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Abstract
In steel bridges fatigue cracks are sometimes repaired by placing drilled holes at the crack tips. From the meaning that the drilled holes stop the propagation of cracks, they are called stop-holes. Since stop-holes are regarded as an emergency measure to delay crack propagation, usually some substantial repair follows. However, if the stress at the stop-holes is below their fatigue limit, fatigue cracks would not be expected to occur. The purpose of this study is to present the conditions under which stop-holes prevent the re-initiation of fatigue cracks. The fatigue limit of stop-holes and the equations necessary to estimate the maximum stress on the circumference of the stop-holes are provided.

Key Words
fatigue; stop-hole; crack; stress concentration.

Address
Osaka University, Department of Civil Engineering, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan

Abstract
This study focuses on the influence of a composite external strengthening on the natural frequencies of a steel beam with open cracks. In a first step, the leading parameters associated with the effect of the composite strengthening are experimentally identified. An analytical model is developed in order to quantify the importance of the force transfer within the resin interface. In a second step, the analytical model of a cracked beam with composite external strengthening is compared to experiments.

Key Words
natural frequencies; mode shape; crack; external strengthening; shear stress.

Address
Laboratoire M?canique Mat?riaux et Structures, Universit? Claude Bernard, 82 boulevard Niels Bohr, Domaine scientifique de la Doua, 69622 Villeurbanne, France

Abstract
An analytical-numerical procedure has been presented in this paper to take into account the non-linear effects of concrete cracking and time-dependent effects of creep and shrinkage in the concrete portion of the continuous composite beams under service load. The procedure is analytical at the element level and numerical at the structural level. The cracked span length beam element consisting of uncracked zone in middle and cracked zones near the ends has been proposed to reduce the computational effort. The progressive nature of cracking of concrete has been taken into account by division of the time into a number of time intervals. Closed form expressions for stiffness matrix, load vector, crack lengths and mid-span deflection of the beam element have been presented in order to reduce the computational effort and book-keeping. The procedure has been validated by comparison with the experimental and analytical results reported elsewhere and with FEM. The procedure can be readily extended for the analysis of composite building frames where saving in computational effort would be very considerable.

Key Words
creep; shrinkage; cracking; composite beam; closed form expressions

Address
Department of Civil Engineering, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110016, India

Abstract
Yielding of the internal steel reinforcement is an important mechanism that influences the Intermediate Crack-induced debonding (IC debonding) behavior in FRP-strengthened RC members since the FRP is required to carry additional forces beyond the condition of steel yielding. However, rational design practice dictates an appropriate limit state is defined when steel yielding is assured prior to FRP debonding. This paper proposes a criterion which correlates the occurrence of IC debonding to the formulation of a critical steel yielding length. Once this length is exceeded the average bond stress in the FRP/concrete interface exceeds its threshold value, which proves to correlate with the average bond resistance in an FRP/concrete joint under simple shear loading. This proposed IC debonding concept is based on traditional sections analysis which is conventionally applied in design practice. Hence complex bond stress-slip analyses are avoided. Furthermore, the proposed model incorporates not only the bond properties of FRP/concrete interface but also the beam geometry, and properties of steel and FRP reinforcement in the analysis of IC debonding strength. Based upon a solid database, the validity of the proposed simple IC debonding criterion is demonstrated.

Key Words
fiber reinforced plastics; reinforced concrete beams; flexural strengthening; IC debonding

Address
Jian-Guo Dai: Department of Civil and Structural Engineering, The Hong Kong Polytechnic University Hung Hom, Kowloon, Hong Kong Kent A. Harries : Department of Civil and Environmental Engineering, University of Pittsburgh 936 Benedum Hall, Pittsburgh PA 15261, U.S.A Hiroshi Yokota : Port and Airport Research Institute, Yokosuka 239-0826, Japan

Abstract
In this study, fracture analysis of orthotropic FGM (Functionally Graded Material) plate having center crack is performed, numerically. Material axis arbitrarily oriented and there is an angle θ° between material and crack axes. Stress intensity factors at the crack tips for Mode I are calculated using Displacement Correlation Method (DCM). In numerical analysis, effects of material properties and variation of angle θ° between material and crack axes on the fracture behavior are investigated for four different boundary conditions. Consequently, it is found that the effect of θ° on stress intensity factor depends on variation of material properties.

Key Words
orthotropic functionally graded materials; displacement correlation method; finite element analysis; stress intensity factor; aerospace structures

Address
Mete Onur Kaman : Firat University, Department of Mechanical Engineering, Elazig, Turkey Fatih Cetisli : Firat University, Department of Civil Engineering, Elazig, Turkey

Abstract
In order to develop a verification method for extremely low cycle fatigue (ELCF) of steel structures, the initiation mechanism of ductile cracks is investigated in the present study, which is the first step of brittle fracture, occurred in steel bridge piers with thick-walled sections. For this purpose, a total of six steel columns with small width-thickness ratios were tested under cyclic loading. It is found that ductile cracks occurred at the column base in all the specimens regardless of cyclic loading histories subjected. Moreover, strain history near the crack initiation location is illustrated and an index of energy dissipation amount is proposed to evaluate deformation capacity of structures.

