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Due to the unavoidable stress concentrations and welding defects, the trapezoidal rib-to-deck welded joints of Orthotropic Steel Bridge Decks (OSBD) are prone to fatigue cracking. It is a new exploration to apply Carbon Fiber Reinforced Polymer (CFRP) sheets to strengthen the complex welded joints to improve the fatigue strength. In this paper, the comparative tests of un-strengthened and CFRP strengthened welded joints were carried out. The hot spot stress distribution under static load and fatigue behavior under cyclic load were studied, considering the influence of the number of CFRP sheet layers. The experimental results indicate that CFRP reinforcement can effectively reduce the stress concentrations at weld toe of the deck, and three-layers reinforcement has a greater effect than one-layer reinforcement. There was no significant difference in failure modes between the un-strengthened and CFRP strengthened welded joints. The fatigue cracks of all joints originated from weld toe of the deck, and then expanded along the width and thickness of the deck which caused the deck to break. CFRP reinforcement can significantly improve the fatigue life of trapezoidal rib-to-deck welded joints under the same nominal stress range and three-layers reinforcement presented a longer fatigue life. The S-N curve log N = 12.059 − 3 ⋅ log Δ σ h s based on the hot spot stress range at weld toe of the deck was established, which can be applied to the fatigue strength design of un-strengthened and CFRP strengthened rib-to-deck welded joints. The mechanism that CFRP reinforcement can significantly improve the fatigue performance of welded joints can be concluded to less stress concentrations and slower growth rate of the crack. [ABSTRACT FROM AUTHOR]

The study aims to propose a methodology to predict the fatigue performance of carbon fiber-reinforced polymer (CFRP) bars under the seawater sea-sand concrete (SWSSC) environment. The effects of exposure in SWSSC pore solution on the fatigue damage mechanisms and fatigue performance of CFRP bars are presented. Firstly, the tensile fatigue tests of reference CFRP bars were performed under various stress levels with a constant stress ratio of 0.1. Secondly, the tensile fatigue tests of exposed CFRP bars subject to the simulated SWSSC pore solution were carried out. The tensile failure modes and fatigue damage mechanisms corresponding to different stress levels, were identified using scanning electron microscopy (SEM). Besides, the relationship between fatigue life expectancy and underlying fatigue damage mechanisms was carefully analyzed. Finally, the effective stress level method (ESLM) was proposed to evaluate the residual fatigue life of exposed CFRP bars based on the benchmark S–N curve. Combined with the degradation models (chemical etching/diffusion models) proposed in our previous research, ESLM could bridge corrosion fatigue life prediction of the exposed CFRP bars and the benchmark fatigue data of the non-exposed CFRP reinforcement. The predicted results have good correlations with the experimental data. This method can also provide valuable insight for the corrosion fatigue life prediction of other types of FRP composites. [Display omitted] [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced polymer (CFRP) composites have exhibited a great potential for strengthening of steel structures. In the current study, an innovative prestressed unbonded reinforcement (PUR) system is introduced for fatigue strengthening of existing steel members. The system relies on a pair of mechanical clamps; each holds multiple CFRP plates and anchors their prestressing forces to the steel substrate via friction. A finite element model was established to optimize the design of the required mechanical components of the system. A set of static and fatigue tests was conducted on the developed mechanical clamps as the key elements of the proposed PUR system. The performance of the PUR system was then evaluated using a set of fatigue tests on two precracked steel plate specimens, one without any strengthening system and the other one strengthened with the proposed PUR system. In the latter specimen, the CFRP plates were prestressed up to about 800 MPa (approximately 30% of the CFRP tensile strength), which resulted in a complete fatigue crack arrest in the precracked steel plate. Furthermore, neither slippage of the mechanical clamps nor any prestress loss in the CFRP plates was observed after 7.5 million fatigue cycles. Based on the promising experimental results, obtained from the sets of fatigue tests performed in the current study, it can be concluded that the proposed PUR system can be considered as an efficient alternative to the conventional bonded reinforcement solutions for fatigue strengthening of damaged steel members. [ABSTRACT FROM AUTHOR]

