Critical Loading Conditions Research Articles

Computational stress analysis of Chicken Feather Fibre (CFF) with Epoxy-Resin matrix composite material

Nowadays, polymer composites have wide applications from the soft electronic devices like actuator, sensors, medical equipment to the parts of aerospace etc. In this study, we are considering Chicken Feather Fibre (CFF) with Epoxy-Resin (CY-230) matrix as a beam structure. The initial characterizations of CFF reinforced composites were done experimentally. The samples were prepared from hand layup techniques. The computational structural analysis is carried to understand the stress and deformation behaviour of the material. In this study, we have developed a model of cantilever beam of the polymer composite using SolidWorks 2018 and has been analyzed for the critical permissible stress using the ANSYS 2019 software. Four material compositions CFF-Epoxy Resin Matrix for (CFF%0), (CFF%3), (CFF%5) and (CFF%7) have been studied for critical loading conditions for uniformly distributed load and point load. These results will be helpful to determine the safe working condition while working with these materials.

Unveiling damage mechanisms of chromium-coated zirconium-based fuel claddings by coupling digital image correlation and acoustic emission

Coated nuclear fuel claddings offer a promising, near-term solution to address the demand for next-generation, accident-tolerant fuel systems. It is expected that they will possess superior mechanical properties and greater oxidation resistance compared to current cladding technology, allowing for improved performance during beyond design-basis accident conditions. Here, we present a methodology to determine the failure mechanisms of chromium-coated zirconium-alloy claddings under expected, critical loading conditions. Three-dimensional digital image correlation and acoustic emission techniques were used in situ to monitor spatial strain and crack development of the claddings under two key, de-coupled loading conditions: expanding plug and four-point bending. Critical strain levels, at which cracking initiated, were determined to be 0.4% εhoop and 0.4% εxx for expanding plug and four-point bending, respectively. A two-dimensional fracture model was also developed for the expanding plug loading condition based on inputs determined from mechanical testing. It was concluded that coating fracture of expanding plug specimens was axisymmetric across the specimen circumference and occurred rapidly through the thickness of the chromium coating. Subsequent high temperature steam oxidation experiments on tested (cracked) expanding plug specimens showed no signs of oxidation degradation to the underlying zirconium alloy, thereby showcasing the effectiveness of the chromium coating. This comprehensive, multi-scale study is intended to inform future testing of next-generation, coated claddings and identify the resulting failure mechanisms that arise in beyond design basis accident conditions.

Inelastic lateral buckling of steel cantilevers

Steel cantilevers are very different from the simply supported beams under uniform bending which provide the basis for designing steel members against lateral buckling. Cantilevers have different end restraint conditions, while the most critical loading condition for cantilevers is one of concentrated end load leading to a linear moment distribution. Thus the elastic lateral buckling formulations for uniform bending of simply supported beams used more generally for design are inappropriate for cantilevers.The mono-symmetric yield patterns in simply supported beams with residual stresses in uniform bending are of opposite sense to those of cantilevers because of the opposite sense of the bending moments. Thus formulations which take account of the effect of mono-symmetry on the inelastic lateral buckling of simply supported beams are inappropriate for cantilevers. Further, the linear cantilever moment distribution leads to very localised yielding instead of the uniform yielding of beams under uniform bending.This paper studies the inelastic lateral buckling of cantilevers in order to produce better design guidance than that currently based on the inelastic buckling of simply supported beams in uniform bending. The effects of mono-symmetry on the elastic buckling of beams and cantilevers are first investigated, and then the effects of the mono-symmetric non-uniform yielding patterns which complicate the prediction of the inelastic buckling resistances of cantilevers are studied. Cantilevers with reduced (instead of rigid) end warping restraints occur in overhanging beams, and so the effects of yielding on overhanging segments are also studied.These studies are used to develop simple approximate methods for designing cantilevers and overhanging segments against inelastic lateral buckling.

