Elasto-plastic Finite Element Analysis Research Articles

The influence of welding-induced 3D residual stress and distortion on the ultimate strength of multi-stiffened thin plate structures under cyclic load

Stiffened thin plates made of high-strength steel are widely employed in ship hull structures. Their strength capacity is highly sensitive to the welding imperfections and potential extreme cyclic loading from harsh sea state. In this work, a thermal elasto-plastic finite element analysis is conducted to simulate the 3D distribution of welding-induced residual stress and distortion of stiffened thin plate structures with triple spans and triple bays varying the plate thickness and heat input. Experimental results provided by other scholars are used to validate the welding simulation. Afterwards, the ultimate strength and collapse behaviour of the welded structures under different load patterns are dealt with, including monotonic compression load, cyclic compression-tension load and cyclic compression load. Observation from numerical results indicates that, in general, the compressive strength is reduced in each considered cycle by the residual stress as the amplitude of cyclic compression-tension load approximates to the ultimate compressive strain. When this parameter is much greater than the ultimate compressive strain or the stiffened plate is subjected to cyclic compression load, the reduction in ultimate strength is significant in the first cycle, whereas the reduction is ignorable in the later cycles. Besides, if the buckling mode of initial deflection with antisymmetric configuration is superposed to the symmetric out-of-plane welding distortion, impact of the residual stress will be diminished as the asymmetry becomes notable.

Numerical simulation of the uplift bearing behavior of pile foundations for transmission towers in expansive soil area

Piles are commonly used as foundations for power transmission towers located in expansive soil areas. The strength of expansive soil is highly sensitive to the changing water content, which complicates the pile–soil interaction. In this study, a 3D elastoplastic finite element analysis of a typical pile foundation for a transmission tower in an expansive soil area was conducted to understand the effect of water content in expansive soil on the uplift bearing capacity of the pile foundation. The results that the failure mode of the pile foundation is almost unaffected by the water content. Shear failure was observed at the pile–soil interface, and, when the pile was pulled out, it lost its uplift bearing capacity. With the increasing water content of the soil, the ultimate uplift bearing capacity of the pile foundation exhibited a significant decreasing trend, whereas the displacement of soil around the pile became remarkable. The distribution of pile axial force and lateral frictional resistance during the uplift process was also evaluated. The pile axial force decreased gradually and at a higher rate with increasing depth. Once the pile reached the ultimate uplift bearing state, the pile lateral friction increased almost linearly with depth.

Simulation of crack closure effect on cracked beam vibrations

Vibrations of beams containing cracks are usually investigated by using open or breathing crack models. Neither one of these approaches considers the plasticity induced crack closure (PICC) phenomena which occurs in fatigue crack growth. In this study, PICC is integrated into the vibration analysis of a cracked beam. First, fatigue crack growth at a notch is simulated with elastoplastic finite element analysis. Residual deformations, stresses and the contact pressures on crack faces are obtained. In the second phase, the model which includes these quantities is used for forced vibration analysis under harmonic loading by using implicit finite element method. For comparison, analyses are also running without PICC. Simulation results indicate that PICC has a significant effect on vibrations of cracked beams. For low load amplitude levels, cracked beam behaves like an intact beam. Nonlinear behavior appears only at rather high amplitude levels. It is concluded that PICC could suppress the crack breathing behavior and this could make it difficult to detect a fatigue crack in a beam from its nonlinear vibration response.

Basal stability analysis of braced excavations with flexible embedded walls in undrained clay

An upper bound mechanism for the basal stability analysis in non-homogeneous undrained clay is presented. The current streamline-based velocity field formulas are extended into broader forms, allowing for the deformation compatibility between the wall and the soil. The proposed mechanism, comprising three shear zones, integrates consideration of the plastic bending moment of the embedded wall. The present study is validated by comparisons with solutions from the finite element limit analysis and elastoplastic finite element analysis. Results demonstrate that the proposed mechanism yields lower upper bounds compared to existing analytical methods, with predicted failure surfaces aligning well with elastoplastic finite element analysis and finite element limit analysis outcomes. Furthermore, a novel dimensionless number is proposed to quantify the relative strength of the plastic bending moment to soil undrained strength. Investigation into the influence of wall plastic bending moment, soil non-homogeneity, and wall penetration depth reveals beneficial effects. Following the analysis of case studies, it becomes evident that the proposed mechanism offers more dependable predictions compared to the existing circular upper bound mechanism.