Advanced carbon fiber reinforced polymer (CFRP) demonstrates promise for the fatigue strengthening of steel structures. By decreasing the stress field at the crack tip, the stress intensity factors (SIFs) can be effectively reduced by CFRP reinforcement. In this paper, the mode I SIF of CFRP-reinforced center-cracked tensile (CCT) steel plate is proposed based on a series of fatigue tests. The selected fatigue tests include experiments conducted by the authors as well as fatigue tests reported in the literature, covering different CFRP systems (low/high modulus, CFRP sheeting/plate) with various CFRP strengthening dimensions. The classical mode I SIF of CCT steel plate without CFRP strengthening is selected as the basis of the proposed solution. Then two reduction factors, similar to the correction factors given in the Japanese Society of Steel Construction (JSSC) standard, are introduced to study the effects of the mechanical properties of CFRP composites and the geometries of the CFRP reinforcement, respectively. Modified SIFs for both single-side CFRP-reinforced and double-side CFRP-reinforced CCT steel plates are proposed. It is found that the experimental SIFs of CFRP-reinforced CCT steel plates can be reasonably captured by the proposed mode I SIF formula. Finally, parametric studies for investigating the sensitivity of SIF to various mechanical and geometric factors are presented. [ABSTRACT FROM AUTHOR]

Cracked cross-beam connections made of thin-walled rectangular hollow sections (RHS) are repaired with carbon fiber-reinforced polymer (CFRP) sheets. Constant amplitude fatigue experiments are conducted on the repaired specimens. A pilot test is conducted on a T-connection of square hollow sections (SHS) to explore the effective method of applying CFRP. Due to the peeling effect at the corner region of the SHS-to-SHS connection, early debonding happens in the pilot test that only results in a slight extension in fatigue life. In the following experiments of repairing RHS-to-RHS cross-beam connections, circumferential or transverse restraining CFRP patches are applied in the corner region that prevents early debonding successfully and leads to significant increase in fatigue life. Finally, an improvement is made with the addition of steel strengthening plates that increase both the flexural stiffness and fatigue life of the cracked specimens significantly. The proposed retrofitting method may be useful for repairing other similar fatigue-cracked welded tubular connections. [ABSTRACT FROM AUTHOR]

Abstract: In recent years, there has been an increased use of concrete-filled composite tubular joints in bridge construction. The reliability of these joints under fatigue therefore need to be investigated to avoid any catastrophic failures. In this paper, hollow section T-joints made up of square hollow section (SHS) chords and braces are investigated. The chords of the SHS–SHS T-joints were concrete-filled thereby forming welded composite tubular T-joints. The SHS–SHS T-joints with concrete-filled chords were strain gauged and tested under static loading to determine stress concentration factors (SCFs) at hot spot locations, where cracks are likely to propagate. Fatigue tests of the welded composite joints were also carried out under cyclic in-plane bending in the brace to obtain stress range vs. number of cycles (S–N) data. The maximum stress concentration factor (SCF) at hot spot locations in a welded composite tubular T-joint was found to be generally lower than the maximum SCF in an empty (or called unfilled) hollow section SHS–SHS T-joint. Fatigue failure in welded composite tubular T-joints occurred through the initiation and propagation of cracks at weld toes in either the chord or brace member, with the majority of tests exhibiting the first visual crack at weld toes in the chord. The welded composite tubular T-joints were found to have better fatigue strength compared to the empty hollow section SHS–SHS T-joints. Design rules are recommended for the SHS–SHS T-joints with concrete-filled chords through analysis of fatigue test data using the hot spot stress method. [Copyright &y& Elsevier]

Abstract: The use of FRP (Fibre Reinforced Polymer) to strengthen steel structures has become an attractive option which may produce confident retrofitting of existing structures. This paper reviews the following areas that have received only small coverage in previous review articles, but have developed rapidly: the bond between steel and FRP, the strengthening of steel hollow section members, and fatigue crack propagation in the FRP–Steel system. Future research topics have also been identified, such as the as bond–slip relationship, the stability of CFRP strengthened steel members, and fatigue crack propagation modeling. [Copyright &y& Elsevier]

