PSI - Issue 47

Jaroslav Václavík et al. / Procedia Structural Integrity 47 (2023) 282–289 Author name / Structural Integrity Procedia 00 (2019) 000–000

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reinforce the structure and make it more resistant also to roll-over. A review of strategies for composite-metal joining technologies is given by Jahn at al. (2016). The bus body is typically made of beams with hollow rectangular cross sections joined together by means of welding technique. These joints are sensitive to fatigue damage which is one of the primary failure modes for steel structures under cyclic loading. Relatively new way for straightening the bus structure with CFRP without the necessity substantially change the design is based on bonding a thin layer of CFRP plates to the steel element. This technique is widely used in the civil engineering. A review of CFRP/steel strengthening techniques under fatigue and crack propagations was performed by Kamruzzaman, M. et al (2014). The study also covered the surface treatment techniques, adhesive curing, and support condition under cyclic loading including fatigue performance, crack propagation, and failure modes with FE simulation. The main conclusion of this work is that the application of CFRP strengthening composites not only delays the initial crack, reduces the crack growth rate, and extends the fatigue life, but also decreases the stiffness decay with residual deflection. Fatigue test-based study of steel beams with welded cover plates retrofitted using pre-stressed CFRP strips was performed by Vatandoost (2010). The maximum increase in fatigue life observed in this work was 125 %, for a specimen strengthened using high modulus CFRP strips bonded onto the cover plates with the highest level of prestressing. End clamping mitigates the debonding of the CFRP strips and maintains the prestressing force, hence also reducing the transfer length. The CFRP strip elastic modulus is important: specimens with higher CFRP elastic modulus exhibited higher fatigue lives. Results of some test on samples with bonded pultruded CFRP strips Sika CarboDur M514 bonded with thixotropic epoxy resin Sikadur 30 were done by Bocciarelli at al. (2014). Static tests after artificial aging had shown that even if the stiffness and strength capability of the bond is not affected, the ductility of the reinforcement drops to 50 %. During fatigue tests it was observed that with increasing cycles the stiffness drops but the S-N curves are comparable with those using welded joints. The fatigue tests on DSR (double side reinforced) joints showed a significant specimen stiffness reduction due to the progressive adhesive debonding at the reinforcement ends. Results show that the fatigue resistance of DSR joint is comparable to the butt welded category 112 MPa when fast delamination growth takes place and category 90 MPa, when debonding reaches the mid-section of the specimen. Above the fatigue limit, debonding onset and propagation started at the CFRP strip ends leading to a sudden fatigue failure. Crack growth tests of CFRP-strengthened steel plates was performed by Colombi et al. (2015) to investigate increased fatigue life of cracked structures. Single edge notched rectangular tension specimens reinforced on a single side by using CFRP strips were considered. Failure was mainly observed due to debonding at the steel/adhesive interface and due to sudden static failure of the steel plate. The crack size in the steel plate at debonding was of about 70–80% of the specimen width. The experimental results showed that the reinforcement application can effectively reduce the crack growth rate and significantly extend the fatigue life. Investigation of the bond behavior of the interface between CFRP sheets and steel plates on DSL (double side lap) specimens during fatigue loading was performed by Zang at al. (2019). The fatigue crack of CFRP-to-steel bonded interface propagates from the mid-span to the loaded end with the increasing numbers of cycles in the testing part. Fatigue behavior of boned lap joints steel-steel (S355J2) with structural adhesive SikaPower® 1277 was presented by Laubrock (2022) with the access used in FKM-guideline. She found the slope of S-N curve k =6.33 and the stress amplitude fatigue limit at N = 2E6 cycles about 4.5 MPa. She observed the increased fatigue life with decreasing the adhesive thickness. In comparison with welded joints (plug or fillet welds) the fatigue limit at higher number of cycles is substantially higher. An extension work was made by Meschut at al. (2020) giving the comprehensive publication concerning the lifetime prediction of hybrid joints, where among others the unidirectional CFRP plates with the thickness of 1.2 mm and adhesive SikaPower® 1277 were used however as the reinforced material aluminum alloys were used. The first fatigue cracks appear in the early stages of the test, so that further behavior is in principle dominated by dynamic crack propagation. This does not change the slope of the S-N curves significantly, but it should be noted that the stress distribution in the specimens and the joined structures changes as a result of the crack growth. Consequently, a