Implementation of the c-phi reduction procedure in Cast3M code for calculating the stability of retaining walls in the layered backfill with strength parameters reduction by elasto-plastic finite element analysis using fields data

Implementation of the c-phi reduction procedure in Cast3M code for calculating the stability of retaining walls in the layered backfill with strength parameters reduction by elasto-plastic finite element analysis using fields data

Burst Speed Prediction of Impellers With Critical Strain Criteria

Abstract Accurate prediction of burst speed and critical location of a rotating disk is a great concern for turbomachinery designers and enables the reduction of test numbers required to show rotor integrity during turbo-engine certification. Existing burst methodologies mainly focused on axial disks with well-defined bore, web, and rim regions and mostly depended on correction factors that require previous test data of rotors with similar sizes and features. Furthermore, variation of radial and hoop stresses in a radial disk is less predictable than in axial disks due to the asymmetric nature of impellers, which results in uneven distribution of centrifugal and bending loads. This paper aims to explain the experimental and numerical effort dedicated to building an accurate burst methodology for impellers. Elastoplastic finite element analyses are performed by incorporating geometric and material nonlinearities. Analysis results allowed the prediction of failure locations and observation of redistribution of stresses due to material plasticity as rotor speed increases. An extensive test campaign is carried out to validate the numerical analysis. Critical strain criteria that correspond to the plastic strain of true ultimate tensile strength (UTS), are used to determine burst speeds before tests. Experimental results are compared with Robinson's average hoop stress criteria, Hallinan's criteria, and critical strain criteria to show the strong and weak sides of these methodologies for an impeller application. The numerical burst speeds and crack initiation locations associated with critical strain criteria are found to be in good agreement with the spin test results.

Model experiment and numerical analysis of horizontal subgrade reactions acting on flexible walls

A method of reinforcing breakwaters with piles has been proposed. In this structure, designing piles as passive piles is important. Although most previous studies on horizontal subgrade reactions have been limited to the deformation mode of walls, the subgrade reaction to the wall is known to depend on the deformation mode of the wall. Herein, we investigated the changes in the subgrade reaction due to different deformation modes of the wall using model loading experiments and finite element analysis. The model ground was simulated using an aluminum rod mass, and an aluminum plate was used for the wall model. The model ground height was set to the horizontal ground condition with a 200 mm step at the front and rear of the wall. Lateral loads were applied simultaneously to the wall at two different depths. By controlling the ratio of the two levels of the lateral load applied to the wall, the deformation mode of the wall was altered. The experimental results indicated that the magnitude of the subgrade reaction at the same deflection depended on the deformation mode of the wall. The elastoplastic finite element analysis results of the lateral pile-head loading experiment in the horizontal ground condition exhibited good agreement with the experimental results. Under the two-point loading condition, the behavior of the wall was significantly different for cases in which the deformation mode differed from that of the lateral pile-head loading condition. By comparing the behavior of the ground around the wall, we discovered that the shear deformation of the ground differed depending on the deformation mode.

Curvature evaluation of metal wires in plain weave structure via the elasto-plastic finite element analysis

Macroscopic deformation characteristics of plain weave structure, the most basic woven fabric, are significantly influenced by its mesoscale shape and internal stress state, depending on weaving process factors. This study analyses numerically the effects of the design variables, i.e. the wire pitch on the heddle/shuttle and the plasticity of the metal wires of plain weave, on the deflection state of the warp and weft wires. A model with continuity of the weave structure simulates the simple weaving process using the finite element method in anticipation of its application to homogenisation methods. Furthermore, the continuous curvature between contact points is analysed mathematically to evaluate the finished shape in plain woven structures. Considering internal stress by the weaving process and elastoplasticity of metal wires, the deflection curve is closer to straight lines than the ideal deflection curve in elasticity, owing to local yielding and tension caused by the plasticity effect and setting, respectively.

