Elastic-plastic Finite Element Research Articles

Geomechanical modelling of fault-propagation folds: Estimating the influence of the internal friction angle and friction coefficient

The interplay between faulting and folding mechanisms has been recognised in extensional and contractional settings. Numerous investigations have documented that the analysis of structural aspects of fault-related folds is important for the exploration and exploitation of hydrocarbon resources and in seismic data interpretation. In this study, we used four series of 2-D elastic-plastic finite element models (A, B, C, and D) to investigate the influence of internal friction angle, friction coefficient, and an additional detachment on the fault slip and uplift gradients, stress-strain patterns, and structural evolution of fault-propagation folds (FPFs). We found that: 1) the stress-strain patterns are similar in different sections of a FPF; 2) the plastic strain is concentrated at the fault tip, fault surface, and forelimb of the FPF; 3) among the internal friction angle, friction coefficient, and additional detachment parameters, only additional detachment affects the stress-strain patterns at the crest and backlimb of the FPF; 4) to create a FPF, not only must the fault friction be less than the layers, but the layer friction also has to be less than a threshold; 5) increase in the internal friction angle, decrease in the friction coefficient, and generation of an additional detachment lead to the formation of tight folds whereas, decrease in the internal friction angle and increase in the friction coefficient create wide folds; and 6) the internal friction angle and friction coefficient are two key factors of the fault-related fold style.

EFFECTS OF INTERFACE FRACTAL PARAMETERS ON NORMAL CONTACT STIFFNESS

In this study, the complex rough surface is simplified as the superposition of a series of trigonometric curves based on the least square method. In order to calculate the interface contact stiffness, the optimal substrate modeling thickness is determined by the elastic-plastic finite element method. The relationship between normal contact pressure and normal contact deformation and normal contact stiffness is quantitatively investigated. The mechanism of fractal dimension and characteristic scale parameters of rough surface on normal contact stiffness is revealed. The results show that there is an optimal thickness of the substrate, which can effectively improve the calculation efficiency of the contact stiffness of the rough surface. The normal contact stiffness increases nonlinearly with the increase of normal contact deformation and normal contact pressure. The surface fractal dimension and characteristic scale parameters have significant effects on the normal contact stiffness, and the normal contact stiffness increases with the increase of the fractal dimension. However, the normal contact stiffness decreases with the increase of characteristic scale parameters.

Estimation of elastic–plastic notch strains and stresses using artificial neural networks

AbstractThe prediction of fatigue lives of metallic components using the notch strain approach is based on local stresses and strains. These can be determined either by analytical approximation formulas such as Neuber's rule based on linear elastic stresses or by finite element analysis with elastic–plastic material behavior. The latter has the disadvantage of longer computation times.This paper discusses a draft for application of artificial neural networks as an alternative approach for estimating uniaxial and multiaxial proportional elastic–plastic stresses and strains in notch roots based on linear elastic stresses. These networks are trained on results from elastic–plastic finite element analyses on flat bars notched on both sides and circumferentially notched shafts to predict local stresses. The resulting accuracy is compared to the analytical approximation formulas.The research findings confirm that neural networks can estimate local stresses with similar scatter and better mean value compared to analytical formulas.

Structural integrity assessment of plates containing embedded cracks-part I: Finite element fracture analyses

In this two-part paper, guidance for assessing plates containing embedded elliptical cracks under complex loading conditions is developed, based on the fracture mechanics validation of the limit load solutions using the finite element (FE) J solutions. In Part I of this paper, elastic-plastic 3D FE analyses are performed for plates with embedded off-set elliptical cracks under combined biaxial stresses and through-thickness bending for developing a FE J database containing 612 cases. Totally 18 cracked plate models are created. For each cracked plate, 17 load combinations are considered and two material strain hardening exponents are analysed. The obtained plastic J solutions are correlated as the EPRI type fully-plastic J factor h1. The effect of the applied normal stress parallel to the crack plane, σ2, on the elastic-plastic J behaviour is investigated and it is found that the EPRI linear addition J estimation scheme may not describe the elastic-plastic J behaviour very well for cases under combined bending moment and σ2. The limit load validation using the FE J solutions is detailed in Part II of this paper.

