Accurate Stress Research Articles

An edge-based smoothed three-node composite plate element with refined zigzag kinematics

The so called Refined Zigzag Theory (RZT) has proven to be one of the most promising theories for analyzing shear-elastic, laminated structures. Since its first appearance considerable efforts have been made in the development of numerical tools and the assessment of the theory. The present contribution focuses on the numerical assessment of a variant of a triangular plate element, which was presented for the first time in 2013 by Versino and co-workers. This element uses low order interpolation functions for the seven kinematic variables. While the deformations show a good convergence a very dense mesh is required to get stresses of satisfying accuracy especially near zones of warping constraints. An edge-based smoothed element technique (ES-FEM) is applied to improve the performance of this element. Several numerical tests are performed with regular and irregular meshes which have shown a significant improvement of the convergence rate of deflections for thick and thin plates respectively. In relation to the standard FEM the ES-version shows a lower sensitivity to mesh irregularities and more accurate stress results.

Accurate stress calculations based on numerical atomic orbital bases: Implementation and benchmarks

We present the detailed formalism for stress calculations within the framework of numerical atomic orbital (NAO) bases, as implemented in the first-principles electronic code package ABACUS. The validity and numerical precision of our implementation are benchmarked against reference results obtained using the finite difference method as well as those calculated using plane wave (PW) bases. The equation of state for α-quartz calculated using our NAO-based implementation is in excellent agreement with the experimental data. We further show that the stresses calculated in terms of NAO bases converge much faster with respect to the energy cutoff than those calculated using PW bases. This enables accurate stress calculations at relatively low cost. An analysis is provided to elucidate the origin of this behavior.

Accurate stress intensity factors for kinked interface crack in bonded dissimilar half-plane

In this study, the stress intensity factor (SIF) of an interface kinked crack is analyzed by the singular integral equation of the body force method. The problem can be expressed by distributing the body force doublets of the tension and shear types along all the boundaries of the kinked and interface crack parts. The SIFs can be obtained directly from the densities of the body force doublets at the crack tips. Although the problem has already been calculated using the crack connection model, the accuracy of the analysis has not been clarified. From the analysis results in this study, it can be seen that the SIFs calculated by the crack connection model have a nonnegligible error, and the present method gives more accurate results. The advantage of the present method is that the SIFs of the kinked and the interface crack tips can be obtained at the same time with high accuracy.

Adaptive analysis for phase-field model of brittle fracture of functionally graded materials

Gradient distributions of material properties are taken into account in phase-field model of fracture and the adaptive consistent element-free Galerkin method (EFG) for the fracture analysis of functionally graded materials (FGMs) is presented. Considering the fact that both the moving least-square approximation and the consistent integration scheme contribute to producing the accurate stress fields, the phase-field model of fracture for FGMs is numerically solved by the consistent EFG method in this paper. The corresponding adaptive criterion is established and 2D and 3D crack propagation of FGMs is simulated. Numerical results indicate that the proposed method can accurately reflect the influence of different distributions of material parameters on the crack propagation and has high efficiency compared with the standard EFG method and linear FEM. At some level, the fracture mechanism that crack propagation is driven by both the strain energy history and the critical energy release rate is revealed using phase-field model.

Two dimensional kinematic models for CNT reinforced sandwich cylindrical panels with accurate transverse interlaminar shear stress estimation

A comparative assessment of two sets of refined higher order theories, one with only shear deformation and another with both shear and normal deformations, is presented for the first time for the thin and moderately thick, CNT-reinforced cylindrical shell panels. The comparisons are also made with simple first order shear deformation theory (FSDT). The governing equations for static and free vibration analyses are systematically derived using a variational principle and solved analytically using Navier’s method. Effective material properties are evaluated using extended rule of mixture for continuously graded CNT fibres in thickness direction. Efficiency parameters are adopted from existing literature to effectively map the nano-scale CNT properties to macro-scale polymer matrix. Transverse shear stresses are recovered using the popular three-dimensional (3D) elasticity equilibrium equations, though there are issues with this scheme which are documented elsewhere, and a comparison with direct constitutive relations’ based evaluation is made. Results highlight the utility of thickness stretching kinematic models and advantage of one-step recovery procedure over a priori-constrained shear traction model. The computational advantage of higher order shear deformation theory make them more suitable for thin and low volume fraction nanocomposites. The estimates of FSDT suffer from significant errors for these heterogeneous materials. The benchmark results using refined shear and normal deformation model are presented for CNT-reinforced sandwich cylindrical panels. The dependence of volume fraction and geometrical parameters on the stress and free vibration response is also studied.

