Refined Analysis of Fatigue Crack Initiation Life of Beam-to-Column Welded Connections of Steel Frame under Strong Earthquake

This paper presents a refined analysis for evaluating low-cycle fatigue crack initiation life of welded beam-to-column connections of steel frame structures under strong earthquake excitation. To consider different length scales between typical beam and column components as well as a few crucial beam-to-column welded connections, a multiscale finite element (FE) model having three different length scales is formulated. The model can accurately analyze the inelastic seismic response of a steel frame and then obtain in detail elastoplastic stress and strain field near the welded zone of the connections. It is found that the welded zone is subjected to multiaxial nonproportional loading during strong ground motion and the elastoplastic stress-strain field of the welded zone is three-dimensional. Then, using the correlation of the Fatemi-Socie (FS) parameter versus fatigue life obtained by the experimental crack initiation fatigue data of the structural steel weldment subjected to multiaxial loading, the refined evaluation approach of fatigue crack initiation life is developed based on the equivalent plastic strain at fatigue critical position of beam end seams of crucial welded connections when the steel frame is subjected to the strong earthquake excitation

Multiaxial Cycle Deformation and Low-Cycle Fatigue Behavior of Mild Carbon Steel and Related Welded-Metal Specimen

The low-cycle fatigue experiments of mild carbon Q235B steel and its related welded-metal specimens are performed under uniaxial, in-phase, and 90° out-of-phase loading conditions. Significant additional cyclic hardening for 90° out-of-phase loading conditions is observed for both base metal and its related weldment. Besides, welding process produces extra additional hardening under the same loading conditions compared with the base metal. Multiaxial low-cycle fatigue strength under 90° out-of-phase loading conditions is significantly reduced for both base-metal and welded-metal specimens. The weldment has lower fatigue life than the base metal under the given loading conditions, and the fatigue life reduction of weldment increases with the increasing strain amplitude. The KBM, FS, and MKBM critical plane parameters are evaluated for the fatigue data obtained. The FS and MKBM parameters are found to show better correlation with fatigue lives for both base-metal and welded-metal specimens

Analytical solutions of 3D anisotropic magneto-electro-elastic bi-materials under extended non-uniform dislocations and tractions over a circular area

In this paper, we derive the analytical solutions in a three-dimensional magneto-electro-elastic (MEE) bi-material under extended dislocations and tractions generally distributed over a horizontal circular area by virtue of the extended Stroh formalism and Fourier transformation. Explicit analytic results of the Hankel transform integrals of different orders and three kernel integrals are obtained so that the final solutions in the physical domain can be expressed in simple line-integral form. Some existing solutions in MEE materials can be reduced directly from the general solutions presented in this paper. The effect of different non-uniform loadings (dislocations and tractions) on the induced fields in MEE bi-material is investigated. Our analytical solutions can serve as benchmarks for future numerical analysis of various problems in MEE bi-materials. (C) 2014 Elsevier Ltd. All rights reserved

Fatigue Properties Estimation and Life Prediction for Steels under Axial, Torsional, and In-Phase Loading

In this study, several estimation methods of fatigue properties based on different monotonic mechanical parameters were first discussed. The advantages and disadvantages of the Hardness Method proposed by Roessle and Fatemi were investigated and improved through the analysis of a total of 92 fatigue test data. A new Segment Fitting Method from Brinell hardness was then proposed for the fatigue properties estimation, and a total of 96 pieces of fatigue test data under axial, torsional, and multiaxial in-phase loading were collected to verify the applicability of the new proposal. Finally, the prediction accuracy of the new proposal and three exciting estimation methods was compared with the predictions based on the experimental fatigue properties. Based on the results obtained, the newly proposed estimation method has a significant improvement on the relation between fatigue ductility coefficient and Brinell hardness, which consequently improves the fatigue life prediction accuracy with the scatter band of 2, particularly for the materials with low Brinell hardness. The present study can provide a simplified analysis of the preliminary fatigue design of engineering structures

Experimental and comparative investigation on fatigue performance of three typical welded joints of Q370qNH bridge weathering steel

The weathering index of welded details, including butt-welded, non-load-carrying T-shaped and cruciform welded joints, fabricated with Q370qNH weathering steel (WS) was evaluated, and fatigue tests were conducted on the three types of welded details with a stress ratio of − 1 to determine the fatigue modes and fatigue properties. The fatigue S-N curves of three typical welded joints are fitted using the fatigue data and compared with the existing fatigue specifications, such as Eurocode 3, IIW 2007 and China Standard for Design of Steel Structures GB50017–2017. A total of 215 fatigue data are collected form the existing literatures, the difference of fatigue performance between the WS welded joints and that of traditional structural steels was analyzed. Results show that the slope parameter m of S-N curve of Q370qNH butt-welded joint is 5.50, which is quite different from the recommended value of 3. The fitting S-N curves of T-shaped welded joints and cruciform welded joints are in relatively well correlation to the S-N design curves. The fatigue performance of non-load-carrying T-shaped welded joints is significantly better than that of butt-welded joints and load-bearing cruciform welded joints. FRI analysis based on the collected fatigue data of 13 types of metallic materials in this study indicates that the actual fatigue strength of Q370qNH welding details demonstrates a certain fatigue safety margin, especially for the non-load-carrying T-shaped welded joints

Fundamental solutions of uniform loads over triangular elements in a three-dimensional piezoelectric medium

In this paper, fundamental solutions of uniform loads over triangular elements in an infinite transversely isotropic piezoelectric three-dimensional space are derived. The triangle element can be parallel or vertical to the plane of isotropy and the uniform load can be mechanical and electric types, oriented in an arbitrary orientation. The solutions are expressed simply as a linear combination of three kinds of elementary functions – linear, trigonometric and logarithm functions. Three methods of superposition are employed to verify the obtained fundamental solutions. Numerical examples are also presented for the extended displacements and stresses induced by both mechanical and electric loads on the vertical and horizontal triangles

A nonlinear bilayer beam model for an interfacial crack in dielectric bimaterials under mechanical/electrical loading

A bilayer beam model is extended to study the fracture behavior of dielectric interfacial cracks. In this model, a semi-infinite crack with an original opening value is oriented along the interface between two dielectric layers which are under mechanical/electrical loading. Taking into account the effect of the electrostatic traction on the interfacial crack, a nonlinear analytical solution is derived, along with also a developed finite element analysis method where a special constitutive equation for the capacitor element in ANSYS is utilized to simulate the electrostatic tractions. Both the analytical and numerical solutions predict the same results which further show that the elastic and dielectric mismatches can play a significant role in the interfacial cracking behavior under mechanical and electrical loading. Furthermore, the electrostatic tractions may cause hysteresis loops in the curve of crack opening versus applied mechanical displacement or versus applied electric voltage. An applied mechanical load is the driving force for the interfacial cracking, while an applied electric field retards it

Indentation stress in multi-layer delaminated thin films induced by a microwedge indenter

Indentation stresses in single- and multi-layer delaminated thin films made of elastic-perfectly plastic materials in microwedge indentation delamination tests are analyzed via finite element calculations with different wedge angles and other geometrical and mechanical parameters. Based on the formula for a single-layer thin film under indentation loading [Zhao et al. J Mater Res 2009;24:1943] and by introducing the equivalent material parameters, we developed simple analytical formulae for the loading indentation stress (as well as the energy release rate) in each layer of the multi-layer thin film, in terms of the residual stress, elastic modulus, Poisson's ratio, yielding strength, and thickness of each layer. Our analytical solution is validated by the finite element calculations and should be useful to thin-film delamination tests. (c) 2012 Elsevier Ltd. All rights reserved