rough surface condition and the presence of manufacturing defects can cause significant scatter, leading to the adoption of large conservative safety factors. To robustly model the fatigue resistance of defected AM materials, there is a need to implement defect-tolerant designs and deal with these uncertainties. The present research activity aims at developing a model for fatigue life estimation in a large range of loading conditions, from high cycle fatigue to low cycle fatigue. This is achieved by adopting a fracture mechanics approach, through elastic–plastic fatigue crack growth calculations based on the known defect population inside the material. The model has been applied to AlSi10Mg produced by three slightly different selective laser melting processes, showing a robust estimation of fatigue life and scatter with limited experimental effort. The life predictions performed are then summarised in design maps where the allowable stress (or strain) depends on the defect size and on the HCF/LCF design upon the number of cycles selected.

Simulation and Modeling of Rigid Aircraft Aerodynamic Responses to Arbitrary Gust Distributions

The stresses resulting from wind gusts can exceed the limit value and may cause large-scale structural deformation or even failure. All certified airplanes should therefore withstand the increased loads from gusts of considerable intensity. A large factor of safety will make the structure heavy and less economical. Thus, the need for accurate prediction of aerodynamic gust responses is motivated by both safety and economic concerns. This article presents the efforts to simulate and model air vehicle aerodynamic responses to various gust profiles. The computational methods developed and the research outcome will play an important role in the airplane’s structural design and certification. Cobalt is used as the flow solver to simulate aerodynamic responses to wind gusts. The code has a user-defined boundary condition capability that was tested for the first time in the present study to model any gust profile (intensity, direction, and duration) on any arbitrary configuration. Gust profiles considered include sharp edge, one minus cosine, a ramp, and a 1-cosine using tabulated data consisting of gust intensity values at discrete time instants. Test cases considered are a flat plate, a two-dimensional NACA0012 airfoil, and the high Reynolds number aero-structural dynamics (HIRENASD) configuration, which resembles a typical large passenger transport aircraft. Test cases are assumed to be rigid, and only longitudinal gust profiles are considered, though the developed codes can model any gust angle. Time-accurate simulation results show the aerodynamic responses to different gust profiles including transient solutions. Simulation results show that sharp edge responses of the flat plate agree well with the Küssner approximate function, but trends of other test cases do not match because of the thin airfoil assumptions made to derive the analytical function. Reduced order aerodynamic models are then created from the convolution integral of gust amplitude and the time-accurate responses to sharp-edge gusts. Convolution models are next used to predict aerodynamic responses to arbitrary gust profiles without the need of running time-accurate simulations for every gust shape. The results show very good agreement between developed models and simulation data.

Modelling of interactions between Barely Visible Impact Damages and Lamb waves in CFRP laminates.

The improvement of the current design practice, especially for conventional materials, has allowed the production of structures able to better tolerate the presence of cracks and, in general, damages under the in-service loading conditions. As a result, especially in the aerospace field, nowadays, the damage tolerance philosophy is the core of the current design practice. However, conversely to the conventional materials, composite materials still cannot count on established prediction models supporting the damage tolerance approach. Among the critical damages which could affect composite materials, BVIDs (Barely Visible Impact Damages) due to accidental Low Velocity Impact (LVI) phenomena play a critical role. Since they cannot be easily detectable during the inspection intervals, in order to fulfil the damage tolerance targets, large safety factors are applied during the design current practice, resulting in the oversizing of the structure. For these reasons, Structural Health Monitoring (SHM) techniques are being widely used for the improvement of the current design practice, allowing the damage detection. Thanks to the possibility to assess the structural health during the in-service loading conditions, Lamb waves, among the several non-destructive testing (NDT), appear to be the best candidate for damage detection. Studies on this topic are never enough and the literature cannot still count on a Finite Element (FE) procedure able to simulate Lamb wave propagation on LVI damaged plates. This work deals with a FE procedure, developed by using the FE code Abaqus v.6.14®, for the simulation of the interaction between ultrasonic guided waves and LVI damages in composite laminates, whose damages have been modelled by means of Hashin criteria in a previous impact simulation.

Geodesic acoustic mode in a reduced two-fluid model

A reduced two-fluid model is constructed to investigate the geodesic acoustic mode (GAM). The ion dynamics is sufficiently considered by including an anisotropic pressure tensor and inhibited heat flux vector, whose evolutions are determined by equations derived from the 16-momentum model. Electrons are supposed to obey the Boltzmann distribution responding to the electrostatic oscillation with near ion acoustic velocity. In the large safety factor limit, the GAM frequency is identical with the kinetic one to the order of when zeroing the anisotropy. For general anisotropy, the reduced two-fluid model generates the frequency agreeing well with the kinetic result with arbitrary electron temperature. The present simplified fluid model will be of great use and interest for young researchers and students devoted to plasma physics.

Collisional damping of the geodesic acoustic mode with poloidal rotation

The collisional damping of the geodesic acoustic mode is investigated by using the gyrokinetic equation with a poloidal rotation flow and a number-conserving Krook collision operator. A dispersion relation of the geodesic acoustic mode with poloidal rotation and collision terms is derived and solved both analytically and numerically. It is shown that poloidal rotation has a significant influence on the collisional damping rate of the geodesic acoustic mode within a certain collision rate and safety factor domain. The variation of the damping rate induced by poloidal rotation depends on the safety factor and collision rate. In addition, the effect of poloidal rotation on the geodesic acoustic mode would be weakened by the safety factor, especially when the safety factor is relatively large. The ratio of the collisional damping rate to the collisionless damping rate of the geodesic acoustic mode in the large safety factor domain is performed, and the critical collision rate is estimated. Besides, the physical mechanism of the results is briefly discussed.