Continuous mental stress level assessment using electrocardiogram and electromyogram signals

Stress endangers the mental and physical health of individuals and also has a major impact on society. Accurate, reliable and quantitative measurement of stress leads to detect the person’s stress level in earlier stages and reduce the intervention time to manage it. Individuals’ stress gradually changes from one level to another. That’s why traditional stress recognition approaches are not useful in real-life. Besides, the physiological stress response is subject-dependent. We proposed a continuous personalized stress assessment method. For this purpose, a fuzzy-based model was employed which uses the electrocardiogram and electromyogram signals together to achieve a highly reliable and accurate mental stress index. A total of 34 healthy participants were recruited during a pre-designed stress-inducing protocol. The results of the experiments illustrated the feasibility of the proposed approach. The perceived stress score showed a high correlation (more than 0.9) with the mental stress index. Moreover, the average stress classification accuracy across all subjects for two-level and three-level achieved as 96.7 % and 75.6 %, respectively. The acceptable accuracy of multi-level stress detection, along with the obtained personalized stress index and high correlation between the perceived and estimated stress, emphasize the proposed methodology as an innovative approach, in line with state-of-the-art research.

Research on Deformation Mechanism of Cutting Nickel-Based Superalloy Inconel718 Based on Strain Gradient Theory

Abstract The traditional material constitutive model can effectively simulate the mechanical properties during the cutting process. However, the scale characteristics contained in materials are not considered in the traditional cutting model, and the inherent scale effect of materials is also ignored. Therefore, the traditional cutting constitutive model cannot effectively reflect the size effect in the cutting process, and then cannot obtain the accurate stress, strain, and temperature. In this present paper, a material constitutive model which can reflect the scale effect is established based on the strain gradient plasticity theory. Through the established model and secondary development of abaqus, the two-dimensional dynamic finite element simulation model of cutting Inconel 718 is established. By comparing the cutting experiment results with the simulation results, the established simulation model can more accurately reflect the effects of temperature, strain gradient effect, equivalent stress, and its scale effect on cutting deformation during the machining process.

Interlaminar stress analysis of functionally graded graphene reinforced composite laminated plates based on a refined plate theory

Nowadays, graphene is considered as one of the most ideal reinforcements for composite materials due to the outstanding mechanical properties. From the literature survey, it is found that the researches on interlaminar stress analysis of functionally graded graphene reinforced composite (FG-GRC) laminated structures are scarce in literature. If transverse shear deformations are unable to be described accurately, the reasonable design of FG-GRC laminated structures will meet severe challenges due to the differentiation of material properties at the adjacent layers. Thereby, such issue is less studied by using the efficient models, so an advanced mixed-form plate theory is to be proposed for transverse shear stress analysis of FG-GRC laminated plates. The proposed theory can meet beforehand compatible conditions of transverse shear stresses at the interfaces of adjacent laminates and only contains seven independent displacement variables. By applying the three- dimensional (3D) equilibrium equations and the Reissner mixed variational theorem (RMVT), the accurate transverse shear stresses are obtained. The 3D elasticity solutions and the results computed by using the chosen models are selected to appraise the capability of the proposed model. Numerical results show that the proposed plate model can yield accurately transverse shear stresses without any post-processing procedure. In addition, the effects of graphene volume fraction, graphene distribution pattern, lamination sequence, span-to-thickness ratio and aspect ratio on the displacements and stresses of the FG-GRC laminated plates are thoroughly investigated.

Multiple ellipsoidal/elliptical inhomogeneities embedded in infinite matrix by equivalent inhomogeneous inclusion method

Traditional equivalent inclusion method provides unreliable predictions of the stress concentrations of two spherical inhomogeneities with small separation distance. This paper determines the stress and strain fields of multiple ellipsoidal/elliptical inhomogeneities by equivalent inhomogeneous inclusion method. Equivalent inhomogeneous inclusion method is an inverse of equivalent inclusion method and substitutes the subdomains of matrix with known strains by equivalent inhomogeneous inclusions. The stress and strain fields of multiple inhomogeneities are decomposed into the superposition of matrix under applied load and each solitary inhomogeneous inclusion with polynomial eigenstrains by the iteration of equivalent inhomogeneous inclusion method. Multiple circular and spherical inhomogeneities are respectively used as examples and examined by the finite element method. The stress concentrations of multiple inhomogeneities with small separation distances are well predicted by equivalent inhomogeneous inclusion method and the accuracies improve with the increase of eigenstrain orders. Equivalent inhomogeneous inclusion method gives more accurate stress predictions than equivalent inclusion method in the problem of two spherical inhomogeneities.