Key Words
ductile crack initiation; brittle fracture; steel bridge; cyclic loading

Address
Xiaoqun Luo : Dept. of Building Engineering, Tongji University, Shanghai 200092, P.R.China Dept. of Civil Engineering, Meijo University, Nagoya 468-8502, Japan Hanbin Ge : Dept. of Civil Engineering, Meijo University, Nagoya 468-8502, Japan Masatoshi Ohashi : Central Japan Railway Company, Nagoya 450-6101, Japan

Abstract
Stress intensity factors are numerically investigated for interfacial edge crack between two dissimilar composite plates jointed with single side composite patch. Variation of stress intensity factor under Mode I loading condition is examined for different material models and fiber orientation angles of composite plates and patch. ANSYS 12.1 finite element analysis software is used to obtain displacements of crack surfaces in the numerical solution and repaired plates are modeled in three dimensions. Obtained results are presented in the form of graphs. It is found that fiber orientation angle of composites is an effective parameter on interfacial stress intensity factor.

Key Words
interface crack; composite patch; stress intensity factor; finite element method

Address
(1) Fatih Cetisli: Department of Civil Engineering, Pamukkale University, Denizli 20070, Republic of Turkey; (2) Mete O. Kaman: Department of Mechanical Engineering, Firat University, Elazig 23119, Republic of Turkey.

Abstract
This study aimed to investigate the fatigue crack growth behavior of a kind of fiber metal laminates (FML) under four different stress levels. The FML specimen consists of three 2024-T3 aluminum alloy sheets and two layers of glass/epoxy composite lamina. Tensile-tensile cyclic fatigue tests were conducted on centrally notched specimen at four stress levels with various maximum values. A digital camera system was used to take photos of the propagating cracks on both sides of the specimens. Image processing software was adopted to accurately measure the length of the cracks on each photo. The test results show that: (1) a-N and da/dN-a curves of FML specimens can be divided into transient crack growth segment, steady state crack growth segment and accelerated crack growth segment; (2) compared to 2024-T3 aluminum alloy, the fatigue properties of FML are much better; (3) da/dN-ΔK curves of FML specimens can be divided into fatigue crack growth rate decrease segment and fatigue crack growth rate increase segment; (3) the maximum stress level has a large influence on a-N, da/dN-a and da/dN-ΔK curves of FML specimens; (4) the fatigue crack growth rate da/dN presents a nonlinear accelerated increasing trend to the maximum stress level; (5) the maximum stress level has an almost linear relationship with the stress intensity factor ΔK.

Key Words
fatigue; experimental study; fiber metal laminates; crack growth rate; stress intensity factor

Address
(1) Zonghong Xie, Fei Peng: School of Astronautics, Northwestern Polytechnical University, Xi'an, China; (2) Tianjiao Zhao: AVIC the First Aircraft Institute, Xi'an, China.

Abstract
A great number of moment-resisting steel structures collapsed due to ductile crack initiation at welded beam-column connections, followed by explosive brittle fracture in the Kobe (Hyogoken-Nanbu) earthquake in 1995. A series of experimental and numerical studies on cracking behaviors of beam-column connections in steel bridge piers were carried out by the authors

Key Words
ductile fracture; post weld treatment; burr grinding; R-finish; beam-column connection; steel bridge pier

Address
(1) Liang-Jiu Jia: Advanced Research Center for Natural Disaster Risk Reduction, Meijo University, 1-501 Shiogamaguchi, Tenpaku-ku, Nagoya, 468-8502, Japan; (2) Hanbin Ge: Deptartment of Civil Engineering, Meijo University, 1-501 Shiogamaguchi, Tenpaku-ku, Nagoya, 468-8502, Japan; (3) Toshimitsu Suzuki: Hiroshima Machinery Works, Mitubishi Heavy Industries, Ltd., 5-1 Ebaokimachi, Naka-ku, Hiroshima, 730-8642, Japan.

Abstract
Concrete-filled circular t steel tubular joints (CFSTJs) in practice are frequently subjected to fluctuated loadings caused by wind, earthquake and so on. As fatigue crack is sensitive to such cyclic loadings, assessment on performance of CFSTJs with crack-like defect attracts more concerns because both high stress concentration at the brace/chord intersection and welding residual stresses along weld toe cause the materials in the region around the intersection to be more brittle. Once crack initiates and propagates along the weld toe, tri-axial stresses in high gradient around the crack front exist, which may bring brittle fracture failure. Additionally, the stiffness and the load carrying capacity of the CFSTJs with crack may decrease due to the weakened connection at the intersection. To study the behaviour of CFSTJs with initial crack, experimental tests have been carried out on three full-scale CFCST T-joints with same configuration. The three specimens include one uncracked joint and two corresponding cracked joints. Load-displacement and load-deformation curves, failure mode and crack propagation are obtained from the experiment measurement. According to the experimental results, it can be found that he load carrying capacity of the cracked joints is decreased by more than 10% compared with the uncracked joint. The effect of crack depth on the load carrying capacity of CFCST T-joints seems to be slight. The failure mode of the cracked CFCST T-joints represents as plastic yielding rather than brittle fracture through experimental observation.

Key Words
concrete-filled circular steel tubular T-joint (CFCST T-joint); experimental study; fatigue crack; residual static strength; failure mode

Address
School of Civil Engineering, Yantai University, Qingquan Road 32#, Laishan District, Yantai, Shandong Province, PR China, 264005.

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