Three-dimensional Elasto-plastic Finite Element Research Articles

Finite element analyses of burst capacities of corroded pipelines containing pinhole-in-corrosion defects

This study carries out extensive parametric three-dimensional elasto-plastic finite element analysis to evaluate the burst capacity of oil and gas pipelines containing pinhole-in-corrosion (PIC) defects. The analysis results reveal that the burst capacity of the PIC defect is insensitive to the pinhole diameter but largely affected by the depth of the pinhole and length of the general corrosion. A pinhole located at the centre of the general corrosion is found to have a larger impact on the burst capacity than the same pinhole located near the edge of the general corrosion. The observed size and location effects on the burst capacity are attributed to the bulging deformation of a corroded pipeline under internal pressure. The findings of this study provide the basis for developing a practical, accurate engineering burst capacity model for PIC defects.

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.

Study on Damage Evolution of Tunnel Lining under Strike-Slip Fault Movements

For the purpose of investigating the damage mechanism and failure characteristics of tunnel lining under the strike-slip fault movements, a three-dimensional elastoplastic finite element model was established in the present study including a railway tunnel across an active strike-slip fault. With the assistance of this numerical model, the tensile and compressive damage, plastic strain development process, and shear failure of the secondary lining at different fault plane positions were analyzed, and further, the damage laws of the secondary lining at different fault displacement and tunnel-fault intersection angles were summarized. The simulation results showed that when the maximum imposed fault displacement is 30 cm, the most unfavorable fault plane appears at the junction between the moving block and the fracture zone, and serious tensile cracks and shear failures occur on each fault rupture plane. Besides, the maximum plastic strain and compressive damage are distributed in the vault and invert. In addition, with the increase of the fault displacement, the axial strain of the secondary lining increases as well, of which the main part shows the tensile strain. Interestingly, with the decrease of the crossing angle, the axial strain gradually changes from the tensile strain to the compressive strain, which is consistent with the direction of the fault angle. Furthermore, the tensile and compressive damage of the secondary lining increases with the increase of movement distance. At the same time, the tensile damage along the tunnel ring develops, and the compressive damage propagates mainly in tunnel vault and invert. With the decrease of the crossing angle, the degree of the tensile damage in the lining reduces, as well as its range. The compressive damage area of the lining develops from the vault and inverted arch to the tunnel wall on both sides, which has a trend of penetrating the side walls on both sides. The distribution area and maximum value of the overall lining damage indices in tensile (OLDT) and overall lining damage indices in compressive (OLDC) increase with the increase of fault movement distance. With the decrease of the crossing angle, the distribution range and maximum value of OLDT decrease, and the distribution range of OLDC increases, while the maximum value basically remains the same. The research results of the present work can provide reference for the antidislocation design of railway tunnels crossing strike-slip faults.

Interaction Assessment of Diagonally Spaced Identical Corrosion Defects and Fully Aligned Unequal-Sized Corrosion Defects Under Combined Loads

Abstract In this study, the interaction effects of closely spaced corrosion defects on the burst capacity of oil and gas pipelines under combined internal pressure and longitudinal compression are investigated by using parametric three-dimensional elasto-plastic finite element analyses. Full-scale burst tests reported in the literature are used to validate the finite element model. It is observed that the interaction effects of diagonally spaced defects on the burst capacity are strongly related to the overlapping portion of the defect width or circumferential spacing between the two defects. The analysis results indicate that the strongest interaction between diagonally spaced defects under combined loads occurs if the defects have zero circumferential separation. The interaction weakens as the defects are more and more overlapped or separated circumferentially. It is also observed that the interaction effect associated with longitudinally or circumferentially aligned, unequal-sized corrosion defects is negligible under the internal pressure only or combined loads.