Subdiffusive Load Balancing in Time-Varying Queueing Systems

The degree to which delays or queue lengths equalize under load-balancing algorithms gives a good indication of their performance. Some of the most well-known results in this context are concerned with the asymptotic behavior of the delay or queue length at the diffusion scale under a critical load condition, where arrival and service rates do not vary with time. For example, under the join-the-shortest-queue policy, the queue length deviation process, defined as the difference between the greatest and smallest queue length as it varies over time, is at a smaller scale (subdiffusive) than that of queue lengths (diffusive).

Aeroelastic dynamic response of elastic aircraft with consideration of two-dimensional discrete gust excitation

Design loads generally require a one-dimensional discrete gust profile without consideration of the spanwise effect, and this profile cannot represent the true gust field exactly. For a high aspect ratio aircraft, two-dimensional gusts may cause critical load conditions, and approaches for calculating dynamic responses under two-dimensional discrete gust excitation are rarely presented. In this paper, a spanwise non-uniform vertical discrete gust field is established based on a one-dimensional ‘1-cos’ gust profile in reference to a DARPA proposal, while frequency and hybrid approaches to the dynamic response analysis of flexible aircraft under this two-dimensional gust excitation are presented. Solution techniques have been applied to a high aspect ratio aircraft to assess the different response characteristics with a comparison between one-dimensional and two-dimensional discrete gust field conditions. The results show that the two-dimensional discrete gust model produces a higher bending moment than that of the one-dimensional condition. Therefore, the critical load conditions that are derived from the two-dimensional discrete gust for high aspect ratio aircraft should be seriously considered. According to the analysis, an active control scheme to alleviate the bending loads caused by the two-dimensional gust is designed, and alleviation effects in different gust conditions are compared.

Liquid hydrogen fuel tanks for commercial aviation: Structural sizing and stress analysis

LH2 fuel tanks are one of the main drivers in the development of a commercial airplane powered with hydrogen. This article discusses the implementation of liquid hydrogen fuel tanks in future commercial airplanes focusing on the sizing of the fuel tank structure and its behavior under critical loading conditions. Fuel tanks are sized according to the mission requirements and geometrical restrictions of a conventional mid-range commercial airplane. Critical loading cases for symmetrical maneuvers and landing conditions are estimated following EASA CS-25 airworthiness specifications for large airplanes. The stress distribution in each tank is evaluated using linear Finite Element Analysis (FEA) in MSC. NASTRAN/PATRAN to ensure that the structural design complies with strength and stiffness requirements

The Effect of the Static Load in the UNSM Process on the Corrosion Properties of Alloy 600.

To suppress stress corrosion-cracking, compressive residual stresses, such as shot peening, laser peening, water jet peening, ultrasonic peening, and ultrasonic nanocrystal surface modification (UNSM) are utilized. However, among the numerous techniques, there is little research about the corrosion effect of detailed conditions, such as static load or amplitude in UNSM. A study on UNSM among various techniques of adding compressive residual stress to Alloy 600 was conducted. The focus of this study was on the effect of the static load in UNSM on the corrosion properties of Alloy 600. Microstructure analysis was conducted using an optical microscope (OM), a scanning electron microscope (SEM), and electron backscattering diffraction (EBSD), while compressive residual stress was measured using a nano-indentation technique. A cyclic polarization test and the AC (Alternating Current)-impedance measurement were both used to analyze the corrosion properties. An increase in static load under critical static load enhanced the grain boundary diffusion, consequently strengthened the passive film, and facilitated the surface diffusion, thereby improving the passivation of Alloy 600. However, higher static loads over the critical value can lead to an increase in the friction between the striking tip and the surface, thereby creating an overlapped wave, which reduces the corrosion properties.

