고무부품의 유한요소해석을 위한 재료시험 및 비선형 재료물성에 관한 연구

This paper presents results of large deformation finite element (LDFE) analyses that investigate the penetration response of deep-penetrating spudcan foundations in stratified soil comprising a strong clay layer overlying weaker clay. Such strength profiles give rise to potential punch-through failure, with a local maximum penetration resistance followed by some reduction. The paper is a companion paper to one that presents results from centrifuge model tests of the same problem, with the overall aim being to investigate the potential for punch-through failure and its severity, as a basis for improving design methods for evaluating spudcan penetration. The LDFE analyses have simulated continuous penetration of smooth and rough spudcan foundations from the sea-bed surface. A detailed parametric study has been undertaken, exploring the relevant range of layer thickness (relative to the spudcan diameter), strength ratios and spudcan base roughness. The results have been validated against previously published results and the centrifuge test data presented in a companion paper. Excellent agreement was obtained between the results from LDFE analyses and centrifuge tests. As the ratio of the shear strength of the bottom layer to that of the top layer decreased, upwards soil flow around the spudcan shoulder became less pronounced and the failure mechanism was dominated by a steep-sided plug of soil from the top layer being forced deep into the lower layer. The potential for severe punch-through was demonstrated by a significant reduction in the resistance profile as the spudcan approached the interface between the two layers. A preliminary design chart is proposed to estimate the cavity depth above the penetrating spudcan, which was significantly greater than that for a single layer clay. The penetration responses have been presented in terms of profiles of normalised net bearing pressure for a range of layer geometries and soil properties. Potential punch-through occurred for all cases where the strength ratio was less than or equal to 0·6, with the reduction in resistance being greater the lower the strength ratio of the underlying layer to the upper layer, and the thicker the upper layer, but reducing as the strength gradient in the lower layer increased. The current approach suggested in offshore design guidelines is shown to underestimate the penetration resistance significantly, in most cases, and give poor estimates of the likelihood and severity of spudcan punch-through.

A stress softening known as the Mullins effect is observed usually in rubberlike material after the first load. This paper describes an experimental test method for defining the nonlinear properties of rubber materials used for finite element analysis. Experimental observations have shown that the Mullins effect induces a permanent set and some anisotropy. To test the Mullins effect the mechanical preconditioning is suggested to stabilize the properties of rubber material. A stress-strain curve will change significantly when the rubber material is strained greater than the previous stabilized level. Therefore, material properties at maximum strain level are obtained to predict behavior of rubber products. To obtain the rubber material constants used for finite element analysis to understand the characteristics of automotive rubber parts, mechanical properties tests such as uniaxial tension, equ-biaxial tension and pure shear tests are required. When the load was repeatedly applied to the rubber specimen, the stress-strain relationship was greatest in the first and second cycles, and the larger the strain range, the more the stress was reduced. The material constants were obtained using the stress-strain data after the rubber specimen was stabilized. The value of stiffness decreased as the maximum strain range increased. The static stiffness of an automotive engine mount is calculated by nonlinear finite element analysis using the experimentally determined material constants and compared with the experimental results considering the mechanical preconditioning effect resulting in a good correlation.

Avoiding the material nonlinearity in an external fixation device

The material modeling of hyper-elastic properties in rubber is generally characterized by the strain energy function. The strain energy functions have been represented either in term of the strain in variants that are functions of the stretch ratios, or directly in terms of the principal stretch. Successful modeling and design of rubber components relies on both the selection of an appropriate strain energy function and an accurate determination of material constants in the function. Material constants in the strain energy functions can be determined from the curve fitting of experimental stress-strain data. The uniaxial tension, equi-biaxial tension and pure shear test were performed to acquire the constants of the strain energy functions which were Mooney-Rivlin and Ogden model. Nonlinear finite element analysis was executed to evaluate the behavior of deformation and strain distribute by using the commercial finite element code. Also, the fatigue tests were carried out to obtain the fatigue failure. Fatigue failure was initiated at the critical location was observed during the fatigue test of rubber component, which was the same result predicted by the finite element analysis.