Rock Triaxial Tests: Global Deformation vs Local Strain Measurements\u2014Implications

Accurate stress–strain measurements in triaxial tests are critical to compute reliable mechanical parameters. We focus on compliance at the interfaces between the specimen and endcaps, and test specimens under various triaxial conditions using different instrumentation protocols. The tested materials include aluminum, Eagle Ford shale, Berea sandstone, and Jubaila carbonate. Results obtained following common practice reveal that surface roughness at the specimen-endcap interfaces leads to marked seating effects, affects all cap-to-cap based measurements and hinders ultrasonic energy transmission. In particular, cap-to-cap deformation measurements accentuate hysteretic behavior, magnify biases caused by bending and tilting (triggered by uneven surfaces and misalignment), and affect the estimation of all rock parameters, from stiffness to Biot’s α-parameter. Higher confining pressure diminishes seating effects. Local measurements using specimen-bonded strain gauges are preferred (Note: mounting strain gauges on sleeves is ill-advised). We confirm that elastic moduli derived from wave propagation measurements are higher than quasi-static moduli determined from local strain measurements using specimen-bonded strain gauges, probably due to the lower strain level in wave propagation and preferential high-velocity travel path for first arrivals.

Stress–strain model for high-strength concrete tied columns under concentric compression

Providing accurate constitutive stress–strain relationships for confined concrete can increase the reliability of the moment curvature (MC) analyses in the displacement-based seismic design of reinforced concrete (RC) columns. This paper presents a new stress–strain model for high-strength concrete (HSC) tied square columns which exhibit a more brittle behavior than normal strength concrete (NSC) members. Introducing the concept of least confined volume in the damage localization zone at the middle of the concrete core, a new approach is developed for the confinement stress distribution of lateral ties. In order to determine the lateral stresses acting on the vertical surfaces of the least confined volume, the effective confinement stresses are reduced considering the tie configuration. The peak strength and the ductility of the column should therefore be calculated for the confined concrete in this region. A concrete failure criterion applicable to multi-axial compression due to the reduced confinement stresses in two orthogonal directions, is then used to determine the ultimate strength of HSC columns. Moreover, plotting the compression meridian of the failure criterion, a simple and design-oriented formula is proposed for the ultimate strength of HSC tied columns. The validity of the proposed approach and the performances of two well-known analytical models are verified against the test results of eighty-six HSC columns from five different experimental studies for both the axial stress–strain behavior and the ultimate strength. Besides, the implementation of the concept of the reduced confinement stress in the Mander’s model leads to a significant improvement in its capability of predicting the triaxial strength of HSC.

A numerical insight into stress mode shapes of rectangular thin-plate structures

Thin-plate structures are widely exploited in many areas due to their excellent structural properties. This study numerically investigates the mechanics characteristics of the stress mode shapes (SMSs) in thin plates. First, theoretical basis of the SMSs is concisely illustrated for thin-plate structures. Then, finite element models are constructed in numerical simulations. The SMSs of the flat plate are compared with the conventional displacement mode shapes (DMSs). The modal participation factors (MPFs) are used to extract the predominant SMSs. The SMSs in x-, y-, xy- directions and von Mises stress are evaluated in detail. Based on results of the flat plate, local thickness and elastic modulus modifications are performed to examine the changes in SMSs. Modal assurance criterion (MAC) is utilized to evaluate the modal data. The information of SMSs from the modified plates indicates the powerful capability for detecting the local variations of structural properties. Lastly, modal order determination via modal effective mass is numerically verified for accurate dynamic stress calculation.

