APPLICATION OF THE PROBABILISTIC FRACTURE MECHANICS METHOD OF PREDICTING THE FATIGUE LIFE OF TUBULAR JOINTS

In this paper .fatigue strength of T-type tubularjoints subjected to in-planebending or out-of-plane bending load is investigated By, considering material constantsand initial crack sizes as random variables and applying Monte Carlo simulation method, we have giyen a statistical analvsis on fatigue life. Simultaneously, linearregression analyses of computed resuldts are performed and compared with that of theknown experimental data.

An Improved Formulation for the Ultimate Static Strength of Tubular Joints subjected to Combined Loads

-'The effect of interaction of loads on the ultimate static strength of tubular joints of offshore fixed platforms, is a practical problem. But there is still absence of rigorous theory to explain available experimental data and empirical criteria for the static strength of tubular joints. The idea of yield at hot spot of tubular joints is introduced in this paper. The interaction equations of plastic capacity for the tubular joints under combined loads (two and three different kinds) are derived. Thereafter the Yura's test data and empirical criteria of ultimate static strength for the tubular joints can be explained. The idea of classification of category of loads in accordance with experimental data and the present theory is suggested. Finally, the improved ultimate capacity equations for tubular joints are recommended. The physical significance of the coefficient of plastic reservation Qp is discussed.

AN INVESTIGATION OF RANDOM FATIGUE STRENGTH OF K-TYPE TUBULAR JOINTS AND CRACK PROPAGATION BEHAVIORS

Random fatigue of welded K-type tubular joints subjected to axial or out-of-plane bending load is analyzed. By considering the sizes of initial surface cracks and material constants as random variables with some probabilistic distributions, incorporating the effect of the weld, five hundred random samples are generated. Statistical computational results of life of crack propagation and effect of change of crack shape are finally obtained and compared with experimental data available based on a regression analysis. Meanwhile, crack propagation behaviors are also investigated.

The Stress Concentration of Multiplanar Tubular Joints of Offshore Structure

The formulation for the stress concentration matrix of multiplanar tubular joint with braces subjected to combined load is established in this paper. The formulations for SCF values of multiplanar tubular joints are compared with those by Wordsworth and Efthymiou. The stiffening effect and load-interaction effect are discussed. A spatial offshore jacket with braces subjected to combined load is computed as a numerical example. The maximum stress of each multiplanar tubular joint and its location are found through SCF matrix. Finally, the difference between the maximum stresses of multiplanar tubular joints and their corresponding uniplanar tubular joints are discussed.

A Ring Model for Limit Analysis of Spatial XX Tubular Joints

-A ring model is developed to investigate the ultimate strength of tubular X and XX joints in the case that the brace is compressed. In the present analysis, the tubular joint is made of elasto-perfectly-plastic material, and the arch element of the chord section intersecting with the brace is assumed to be rigid. It is found that when 6 plastic hinges for X joint and 8 plastic hinges Tor XX joint appear in the ring, the limit state is reached, and by means of the equivalent ring width Be formula proposed in a previous paper by the authors, the ultimate strength of tubular X and XX joints subjected to compression can be obtained.

A review of stress concentration factors in tubular and non-tubular joints for design of offshore installations

Tubular structures are widely used in offshore installations,trusses,high rise buildings,towers for wind turbines,ski-lift installations,lightning,road pole signals etc.,owing to their excellent structural performance and attractive appearance.Stress concentration,especially in the welded joints of these structures,is an important design consideration particularly for fatigue design.In the context of tubular and non-tubular joints,this paper provides a review of the experimental and numerical studies that have been carried out so far to determine the stress concentration factor(SCF).Emphasis is also placed on the complexity of capturing different types of stresses in tubular/non-tubular joints for estimation of SCF.Present code provisions for evaluation of SCF are also discussed.Further,a few issues,which require significant research effort to advance our understanding and to improve the current design guidelines,have been identified.©2016 Shanghai Jiaotong University.Published by Elsevier B.V.

Prediction of Static Strength of Tubular T and X Joints Subjected to Axial Compression

-The formulation of ring analogy method for the prediction of static strength (ductile collapse) of tubular T, X joints under axial compression based on the limit analysis of the ring with some assumptions is presented in this papaer. The regression formula for the effective length of the chord based on test results is established by means of the least square method. The results computed by the present semi-analytic formula are compared with previous results and test data. They are quite close to each other. The accuracy of the present formula depends on the reasonable selection of the effective length of the chord, which requires numerous test data.

Inspection system for welded tubular joint based on ultrasonic phased array

A manual inspection of large-diameter tubular joints is difficult. As a result a scanner with three degrees of freedom （ DOFs） was developed based on the scanning principle of ultrasonic phased array. The weld tracing is realized by a 2DOF motion of scanner. The pose of ultrasonic probe is controlled by the third one. The control strategy is put forward based on a programmable multi-axis controller. Four kinds of scanning modes can be implemented simultaneously employing this ultrasonic inspection system. Experiments on reference blocks of tubular joints reveal that the automatic ultrasonic phased array inspection system has the same inspection accuracy as a manual ultrasonic inspection. This system is superior to the manual ultrasonic system in terms of reliability and repeatability. The artificial dejects of weld at tubular joint can be detected accurately with the presented inspection system.

Design equations for maximum stress concentration factors for concrete-filled steel tubular K-joints

Stress concentration factors(SCFs)for welded tubular joints can be decreased by filling the chord with concrete leading to a longer fatigue life.However,there are currently no design formula available in guidelines to predict the SCF of concrete-filled circular hollow section(CFCHS)K-joints,thus limiting their applicability in bridge design.To address this gap,finite element models for CFCHS K-joints were developed and compared against test results to ensure their accuracy.Then,a comprehensive parametric study was conducted to establish relationships between maximum SCFs and four variables:brace-to-chord diameter ratio(b),chord diameter-to-thickness ratio(2g),brace-to-chord thickness ratio(t),and the angle between braces and chord(q).A total of 480 FE models were examined under three loading conditions including brace and chord loading:balanced axial force,chord axial force,and chord bending.Design equations to predict the maximum SCF for CFCHS Kjoints were established by multiple regression analyses of the numerical results.A comparison of maximum SCFs between circular hollow section(CHS)and CFCHS K-joints was made,and it was concluded that average reductions of 42%and 33%in maximum SCFs in CFCHS K-joints at the locations of the chord and brace were found compared to CHS joints for balanced axial force,respectively.Finally,a case study illustrating how to use the proposed equations for fatigue safety verification was presented.

A toughness based deformation limit for X- and K-joints under brace axial tension

This study reports a deformation limit for the initiation of ductile fracture failure in fatigue-cracked circular hollow section （CHS） X- and K-joints subjected to brace axial tension. The proposed approach sets the deformation limit as the numerically computed crack driving force in a fatigue crack at the hot-spot location in the tubular joint reaches the material fracture toughness measured from standard fracture specimens. The calibration of the numerical procedure predicates on reported numerical computations on the crack driving force and previously published verification study against large-scale CHS X-joints with fatigue generated surface cracks. The development of the deformation limit includes a normalization procedure, which covers a wide range of the geometric parameters and material toughness levels. The lower-bound deformation limits thus developed follow a linear relationship with respect to the crack-depth ratio for both X- and K-joints. Comparison of the predicated deformation limit against experimental on cracked tubular X- and K- joints demonstrates the conservative nature of the proposed deformation limit. The proposed deformation limit, when extrapolated to a zero crack depth, provides an estimate on the deformation limits for intact X- and K-joints under brace axial loads.