Effect of indenter tip radius on indentation hardness by finite element analysis

In this study, the indentation hardness test is performed by elastic-plastic finite element (FE) analysis. In order to investigate the effect of the wear of indenter tip on the load-penetration depth curve ([Formula: see text] curve), indentation simulation is made by changing the indenter tip radius. The [Formula: see text] curve obtained by finite element method (FEM) is in good agreement with the experimental results. The calculation shows that the indentation plastic work [Formula: see text] corresponding to the area in the [Formula: see text] curve is hardly affected by the indenter tip radius.

A Methodology to Determine the Effective Plastic Zone Size Around Blunt V-Notches under Mixed Mode I/II Loading and Plane-Stress Conditions

The determination of the ductile failure behavior in engineering components weakened by cracks and notches is greatly dependent on the estimation of the plastic zone size (PZS) and, particularly, the effective plastic zone size (EPZS). Usually, time-consuming complex elastic–plastic analyses are required for the determination of the EPZS. Such demanding procedures can be avoided by employing analytical methods and by taking advantage of linear elastic analyses. In this sense, this work proposed a methodology for determining the PZS around the tip of blunt V-notches subjected to mixed mode I/II loading and plane-stress conditions. With this aim, firstly, existing approximate mathematical expressions for the elastic stress field near round-tip V-notches reported in the literature are presented. Next, Irwin’s approach (fundamentally proposed for sharp cracks) and a yield criterion (von Mises or Tresca) were applied and are presented. With the aim of verifying the proposed methodology, elastic–plastic finite element analyses were performed on virtual AISI 304 steel V-notched specimens. It was shown that the analytical formulations presented cannot estimate the complete shape of the plastic zone. However, the EPZS, which is crucial for predicting the type of ductile failure in notched members, can be successfully estimated.

The effect of axial tension and borehole curvature on torsion limit of drill string threaded connections

Threaded connections of drill string have to withstand huge axial tension, large working torque, and even severe bending. When these loads exceed a certain value, threaded connections often fail and may lead to enormous losses. There is no formula for calculating the torsional capacity of threaded connections under complex loads, even in API standards. In this study, the stress characteristics of threaded connections under complex loads were analyzed with 3D elastic–plastic finite element mothed, and a new method was presented to determine the torsion limit of threaded connections under tension and bending. The results show that the torsion limit of threaded connections increases first and then decreases with the increase of axial tension, while decreases gradually with the increase of bending moment. The safety factor 1.1 in API Standard for bending moment is incomplete and a reasonable one can be determined by using the proposed method. The prediction model and 3D diagram allow engineers and operators to easily obtained torsion limit of threaded connections under arbitrary axial tension and borehole curvature.

Deformation mechanism of Mg-Gd-Y-Zr alloy during hot ring rolling

The strain level-dependent deformation mechanisms during hot ring rolling (HRR) were clarified by microstructural and textural heterogeneities of a large-size Mg-Gd-Y-Zr alloy in this paper. Results show that non-uniform strain distribution along the radial direction of the Mg alloy ring caused the variation in practically activated deformation mechanisms among various regions of the ring. According to the result of 3D elastic–plastic finite element model (FEM), the strain level is highest in the outer region, slightly lower in the inner region, and lowest in the middle region. We found that tension twinning and prismatic <a > slip dominate deformation in the region with a relatively low strain level region, such as in the middle region. In the edge region with a high strain level, the main deformation behaviors include the activation and propagation of tension twins, high degree dynamic recrystallization (DRX), and multiple slip mechanisms. This work clarified the origin of the microstructural and textural heterogeneities of large-size Mg alloy ring fabricated by hot ring rolling, and revealed the deformation mechanism during the process, which sheds a light on the manufacturing of large-size Mg alloy structural components and provides a clue for improving their microstructural and mechanical homogeneity.

