Crack modeling in FE analysis of circular tubular joints

By mapping circles in two-dimensional planes to three-dimensional intersection curves between tubular members, a complicated three-dimensional mesh generating procedure for tubular joints is changed to a procedure similar to a two-dimensional case. More detailed modeling of welds and cracks can be included to analyze the fracture behavior of cracked tubular joints. Different types of crack tip models are discussed and a four-tip crack model is introduced to model crack propagation. These crack models can be applied to both through-thickness cracks and surface cracks. A procedure for transforming crack elements around a plane curve into crack elements for a doubly curved semi-elliptical surface crack around an intersection is also introduced. High-quality meshes can be obtained.     

Numerical models verification of cracked tubular T, Y and K-joints under combined loads

This paper summarizes the key steps involved in the construction of an accurate and consistent finite element model for general cracked tubular T, Y and K-joints. The joint under consideration contains a surface crack which can be of any length and located at any position along the brace-chord intersection. Welding details along the brace-chord intersection, compatible with the American Welding Society (AWS) specifications [American Welding Society. Structural welding code-steel, ANSI/AWS, 15th ed., Miami, 2000], are included in the geometrical model. In order to develop a systematic and consistent modelling procedure, the whole process is divided into four key steps. They are, namely, (1) construction of a consistent geometrical model of the joint with welding details, (2) determination of cracked surface to define the semi-elliptical surface crack profile, (3) generation of well-graded finite element meshes, and (4) stress intensity factor studies around the crack front. To produce a well-graded finite element mesh, a sub-zone technique is used in the mesh generation whereby the entire structure is divided into several sub-zones with each zone consisting of different types of elements and mesh densities. The stress intensity factors (SIFs) are evaluated using the standard J-integral method. Two full-scale T and K-joint specimens were tested to failure under axial load (AX), in-plane bending (IPB), and out-of-plane bending (OPB). In the tests, the rate of crack propagation was monitored carefully using the alternating current potential drop (ACPD) technique. Using the known material parameters C and m, the experimental SIFs were obtained, and they are found to be in complete agreement with the computed SIFs obtained from the generated models. Hence, the proposed finite element models are both efficient and reliable.     

Fatigue behaviors of square-to-square hollow section T-joint with corner crack. II: Numerical modeling

Based on the experimental results obtained in the first part of the study, a geometrical model and a mesh generation procedure are suggested for the numerical modeling of the uncracked and cracked square-to-square hollow section (SSHS) T-joints. In the proposed model, the surface crack is represented as an unsymmetrical 3D curved surface with the deepest point located at the corner of the brace-chord intersection. The mesh generation scheme developed is capable to generate well graded finite element (FE) meshes for cracked SSHS T-joints with multiple cracks. The validity of the proposed geometrical model and the reliability of the proposed mesh generation scheme are checked by comparing the predicted SIFs and the residual life against those computed based on the actual measurements. It is found that the suggested modeling method is reliable in the sense that conservative predictions for the SIFs and the residual life are obtained from the numerical simulations.     

Adaptive mesh generation based on multi‐resolution quadtree representation

An advanced CAD model is required for efficient, real‐time adaptive generation of FE meshes. In this paper, a discrete level of detail (LOD) method for reconstructing progressive multiresolution models is proposed. With this approach, the model is reconstructed a priori so that any level of detail can be accessed directly, in real time, according to application requirements. The mesh is generated adaptively according to geometrical or analysis error indicators, where even at lower levels of resolution, critical areas are preserved. The method has been extended to progressive time and geometrical models for simulation and is demonstrated by several examples. Copyright © 2000 John Wiley & Sons, Ltd.     

Automatic metric 3D surface mesh generation using subdivision surface geometrical model. Part 2: Mesh generation algorithm and examples

In this paper, a new metric advancing front surface mesh generation scheme is suggested. This new surface mesh generator is based on a new geometrical model employing the interpolating subdivision surface concept. The target surfaces to be meshed are represented implicitly by interpolating subdivision surfaces which allow the presence of various sharp and discontinuous features in the underlying geometrical model. While the main generation steps of the new generator are based on a robust metric surface triangulation kernel developed previously, a number of specially designed algorithms are developed in order to combine the existing metric advancing front algorithm with the new geometrical model. As a result, the application areas of the new mesh generator are largely extended and can be used to handle problems involving extensive changes in domain geometry. Numerical experience indicates that, by using the proposed mesh generation scheme, high quality surface meshes with rapid varying element size and anisotropic characteristics can be generated in a short time by using a low‐end PC. Finally, by using the pseudo‐curvature element‐size controlling metric to impose the curvature element‐size requirement in an implicit manner, the new mesh generation procedure can also generate finite element meshes with high fidelity to approximate the target surfaces accurately. Copyright © 2003 John Wiley & Sons, Ltd.     

