Seng Tjhen Lie

Model and mesh generation of cracked tubular Y-joints

Abstract In this study, the methods for constructing accurate and consistent geometrical and finite element (FE) models for general cracked tubular Y-joints are described. Firstly, geometrical analysis of welded tubular joint is given and it is then extended to the modelling of general cracked Y-joints. The concept of crack surface and a simple mapping approach are suggested to model either through-thickness or surface cracks which can be of any length and located at any position along the brace–chord intersection. Secondly, the geometrical model developed will be used in the generation of consistent FE meshes. The basic concepts used for the design and generation of three-dimensional FE meshes will be described. This will include the meshing procedures for discretization of tubular joints with through-thickness and surface cracks which are frequently regarded as one of the most difficult steps in the construction of tubular joint models. Finally, some mesh generation examples for uncracked and cracked Y-joints will be presented to demonstrate the use of the purposed geometrical model and mesh generation scheme developed.

A simulation of fatigue crack propagation in a welded T-joint using 3D boundary element method

A general procedure to investigate the fatigue propagation process of a 3D surface crack based on multi-region Boundary Element Method is detailed in this paper. The mesh can be automatically regenerated as the crack propagates. A new formula for estimating the effective stress intensity factor is used to calculate the crack extension. The maximum principal stress criterion is then employed to predict the crack growth direction. Comparison between numerical and experimental results of a welded T-joint shows that the proposed procedure is reliable.

Numerical analysis of dynamic behavior of stream turbine blade group

This paper considers the stream turbine blade group as a rotationally periodic structure and the complex constraint method has been induced. The effect of the centrifugal force to dynamic vibration frequency has been considered by introducing a nonlinear large deformation equation. A method has been given to introduce the special constraint between the fin heaves of every blade during dynamic analysis.

Spline shell element and plane-wave approximation for dynamic response of submerged structures

Abstract This paper presents a performance study on the plane-wave approximation for dynamic responses of shell structures using spline finite elements, in particular, the interaction between submerged shell structures and the surrounding infinite fluid. The shell structures are discretized by using quadrilateral spline shell elements. The element is doubly curved. Each element consists of nine primary nodes and eight auxiliary nodes, and has a total of 63 degrees of freedom. The inter-element is C 1 continuous. A non-reflecting boundary based on acoustic approximations is used to model the infinite fluid medium. Two examples are presented. Results demonstrated good performance.

The ultimate behaviour of cracked square hollow section T-joints

Publisher Summary The chapter proposes an approach to predict the ultimate strength of cracked square hollow section (SHS) tubular (T) joints. The ultimate static strength of tubular joints is usually calculated at the design stage based on empirical formulae incorporating the joint geometry, loading mode, and materials strength. However, fatigue cracks are detected in some aging structures that tend to reduce the static strength. Methods for predicting the loss of strength of cracked tubular joints are therefore important in practice. Very few published information is available concerning the residual strength of SHS joints containing cracks. To develop guidelines on assessing the static strength of fatigue-cracked SHS joints, a range of numerical analyses and full-scale tests are carried out on cracked and uncracked T-joints. The nonlinear elastic-plastic finite element (FE) technique is employed successfully for calculating the plastic collapse loads of uncracked and cracked T-joints under axial load at the brace end.

Safety Assessment Procedure for a Cracked Square Hollow Section (SHS) Y-joint

In this paper, an accurate crack model of a square hollow section (SHS) welded Y-joint is proposed, and a high quality finite element mesh is generated. Based on this numerical model, the load-displacement curves and the stress intensity factors (SIFs) along the crack front are calculated. It is shown that the highest value of the stress intensity factor is always located at the chord corner. From the full-scale static strength test carried out on the cracked SHS Y-joint, it was found that ductile crack tearing initiated at the crack front parallel to the chord side wall where the fracture toughness is smaller, and not from the crack front around the corner where the SIF is the highest. Using the plastic collapse load obtained via the twice elastic compliance technique and the fracture toughness obtained from the Charpy V-notch and CTOD tests, the assessment points of Kr and Lr are calculated and plotted on the BS7910 (2005) Level 2A Failure Assessment Diagram (FAD) curve, thus demonstrating its usage. It shows that the standard curve gives a conservative assessment for the cracked SHS Y-joint subjected to brace end axial loads.

Plastic Collapse Load Investigation for Safety Assessment of Cracked Square Hollow Section (SHS) T-, Y- and K-Joints

The plastic collapse load play a critical role in the entire procedure of constructing Failure Assessment Diagrams (FADs) of cracked tubular structures. In this study, the models of square-to-square hollow section (SHS) T-, Y- and K-joints with the actual 3D surface cracks obtained from previous fatigue tests are generated, and their general plastic collapse behavior has been studied carefully using the elastic-plastic finite element analysis. Based on the extensive FE analyses, the fully plastic collapse load solutions for the cracked SHS joints under brace end axial loads are proposed and derived. The load-displacements curves are plotted for predicting the plastic collapse loads based on the twice-elastic compliance criterion. Compared with formulae proposed in BS7910 [1], it was found that a conservative plastic collapse load for the SHS joints under brace end axial loads is produced using the proposed formulae. Future works are recommended for validation of BS7910 [1] FADs for damaged SHS T-, Y- and K-joints based on the numerically calculated plastic collapse load.Copyright

Finite Element Validation Studies of BS7910 FAD Method Applied to Welded SHS Joints

The failure assessment diagram (FAD) has now been widely accepted and used for the assessment of defects found in metallic structures. In BS7910 (2005), the use of this method for offshore structures has been validated for a range of joint geometries. But these validations are only applicable for circular hollow section (CHS) welded joints. For rectangular or square hollow section (RHS or SHS) joints, there are very few references available in the literature. In this paper, systematic investigations have been carried out for the validation and verification of the FAD curves for SHS T-joints. FAD curves for a wide range of welded SHS T-joints containing surface cracks have been established using the fracture mechanics data generated from the finite element analyses. The range of β ratio of these joints is from 0.3 to 0.8. Therefore, the failure mode is constrained in the chord face yielding. The influence of residual stresses on the plastic collapse load and the FAD curves has also been analyzed. The reduction factor used to calculate the plastic collapse load of the SHS T-joints containing cracks have been quantitatively examined, and the use of the BS7910 (2005) Level 2A FAD for SHS joints containing surface cracks has been validated accordingly.Copyright