Quasi-static strength of bridge fingerplates under various design parameters

Fingerplate expansion devices are commonly used to account for movements and rotations of two bridge deck slabs. Under high traffic volumes, these devices experience premature damage that affects the structural integrity of the bridge superstructure. In this paper, a group of experimental tests and 3D elastic–plastic finite element models (FEMs) were conducted to study the structural behavior and modes of failure of this type of joints. The experimental results were also used to validate the FEMs, from which additional FEM models were employed to study the effects of various design parameters. The parametric study results showed that the finger thickness did not influence the response in the absence of stiffeners. On the other hand, the presence of stiffeners increased the finger joint stiffness while decreased the device ductility. Welding zones were the sites of crack initiation in most of the studied cases. The numerical results showed that the locations of the highest stress concentrations were in the weld between the fingerplate and the supporting beam top flange and in the weld between the stiffeners and the top flange.

Influence of thickness reduction on the forming results in the three-dimensional surface rolling process with rigid arc-shaped rollers

Three-dimensional surface rolling with rigid arc-shaped rollers (TSRRAR), a new method for manufacturing double-curved metal sheets, can be used to realize the rapid forming of 3D surface sheets. The non-uniform roll gap is generated by a pair of rollers with different axial radii with appropriately reduced sheet thickness. After passing through the roll gap, a flat sheet is bent in the rolling direction. Meanwhile, it deforms perpendicular to the rolling direction, resulting in a three-dimensional surface part with curvatures in both directions. The work adopted the elastic-plastic finite element (FE) method to simulate aluminum alloy sheets with different maximum thickness reductions. The results showed that the longitudinal curvature of sheets rose as the reduction increased. Moreover, a slightly increased reduction caused a significant variation in the longitudinal curvature radius of sheets. Different bending curvatures of sheets could be obtained by changing the reduction, indicating the flexible forming of three-dimensional curved surface metal sheets. Also, the changing trend of longitudinal curvatures of testing parts obtained by rolling experiments was substantially in agreement with the FE simulation results. Therefore, the established FE models can be used for exploring the manufacturing process of TSRRAR.

Investigating Welding Distortion of Thin-Plate Stiffened Panel Steel Structures by Means of Thermal Elastic Plastic Finite Element Method

In the past decades, thin-plate weldments have been increasingly used in various engineering structures to meet the requirements of lightweight. However, deformation induced by welding process has a negative effect on assembly accuracy and structural integrity. Therefore, it is critical to predict welding deformation and then to control or reduce the total deformation. Because the thermal elastic plastic FEM has the advantage of high computational accuracy, an attempt was made to predict the welding deformation of a large-scale thin-plate welded structure in this study. Based on the commercial software MSC. Marc, a thermal elastic plastic FEM with the consideration of material nonlinearity, geometrical nonlinearity, and boundary nonlinearity was developed to simulate the welding deformation of large-scale welded structures. The effectiveness of the developed computational approach was verified by the experimental measurements. Moreover, the mechanism of buckling distortion in a large-scale stiffened panel structure was investigated based on the simulated results. In addition, the influences of heat input, welding procedures, and constraint condition on welding deformation were also examined numerically.

Role of the mandrel in the variable curvature local-induction-heating bending process of B1500HS thin-walled rectangular tubes

The variable curvature local-induction-heating bending forming (VC-LIHBF) technology is commonly used in the automotive industry to lighten and stiffen the structure parts. This technology allows the simultaneously bending forming and hardening of ultrahigh strength steel tubes, and allows the tensile strength of B1500HS steel to reach 1500 MPa. By adjusting the bending curvature parameters of the experimental facility through the numerical control system, the production of the tubes with variable curvatures could be achieved without changing dies. However, the more major problem of wrinkling occurs when a small-radius bending is applied on thin-walled rectangular tubes. This paper focuses on improving of forming quality and forming limit of the thin-walled rectangular steel tube (TWRST) with mandrel supported. In this study, an analytical model of the mandrel is established and some reference formulas for the selection of the mandrel parameters are deduced. Based on the above analysis, a three-dimensional elastic-plastic finite element method (FEM) model of the LIHBF process of the B1500HS TWRST is developed using the dynamic explicit FEM code ABAQUS/Temp-disp/Explicit, and key technological problems are solved. Experiments are carried out to verify the accuracy of the analytical model and confirm the reliability of the FEM model. The influences of the mandrel on stress distribution during the local-induction-heating bending process are also investigated. The influence mechanism of the mandrel parameters on the minimum radius of the LIHBF without wrinkling and the forming quality is revealed. The appropriate process parameters for the B1500HS TWRST are obtained through experiments and simulations.