No abstract received. [ABSTRACT FROM AUTHOR]

In this paper, steel plates with longitudinal weld attachments strengthened by carbon fiber reinforced polymer (CFRP) laminates on one side were analyzed based on the boundary element method and compared with test data from the literature. Good agreement with the data indicated that the numerical analysis was reliable for estimation of the fatigue crack propagation of CFRP-bonded steel plates with longitudinal weld attachments. The effects of double-sided strengthening, double-sided weld attachment and CFRP stiffness on the fatigue behavior of retrofitted welded joints were also investigated. The results showed that double-sided strengthening was much more efficient than single-sided application. It was observed that the crack propagation of steel plates with weld attachments on both sides was accelerated compared with those with attachments on only one side. In comparison with steel plates without a weld attachment, the retrofitting efficiency, in terms of the fatigue life extension ratio, was significantly lowered in welded plates with single-sided repair, whereas only a slight difference was observed in those with double-sided strengthening. The effect of an increased modulus of the composite materials could result in better fatigue performance, especially with double-sided application. [ABSTRACT FROM AUTHOR]

Bird-beak joints are a type of welded square hollow section (SHS) joint, the static strength of which is higher than that of conventional joints. Experimental analyses were conducted on the stress concentration factors (SCFs) of square bird-beak joints to obtain more information regarding their fatigue behavior. Twelve specimens, i.e., three conventional joints and nine square bird-beak joints, are separately tested by considering chord and brace axial force load cases. Both conventional and square bird-beak joints tested have a T-shape and are simply supported at their chord ends. Elastic strain distributions near the crown and saddle areas are measured, and strain concentration factors (SNCFs) at potential hot spots are calculated using an extrapolation approach. Quadratic extrapolation is suggested for the square bird-beak joints because linear extrapolation may underestimate SNCFs by up to 35%. Furthermore, critical locations are identified for square bird-beak joints subjected to chord and brace axial forces. A comparison indicates that the square bird-beak joints can produce lower stress concentration factors than the conventional or the diamond bird-beak joints with identical nondimensional parameters, especially when brace-to-chord width ratios are relatively small. [ABSTRACT FROM AUTHOR]

There is an increased use of steel reinforced concrete (SRC) beams in civil engineering construction where fatigue loading could be critical. Most of the research on SRC beams so far was focused on static or earthquake loading, with very limited work on SRC beams under fatigue loading. This paper presents a numerical study on SRC beams subjected to fatigue loading, which follows a recent experimental study carried out by the authors. ABAQUS software was utilized to perform numerical simulation which considers the crack propagation in the H-steel using fracture mechanics theory and the damage evolution of the reinforcement and concrete. The predicted fatigue life of SRC beams was in good agreement with experimental data. The FE analysis revealed the influence of stress ranges, reinforcement ratios, sizes of H-steel beam and stud shear connectors on fatigue characteristics of SRC beams, such as stress intensity factors, crack propagation, damage in reinforcement bars and residual strength of concrete. [ABSTRACT FROM AUTHOR]

Different failure criteria used to prevent fatigue crack initiation in metallic members are described. A series of closed-form analytical solutions were developed to predict the fatigue resistance of metallic beams after strengthening with carbon fiber-reinforced polymer (CFRP) laminates. The model was used to determine Young’s modulus, pre-stress level and dimensions of the CFRP laminates such that the metallic detail is shifted from a ‘finite-life’ regime to the ‘infinite-life’ regime. This method is capable of predicting changes in both the stress range and the mean stress after strengthening. The results show that increasing the stiffness and pre-stress levels of the CFRP laminate can affect the fatigue life of the retrofitted member through different mechanisms. The latter preserves the alternating stress and decreases the mean stress level, whereas the former decreases both the mean and alternating stresses proportionally. To validate this model, a series of fatigue tests were performed on five steel beams, including one reference beam and four strengthened beams. The beams were strengthened with normal modulus (NM), high modulus (HM) and ultra-high modulus (UHM) laminates. Finally, a design example for the fatigue strengthening of a typical riveted metallic girder is presented. The developed analytical model was used to find the most effective fatigue strengthening solution under different cyclic load scenarios. Although the major focus in this paper is on steel members, it also describes others, such as wrought iron and cast iron. [ABSTRACT FROM AUTHOR]