Influence of soil-structure interaction on the dynamic response of continuous and integral bridge subjected to moving loads

ABSTRACT In this paper, the effects of high-speed moving train loads over a non-prismatic continuous and integral bridge are investigated, in which the dynamic soil-structure interaction (SSI) analysis is comprehensively considered. Initially, a complete-pile model and an equivalent spring model of the continuous bridge are analyzed and compared. Two complex cases of track loading for the complete-pile model Case-I (C-I): one train moving on the single track, and Case-II (C-II): two trains moving on both the tracks entering at same instant but in opposite direction), under the CRH3 train loading, are studied. The variations in dynamic responses of the bridge for these cases confirm the influence of soil-structure interaction. C-II represents higher vertical displacement and acceleration bridge response compared to the C-I for both, continuous and integral bridge. It is proposed to analyze a detailed 3D model under critical loading condition for acquiring an accurate vertical dynamic response of bridge structure.

Design of a Beam Structure for Failure Prevention at Critical Loading Conditions

Design of a Beam Structure for Failure Prevention at Critical Loading Conditions

Estimation of strains in composite cylindrical shells in a statistical energy analysis framework

Broad band Acoustic excitation is one of the critical loading conditions for launch vehicle/spacecraft structures. Estimation of responses of such multimodal systems in a wide band dynamic environment is commonly performed using Statistical Energy Analysis (SEA). In SEA the strains are determined from known velocity responses. These relations are available for plate type structures but not reported for composite cylindrical shells. Empirical relations based on experiments only are reported for isotropic cylindrical shells. In this work, expressions for estimating the strains in the higher order modes of a composite cylindrical shell from its velocity responses are derived. It is seen that the strains estimated using the existing empirical relations of isotropic cylinders, with the elastic properties corresponding to the composite cylinder, will be in large error. Numerical simulations carried out using a finite element model are in good agreement with those determined using the relations presented in this work.

Investigation on Composite Energy Absorbers of a Composite Fuselage Section Subjected to Vertical Drop Test

In the aircraft industry, crashworthiness design and certification phases have been and are going to be the most attractive topics for designers, mostly because of the increasing use of composites for primary structural components. It is well known that the cargo subfloor elements of the fuselage structure play a crucial role in absorbing the kinetic energy during a crash. In particular, the stanchions, or struts, are important parts for the structural response; as a matter of fact, they connect the fuselage frames to the cabin’s floor and, ideally, are expected to crush under a compressive force in order to dissipate the impact energy in a controlled way and, consequently, to minimize the energy transferred to the passengers. The aim of this work is to demonstrate, experimentally and numerically, the energy absorption capability of the stanchions, made of both composite material and aluminium alloy, of a full-scale 95% composites made fuselage section under a critical load condition, such as an emergency landing. A Finite Element model allowing estimating the passive safety capabilities of the designed struts has been developed and herein proposed.

Investigation on the Crack Evolution in Glass-Fiber Laminates Depending on the Stacking Sequence

In this paper experiments on the fatigue behavior of flat and tubular Glass Fiber Reinforced Plastics (GFRP) specimens with three different layups ([0/905/0], [0/902/01/2]s and [0/±45/01/2]s) are presented. The experiments are conducted to study the mechanics under cyclic tension-tension loading (R=0.1) until crack saturation (CDS). Fatigue testing is performed below the critical static load level where first matrix cracks can be observed. Therefore Load Levels (LLs) are derived from crack evolution curves obtained by static tests under usage of transmitted light photography. The shear lag model of Berthelot [1] is applied to the two cross-ply specimens to predict crack evolution. The results show good agreement between the prediction and the experimental data. Deviations can be found in prediction of crack evolution in [0/902/01/2]s-specimens. For predicting fatigue stiffness degradation the phenomenological model of Adden [2] is used. The results show good capabilities for predicting stiffness degradation after crack onset.