Fatigue study of partially overlapped circular hollow section K-joints: Part 2: Experimental study and validation of numerical models

In the second part of this study, the reliability of the geometrical models and the mesh generation procedure developed in Part 1 are validated by comparing the modelling results with full scale tests results. Static tests were applied to study the stress concentration factors while fatigue tests were applied to study the stress intensity factor and the fatigue life of the joints. The results obtained indicated that the uncracked joint model could lead to reliable stress concentration factor estimations while the cracked joint models could lead to conservative stress intensity factor and residual fatigue life predictions close to experimental results.     

Octree‐based reasonable‐quality hexahedral mesh generation using a new set of refinement templates

An octree‐based mesh generation method is proposed to create reasonable‐quality, geometry‐adapted unstructured hexahedral meshes automatically from triangulated surface models without any sharp geometrical features. A new, easy‐to‐implement, easy‐to‐understand set of refinement templates is developed to perform local mesh refinement efficiently even for concave refinement domains without creating hanging nodes. A buffer layer is inserted on an octree core mesh to improve the mesh quality significantly. Laplacian‐like smoothing, angle‐based smoothing and local optimization‐based untangling methods are used with certain restrictions to further improve the mesh quality. Several examples are shown to demonstrate the capability of our hexahedral mesh generation method for complex geometries. Copyright © 2008 John Wiley & Sons, Ltd.     

承受平衡轴力的空间KK管节点应力集中系数研究

采用有限元法分析了空间KK型管节点在承受平衡轴力作用下的应力集中系数。在有限元分析中,首先采用分区网格法产生KK节点的有限元网格,这种方法可以针对不同应力梯度的区域形成不同质量和精度的网格。然后在分区网格法的基础上,采用ABAQUS(2000)通用软件分析计算了KK型管节点在平衡轴力作用下的应力集中系数的大小和分布。最后,通过对30个KK节点模型的有限元分析,研究了KK节点几何参数对其应力集中系数大小以及分布情况的影响。     

SIMPLE MODELS FOR PREDICTING STRESS INTENSITY FACTORS FOR TUBULAR JOINTS

Offshore structures are subjected to onerous environmental conditions. These conditions can cause cracking at tubular joints in these jacket structures and increasingly fracture mechanics is being applied to assess the cracks. There are a variety of fracture mechanics approaches to assessing cracks in tubular joints, all of which predict similar trends but give a wide variation of stress intensity factors. This variation is caused by the different models and inconsistent input parameters used. This paper presents a finite element study of a cracked tubular joint using several different structural models and consistent input parameters. The analysis includes both simple representations of a tubular joint, e.g. a plate model, and a full-scale analysis on the joint. A comparison of the full-scale analysis and the simple representations allow conclusions to be drawn on the applicability of simplifications to the problem and also allows errors to be quantified.     

An efficient approach to converting three-dimensional image data into highly accurate computational models

Image-based meshing is opening up exciting new possibilities for the application of computational continuum mechanics methods (finite-element and computational fluid dynamics) to a wide range of biomechanical and biomedical problems that were previously intractable owing to the difficulty in obtaining suitably realistic models. Innovative surface and volume mesh generation techniques have recently been developed, which convert three-dimensional imaging data, as obtained from magnetic resonance imaging, computed tomography, micro-CT and ultrasound, for example, directly into meshes suitable for use in physics-based simulations. These techniques have several key advantages, including the ability to robustly generate meshes for topologies of arbitrary complexity (such as bioscaffolds or composite micro-architectures) and with any number of constituent materials (multi-part modelling), providing meshes in which the geometric accuracy of mesh domains is only dependent on the image accuracy (image-based accuracy) and the ability for certain problems to model material inhomogeneity by assigning the properties based on image signal strength. Commonly used mesh generation techniques will be compared with the proposed enhanced volumetric marching cubes (EVoMaCs) approach and some issues specific to simulations based on three-dimensional image data will be discussed. A number of case studies will be presented to illustrate how these techniques can be used effectively across a wide range of problems from characterization of micro-scaffolds through to head impact modelling.     