In this study, fatigue tests of square bird-beak square hollow section (SHS) joints were conducted under out-of-plane bending. Nine orthogonally designed T-joints were fabricated. Static loading was first conducted, during which elastic strain distributions near the potential hot spots were measured. Stress concentration factors (SCFs) at weld toes were calculated using the quadratic extrapolation approach. Critical locations were also identified. Based on the high-cycle fatigue testing that followed, the fatigue crack initiation, propagation and fatigue failure mode were obtained. Variations of the surface crack length and out-of-plane rigidity against number of load cycles were also revealed. The cycle numbers that correspond to through-thickness cracks were observed to be greater than the numbers obtained from S–N curves provided by the IIW code, which indicates that the IIW curves/formulations for conventional joints could also be used to estimate the fatigue lives of square bird-beak joints if the hot spot stress ranges have been properly determined. [ABSTRACT FROM AUTHOR]

Abstract: This paper investigated the fatigue behavior of very high strength (VHS) steel tubes (t<2mm) to steel plate T-joints under cyclic in-plane bending. VHS tubes, with a yield stress of 1350MPa and an ultimate tensile strength of 1500MPa, were welded to G350 steel plates using the GTAW welding method. Three different failure modes were observed. Stress concentration factors (SCFs) ranging from 1.2 to 2.2 were obtained through linear extrapolation at the weld toe. The fatigue life of VHS (t<2mm) to plate T-joints subject to in-plane bending was determined using the hot spot stress approach with a stress concentration factor SCF=2.0 and the existing fatigue design equation in CIDECT Design guide No. 8. VHS tubes (t<2mm) showed better fatigue life than cold formed CHS (2mm≤t<4mm) to plate T-joints when welded to plates and loaded under in-plane bending. A class of 102 was proposed for fatigue design of VHS-plate T-joints under in-plane bending based on the current steel design standards. [Copyright &y& Elsevier]

Abstract: Three methods of fatigue strength evaluation, i.e., the hot spot stress (HSS) method, 1mm stress method and linear elastic fracture mechanics (LEFM) method, are used to evaluate the fatigue strength of transverse fillet welded joints in bending and the results are compared with those of fatigue experiments. It is found that the HSS is overly conservative in predicting the fatigue strength of fillet welded joints in bending. The 1mm stress catches the geometric influence of attachments very well. A reference strength range is suggested for applying the 1mm stress method to the fatigue strength evaluation of fillet welded joints in bending on the base of analysing available test data. LEFM is an established method for fatigue strength evaluation of welded joints and its applicability is much dependent on the ease and accuracy of quantifying the stress intensity factor (SIF) along the crack front. In this study, the SIF of fillet welded joints in bending is estimated in different ways and the resulting fatigue strengths are compared with fatigue test data. [Copyright &y& Elsevier]

Abstract: Carbon fibre reinforced polymer (CFRP) composites have proven to be effective in enhancing the load-carrying capacity and in extending the fatigue life of structural steel elements. Most studies have relied extensively on experimental testing and numerical simulations. It is important to develop some efficient analytical methods for predicting the fatigue behaviour of the composites repaired structures. In this paper, a simple analytical method is presented for estimating the fatigue crack growth and fatigue life of the CFRP repaired steel plates. Two types of fibre sheets are studied. These are normal modulus (E=240GPa) and high modulus CFRP (E=640GPa) sheets. Both double-sided and single-sided repairs are investigated. The analytical models are verified by experimental results. [Copyright &y& Elsevier]