Prediction and mitigation of out-of-plane welding distortion of a typical block in fabrication of a semi-submersible lifting and disassembly platform

Out-of-plane welding distortions of block structures during fabrication of offshore structure will significantly influence its dimensional accuracy and production schedule. Taking a B514 block of a semi-submersible lifting and disassembly platform as research object, typical welded joints and their welding conditions were summarized based on actual welding procedure specification (WPS). Effective thermal elastic plastic (TEP) finite element (FE) analysis with parallel computation technology was carried out to examine thermal-mechanical response. Welding inherent deformations, which are considered as the elementary cause of welding distortion, were then evaluated. With welding inherent deformations as mechanical loading, elastic finite element (FE) analysis was then employed to predict dimensional accuracy of examined B514 block, which has a good agreement with measurement data. In order to ensure the fabrication accuracy with less out-of-plane welding distortion, inverse deformation approach was applied to reduce the out-of-plane welding distortion, and influence of welding sequence on out-of-plane welding distortion was also examined. Both mitigation practices have obvious effect on dimensional accuracy of examined B514 block, while corresponding mechanical mechanisms were also clarified.

Numerical simulation of peel test for ductile thin film along ceramic substrate: Elasto-plastic analysis

Due to extensive applications of the thin film/substrate systems in engineering, the research on strength, ductility and reliability of these systems have attracted great deal of interest in recent years. The peel angle of debonded film on the ceramic substrate has a very important effect in the mechanical resistance of film/substrate bi-material. Among critical debonding parameters, peeling angle and thermal residual stresses can be a potential risk of brutal propagation causing the film/substrate composite failure under tensile loading. This study is carried out to analyze the peeling angle and residual thermal stresses effects with crack growth in the specimen. A two dimensional elastic-plastic finite element model is used to compute the J-integral and estimate the plastic zone size at the interfacial crack tip of film/substrate composite. Results show that the peeling phenomena is a fracture mixed mode where the dominance of either mode I or mode II is influenced by the peeling angle while delamination of thin film is greatly dependent on thermal residual stresses.

Low-Cycle Fatigue Properties of Ti6Al4V Laser-Welded Joints Based on a Local Strain Approach

The low-cycle fatigue (LCF) properties of Ti6Al4V and laser-welded joints with different welding speeds are studied using smooth and notched flat specimens. A local strain approach is proposed to estimate the LCF properties of the notched flat specimen based on elastic–plastic finite element analysis (EPFEA). The local strain used in the approach is defined as the average strain in the plastic zone at the notch root and critical section of the notched flat specimen. The results indicate that the local strain approach is suitable for estimating the LCF properties of the notched flat Ti6Al4V titanium alloy specimen. The stress–strain relationship derived from the microhardness can be used in the EPFEA. The LCF properties of the Ti6Al4V laser-welded joints are worse in the heat-affected zone (HAZ) than in the base metal (BM) due to the martensite structure in the HAZ and the mechanical heterogeneity in the welded joints. The LCF properties of the Ti6Al4V laser-welded joints with different welding speeds are close under low strain levels and worse with higher welding speeds under high strain levels.

A constraint correction method based on use of a single test specimen

Commonly, fracture toughness tests on deeply cracked specimens are used to assess defects in large-scale components. The paper presents a method for selection of test specimen type, size and crack length in order to obtain fracture toughness estimates relevant to defects in cracked pipes. The method uses available closed-form T-stress, stress intensity factor and limit load solutions to determine the required specimen dimensions. The paper reports elastic–plastic finite element analyses for single edge notched bend (SENB) and single edge notched tension (SENT) specimens and cracked pipes which demonstrate good agreement of the matching approach, although care is needed in selecting the appropriate limit load solution for SENT geometries.