Assessment of the interaction of corrosion defects on steel pipelines under combined internal pressure and longitudinal compression using finite element analysis

Extensive parametric three-dimensional elasto-plastic finite element analyses are carried out to investigate the interaction effect on the burst capacity of oil and gas pipelines containing closely-spaced corrosion defects under combined internal pressure and load-controlled longitudinal compression. The analysis considers two identical, semi-ellipsoidal-shaped corrosion defects aligned circumferentially or longitudinally on the pipeline. The analysis results reveal that the interaction between circumferentially-aligned defects under combined loads is marked: the burst capacity corresponding to the two-defect case can be lower than that corresponding to the single-defect case by as much as 18% as a result of the enhanced bulging of the defect region due to the presence of the longitudinal compression. This is in direct contrast to the observation that the interaction effect is marginal for circumferentially-aligned defects subjected to internal pressure only. On the other hand, the interaction between longitudinally-aligned defects under combined loads is negligible due to the so-called shielding effect; this is again in direct contrast to the observation that the interaction between longitudinally-aligned defects under internal pressure only is marked. The adequacy of four practical interaction rules, which are developed for the loading condition of internal pressure only, for the combined loading condition is also examined. The findings of this study have important practical implications for the integrity assessment of corroded pipelines under combined loads.

Modeling and simulating nonhomogeneous non-Gaussian corrosion fields on buried pipelines and its use in predicting burst capacities of corroded pipelines

The present study proposes an iterative algorithm to simulate nonhomogeneous non-Gaussian corrosion fields on buried oil and gas pipeline's external surfaces. The algorithm incorporates the concepts of the discrete orthonormal S-transform and the iterative amplitude adjusted Fourier transform algorithm. This results in the space-wavenumber power spectral density function and marginal distribution function of the simulated corrosion field matching, respectively, their corresponding targets that could be determined from the high-resolution corrosion measurement data obtained from naturally corroded pipe segments. The simulation results demonstrate that the use of the proposed algorithm is able to capture the statistical characteristics of naturally occurring corrosion fields on pipelines. Three-dimensional elasto-plastic finite element analyses are carried out to evaluate the burst capacities of pipe segments containing simulated corrosion fields. Combined with finite element analyses, this algorithm can be employed to conduct virtual full-scale burst tests of corroded pipe specimens, which is of great value to the pipeline corrosion management practice.

Load-settlement response of piled raft foundations in sand

ABSTRACT The load-settlement behaviour of piled raft foundations (PRFs) resting on sand subjected to vertical uniformly distributed load is studied using three-dimensional elasto-plastic finite element (FE) analyses. The unified Clay and Sand Model (CASM), based on the critical state soil mechanics, is used as the soil constitutive model. In the present study, 38 rectangular, strip, and circular raft connected to different pile configurations resting on four different types of sands in their loose, medium, and dense states are considered. The accuracy of the finite element analysis is confirmed through verification and validation with other numerical and experimental studies. A detailed parametric study is conducted, which includes soil elastic and critical state parameters as well as different dimensions of PRFs. A guideline chart is provided, which will be helpful for a design practitioner to choose the suitable PRF configurations considering serviceability limit states of safety and economical design aspect. This flow chart is based on (i) combined effects of various parameters on PRF load settlement behaviour, (ii) load sharing between the raft and the pile groups, and (iii) a preliminary cost analysis.

Effect of Specimen Thickness and Stress Intensity Factor Range on Plasticity-Induced Fatigue Crack Closure in A7075-T6 Alloy.

Fatigue crack growth experiments are performed using A7075-T6 compact tension (CT) specimens with various thicknesses t (1–21 mm). The stress intensity factor at the crack opening level Kop is measured, and the effects of t and the stress intensity factor range ΔK on Kop are investigated. In addition, the change in Kop value due to specimen surface removal is investigated. Furthermore, we clarify that the radius of curvature of the leading edge of the fatigue crack decreases as t becomes thinner. Using the three-dimensional elastoplastic finite element method, the amount of plastic lateral contraction (depression depth d) at the crack tip after fatigue loading is calculated quantitatively. The following main experimental results are obtained: In the region where ΔK is 5 MPam1/2 or higher, the rate of fatigue crack growth da/dN at a constant ΔK value increases as t increases from 1 to 11 mm. The da/dN between t = 11 and 21 mm is the same. Meanwhile, in the region where ΔK is less than 5 MPam1/2, the effect of t on da/dN is not observed. The effects of t and ΔK on the da/dN–ΔK relationship are considered physically and quantitatively based on d.

