Reduction Of Stress Concentration Factor Research Articles

Investigating surface effect on stress concentration in amorphous carbon materials with nano-scale pores: A molecular dynamics study

This article investigates the impact of nano-scaled pore on the mechanical properties of amorphous carbon (a-C) as a potential anode material for energy storage batteries, capacitors, and gas storage. Classical mechanics theory suggests that the surface tension caused by surface-bulk elastic mismatch can be used as a design parameter to offset stress concentration around pores and suppress crack growth. In this work we use molecular dynamics simulations to prepare and test 3D a-C specimens with cylindrical pores of varying radii to test this theory. Our results show a significant reduction in stress concentration factor from continuum prediction of 3 to a much lower value between 1.7 and 2.3, which can be attributed to local compressive hoop stress at the pore surface due to surface elastic mismatch in conjunction with nanoscale pore radius. These findings support the idea that pore size and shape can play an important role in improving the mechanics and strength of porous carbon materials, which could have significant implications for the development of high-performance energy storage devices.

Attenuating liquid crystal elastomers’ stress concentration by programming initial orientation

Liquid crystal elastomers (LCEs) are a kind of soft smart materials with fascinating programmability and have a broad application prospect. The monodomain LCEs under loading exhibit the semi-soft elasticity due to the stress-induced director rotation and the resulting spontaneous strain. In this article, we aim to reduce the severe stress concentration of LCEs by programming the initial director orientations. We numerically studied the stress concentration behaviors of a LCE sheet containing a small circular hole with different distribution of initial director under uniaxial loading. For monodomain LCE sheets, the stress concentration factor monotonically decreases as the initial director orientation deviates from the loading direction, and achieves its minimum when the deviation angle reaches 45°. This reduction of stress concentration is attributed to the smaller relative net director rotation and the lower resulting spontaneous strain near the hole edge as the deviation angle increases. Since the director rotation near the hole edge is critical for the attenuation of stress concentration, we only program the initial director orientation near the hole edge, and achieve up to 50% reduction in stress concentration factor. Increasing the size of programming area also enhances the attenuation of stress concentration. This work provides a new strategy to reduce the stress concentration in LCEs with geometric defects.

Stress concentration in shape memory alloy sheets with circular cavities: Effects of transformation, saturation, and plasticity

The analysis of stress concentration in geometrically heterogeneous smart structures is of great importance. In this study, by utilizing a recent constitutive model which considered both transformation and plasticity of shape memory alloys (SMAs), the stress concentration factor (SCF) in plates with circular cavities is investigated and the effect of phase transformation, saturation, and plasticity which may occur locally is studied. The results show that the conversion of the austenitic phase to the martensite leads to a reduction in SCF. After saturation of phase transformation at the stress concentration point, the SCF increases until the entire sheet enters the martensite phase. In the example under study, the SCF reaches 5.8 which is greatly higher than the elastic SCF. By entering the plastic region locally, the SCF reduces. Also, the modeling of sheets with more than one cavity has been done. It is concluded that extra hole, as a stress relief method, has a stronger effect on decreasing maximum stress concentration of shape memory alloys (considering transformation and plasticity) compared to purely elastic stress concentration studies.

Reduction of Stress Concentration Factor (SCF) on the Bolted Joint Connection for a Large Wind Turbine Rotor Blade through Various Design Modifications

The importance of a reliable blade root connection has grown due to the higher-gravity-induced edgewise loads on the blade root that resulted from the recent increased size and weight of a wind turbine rotor blade. To avoid the loosening of a bolt joint connection or even consecutive blade failures, the stress concentration factor (SCF) at the bolt thread root that is sensitive to fatigue should be understood comprehensively. In this work, two-dimensional and three-dimensional finite element (FE) analysis methods were used to determine the SCF at the bolt threads both between an insert and a M42 bolt used for a large offshore blade, and between a M42 bolt and a nut. The effect of various geometric parameters on the SCF were also investigated, which included shank diameter, nut height, nut type, and relief cone. Results showed that the decreased diameter of a M42 bolt shank diameter was the dominant design driver in reducing the stress concentration factor by 40%, from 3.94 to 2.32. The round nut type was also a recommended factor to be implemented to connect bolts and inner pitch bearing with an additional 10% SCF reduction. The relief cones applied to bolt threads and insert threads also contributed to the reduction of SCF to 2.01, a 49% reduction in total. This work not only provides guidelines by which to choose the proper geometry of the bolt and nut for a large blade, but also could be beneficial in designing bolted joint connections of segment or modular blades.