Welded steel joints in structures are susceptible to fatigue failure. In this context, advanced carbon fiber reinforced polymer (CFRP) possess a demonstrated potential for the fatigue strengthening of steel structures. However, limited information is available on CFRP strengthening of butt-welded joints, which are the most common joint types in steel structures. This paper presents a study on fatigue behavior of butt-welded thin-walled steel plates strengthened using CFRP sheets. A total of 34 specimens with both single-sided and double-sided strengthening were performed via constant amplitude tensile fatigue loading, and another four specimens were employed for hot-spot stress measurement. The effect of CFRP strengthening on fatigue behavior was investigated by varying the strengthening scheme (single-sided or double-sided) and the number of CFRP sheet layers. Test results showed that the fatigue life of butt-welded steel plates with CFRP strengthening increased by approximately one to ten times when compared to the fatigue life of butt-welded steel plates without CFRP strengthening. Triple-layered double-sided specimens seemed to exhibit the best effect on fatigue life; their fatigue strength at 2 million cycles increased by 34% compared to that of specimens without CFRP strengthening. Specific fatigue design S–N curves of butt-welded thin-walled steel plates with and without CFRP strengthening were proposed directly. The fatigue test results were used to calibrate a fatigue design approach for CFRP strengthened butt-welded steel plates. In addition, the effect of CFRP strengthening on stiffness degradation was also discussed. • Thirty-four fatigue tests on CFRP strengthened butt-welded thin steel plates were conducted. • Stress concentration factors for CFRP strengthened butt-welded steel plates were measured. • Fatigue design approach for CFRP strengthened butt-welded steel plates was calibrated. • The effect of CFRP strengthening on stiffness degradation was discussed. [ABSTRACT FROM AUTHOR]

• A design approach for the mixed mode I/II fatigue crack arrest using prestressed CFRP plates. • Sets of stepwise HCF tests on precracked steel plates under various mode mixities. • Performance evaluation of the existing mixed mode I/II fatigue crack criteria. • Paris' law parameters of grade S355J2+N steel is independent of the material rolling direction. • A simple linear mixed mode I/II model is proposed for the safe design of strengthening solutions. In the present study, a strengthening design approach is proposed for the mixed mode I/II fatigue crack arrest in existing structural steel members using prestressed unbonded carbon fiber reinforced polymer (CFRP) composites. Through the analytical formulation of mode I and II stress intensity factor ranges, a design model is proposed to determine the strengthening solution, including the required prestressing level and/or the cross-sectional area of the reinforcement, which would ensure the complete arrest of an existing mixed mode I/II fatigue crack in a steel member. In parallel, sets of stepwise high-cycle fatigue tests were carried out on reference unstrengthened and prestressed CFRP-strengthened precracked steel plates of grade S355J2+N under various mode mixities. The experimental results revealed that the maximum tangential stress (MTS) criterion fairly predicts the state of the mixed mode I/II fatigue cracks (i.e., crack arrest or growth) in unstrengthened specimens, while the proposed design model provides a conservative estimation of the mixed mode I/II fatigue threshold in prestressed CFRP-strengthened specimens. Furthermore, the crack propagation characteristics of grade S355J2+N steel, i.e., Paris' law parameters (C and m) and the crack closure parameter (U), were determined and demonstrated to be independent of the material rolling direction. Based on the analytical and experimental results of the current study, it can be concluded that the proposed model can be used for the safe design of strengthening solutions, which is an increasing need to extend the service life of existing fatigue-damaged steel structures; certain recommendations are provided in this regard for practical strengthening applications. [ABSTRACT FROM AUTHOR]

Steel reinforced concrete (SRC) beams have increasingly been adopted in civil engineering construction. There has been a paucity of studies on the fatigue behavior of SRC beams under high-cycle fatigue loading. To fill the knowledge gap in this area, this paper reports on an experimental investigation of SRC beams subjected to fatigue loading. Fatigue tests were carried out on 18 H-steel reinforced concrete beams and two pure H-steel beams. Key issues addressed in the paper include failure modes and failure sequences, crack initiation and crack propagation, as well as stiffness. The key parameters influencing the fatigue strength of SRC beams are identified, including the steel ratio of H-steel components, the steel reinforcement ratio of components and stud shear connectors. A comparison of fatigue behavior between H-steel components inside SRC beams and pure H-steel beams has been made. It seems that the fatigue failure modes of H-steel inside SRC beams are the same as those displayed by pure H-steel beams, but the former have significantly higher fatigue strength than the latter. Fatigue design (in terms of stress range – fatigue life (S-N curves)) is recommended for fatigue assessment of welded H-steel components and reinforcement components inside SRC beams, respectively. [ABSTRACT FROM AUTHOR]