Dynamic mechanical behaviour and energy absorption of aluminium honeycomb sandwich panels under repeated impact loads

Sandwich structures are widely applied in the field of marine structure lightweight design and impact protection due to their excellent mechanical properties. In this paper, the dynamic mechanical behaviour and energy absorption characteristics of aluminium honeycomb sandwich panels (AHSPs) under repeated impact loadings are investigated. A three-dimensional elastoplastic finite element model of AHSP under repeated impact loadings is established by using the commercial software ABAQUS Explicit. The accumulation processes of plastic deformation, the impact force-displacement curves as well as the energy absorption performances are numerically calculated. Besides, the repeated impact experiments of AHSPs were conducted by using the drop-weight impact testing machine INSTRON CEAST 9350. Furthermore, the influences of geometrical parameters on the dynamic behaviours of AHSPs under repeated impact loadings are further performed. Results show that the numerical calculated results are in good agreement with the experimental results. With the increase of the repeated impact numbers, the bending deflections of both top and bottom face sheets of AHPSs accumulate gradually, the compressive deformations of the honeycomb cores enlarge gradually and finally the densification phenomenon appears. The deformation mode of bottom face sheet experiences the mode transition from the global bending deformation model to the coupling deformation mode of the global bending deformations and the local indentations.

Assessment of effects of idealized defect shape and width on the burst capacity of corroded pipelines

Abstract This study investigates the burst capacities of corroded pipelines using three-dimensional elasto-plastic finite element analyses validated by full-scale burst tests of corroded pipe specimens reported in the literature. The semi-ellipsoidal idealization of naturally-occurring corrosion defects in FEA is found to lead to more accurate predictions of the burst capacity than the rectangular idealization for defects that are less than 70% through the pipe wall thickness. Results of extensive parametric FEA based on the semi-ellipsoidal idealization of the corrosion defects indicate that the burst capacity in general increases as the defect width increases if the defect depth and length remain the same. The defect width effect is marked for deep, relatively short defects, and should therefore be taken into account accordingly in the empirical or semi-empirical burst capacity models. The defect width effect is marginal for long defects with w/l ≤ 1.5. The observed width effects are explained by considering the change in the membrane and bending stresses in the defect region as the defect width increases.

Nonlinear subgrade modulus of sandy soils for analysis of piled raft foundations

Continuum-based three-dimensional (3D) elasto-plastic finite element (FE) analyses of piled raft foundations (PRFs) resting on sandy soils and subjected to uniformly distributed vertical loads are performed using the software Abaqus, and equations are developed for estimating the nonlinear soil subgrade moduli (spring stiffnesses) beneath the raft, beneath the pile base, and along the pile shaft. The unified clay and sand model (CASM) is used as the soil constitutive model in the continuum-based 3D FE analysis. Four different types of sand with three different states (loose, medium and dense) and twenty different configurations of PRFs with rectangular, square, and strip rafts are considered in this study. The developed equations of the raft and pile spring stiffnesses are subsequently used in a soil spring-based nonlinear FE analysis of PRFs performed using a structural engineering software. It is shown through verification and validation studies that the developed equations of the raft and pile spring stiffnesses can be reliably used in spring-based FE analysis of PRFs, instead of the computationally expensive continuum-based 3D FE analysis, for quick and accurate estimate of PRF responses. The developed equations of spring stiffnesses are applicable to PRFs that are similar in dimensions considered in this study.