Being the latest representative of gravity installed anchors (GIAs), the OMNI-Max anchor can perform comprehensive behaviors in the seabed, like keying and diving, by adjusting orientation and position to derive higher capacity and to avoid anchor failure. During the keying process of GIAs, many factors may influence the keying capacity, such as the embedment depth, the anchor orientation, the rotational center, the bearing area, and the soil strength. Based on the coupled Eulerian–Lagrangian (CEL) technique, large deformation finite element analyses combined with a bounding-surface plasticity constitutive model are performed to investigate systematically the keying behavior of GIAs in sand. A series of analytical cases involving multiple factors are designed and analyzed to explore the effects of various factors on the keying capacity of GIAs, defined by the soil resistance coefficient during keying. The soil resistance coefficient increases with increasing soil density, while it tends to be stable with the increase of the embedment depth. The closer the rotational center approaches to the two ends of the anchor, the greater the soil resistance coefficient becomes. When the loading arm angle is in π/6–π/4, the soil resistance coefficient takes a relatively small value, while the degree of influence decreases with increasing embedment depth. An explicit expression of the soil resistance coefficient during keying is derived to quantify the effects of various factors, whose applicability and accuracy are validated by different ways. These findings are helpful to understand further the comprehensive anchor behaviors and to promote the application of GIAs in offshore engineering.

Deformation of trenched multilayer pipelines during large lateral displacement due to subsea geohazards

To eliminate the residual stress formed during the process of cold rolling Al board, the distribution of stress in the deformation region is simulated by elas-plastics large deformation finite element analysis. On the basis of analyzing the effects towards residual stress from different rolling loadcase and stretching the parameters toward different rolling loadcase board, the value of cold rolling residual stress caused by different compress and the effect of the residual stress elimination influenced by stretching force and stretching deformation can be figured out. The simulative results show that the method of the proper stretching can evenly and partly eliminate the residual stress, reduce the effect toward the following machining procedure induced by the residual stress.

As offshore oil and gas fields mature on the Grand Banks, offshore Newfoundland and Labrador, marginal field subsea tie-backs are necessary to maintain production levels. Existing untrenched flowline lengths have been limited by the assumption that iceberg contact equates to flowline failure. However, extended tie-backs will be necessary to develop stranded resources. To potentially reduce the number of failure cases, we can consider a better definition of failure that accounts for the pipeline response due to iceberg-soil-pipeline interaction events. Reducing the failure rate from free-floating iceberg contacts alone can significantly increase safe tie-back lengths. This paper examines the flowline response from impacts with free-floating icebergs using large deformation finite element analysis. The plane strain pipe-soil interaction response is first simulated for pure vertical loading and compared against analytical bearing capacity theory. The influence of non-associativity in the soil constitutive model is demonstrated with respect to predicting the pipe drained penetration resistance in dense sands. Oblique vertical-horizontal plane strain pipe-soil interaction is also investigated, and it is shown that the vertical penetration resistance is reduced when the pipe trajectory deviates from pure vertical, consistent with published interaction diagrams. Lastly, the fully coupled interaction scenario of free-floating iceberg-pipe-soil interaction is simulated, showing the effects of the pipe wall thickness and soil strength. The numerical modelling procedures are described and the soil constitutive model that incorporates dense sand behavior is detailed.

Normalizing the influence of flaw length on failure pressure of straight pipe with wall-thinning

Effect of padeye offset ratio on keying behaviours and capacity of square plate anchors in clay

: This research program is aimed at the development of a soil-tire interaction model based on Contact Mechanics Theory. The model developed obviates the need for a priori knowledge of soil-tire contact properties and instead relies on directly and accurately measurable quantities. Other major research outcomes of this program include: 1) the first complete solution to the frictional contact problem between a granular material and a deformable solid, 2) extension of the Double Shearing theory for dilatant granular flow to three dimensions and 3) the development of a new technique for the micro-mechanical analysis of granular media.

