This study presents a design criterion developed for fatigue strengthening of a 120-year-old metallic railway bridge in Switzerland and presents a pre-stressed un-bonded reinforcement (PUR) system developed to apply the strengthening. The PUR system uses carbon fiber reinforced polymer (CFRP) plates; however, unlike conventional pre-stressed CFRP reinforcement methods, preparation of the existing metallic bridge surface is not required. This decreases the time required for on-site strengthening procedures. The principle of the constant life diagram (CLD) and two fatigue failure criteria (Johnson and Goodman) are described. Analytical formulations are developed based on the CLD method to determine the minimum CFRP pre-stress level required to prevent fatigue crack initiation. The PUR system uses an applied pre-stress force to reduce the mean stress level (and stress ratio) to shift an existing fatigue-susceptible metallic detail from the 'at risk' finite life regime to the 'safe' infinite life regime. The applied CLD method is particularly valuable when the stress history of the detail is not known and it is difficult to assess the remaining fatigue life. Moreover, it is shown that the currently adopted approach in many structural codes which emphasizes stress range as the dominant parameter influencing fatigue life are non-conservative for tension-tension stress patterns (i.e., stress ratios of 0

Numerous studies in the literature have shown that the strengthening of steel members using carbon fiber reinforced polymer (CFRP) composites can significantly extend the fatigue life of these structures. However, not enough attention has been focused on the potential of prestressed CFRP reinforcements for fatigue crack arrest in such members. In the current study, a simple analytical model is proposed to calculate the required prestressing level in the CFRP reinforcements in order to arrest the propagation of an existing fatigue crack in tensile steel members. Furthermore, a novel mechanical unbonded system is developed to anchor the high prestressing forces in CFRP reinforcements to the steel substrate using friction. A set of fatigue tests are performed on unstrengthened and strengthened precracked steel plates to verify the proposed model. The experimental results of the current study showed that the application of nonprestressed ultra-high modulus CFRP plates as externally bonded reinforcements can increase the fatigue life of precracked steel plates by a factor of 4.3. However, fatigue crack arrest is only possible when prestressed CFRPs of a certain prestressing level are used. Based on the analytical, numerical, and experimental results of the current study, it can be concluded that existing fatigue cracks in tensile steel members can be arrested using the proposed prestressed unbonded reinforcement system with the initial prestressing level calculated using the proposed model. In addition, some design recommendations are provided for fatigue crack arrest in practical cases.

Pre-stress bonded composite patch is a promising technique to reinforce steel member damaged by fatigue. ifhe effectiveness of this technique was verified by fatigue tests on notched steel plâtes. Results showed that the application of carbon fibre reinforced plastic (CFRP) strips and, eventually, the introduction of a compressive stress by pretension of the CFRP strips prior to bonding produced a significant increment of the remaining fatigue life. In this paper, the stress intensiqr factor in the notched plates is computed by a two-dimensional finite element model in connection with the three-layer technique in order to reduce the computational effort. Due to high stress concentrâflon at the plate crack tip, debond is assumed at the adhesive-plate interface. The goal is to illustrate the influence of some reinforcement parâmeters such as the composite strip stiffness, the pre-stress level, the adhesive layer thickness and the size of the debonded region on the effectiveness of the composite patch reinforcement.Keyvords composite patch; fatigue crack reinforcement; parametric analysis.In Bassetti5 a novel technique was proposed to reinforce steel member damaged by fatigue. It consists in the appiication of carbon fibre reinforced plastic (CFRP) strips and, eventually, the introduction of a compressive stress by pretension of CFRP strips prior to bonding. Note that this new methodology applied to riveted steel members avoids the drawbacks of standard reinforcement techniques such as hole drilled at crack fiont, cover plates application, replacement of rivets by high strength bolts, cold expansion of the rivet hole and welding of the detected cracks. effectiveness of pre-stress CFRP-strips to stop fatigue crack emanating from the rivet hole or to prevent further cracking at other locations. The carbon fibre reinforced plastic laminates have physical and mechanical properties particularly interesting for reinforcement of fatigue damaged steel elements. The high fatigue resistance of CFRP avoids crack propâ-gation from the cracked steel section into the patch. The high stiffness of CFRP reduces the stress range in the cracked steel section and promotes crack bridging. llhe high tensile strength of CFRP allows also the application of a pretension to composite strips in order to increase the effectiveness of the bonded patch on thicker steel section. Finally, the low self-weight of CFRP plates limits the dead load increment and simplifies the strengthening operations. Composite patch reinforcement technique is a standard and reliable procedure in different engineering branches to reinforce structural elements subjected to extreme actions (high fatigue loads, high temperature ranges and exposure to aggressive agents) and nowadays, is also a standard reinforcement methodology in aircraft industries.T In fact the CFRP laminates is becoming a familiar procedure for O 2003 B ackwe I Publ sh ng Ltd. Fatigue Fract Engng Mater Struct 26, 59-66