An interactive procedure to reduce stress concentration factor

Reducing the stress concentration factor (SCF) in a design has been always a major challenge for mechanical designers. In this paper, a comprehensive investigation is conducted on the SCF reduction problem. Using a novel interactive two-step procedure, the benchmark SCF reduction problem of an axially loaded plate with a central circular hole is studied. In the first step of the procedure, the reliever topological material elimination approach is used. Numerical results show a considerable permanent reduction in SCF and the weight of the plate, simultaneously. In the second step of the procedure, piezoelectric actuators arranged by Particle Swarm Optimization Algorithm are used to perform a smart and controllable decrease in the SCF. Furthermore, the effect of the number of actuators used, the optimal pattern for locating actuators in each case, and the values of optimal voltages for each pattern are investigated. Finally, some experimental tests are carried out to validate the numerical results. As the final results show, this procedure decreases the SCF and the weight of the plate up to 54.4% and 25.2%, respectively. Making comparisons between the procedure and two previous techniques, the procedure is more effective in satisfying the problem objects. Finally, an interactive procedure helping designers in problem-solving and decision making in an SCF reduction problem is proposed.

Stress concentration factors in joints of square hollow section (SHS) brace and concrete-filled SHS chord under axial tension in brace

It has been shown that concrete filling of square hollow section (SHS) chords in trusses increases fatigue life at the joint between the chord and the SHS brace. Nonetheless, currently there are no expressions for the stress concentration factor (SCF) at these joints. In this study, a robust finite element investigation is carried out to examine SCF in 90° joints made up of empty SHS braces and concrete-filled SHS (CFSHS) chords under axial tension in the brace. Results were validated against experimental data. A comprehensive parametric study was then carried out on 80 FE models to generate a large database for SCFs, considering the effects of key non-dimensional parameters, namely; width ratio between brace and chord of 0.4–1.0, width to wall thickness ratio of the chord of 12.5–25, and wall thickness ratio between brace and chord of 0.25–1.0. Using multiple regression analyses, a series of formulae for SCF were developed for practical design applications. A comparison of SCFs in joints with hollow and concrete filled chords shows a 10–26% reduction in SCFs in concrete-filled chords, leading to better fatigue behavior.

Best pattern for placement of piezoelectric actuators in classical plate to reduce stress concentration using PSO algorithm

ABSTRACTThe purpose of this article is using piezoelectric actuators to reduce stress concentration factor in classic plates under tensile force. There are three classic plates in this study including plates with hole, notched plates, and filleted plates. Here we look for optimal models for placement of the introduced piezoelectric actuators at different ratios of stiffness to maximize reduction in stress concentration factor. In order to optimize piezoelectric actuators placement, particle swarm optimization algorithm is used. The results show that the location of patches is changed by different values of stiffness ratio (stiffness of plate/stiffness of patches). The findings indicate that the piezoelectric actuators placement models are different for stiffness values at smaller and greater than 1 in classical plates. For stiffness ratios smaller than 1, the patches can reduce the stress directly. However, the actuators follow a certain model in all classic plates at stiffness ratio greater than 1, and reduce stress stream indirectly by affecting the stress stream. The results show that there is a certain model for placement of piezoelectric actuators at any certain stiffness ratio in all classical plates. Three empirical tests have been used to validate the results.

On the reduction of stress concentration factor in an infinite panel using different radial functionally graded materials

On the reduction of stress concentration factor in an infinite panel using different radial functionally graded materials

On the reduction of stress concentration factor in an infinite panel using different radial functionally graded materials

This work deals with a parametric study of the stress concentration factor in different functionally graded material panels having a central circular hole subjected to a uniaxial tensile, biaxial tensile and shear load. The extended finite element method along with isoparametric graded finite element material properties modelling scheme is used to evaluate the stress concentration factor. Three candidate material gradation models are tested in this parametric study and various metal ceramic functionally graded materials are also tested for stress concentration. The stress concentration factor evaluated numerically for a wide range of Young's modulus ratio and power law index are presented. It has been concluded that the low stress concentration factor can be achieved by choosing the proper material gradation model and their power law index for a fixed value of Young's modulus ratio.