Utilisation of fuzzy logic and genetic algorithm to seek optimal corrugated die design for CGP of AZ31 magnesium alloy

ABSTRACT Magnesium alloys are the first choice among other lightweight structural metals like aluminium, titanium and beryllium due to low density and excellent corrosion resistance. However, low slip and cold plastic processing ability limits its applications. Constrained groove pressing (CGP), one among the SPD techniques, is well suited for improving the material properties. Corrugated dies are designed to investigate the deformation behaviour of AZ31 Mg alloy samples. To minimise the number of numerical simulations, Taguchi’s L9 orthogonal array is selected for the grooved die dimensions (viz. groove angle, groove width and coefficient of friction). Elasto-plastic finite element analysis is performed by implementing a multi-criterion-based genetic algorithm optimisation tool for obtaining the optimal die geometry. Reduction in total deformation, increase in the equivalent stress and elastic strain of AZ31 sheet were possible with 50° groove angle, 3 mm groove width and 0.22 friction coefficient. Mamdani-based fuzzy logic soft computing tool is implemented for the model to examine the deformation behaviour of AZ31 Mg alloy. 7.86 % deviation is observed in the fuzzy logic model predictions in comparison with simulation results. The suggested die design is well suitable for multiple CGP passes, to improve the grain refinement in the processed sheet.

Experimental characterization and numerical modeling of the influence of a proof load on the fatigue resistance of welded structures

The influence of a proof load, or an initial overload, on the fatigue resistance of welded structures is investigated in this study. Arc welded stiffeners made of S355 steel are considered and fatigue-tested in as-welded and proof-loaded conditions. X-ray diffraction analyses are carried out to evaluate the influence of a proof load on the residual stress field. To better understand the role of a proof load on the mechanical fields in the stress concentration zones, a numerical approach based on elasto-plastic finite element analyses is proposed. The methodology includes a non-local multiaxial fatigue criterion, which integrates a strain hardening contribution, that makes it possible to estimate the number of cycles to failure. The comparison between experimental and numerical results indicates that the beneficial effect of a proof load on the fatigue resistance can be mainly attributed to residual stress and strain hardening effects. The final result is a master SN curve adapted to the fatigue design of stiffener structures, tested under different loading conditions.

Elastoplastic Analysis of Timber Connections with Dowel-Type Fasteners at Ambient and High Temperatures

Three-dimensional elastoplastic finite element (FE) analysis models considering a damage zone and cleavage were developed, including plastic behavior of embedment tests parallel and perpendicular to the grain at ambient and high temperatures, in order to study the embedment behavior of timber. In a numerical analysis of compression tests to define material properties of timber, the post-yield behavior of the X-axis in the axial direction was larger than the test values when the post-yield slope of the test result were set to the plastic modulus. The reason was that the stress on the Y-axis and Z-axis increased after the yield point; thus, the equivalent yield stress decreased. In the numerical analysis of the embedment tests, the effect of the damage zone must be considered in the analytical models because the initial stiffness of the numerical results in compression parallel to the grain was considerably higher than that of the test results when the damage zone was not considered. In the numerical analysis of the embedment tests in compression perpendicular to the grain, the two models reproduced the stiffness and behavior after the yield point of the test results. The model that considered cleaving showed the stress distribution after cleavage.

焼戻マルテンサイト鋼の高C化による粒界破壊強度の向上

Effect of carbon (C) content (0.05 mass% to 0.3 mass%) on critical intergranular fracture stress of tempered martensite steel was investigated using 3 mass% manganese (Mn) steel. The critical intergranular fracture stress was obtained by calculating the maximum principal stress at fracture in a circumferential notched specimen tensile test using elastoplastic finite element analysis. As a result, critical intergranular fracture stress of tempered martensite steel increased with increasing C content. Therefore, the dominant factors of critical intergranular fracture stress were examined from the viewpoints of the amount of segregation of each element on prior-austenite grain boundaries and the grain size of the martensite substructure. The first result was found to be the effect of reducing the amount of Mn segregation by increasing C content. This was thought to be because cementite acts as a solid solution site for Mn, and the amount of Mn in solid solution in the matrix phase is reduced by increasing the C content. The second result was found to be the effect of refining the substructure of martensite surrounded by high angle grain boundaries by increasing the C content. This indicates that grain size also affects crack initiation resistance in intergranular fracture by determining the stress concentration at the grain boundaries as the distance of dislocation accumulation.

Fatigue life prediction considering variability for additively manufactured pure titanium clasps.