Stress state characterization of ductile materials during scratch abrasion

Abrasive wear limits the lifetime of many machine components. Most empirical models relate the abrasive wear resistance to material hardness. In reality, however, other material properties are also influencing as scratch abrasion damage follows from a highly complex stress trajectory upon scratching. Numerical (finite element) simulation of scratch abrasion requires the use of a material damage model, which translates this stress trajectory into material degradation and removal. Most damage models include the first two stress invariants. However, fully incorporating the complex stress trajectories that occur during scratch abrasion may require damage models with dependence of the third deviatoric parameter (Lode angle). This paper serves as an a-priori study to evaluate the stress states that may occur during scratch abrasion. Three mechanisms (ploughing, wedging, cutting) are considered. Hereto, the results of an extensive parametric study using elastic-plastic finite element simulations of a scratch indentation process are discussed. Complex, non-proportional variations in stress state values are observed to occur during scratch abrasion. Distinct stress state trajectories are identified for the three abovementioned mechanisms. These variations are critically discussed to motivate a selection of suitable damage models for rigorous finite element analysis of the wear processes associated with scratch abrasion.

A novel methodology for determining the elastic shakedown limit loads via computing the plastic work dissipation

A methodology that targets determining the shakedown (SD) limit loads via computing the plastic work dissipation (PWD) is presented. The methodology is termed: Shakedown Limit - Plastic Work Dissipation (SDLim−PWD) method. Precisely, the SDLim−PWD method performs a fully automated systematic iterative series of elastic-plastic finite element full cyclic loading simulations till the SD limit load is determined via tracking the PWD history of the applied load cycles. Since the SDLim−PWD method is an energy-based approach for determining the SD limit loads; hence, it permits the incorporation of large displacement formulation and/or cyclic plasticity constitutive material models. The SDLim−PWD method is successfully verified against the closed-form solutions of two classical SD benchmark problems. Additionally, it is validated against the test recordings of a full-scale experimental setup conducted at Berkeley Nuclear Laboratories in the UK concerning a large spherical vessel with a thin radial protruding nozzle subjected to pressurization/depressurization cycles.

Novel three-dimensional multi-objective numerical modeling for hot strip tandem rolling

During the hot strip tandem rolling (HSTR) process, the strip is compressed and elongated continuously, causing significant elastic deformation of rolls. At the same time, the strip temperature undergoes rapid increase and decrease, and distributes unevenly along the strip width direction. Elastic deformation of rolls and strip temperature variations have a significant effect on strip deformation, resulting in various strip shape. In this study, a novel three-dimensional (3D) coupled thermal-mechanical elastic-plastic finite element (FE) model for the HSTR is proposed based on the segmentation modeling strategy, where the finish mill is divided into several sub-models. The data transfer technology is developed to integrate the sub-models into a whole model via transferring the strip crown and temperature among the sub-models. Furthermore, the active and deactive element method, rigid pushing technology, and element remesh are also used to improve the calculation efficiency and accuracy of the model. Multi-objective parameters, such as strip temperature, strip crown, elastic deformation of roll stacks, and rolling force during the HSTR process are studied systematically using the developed FE model. The results show that the calculated strip temperature and strip crown after F7 agree well with the measured values, and the relative error between calculated and measured rolling force at each stand is less than 10%. This work offers a fresh perspective on the HSTR simulation.

Evaluation of Tensile Shear Strength for Lap Joint of Dissimilar Steels

The influence of different thickness combinations was investigated on the strength of the lap joint of dissimilar steels. In this study, lap joints of dissimilar steels were welded by laser welding. The tensile shear test was conducted for the lap joints. Rotational deformation process around the weld bead of the lap joint was observed by a digital video camera during the test. Motion analysis from the video of the tensile shear test indicated that the rotation angle around the weld bead was reduced by overlapping higher strength grade steel. Three-dimensional elastic-plastic finite element analysis was performed for the tensile shear test of the lap joint. The numerically calculated deformation behavior of the lap joint subjected to tensile shear loading showed reasonable agreement with the experimental record. It was found that the rotation angle was reduced and tensile shear strength of the lap joint increase by overlapping higher strength grade steel sheet.