Abstract: The transverse distribution of wheel loads in orthotropic decks generates significant out-of-plane bending moments in the deck plate and rib wall at the rib-to-deck joint. Due to the relatively small thickness of both the deck plate and rib wall, the out-of-plane bending moments result in high local flexural stresses causing fatigue cracks to develop at the joint. In this study, the transverse stresses within the joint region that arise under the action of wheel loads are investigated with finite element analyses. Based on the stress results and the basic theories of linear elastic fracture mechanics, the design fatigue strength is determined for the joint. Factors affecting the stress range are also studied. The analyses show that the surface stresses in the deck plate are much larger than those in the rib wall in the case of 75% weld penetration into the rib wall, indicating that the fatigue strength of the joint is governed by the fatigue cracks propagating into the thickness of deck plate. It is also shown with finite element analyses that increasing the distribution area of wheel load or the thickness of deck plate can reduce the stress range of the deck plate and increase significantly the fatigue life of the joint. [Copyright &y& Elsevier]

This paper presents an experimental investigation on the fatigue behavior of bird-beak X-joints under brace axial force. Two square bird-beak and two diamond bird-beak specimens were fabricated by using 80% partial joint penetration (PJP) welds. Static loading was firstly carried out to measure the hot spot stresses at weld toes, by which stress concentration factors (SCFs) were extrapolated and subsequently the critical locations were identified. A cyclic axial loading was then implemented on the brace end. The fatigue behavior investigated includes crack initiation, crack propagation, failure mode, fatigue life and stiffness degradation. The results show that the SCFs calculated from test data could well predict the location of crack initiation. Furthermore, the through-thickness crack lives (N 3) of all are found to be greater than the fatigue lives derived from the IIW (International Institute of Welding) design S N curves, which indicates that these curves for conventional joints can be conservatively used for estimating the fatigue life of bird-beak SHS X-joints. • High-cycle fatigue tests for square and diamond bird-beak SHS X-joints under cyclic brace axial force were conducted. • Fatigue crack propagation and failure mode of bird-beak SHS X-joints under cyclic brace axial force were investigated. • Variations of crack length and joint stiffness with number of load cycles were revealed. • Comparisons between experimental fatigue data and IIW fatigue design curves were carried out. [ABSTRACT FROM AUTHOR]

This paper deals with the stress concentrations and fatigue behaviour of X-shape bird-beak joints under brace in-plane bending. Static load was first implemented to record the strain distribution along the hot lines, and then stress concentration factors (SCFs) were obtained through quadratic extrapolation method. Numerical analysis was then developed to further determine key parameters affecting SCFs and to propose corresponding SCF design formulas. During the subsequent high-cycle fatigue test, the fatigue cracking process including crack initiation, crack propagation, and fatigue lives, as well as the rigidity degradation were captured. Results show that crack initiation always occurred at the crown area where the SCFs are proved to be the greatest. The through-thickness lives may be utilized to conservatively represent the fatigue design lives of SHS bird-beak X-joints. Bird-beak SHS X-joints demonstrated better fatigue behaviors than those for traditional joints if the same service load is applied. • Stress concentration factors (SCFs) were measured for bird-beak X-joints under brace in-plane loading. • Finite element analysis was conducted to identify key parameters affecting SCFs. • Formulas were proposed for SCFs for bird-beak X-joints. • Fatigue tests were performed to determine fatigue life of bird-beak X-joints. • Joint stiffness degradation was recorded to explain fatigue failure process. [ABSTRACT FROM AUTHOR]