On the creep fatigue and creep rupture behaviours of 9–12% Cr steam turbine rotor

This paper presents the study of creep-fatigue interaction damage and creep rupture limit for 9–12% Cr steam turbine rotor under coupled cyclic thermal-mechanical loadings that alter in phase and out of phase. The investigation is implemented using the Linear Matching Method (LMM) and based on the newly developed creep-fatigue and creep rupture evaluation procedures. Latest experimental creep data of FB2 which belongs to 9–12% Cr heat-resistant steel family is employed to calculate creep-related damage and creep rupture bearing capacity of steam turbine rotor. Various factors that affect creep and fatigue damage of steam turbine rotor are analyzed and discussed, including dwell period and rotating speed of rotor. From the parametric studies, the damage locations and cycles to failure induced by creep and fatigue mechanism are presented respectively. Moreover, through creep rupture analyses for varying desired fracture time, the novel creep rupture curves under multi-type double-cyclic-loading conditions are given and further compared with cyclic plasticity failure curve. These surrogate-model analyses offer a deep understanding of structural responses of steam turbine rotor under long-term high temperature operation and provide critical loading conditions to be referenced for smooth running.

MRAC-Based Voltage Controller for Three-Phase CVCF Inverters to Attenuate Parameter Uncertainties Under Critical Load Conditions

This paper investigates a robust model reference adaptive control (MRAC) method for a three-phase constant-voltage constant-frequency (CVCF) inverter with an output LC filter. The proposed MRAC method is designed to stabilize the error dynamics of the system by a feedback control term in the steady state and attenuate the parameter uncertainties of the system by an updated MRAC term. Unlike the conventional proportional–derivative control (PDC) scheme, the proposed MRAC scheme ensures the fast convergence of the output errors to the exponential trajectories predefined by the reference models. Furthermore, the adaptive state-feedback mechanism can guarantee the fast dynamic response in the transient state without using load current sensors or observers. The asymptotic stability is mathematically proven by a Lyapunov theory. The feasibility of the proposed controller is confirmed through extensive experimental studies on a prototype three-phase CVCF inverter with a TI TMS320LF28335 DSP. Finally, comparative experimental results of three control methods (i.e., conventional PDC, feedback linearization control, and proposed MRAC) are provided to validate the superior performance of the proposed method such as fast transient response, low total harmonic distortion, and robustness to parameter uncertainties under critical load conditions (i.e., abrupt load changes, unbalanced loads, and distorted nonlinear loads).

Energy Feedback Control of Light-Load Voltage Regulation for LLC Resonant Converter

An LLC resonant converter would have light-load voltage regulation problems due to the effect of parasitic capacitances in a high-frequency range. In this paper, an energy feedback control method is proposed to improve the light-load regulation capacity by using synchronous rectifiers (SRs). The SR bridges are controlled ahead of the primary inverter bridges to deliver the energy from load to source, reducing the output voltage independent of the load. Then the desired voltage can be achieved even in no-load conditions by regulating the operation time of the energy feedback mode. The gain, voltage ripple, and efficiency performances of the proposed control method are discussed based on the precise analysis and an accurate model of the converter. Moreover, the light-load operations of the LLC resonant converter are analyzed deriving the critical load condition. Finally, the proposed control method is verified by the experiments of a 12 V/8 A LLC resonant converter prototype.

Tria and quad plate finite elements based on RZT(m) for the analysis of multilayered sandwich structures

Aim of the paper is to develop and to assess a class of plate finite elements for the analysis of multilayered composite and sandwich structures. The adopted model is the mixed Refined Zigzag Theory (RZT(m)), based on the kinematics of the Refined Zigzag Theory (RZT) and on the assumption of transverse shear stresses coming from integration of indefinite equilibrium equations. A triangular and quadrilateral flat finite element are developed by means of the Reissner’s Mixed Variational Theorem and an interpolation strategy to eliminate shear locking. Several numerical examples are discussed to demonstrate the accuracy of RZT(m) and related finite elements for static response, free-vibrations and critical load problems of sandwich structures.