Implementation and validation of true material constitutive model for accurate modeling of thick-walled cylinder swage autofrettage

The fatigue life prediction and associated fail-safe design of autofrettaged tubes require accurate stress modeling of each specific high-strength thick-walled cylinder during and after the autofrettage process, which depends largely on the elastoplastic behavior of the original steel tubes. However, the complex material behavior is dominated by the Bauschinger effect. Modeling swage autofrettage of such steel tubes remains problematic. This is a crucial issue that has not been resolved by any available modeling software. Therefore, the modeling and design of the autofrettage process for current and future materials depends upon extending the capability of current software.This research aims to develop a user-programmable function (UPF), USERMAT, to realize a user-defined material model that features true material constitutive behavior with the corresponding algorithms for accurate stress analysis of high strength thick-walled cylinders during the autofrettage process. The material constitutive model to be implemented can be formulated with high accuracy via an available material characterization, which can adapt to both nonlinear and linear strain hardening during the initial tensile loading, while the elastic modulus can be reduced during reversed-loading. It also incorporates the Bauschinger effect as a function of the prior tensile plastic strain during the nonlinear compressive reversed-loading phase and fits the experimental data of the true material model. Swage autofrettaged thick-walled cylinder case studies were reported. The results for residual radial and hoop stress and for elastic strain components were compared with independent, available neutron diffraction measurements, and they are in good agreement. Developing and integrating such a UPF into a standard finite element analysis software package is potentially the most significant unsolved fundamental modeling issue relating to re-yielding, fracture and fatigue of modern autofrettaged cylinders and pressure vessels. It can not only provide a fundamental understanding of the deformation mechanism and stress development within the high strength steel tubes during the autofrettage process, but also provide guidance for the design and optimization of the autofrettage manufacturing processes and of material selection for high intensity pressure vessels, a potential market of billions of dollars.

The Effects of Shot Distance and Impact Sequence on the Residual Stress Field in Shot Peening Finite Element Model

In order to investigate the effect of shot distance and impact sequence on the residual stress distribution of 42CrMo steel in shot peening (SP) finite element (FE) simulation, 3D dynamic models with order dimple pattern and stochastic dimple pattern were established via ABAQUS/Explicit 6.14, and the simulation results were compared with experiments. The results show that shot overlap has a significant effect on the residual stress distribution of peened parts. Meanwhile, there is a threshold (related to SP parameter) for shot distance in the vertical and horizontal directions. When the shot distance is greater than the threshold in this direction, the residual stress distribution after SP tends to be stable. The impact sequence has almost no effect on the impact of a small number of shots, but this effect will appear when the number of shots increases. It is necessary to avoid shot overlap and continuous impact of adjacent dimples when the FE model is established; on this basis, the distance between shots and the number of layers of the shots can be reduced as much as possible without affecting the residual stress distribution. In addition, the comparison of simulation and experimental results shows that the residual stress evaluation area consistent with the experimental measurement is essential to obtain accurate residual stress distribution in the FE simulation process.

Investigation on Dynamic Stresses of Pump-Turbine Runner during Start Up in Turbine Mode

The startup process occurs frequently for pumped storage units. During this process, the rotating rate that changes rapidly and unsteady flow in runner cause the complex dynamic response of runner, sometimes even resonance. The sharp rise of stress and the large-amplitude dynamic stresses of runner will greatly shorten the fatigue life. Thus, the study of start-up process in turbine mode is critical to the safety operation. This paper introduced a method of coupling one dimensional (1D) pipeline calculation and three-dimensional computational dynamics (3D CFD) simulation to analyze transient unsteady flow in units and to obtain more accurate and reliable dynamic stresses results during start up process. According to the results, stress of the ring near fixed support increased quickly as rotating rate rose and became larger than at fillets of leading edge and band in the later stages of start-up. In addition, it was found that dynamic response can be caused by rotor stator interaction (RSI), but also could even be generated by the severe pressure fluctuation in clearance, which can also be a leading factor of dynamic stresses. This study will facilitate further estimation of dynamic stresses in complex flow and changing rotating rate cases, as well as fatigue analysis of runner during transient operation.

StressGAN: A Generative Deep Learning Model for Two-Dimensional Stress Distribution Prediction

AbstractUsing deep learning to analyze mechanical stress distributions is gaining interest with the demand for fast stress analysis. Deep learning approaches have achieved excellent outcomes when utilized to speed up stress computation and learn the physical nature without prior knowledge of underlying equations. However, most studies restrict the variation of geometry or boundary conditions, making it difficult to generalize the methods to unseen configurations. We propose a conditional generative adversarial network (cGAN) model called StressGAN for predicting 2D von Mises stress distributions in solid structures. The StressGAN model learns to generate stress distributions conditioned by geometries, loads, and boundary conditions through a two-player minimax game between two neural networks with no prior knowledge. By evaluating the generative network on two stress distribution datasets under multiple metrics, we demonstrate that our model can predict more accurate stress distributions than a baseline convolutional neural-network model, given various and complex cases of geometries, loads, and boundary conditions.