This study aims to develop a numerical prediction method for the average and standard deviation values of the largely varied fatigue life of additively manufactured commercially pure titanium (CPTi grade 2) clasps. Accordingly, the proposed method is validated by applying it to clasps of different shapes. The Smith-Watson-Topper (SWT) equation and finite element analysis (FEA) were used to predict the average fatigue life. The variability was expressed by a 95% reliability range envelope based on the experimentally determined standard deviation. When predicting the average fatigue life, the previously determined fatigue parameters implemented in the SWT equation were found to be useful after conducting fatigue tests using a displacement-controlled fatigue testing machine. The standard deviation with respect to stroke and fatigue life was determined for each clasp type to predict variability. The proposed prediction method effectively covered the experimental data. Subsequently, the prediction method was applied to clasps of different shapes and validated through fatigue tests using 22 specimens. Finally, the fracture surface was observed using scanning electron microscopy (SEM). Many manufacturing process-induced defects were observed; however, only the surface defects where the maximum tensile stress occurred were crucial. It was confirmed that the fatigue life of additively manufactured pure titanium parts is predictable before the manufacturing process considering its variability by performing only static elasto-plastic FEA. This outcome contributes to the quality assurance of patient-specific clasps without any experimental investigation, reducing total costs and response time.

Effects of strength and weight of pre-ground improvement on seismic behavior of shallow overburden tunnel

Pre-ground improvement is an auxiliary method for shallow overburden tunnels in which the tunnel excavation is performed after improving the ground on top of the tunnel with/without sides by the replacement method and/or the cement mixing method. Although it has been confirmed in previous studies that pre-ground improvement enhances the tunnel face stability and reduces the settlement of the ground surface during the tunnel excavation, the seismic behavior of tunnels with pre-ground improvement has not been fully investigated. Therefore, the authors conducted a shaking table experiment under the centrifugal acceleration of 50 g in order to understand the effect of the ground-improvement pattern on the seismic behavior of a shallow overburden tunnel. Furthermore, the authors performed a two-dimensional elasto-plastic finite element analysis of the centrifuge model experiment to verify the applicability of the numerical method and to examine the influence of the stress release associated with the tunnel excavation, which cannot be considered in the centrifuge model experiment. In this study, the effects of the strength and weight of the improved ground on the seismic behavior of a shallow overburden tunnel are investigated using the same numerical method. The results confirm that, for a Level 1 earthquake, the increments in bending moment are almost the same regardless of the strength of the improved ground in both ground-improvement patterns. On the other hand, for a Level 2 earthquake, the seismic behavior of a tunnel has different tendencies depending on the ground-improvement pattern because of the expansion of the plastic area in the improved ground and the difference in stiffness between the improved ground and the surrounding ground. Therefore, the optimum strength of the improved ground differs depending on the ground-improvement pattern. It is also found that, although the translational motion of the tunnel with the improved ground is slightly greater when the weight of the improved ground is larger, the increase in weight due to the ground improvement has only a small effect on the seismic behavior of the tunnel because the behavior of the tunnel during an earthquake is dominated by the surrounding ground.

Investigation of the Inhomogeneous Mechanical and Crack Driving Force of Low Alloy Steel SA508 and Its Welded 309L/308L Stainless Steel Cladding

Low alloy steel SA508 autogenously welded with Type 309L/308L austenitic stainless steel cladding is one of the common forms of the dissimilar metal weld joint (DMWJ) in the primary water of a pressurized water reactor (PWR). Accurate evaluation of the inhomogeneous mechanical propriety and crack growth driving force at the corresponding place on the DMWJ is important for integrity analyses of a PWR. In this study, the mechanical propriety of the DMWJ was obtained using a combined Vickers hardness test and the stress-hardness relationship. And, a finite element (FE) model for the DMWJ in a PWR with continuous transition mechanical propriety was built using a predefined temperature field method. Based on consideration of the heterogeneity mechanical properties on the DMWJ with the continuous transition mechanical propriety, the Mises stress distribution and J integral on the crack tip with different crack lengths was analyzed using elastoplastic FE analysis. As shown by the distribution profile of the Mises stress distribution and J integral on the crack tip, the inhomogeneous mechanical propriety distribution is found to significantly affect the crack driving force when the crack tip is close to the fusion boundary.