Serve the shortest queue and Walsh Brownian motion

We study a single-server Markovian queueing model with $N$ customer classes in which priority is given to the shortest queue. Under a critical load condition, we establish the diffusion limit of the nominal workload and queue length processes in the form of a Walsh Brownian motion (WBM) living in the union of the $N$ nonnegative coordinate axes in $\mathbb{R}^{N}$ and a linear transformation thereof. This reveals the following asymptotic behavior. Each time that queues begin to build starting from an empty system, one of them becomes dominant in the sense that it contains nearly all the workload in the system, and it remains so until the system becomes (nearly) empty again. The radial part of the WBM, given as a reflected Brownian motion (RBM) on the half-line, captures the total workload asymptotics, whereas its angular distribution expresses how likely it is for each class to become dominant on excursions. As a heavy traffic result, it is nonstandard in three ways: (i) In the terminology of Harrison (In Stochastic Networks (1995) 1–20 Springer), it is unconventional, in that the limit is not a RBM. (ii) It does not constitute an invariance principle, in that the limit law (specifically, the angular distribution) is not determined solely by the first two moments of the data, and is sensitive even to tie breaking rules. (iii) The proof method does not fully characterize the limit law (specifically, it gives no information on the angular distribution).

Feasibility study of the application of a cooling energy storage system in a chiller plant of an office building located in Santiago, Chile

Energy consumption of commercial buildings has increasingly gained attention worldwide, because of its significant electricity consumption and peak power demand. Chiller plants are mostly used in commercial buildings to generate cold water for air conditioning. However, because the chiller plants and the refrigeration systems tend to be oversized with respect to their critical design load conditions, they certainly lead to higher energy consumption than a properly sized chiller plant. One possible way to reduce the power consumption and redistribute energy use is through the integration of latent heat thermal energy storage (LHTES) systems with air-cooling system in buildings. In the present work, a LHTES system based on ice is implemented along with a conventional chiller system of an existing commercial building located in Santiago, Chile. Different strategies of operation in a design day are assessed by computational simulation with EnergyPlus. A hybrid operation strategy is evaluated for the hottest summer week, and the results of the cooling loads are obtained. Finally, we founded that the implementation of a LHTES reduces by 7.8% the cooling energy consumption of the cooling system during the analyzed period.

A Robust Adaptive PI Voltage Controller to Eliminate Impact of Disturbances and Distorted Model Parameters for 3-Phase CVCF Inverters

This paper presents a robust adaptive PI voltage controller (PIVC) that effectively eliminates the impact of disturbances and distorted model parameters for a three-phase constant voltage constant frequency (CVCF) inverter with an output LC filter. The proposed controller contains an adaptive control stage to enhance its tracking performance by compensating for distorted model parameters and a feedback control stage to stabilize the error dynamics to converge to zero. Unlike a conventional PIVC, the proposed adaptive method automatically updates its gains using the adaptive control stage. The stability of the proposed controller is proven by the Lyapunov stability theory. Even without information regarding the load current, the proposed adaptive PIVC achieves a fast dynamic response, small steady-state error, and low total harmonic distortion under critical load conditions. Experimental results demonstrate the efficacy of the proposed control method on a prototype three-phase CVCF inverter using a TI TMS320F28335 DSP.

A Series Voltage Regulator for the Radial DC Microgrid

The concept of a novel series voltage regulator (SVR) for controlling the dc-bus voltage of a radial dc microgrid is presented in this paper. The proposed SVR uses a dual-active-bridge dc-dc converter followed by a full-bridge dc-dc converter. It injects dynamic voltage in series with the dc grid to compensate resistive drop over the network. As a result, the voltage level at the different points of the grid becomes independent of load variation and stays within the specified limit. Note that the required power rating of the SVR is very low (say 2.7%) compared to the load demand considering 5% voltage regulation. In this paper, the voltage regulator is connected at the midpoint of the grid, but it may be connected in some other locations to get optimal rating of the same. The proposed configuration is simulated in MATLAB/SIMULINK at a 380-V level to check the dynamic performance under various operating conditions. A scaled-down version (at 30-V level) of the proposed system is developed in the laboratory to experimentally validate the concept. The results show the effectiveness of such a voltage regulator for the radial dc microgrid, especially under critical load condition.