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

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Prestressed composite patch bonded on cracked steel section is a promising technique to reinforce cracked details or to prevent fatigue cracking on steel structural elements. It introduces compressive stresses that produce crack closure effect. Moreover, it modifies the crack geometry by bridging the crack lips and reduces the stress range at crack tip. Fatigue tests were performed on notched steel plate reinforced by CFRP strips as a step toward the validation of crack patching for fatigue life extension of riveted steel bridges. A debond crack in the adhesive-plate interface was observed by optical technique. Debond crack total strain energy release rate is computed by the modified virtual crack closure technique. A parametric analysis is performed in order to investigate the influence of some design parameters such as the composite patch YoungÕs modulus, the adhesive thickness and the pretension level on the adhesive-plate interface debond.

A B S T R A C T Prestressed composite patch bonded on cracked steel section is a promising technique to reinforce cracked details or to prevent fatigue cracking on steel structural elements. It introduces compressive stresses that produce a crack closure effect. Moreover, it modifies the crack geometry by bridging the crack faces and so reduces the stress intensity range at the crack tip. Fatigue tests were performed on notched steel plate reinforced by CFRP strips as a step toward the validation of crack patching for fatigue life extension of riveted steel bridges. A crack growth induced debonded region in the adhesive-plate interface was observed using an optical technique. Moreover, the size of the debonded region significantly influences the efficiency of the crack repair. Debond crack total strain energy release rate is computed by the modified virtual crack closure technique (MVCCT). A parametric analysis is performed to investigate the influence of some design parameters such as the composite patch Young's modulus, the adhesive thickness and the pretension level on the adhesive-plate interface debond.Keywords composite patch reinforcement; debond crack; strain energy release rate. I N T R O D U C T I O NWork on reinforcement of cracked aluminium structures by composite patches started in the seventies and today is a standard procedure in aircraft industry. 1 Other applications on structural elements subjected to extreme actions (high fatigue loads, high temperature ranges and exposure to aggressive agents) in the automotive and aeronautical industry showed that it is possible to produce high strength assemblies using composite materials bonded to thin metallic elements.2 At the beginning of the nineties it was shown that the properties of carbon fibre laminates (CFRP) could be exploited also in civil engineering applications and the reinforcement of concrete bridges with CFRP-laminates was achieved.3 This technical realisation showed that composites bonded with epoxy resins are a reliable solution for strengthening of bridges. It was also shown that the very high tensile strength of composite materials could be exploited in order to prestress the CFRP-laminates. 4 More recently, attempts were made to reinforce metallic structures in civil engineering with composite materials in order to increase the load carrying capacity of the steel bridge sections. 5 Finally, CFRP patch, eventually prestressed prior to bonding, was proposed.6±11 as an innovative reinforcement methodology for steel structural elements damaged by fatigue. CFRP-laminates have physical and mechanical properties that are particularly interesting for reinforcement of fatigue damaged steel elements. The high fatigue resistance of CFRP avoids crack propagation 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. The high tensile strength of CFRP also enables a pretension to be applied to composite strips so as to promote crack closure in thi...

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