INELASTIC LINE SPRINGS IN NON-LINEAR ANALYSIS OF CRACKED TUBULAR JOINTS

Abstract— In the present investigation the ultimate capacity of cracked tubular T-joints, loaded in tension or out-of-plane bending, is computed by means of non-linear finite shell element analyses. The cracks are accounted for using inelastic line springs. The calculated results are compared to corresponding test data and other published case computations. Global load-displacement behaviour and local behaviour by means of the J integral are utilised for the cracked joints. The analyses demonstrate the feasibility of the FE analyses in assessing the joint capacity, but some deficiencies in the modelling are pointed out.     

Elastic-plastic fracture mechanics assessment of test data for circumferential cracked pipes

This paper presents experimental validation of two reference stress based methods for circumferential cracked pipes. One is the R6 method where the reference stress is defined by the plastic limit load. The other is the enhanced reference stress method, recently proposed by the authors, where the reference stress is defined by the optimised reference load. Using 38 published pipe test data, the predicted maximum instability loads according to both methods are compared with the experimental ones for pipes with circumferential through-thickness cracks and with part circumferential surface cracks. It is found that the R6 method gives conservative estimates of the maximum loads for all cases. Ratios of the experimental maximum load to the predicted load range from 0.54 to 0.98. On the other hand, the proposed method gives overall closer maximum loads than R6, compared to the experimental data. However, for part through-thickness surface cracks, the estimated loads were slightly non-conservative for four cases, and possible reasons are fully discussed.     

Fatigue behaviors of square-to-square hollow section T-joint with corner crack. I: Experimental studies

Four full-size square-to-square hollow section (SSHS) T-joints were subjected to static and fatigue tests under combined loads. Static tests were first carried out to investigate the hot spot stress (HSS) distribution along the brace-chord intersections. Fatigue tests were then performed until extensive cracks appeared and the cracks development were monitored by the alternating current potential drop (ACPD) technique. The cracked joints were then opened up and the shapes of the crack surfaces were measured. Fatigue test results and crack geometry measurements showed that under cyclic loading, cracks will be initiated at one of the four corners of the brace-chord intersection and propagate mainly in the direction parallel to the chord wall. Furthermore, branches and secondary cracks, which seldom appear in circular hollow section (CHS) joints, were formed near the end of the test. The test results also indicated that a more stringent criterion should be used to define the fatigue life the joint. Finally, the shapes of the surface cracks of the tested joints were found to be more complicated than that for CHS joints and any of the existing geometrical model used in numerical modeling of CHS joint is deemed to be inadequate.     

Advances in tetrahedral mesh generation for modelling of three‐dimensional regions with complex,curvilinear crack shapes

Advances in tetrahedral mesh generation for general, three‐dimensional domains with and without cracks are described and validated through extensive studies using a wide range of global geometries and local crack shapes. Automated methods are described for (a) implementing geometrical measures in the vicinity of the crack to identify irregularities and to improve mesh quality and (b) robust node selection on crack surfaces to ensure optimal meshing both locally and globally. The resulting numerical algorithms identify both node coincidence and also local crack surface penetration due to discretization of curved crack surfaces, providing a proven approach for removing inconsistencies. Numerical examples using the resulting 3D mesh generation program to mesh complex 3D domains containing a range of crack shapes and sizes are presented. Quantitative measures of mesh quality clearly show that the element shape and size distributions are excellent, including in regions surrounding crack fronts. Copyright © 2005 John Wiley & Sons, Ltd.     