Three-dimensional finite element modelling of excavation-induced tunnel wall movement and damage: a case study

A three-dimensional elastoplastic finite element modelling is conducted for the back analysis of a tunnel collapse accident that happened at the Hobart Myer construction site, including the deformation and failure processes of an existing tunnel in close proximity to a basement excavation as well as further excavation-induced tunnel wall damage and collapse. The full three-dimensional tunnelling process and the basement excavation process are modelled using step-by-step approaches through the successive removal and installation of the solid and structural elements, respectively. Furthermore, following the field observations, the storm-induced water pressure in the existing tunnel is taken into consideration and three further small-scale pit excavations without support installation in the sidewall of the basement towards the tunnel are also performed in the numerical modelling process. The obtained results are discussed in comparison to the observed failures at the construction site, to highlight the deformation and failure mechanisms induced by the excavations and stormwater and to examine the trend of the damage and collapse of the tunnel wall and its support system. It is concluded that although the collapse of the tunnel wall is comprehensively the result of nearby basement excavation before and after the heavy rainfall, the storm-induced water pressure is the main factor resulting in the final collapse of the tunnel wall. Further excavations of three small pits only have relatively slight effects on the instability of the tunnel. The well-developed finite element method provides a valuable tool for the study of the stability of shallow tunnels in the presence of nearby excavations under various conditions.

Study on deformation prediction of cement-stabilized soil base

The cement-stabilized soil base has the advantages of high strength, suitable stiffness, water resistance, frost resistance, etc. In this paper, the mechanical behavior and structural characteristics of cement stabilized soil foundation from construction to operation are analyzed by using three-dimensional elasto-plastic finite element method. The process of pavement filling and traffic load is simulated in the analysis. Based on the construction process, the structural characteristics and mechanical properties of cement stabilized soil foundation under asphalt concrete pavement condition are studied. The general rules of deformation and stress of soil-cement foundation under different conditions are discussed.

GROUND STRENGTHENING METHOD BY MULTISTAGE LOCAL CONSOLIDATION

Authors are investigating a ground strengthening method, which effectively consolidates the construction area of requisite minimum in a short time by local consolidation for the soft ground. In this paper, Authors research that the effect of increasing the strength of soft ground by using the three-dimensional elasto-plastic finite element analysis when the area of local loading is gradually changed. First, it was found that the strength of the soft ground just below the local loading surface increased with a short time by the local consolidation and ultimate bearing capacity increases to 2.6 times as much as the initial bearing capacity by the multistage loading. Next, analysis was carried out in the case of the multistage loading with cyclic loading. In the reloading process, loading area is reduced to 1/3 and 1/6 of the loading area of the previous stage. In this case, the ultimate bearing capacity increases to 8.25 times as much as the initial bearing capacity. In conclusion, it was found that the strength of the soft ground just under the loading surface is considerably increased by multistage local loading with reducing the loading area.

Thermomechanical analysis of the wheel-rail contact using a coupled modelling procedure

Abstract This research presents a coupled thermomechanical modelling procedure for the wheel-rail contact problem and computes the flash-temperature and stress-strain responses when thermal effects are present. A three-dimensional elasto-plastic finite element model was built considering the wheel-track interaction. When the wheel is running on rail, frictional energy is generated and converted into heat. To evaluate the contribution of thermal effects and plasticity, five different material models were studied among them TEPS was nonlinear and temperature-dependent including thermal softening. Discussions were made on the effect of solution type and material type. The rail temperature, calculated for a critical creepage case, confirmed the potential of martensitic phase transformation. Thermal effects were also important at lower creepages, where a synchronization effect causes earlier damage.

Automated local line rolling forming and simplified deformation simulation method for complex curvature plate of ships

Abstract. Local line rolling forming is a common forming approach for the complex curvature plate of ships. However, the processing mode based on artificial experience is still applied at present, because it is difficult to integrally determine relational data for the forming shape, processing path, and process parameters used to drive automation equipment. Numerical simulation is currently the major approach for generating such complex relational data. Therefore, a highly precise and effective numerical computation method becomes crucial in the development of the automated local line rolling forming system for producing complex curvature plates used in ships. In this study, a three-dimensional elastoplastic finite element method was first employed to perform numerical computations for local line rolling forming, and the corresponding deformation and strain distribution features were acquired. In addition, according to the characteristics of strain distributions, a simplified deformation simulation method, based on the deformation obtained by applying strain was presented. Compared to the results of the three-dimensional elastoplastic finite element method, this simplified deformation simulation method was verified to provide high computational accuracy, and this could result in a substantial reduction in calculation time. Thus, the application of the simplified deformation simulation method was further explored in the case of multiple rolling loading paths. Moreover, it was also utilized to calculate the local line rolling forming for the typical complex curvature plate of ships. Research findings indicated that the simplified deformation simulation method was an effective tool for rapidly obtaining relationships between the forming shape, processing path, and process parameters.