SCF in Isotopic & Orthotropic Plates Part II: Biaxial Load

This is the second of two companion papers which collectively present a comprehensive 3D finite element analysis on stress concentration in isotropic and orthotropic plates. Any design engineer must know the effect of the various parameters on the stress distribution and strength to successfully design a machine element. The following research makes an attempt to study the effect of cutout orientation and bluntness ratio on the Stress Concentration Factor (SCF) of biaxial loaded isotropic and orthotropic plates. ANSYS APDL environment has been used here to carry out the 3D analysis. The bluntness ratio r/R=0.2 is most effective in terms of percentage reduction in SCF as compared to higher bluntness ratios. The percentage reduction in SCF with r/R=0.2 is about 35% for Mat 1 and Mat 3 and about 50% for Mat 2.

On the Reduction of Stress Concentration Factor Around a Notch Using a Functionally Graded Layer

The aim of this paper is to study the effect of functionally graded material (FGM) layer around a notch in a plate in three dimensions. Using exponential law of variation of Young's modulus and a coefficient of Poisson constant, the stress concentration factor (SCF) depends on the gradation direction of the constituent materials. The finite element method is used to study the performance of FGM layer around a notch in a ceramic plate under in tensile load. A parametric study is performed for several geometric and mechanical parameters such that width of the FGM layer and the ratio of FGM layer components. The effect of notch radius is also studied.

Optimum pattern of piezoelectric actuator placement for stress concentration reduction in a plate with a hole using particle swarm optimization algorithm

This paper focuses on pattern recognition of the best position of piezoelectric patches around the hole in a plate under tension to attain the maximum reduction in stress concentration factor. For this purpose, using particle swarm optimization algorithm, the area and the best location of piezoelectric patches are investigated and the optimum pattern recognition around the hole is presented. Then, the effect of increasing the area of piezoelectric patches and voltage on stress concentration reduction are investigated for all percentage area of piezoelectric patches. To confirm the results, by considering two experimental tests, the results are compared with those reported in presented paper.

Analysis of damage localization in composite laminates using a discrete damage model

Damage localization around stress raisers and material defects in laminated composites is studied using a discrete damage mechanics model augmented by a fiber damage model. The proposed formulation captures the damaging behavior of plates with initial defects and stress raisers such as holes, including damage initiation, evolution, and ultimate fracture of the specimen. It also helps explain the reduction of stress concentration factor when matrix and fiber damage develop. The state variables are the crack density and the fiber failure damage. The formulation is implemented as a material model in Abaqus applicable to laminated composite plates and shells. Material defects are simulated by inserting an initial crack density in a small region of the specimen. Stress raisers are simulated by an open hole. The predictions are shown to be insensitive to mesh density. Further, damage localizes near stress raiser and material defects, thus numerically demonstrating the objectivity of the proposed model. Qualitative and quantitative comparisons with experimental data are presented.

Design Optimization and Analysis of a Steam Turbine Rotor Grooves

A large variety of turbo-machinery rotor groove geometries are used in industry. However, there is no specific attempt made to compare them on the basis of performance. This paper provides an attempt to fill that gap aiming to find an optimum geometry. The main objective of the investigations is to reduce fillet stress concentration factor and associated deformations. The present work carries out the design modification of fillet of a Steam Turbine Rotor. Finite Element Analysis is performed using ANSYS Workbench that is used to determine the fillet stresses effectively to modify the blade rotor grooves. The low pressure steam turbine rotor blades have usually a history of stress failure. They suffer from tensile and bending stresses partly due to the centrifugal force as a result of high rotational speeds and partly due to high pressure, temperature and speed steam loading. The centrifugal force is one of the problems that face the designers of turbine blades especially the long ones. The designer always aims at reducing these stresses. One way to do so is by the reduction of stress concentration factor which takes place between the rotor and its groove. That is to make the rotor blade of variable cross section instead of straight. This paper presents the method of reducing stress concentration factor as well as reduction in the deformation of the rotor.