Automatic parallel generation of finite element meshes for complex spatial structures

Recent developments in experimental techniques are enabling researchers to non-destructively characterize complex spatial structures with multiple constituents, e.g. polycrystalline aggregates. However, a combination of the high level of detail of the experimental data and often extreme geometry complexity make the building of models of such structures highly difficult and demanding. Finite element (FE) pre-processor tools can often be inadequate, especially when the structure contains multiple constituents and when the model building process has to be automatized.This paper proposes a novel framework for automatic and parallelized generation of FE models from discrete spatial data (voxels) procured from experimental techniques, e.g. 3D X-ray diffraction microscopy or X-ray diffraction contrast tomography (DCT). The technique can also be applied to analytical spatial geometries. The framework consists of reconstructing the surfaces of different constituents from the experimental data, generating FE meshes of these surfaces, followed by volume meshing of the constituents interior while enforcing the already generated surfaces meshes. This approach assures a conformal mesh between the adjoining surfaces and at the same time enables a fully independent and parallel meshing of the constituents. The conformal mesh allows for a variety of connectivity models between the constituents, including layers of cohesive elements for simulating the grain boundaries.The applicability of the approach is demonstrated first by creating a FE model of a 400 μm diameter stainless steel wire characterized in 3D by DCT. FE model generation of spatial Voronoi tessellations, representing models of polycrystalline aggregates with up to 5000 grains, is then demonstrated. Here, anisotropic elasticity and crystal plasticity constitutive laws are used to estimate the scatter of the macroscopic responses due the random nature of the grains’ crystallographic orientations. At 2000 grains this influence is shown to be very small.     

基于失效评定图(FAD)研究含疲劳裂纹T型圆钢管节点的安全性

失效评定曲线(FAD)常用来评价焊接结构在出现裂纹后的安全性,为了验证这种曲线在评价焊接管结构在节点部位出现疲劳裂纹后安全性的适用性,采用实验测试和有限元分析的方法研究了3个含疲劳裂纹的T型管节点试件在静力作用下的极限承载能力及破坏过程。3个T型管节点试件首先进行疲劳实验在焊趾处产生表面裂纹,然后通过在支管端部施加轴向拉力作用检测节点的破坏过程。基于自行开发的含表面裂纹T型管节点的有限元网格自动产生程序以及ABAQUS分析软件,研究了在管节点破坏过程中表面裂纹最深点的应力强度因子大小,并通过实验的荷载-位移曲线确定了T节点试件的塑性极限承载力。在这些结果的基础上,验证了FAD在评价含疲劳裂纹的焊接管节点安全性方面的适用性。研究结果标明:FAD在评价含疲劳裂纹管节点的安全性方面是安全可靠的,但偏于保守。     

Extended finite element fracture analysis of a cracked isotropic shell repaired by composite patch

An extended finite element method (XFEM) is developed to study fracture parameters of cracked metal plates and tubes that are repaired on top of the crack with a composite patch. A MATLAB® stand‐alone code is prepared to model such structures with eight‐noded doubly curved shell elements in the XFEM framework. Crack trajectory studies are performed for a diagonally cracked panel under fatigue loading. Verification studies are investigated on different shell type structures such as a cracked spherical shell and cracked cylindrical pipe with different crack orientations. The effects of using patch repairs with different fibre orientations on the reduction of stress intensity factors (SIFs) is also studied which can be useful for design purposes. XFEM is selected as any crack geometry can be embedded in the finite element mesh configuration with no need to coincide the crack geometry with meshed elements and so re‐meshing with fine mesh generation is not needed in the current method.     

Plastic collapse load prediction and failure assessment diagram analysis of cracked circular hollow section T‐joint and Y‐joint

This paper concerns the validation of standard safety assessment procedure given in BS 7910 for cracked circular hollow section T‐joint and Y‐joint, using the finite element (FE) results. A robust and efficient FE mesh generator is developed to produce the 3D models of the cracked joints and to calculate the elastic J‐integral (J_e) and elastic–plastic J‐integral (J_ep) values of the crack respectively. In order to verify its accuracy and convergence, the plastic collapse loads (P_c) obtained from experimental tests and FE predictions are compared; they agree very well with each other. It is also found from experimental tests that the plastic collapse loads (P_c) predicted using the BS 7910 reduction factor (F_AR) are safe and conservative. Subsequently, the failure assessment diagrams (FADs) of five cracked T‐joints and three cracked Y‐joints are constructed using the FE results, following the J‐integral method, which is classified as Level 3C in BS 7910. Thereafter, a comparison between the constructed FAD curves and the standard Level 2A curve is carried out, and it is observed that the safety assessment results using the standard Level 2A curve might be unsafe because some parts of the constructed FAD curves fall inside of the standard one. A penalty factor of 1.15 working on both the elastic–plastic J‐integral and plastic collapse load (P_c) is proposed to move all the constructed FAD curves just outside of the standard Level 2A curve.