Fatigue tests were carried out on welded circular hollow section K-joints typical to bridges. The tests specimens were large-scale (approximately 9 m long and 2 m high) trusses loaded in the plane of the truss. Measured member stresses showed that a significant proportion of the load in a truss member may be due to bending, underlining the importance of considering correctly this load case in the design of these structures. Measured hot-spot stresses in the joints were compared with hot-spot stresses calculated using the current design guidelines. It was found that the measured values are considerably lower than the calculated values, calling into question the applicability of the design guidelines to these types of (bridge) structures.The S-N fatigue results from the current study, on the other hand, showed that the fatigue resistance of the joints that were tested is lower than the corresponding S-N design curves. This means that when the considerably higher calculated hot-spot stress range is applied to the corresponding design curve, the predicted resistance is similar to the resistance predicted using the lower measured hot-spot stresses in combination with the lower measured S-N curve too. This has highlighted the importance of relating hot-spot stresses to the appropriate, corresponding S-N curves.Evidence from the fatigue tests has clearly demonstrated the effect of size on the fatigue strength of welded tubular joints. A comparison of fatigue S-N results from smaller and larger welded circular hollow section (CHS) joints has shown the same trend indicated in design specifications: a thicker failed member results in a lower fatigue strength. The size correction factor integrated into the S-N design curves of the specifications, however, does not seem to represent this significant effect justly. In light of the size effect results presented in this paper and the major influence of this effect on the design of welded CHS joints in general, it is recommended that a soundly based solution with targeted S-N curves and a representative size effect should be sought.

a b s t r a c tThe majority of fatigue strengthening studies focus on reducing propagation rates of existing cracks, ignoring the crack initiation stage. Many existing metallic bridge members however do not contain existing cracks, but rather are nearing their design fatigue life. Limited research exists on the prevention of crack initiation using carbon fiber reinforced polymer (CFRP) materials. In this paper, constant life diagrams (CLDs) are used to determine the minimum level of CFRP pre-stress required to indefinitely extend the fatigue life of existing metallic beams. It is shown that by applying a compressive force to an existing fatigue-susceptible detail using pre-stressed CFRP plates, the mean stress level can be reduced such that the detail is shifted from the 'finite life' regime to the 'infinite life' regime. The proposed fatigue strengthening approach is advantageous particularly when the stress history from the prior traffic loadings is not known. To validate the proposed method, a pre-stressed un-bonded CFRP reinforcement system is introduced and tested on four metallic beams. The proposed un-bonded CFRP system is advantageous over traditional bonded CFRP systems as it can be applied to rough or obstructed surfaces (surfaces containing rivet heads or corrosion pitting for example). Additionally, the new un-bonded CFRP system offers a fast on-site installation (no glue and surface preparation are required) and an adaptive pre-stress level. Experimental results show that strengthening using pre-stressed CFRP plates are capable of shifting the working stresses from a finite fatigue-life zone to an infinite fatigue-life zone preventing crack initiation. Although according to many structural standards, the stress range is the main parameter that affects the fatigue life of a metallic detail, the results of this study clearly show that the mean stress level also plays a significant rule in the detail fatigue life. Based on the proposed CLD approach in this paper, the combined effects of the stress range and mean stress level can be taken into account for prediction of fatigue life of metallic members.

An innovative type of connections for glass components, called laminated connections, has been developed in the last years. Two materials have been used for laminated connections: the transparent ionomer SentryGlas (SG) from Kuraray (former Dupont) and the Transparent Structural Silicon Adhesive (TSSA) from Dow Corning. In this paper, the mechanical behaviour of SG and TSSA bulk materials is studied under uniaxial tensile stress condition. The effects of strain rate and temperature variations are investigated. Particular attention is paid (i) to the study of these polymers in cured condition and (ii) to the computation of true stress and strain field during the tests. Firstly, it is observed that the mechanical behaviour of both SG and TSSA are temperature and strain rate dependent. These effects are quantitatively determined in the paper. Secondly, two additional phenomena are observed. For TSSA, it is observed that the material goes from fully transparent to white colour, exhibiting the so-called whitening phenomenon. For SG, instead, it is observed that the strain field distribution is dependent on the temperature. More specifically, the material exhibits a non-uniform strain field distribution due to the occurring of the necking phenomenon. Measurements along the specimens, using Digital Image Correlation techniques, showed that the localized strain propagates over the full specimen length, resulting in a cold-drawing phenomenon. Finally, it is also shown that engineering and true stress-strain definition exhibits large deviation indicating that the finite deformation theory should be used for the computation of the stress-strain curves to be implemented in numerical modelling.

Connections between structural glass components represent one of the main critical aspects of glass engineering. In the last years, a novel typology of adhesive connections has emerged, known as laminated adhesive connections. Two adhesive materials for laminated connections in glass applications are used in this work: the transparent ionomer SentryGlas® (SG) from Kuraray and the Transparent Structural Silicon Adhesive (TSSA) from Dow Corning. Both SG and TSSA show a complex behaviour dependent on strain rate and temperature. This work presents a study that aims (i) to investigate the mechanical behaviour and strength of this connection typology under shear loading and (ii) to quantify the effects of strain rate and temperature on the strength of the connections. This is done by means of a combined experimental, analytical and numerical study on laminated connections made of circular metal connectors bonded to rectangular glass plates. The experimental investigations presented in this work showed that temperature and strain rate variations have important effects on the mechanical response of the connections. Three-dimensional numerical analyses showed a non-uniform stress field with large gradient over the three dimensions. Through analytical studies, prediction models are finally proposed for the shear resistance of TSSA and SG laminated connections. The models are obtained developing an algorithm for multi-dimensional non-linear models with variable standard deviations. A logarithmic law is proposed for the strain rate effects for both TSSA and SG connections. Linear and inverse hyperbolic-tangent-based laws are instead proposed for the TSSA and SG temperature behaviour respectively.

No abstract