Mechanized tunneling using tunnel boring machines (TBMs) has successfully led to many underground infrastructure projects with various ground conditions. Nevertheless, construction under changing ground conditions is still challenging because it causes uncertainty when optimizing TBM operation parameters. Numerical methods, which simulate the advance and excavation process of TBMs, are effective ways to analyze ground behavior and predict operational parameters by considering complex ground conditions and geometries. However, this has not been well established because it involves a large deformation problem in the surrounding ground. The Coupled Eulerian-Lagrangian (CEL) method can be considered a capable approach for large deformation problems in geotechnical engineering. This study presents a three-dimensional large deformation finite element model for TBM tunneling using the CEL method. Under specific operating conditions, TBM thrust force, cutterhead torque, and working chamber pressure were analyzed. The effect of changing ground conditions on TBM operation parameters was investigated.

Dynamic analyses of steel catenary risers (SCRs) suspended from a floating platform usually show that the vicinity of the zone where the riser touches down on the seabed is important to fatigue assessments, and can be critical in realizing an economical SCR design or in assessing SCR design life extension. The motivations of the paper are to advance the understanding of the complex mechanism of SCR-soil interactions in a comprehensive yet efficient way, to supplement the extensive model test-based SCR-soil interaction studies in the literature, and to offer a numerical tool to predict the SCR-soil interaction in different clay types. The objective is to investigate the fundamental mechanism of SCR-soil interactions using advanced finite element (FE) analyses. The FE analysis is conducted in Abaqus using the coupled Euler-Lagrangian (CEL) technique to capture the local large deformation surrounding a riser. A strain-softening soil model for deepwater soft clays subject to cyclic degradation is developed based on cyclic direct simple shear (DSS) tests to avoid cycle-by-cycle simulations. The stress-displacement relations are proposed in the FE analysis to avoid the size effect associated with strain-softening soils. The FE analysis results are compared to the test data from the independently conducted model tests and the established results from the literature. Good agreements among different studies are observed.

As a widely used taut-wire mooring system for deepwater platforms, the Vertically Loaded Anchor (VLA) has better performance in bearing capacity, angle adaptability, and deepwater installation than other systems. However, the installation process of the VLA and its motion characteristics are significantly impacted by multi-layered seabed soil. In this paper, the coupled Eulerian‒Lagrangian (CEL) large deformation finite element analysis method has been applied to analyse the continuous penetration of a VLA in nonuniform clay with an interbedded stiff layer. A detailed parametric study has been carried out to explore the trajectory, drag angle, movement direction and drag force of the VLA in layered clay with different embedded depths, thicknesses and undrained shear strength of the stiff layer. The CEL numerical analysis results have been validated by comparison with the analytical solutions from the inverse catenary equation. Excellent agreement has been obtained between the results from the CEL analyses and the analytical solutions. The stiff layer leads to concave and convex shapes on the trend lines of the movement direction angle and drag forces, respectively. The embedded depth of the stiff layer determines where the concave and convex shapes appear on the trend lines, while the thickness affects the sizes of the openings of the shapes. The most decisive parameter, an abrupt variation in the undrained shear strength, causes predominant rotation at the interface of layered clay. It diminishes the final embedment depth and ultimate stable drag force, meaning that the bearing capacity of the VLA severely declined in layered clay.

The accurate quantification of the temporal changes in seabed strength allows for more reliable and less conservative geotechnical design. A recently developed effective stress framework, established within a one-dimensional computational domain to quantify changes in soil strength due to pore pressure generation and dissipation, has been extended to a two-dimensional (2D) computational domain to allow for consideration of boundary value problems that are too complex to be simplified to one-dimensional conditions. The work to implement the 2D framework is reported across two companion papers. The first of the two papers utilises large deformation finite element analyses to quantify the spatial distribution of accumulated plastic shear strain. These distributions are encapsulated within a strain influence function that is used within the new 2D framework in this paper to calculate the extent and magnitude of excess pore pressure, and in turn the mobilised soil strength for a number of boundary value problems that represent typical offshore geotechnical processes. The merit of the new 2D framework is explored via retrospective simulations of existing experimental and numerical data. The resulting comparisons demonstrate the potential of the new framework, which is in quantifying the reliability of a range of geotechnical structures under complex loading conditions.