Artificial neural networks-based [formula omitted]-integral prediction for cracked bodies under elasto-plastic deformation state –monotonic loading

Artificial neural networks (ANNs) integrated with a finite element (FE)-based equivalent domain integral method are developed to compute J-integral at the vicinity of crack tips through a time-efficient approach. Robust ANN models are trained to establish nonlinear relationships between FE predicted elastic and elasto-plastic stress, strain, and displacement fields of stainless steel (SS304). Subsequently, elastic–plastic J-integral can be determined by using only elastic FE analysis solution rather than computationally expensive elasto-plastic FE analysis solution. The results show that well-trained ANN models can efficiently and accurately determine J-integral around the crack tips on the basis of numerical elastic FE analysis solution.

Prediction of buckling capacity of liquid-filled steel conical tanks considering field-measured imperfections

Geometric imperfections caused by fabrication and welding reduce the buckling capacity of steel conical tanks. Few studies have considered field-measured geometric imperfections in such type of shell structures. This study carries out elastoplastic finite element analyses (FEA) to evaluate the buckling capacity of a full-scale stiffened conical steel water tank by considering its imperfections extracted from high-resolution laser scan data. Both global and local imperfections are obtained by comparing the laser scan data with the nominal tank geometry. The global imperfection involves deviation of the actual central axis of the tank from its nominal axis and ovalization of the tank circumference, whereas the local imperfection involves local fluctuations of the tank geometry around the circumference at a given elevation. The FEA is conducted using the commercial package ANSYS where both geometric and material nonlinearities are considered in the analysis. A four-node shell element with a six degrees of freedom per node is utilized in the model. The analysis results shed light on the shapes and magnitudes of global and local imperfections in an existing stiffened steel conical tank and their impact on the buckling capacity of the tank. Furthermore, the adequacy of imperfection tolerances recommended in widely used design standards for steel tanks are examined and compared with those extracted from the field measurements. The analysis results suggest that the local imperfections can be more critical than the combination of global and local imperfections in terms of the buckling capacity of the conical steel tank.

Implicit elastoplastic finite element analysis of tube-bending with an emphasis on springback prediction

Implicit elastoplastic finite element analysis of tube-bending with an emphasis on springback prediction

A simplified settlement prediction method for piled rafts in clay

A method for the estimation of the non-linear, time-dependent settlement of piled raft foundations (PRFs) embedded in saturated clayey soils subjected to uniformly distributed load under the single-drainage condition was developed. A total of 20 different rectangular PRFs resting on four different types of normally consolidated to moderately overconsolidated clays (overconsolidation ratios of 1, 2 and 5) were analysed to quantify their short- and long-term settlements. Rigorous three-dimensional, elasto-plastic finite-element (FE) analyses were performed with the critical-state-based modified Cam Clay soil constitutive model. Biot's coupled consolidation theory was used to model the time-dependent dissipation of excess pore pressure under the single-drainage condition. The FE analysis results were used to investigate systematically the influence of time and various soil and PRF parameters on the settlement responses of piled rafts. Fitted equations correlating PRF settlements and the degree of consolidation with various influencing parameters were developed. The proposed equations predicted the immediate and primary consolidation settlements and the degree of consolidation with reasonable accuracy and can be used in initial design stages.

Performance Evaluation of Sandwich Structures Printed by Vat Photopolymerization.

Additive manufacturing such as vat photopolymerization allows to fabricate intricate geometric structures than conventional manufacturing techniques. However, the manufacturing of lightweight sandwich structures with integrated core and facesheet is rarely fabricated using this process. In this study, photoactivatable liquid resin was used to fabricate sandwich structures with various intricate core topologies including the honeycomb, re-entrant honeycomb, diamond, and square by a vat photopolymerization technique. Uniaxial compression tests were performed to investigate the compressive modulus and strength of these lightweight structures. Sandwich cores with the diamond structure exhibited superior compressive and weight-saving properties whereas the re-entrant structures showed high energy absorption capacity. The fractured regions of the cellular cores were visualized by scanning electron microscopy. Elastoplastic finite element analyses showed the stress distribution of the sandwich structures under compressive loading, which are found to be in good agreement with the experimental results. Dynamic mechanical analysis was performed to compare the behavior of these structures under varying temperatures. All the sandwich structures exhibited more stable thermomechanical properties than the solid materials at elevated temperatures. The findings of this study offer insights into the superior structural and thermal properties of sandwich structures printed by a vat photopolymerization technique, which can benefit a wide range of engineering applications.