The reduction of stress concentration in a uni-axially loaded infinite width rectangular isotropic/orthotropic plate with central circular hole by coaxial auxiliary holes

A comprehensive plane stress finite element study is made for reduction of stress concentration factor (SCF) in a uni-axially loaded infinite width rectangular isotropic/orthotropic plate with central circular hole. The finite element formulation was carried out by the analysis section of the package ANSYS. With the help of present work, stress concentration can be reduced up to 24.4 % in an isotropic and 31 % in an orthotropic plate by introducing four coaxial auxiliary holes on either side of main hole. The study reveals that the introduction of these holes helps to smooth flow of the tensile stress past the main hole and result a reduction in stress concentration factor. With such reduction in maximum stress levels, the improvement in fatigue life of a component can be significant.

Stress Concentration and tts Mitigation Techniques in Flat Plate with Singularities - A Critical Review

A number of analytical, numerical & experimental techniques are available for the reduction of stress concentration factor around discontinuities. Using various techniques the SCF around different discontinuities in a rectangular plate made up of different materials under different loading conditions have been reported in literature. Mitigation of stress concentration around different types of discontinuity is also reported in literature. This paper is to present an analysis and overview of emerging techniques developed for analysis as well as mitigation of stress concentration. The proposed methods in literature are compared.

In-situ neutron diffraction study of phase stress evolutions in a Ni-based porous anode solid oxide fuel cells under uniaxial load

Ni/YSZ porous composite is used widely as anode for solid oxide fuel cells. In this study, neutron diffraction patterns were recorded in-situ while a bulk porous Ni-NiO-YSZ anode was loaded under uniaxial compression. Single peak refinement was used to calculate the lattice strains of each phase in the composite, and the local stress state of each phase was derived from the measured lattice strains and the corresponding diffraction elastic constants. An internal triaxial stress state was observed to develop in the bulk of the specimen under plastic deformation, specifically in the Ni phase. Meanwhile, the NiO and YSZ phases are deforming elastically even in the macroscopically plastic regime of deformation. The von Mises equivalent stress was used to quantify the phase stress evolution. A significant stress concentration induced by the presence of pores becomes manifest in all three phase components. The reduction of stress concentration factor in Ni above the yield point of the composite can be attributed to a gradual change of the grains-pores morphology during the plastic deformation.

Finite element analysis of defence hole systems for the reduction of stress concentration in a uniaxially-loaded plate with two coaxial holes

A comprehensive plane stress finite element study is made of the effect of three different types of defence hole systems (A, B and C) upon the stress concentration in a uniaxially-loaded plate with two coaxial holes. Throughout this project, the geometry definition, model creation aspects for meshing the plate and generating elements and nodes, model preparation for applying the appropriate loads and restraints, model checking for coincidence nodes and element distortion, and output display of the many cases investigated were all carried out interactively using SDRC (SUPERTAB-IDEAS) pre- and post-processors. The finite element analysis was carried out in the analysis section of the package, known as SUPERB. The study reveals that the introduction of defence holes on either side of the main holes helps to smooth the flow of the tensile principal stress trajectories past the original holes, and thus effect a reduction in stress concentration factor (SCF), an improvement in component strength and a reduction in its weight. Reductions in maximum SCF ranging from 7.5% to 11% have been achieved by the present technique. With such reductions in maximum stress levels, the improvement in fatigue life of a component can be significant.

Optimum shapes of inner and outer boundaries of circular rings under internal diametral tension

Optimisation of inner and outer boundaries of circular rings under internal diametral tension using two dimensional photoelasticity is presented. It is shown that, when shape optimisation of only one of the two boundaries in the field is carried out, there is an increase in the stresses on the other boundary. However, optimisation of both the boundaries is found to be beneficial with significant reduction in stress concentration factor and weight.

Optimization of inner and outer boundaries of beams and plates with holes

This paper presents optimized shapes of inner and outer boundaries for three specific problems: a long rectangular plate with a central hole subjected to uniaxial tension, a simply-supported slotted beam subjected to a load uniformly distributed over a small area at the centre, and a square plate with a central hole under uniaxial uniform pressure. The two-dimensional photoelastic method is used for optimization. The results indicate a significant reduction in stress concentration factor or in weight, or in both. The examples presented also include cases where the inner and outer boundary stresses are mutually dependent.