Through-Thickness Deformation Gradient in a Part-Pilgered Zirconium Tube: Experimental Measurements and Numerical Validation

Through-thickness microstructure development and residual stress evolution were explored in a part-pilgered Zircaloy-4 tube. Clear gradients in microstructure and residual stress were experimentally established at different locations. Such locations were considered along the axial length and also across the wall thickness. These were naturally subjected to different von Mises effective strains and were reflected on strain/microstructural gradients. The deformation gradients were also simulated with a three-dimensional elastoplastic finite element model. The model used both isotropic and anisotropic yielding. Though deformation gradients were similar with both yielding criteria, the anisotropic yielding provided a better match with experimental residual stress gradients.

Bearing Capacity Characteristics of Steel Pipe Piles with Wings installed in Soil Cement Column

The steel pipe piles with wings installed in soil cement developed in Japan and Vietnam are composite foundation piles made of steel pipes with wings with foundation improvement pillars in soil cement and spiral wings attached to the shaft intermittently. This composite pile has large vertical and horizontal bearing capacities and is a construction method of foundation piles with consideration of environments with little surplus soil, using steel pipes with small diameters. In this study, several full scale loading tests on steel pipe pile with wings installed in soil cement column are presented, and bearing capacity of the composite pile is studied. Especially, combination effect of soil cement column and steel pipe with wings on bearing capacity performance are discussed. Threedimensional elasto-plastic finite element analysis based on the results of loading tests to help understand this composite pile’s mechanical characteristics is also performed.

Short rolling contact fatigue and thermal cracks under frictional rolling – A comparison through simulations

The relative severity of radial (thermal) and inclined rolling contact fatigue surface cracks of equal depth in a railway wheel is investigated by three-dimensional elastoplastic finite element analyses of a cracked wheel sector subjected to contact loading. Response is quantified by relative displacements of nodes close to crack tip and at the crack mouth. Highly inclined cracks give the highest magnitudes of crack tip shear displacements, which is the dominant deformation mode. Braking conditions are found to open the crack mouth. Initially higher temperatures on thermal cracks cause increased crack tip deformation and opening of the crack, whereafter subsequent mechanical load cycles impose crack closure.

The influence of high temperature due to high adhesion condition on rail damage

In wheel–rail contact, the locomotive adhesion variable characterizes the capability of the locomotive to convert available friction into traction at the interface. Recently developed AC (Alternating Current) drive induces a higher adhesion level compared to DC (Direct Current) drive. This can significantly affect the wheel–rail contact conditions such as high contact temperature due to the frictional rolling, wear and damage initiation of the rails.The aims of this paper are to determine the temperature rise due to high adhesion contact and the thermal influence on the wear and rail life. Three-dimensional (3D) elasto-plastic finite element model was applied to evaluate the growth of temperature, residual stress and strain. The numerical model employed the moving heat source code developed by Goldak within ANSYS/LS-DYNA. The mechanical and thermal properties of the rail material were governed by temperature. The influence of multi-passes from multiple wheels attached to the locomotives on one point of the rail was also taken into account.The results indicated that after six wheel passes, the temperature due to the high adhesion condition was sufficiently high (723°C) to form the white etching layer (WEL) known to be associated to the rolling contact fatigue (RCF) on the rail surface. Moreover the rail material would be softened by high temperature, which resulted in the acceleration of wear process. Finally the results of thermal stress and strain from FE model were used as input to Kapoor׳s ratcheting model to determine the number of wheel rolling cycles leading to rail damage.