On the use of neural networks for dynamic stress prediction in Francis turbines by means of stationary sensors

Nowadays, one of the major mechanical issues of hydraulic turbines and particularly Francis turbines are the failures produced by fatigue. Due to the massive entrance of new renewable energies such as wind or solar, hydraulic turbines have to withstand off-design conditions and multiple transients, which greatly increase the risk of fatigue failures. Fatigue damage and crack propagation models are based on the static and dynamic stresses on the turbine blades. Therefore, an accurate and realistic determination of these stresses is of paramount importance although it is yet a challenging task. Numerical simulations have still limitations when predicting static and dynamic stresses in the most harmful conditions such as deep part load conditions and transients, which are highly stochastic. The installation of strain gauges on the blades gives accurate stress measurement but it involves long and very expensive measurement campaigns, as the turbine runner is submerged, confined and rotating. In this paper we propose a neural network-based method to determine the magnitude of static and dynamic stresses based on the measurements of stationary sensors, which greatly reduces the complexity and costs of strain gauge testing. Inputs of the neural networks are selected based on previous experience monitoring Francis turbines. The trained network can be implemented in advanced monitoring systems that could continuously evaluate the stress level of the turbine and determine the risk of possible fatigue damage.

Analytical model for stress and deformation of multiple-winding-angle filament-wound composite pipes/vessels under multiple combined loads

An multi-angle filament wound(FW) model is proposed under pressure, torsion and axial loads (multiple combined loads), which can calculate stresses and displacements of arbitrary multi-layers and bring more uniform strength though FW wall thickness. Based on orthotropic constitutive relation and axisymmetric thick-walled cylinder theory, a three-dimensional (3D) analytical solution for deformations and stresses of multi-angle FW pipes/vessels is presented. First, the stresses of fiber layer in cylindrical coordinate are obtained. Then, the deformations and stresses of each orthotropic winding layers, including longitudinal stresses σ1, transverse stresses σ2, σ3 and shear stresses τ12 are obtained, respectively. Moreover, the effect of shear extension coupling is taken into consideration because it is impossible for the ±ϕ winding angles to exist in the same radius. Subsequently, the proposed formulae are verified by numerical and experimental examples with finite element method (FEM). Results show that our new method calculates accurate stresses of FW pipes/vessels under multiple combined loads, which provides a theoretical foundation for damage analysis and optimization for FW structures.

Bending analysis of simply-supported sandwich beam using Chebyshev quadrature element method

Based on the higher-order sandwich beam theory, the Chebyshev quadrature sandwich beam element is established for the bending analysis of the simply-supported sandwich beam. Gauss Lobatto sampling is adopted as element nodes. The discrete governing equation of the sandwich beam is obtained by the principle of minimum potential energy. A series of numerical examples are carried out on simply-supported sandwich beams under sinusoidal loading. The results are compared with the finite element method, showing that the proposed method can yield very accurate displacements and stresses. This method is suitable for the static analysis of simply-supported sandwich beams with different materials and different geometric parameters.

Effective-notch-stress-based fatigue evaluation of rib-deck welds integrating the full-range S—N curve concept

Compared with the nominal and hot-spot stress, the effective notch stress could present more accurate stress state by involving the notch stress induced by weld bead. Inspired by the concept of full-range SN curve, a new notch stress-based fatigue evaluation framework is proposed in this study. The proposed fatigue evaluation framework reasonably considers the material types, welding residual stress, mean stress effect and notch stress concentration. Fatigue testing data of 60 rib-deck welded specimens are extracted to validate the feasibility of the proposed framework. Field monitoring in a real cable-stayed bridge was also carried out as a case study to present the application of the proposed framework. Results show that the proposed fatigue evaluation framework is feasible and reliable for the evaluation of fatigue testing results of rib-deck welded specimens. Instead of the Goodman method and SWT method, Walker equation is recommended to take into consideration the mean stress effect. In engineering application, the proposed fatigue evaluation framework provides more conservative prediction results compared with code recommended FAT225 SN curve, which is applicable to important components where fatigue cracking is not allowed.