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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design based on the start of fatigue crack growth is recommended. A simplified method was developed to identify the beginning of the crack. Effect of cyclic loading on bond strength was investigated by Liu at al. (2010). It was concluded, that in comparison with static tests the reduction in bond strength is around 20 to 30% even when the load ratio is 0.3 with the preset fatigue cycle of 8 million. There was also found that the fatigue life of CFRP - strengthened steel plates initially increased with the bond length until the effective bond length under cyclic loading was reached, after which any further increase in the bond length did not further increase the fatigue life. Wu et al. (2012) studied the influence of fatigue cycle on the bond between high-modulus CFRP plates and steel. It was revealed that the influence is minimal (less than 4.5%). The effect of fatigue loading on stiffness is less than 10%. A local fatigue damage zone near the joint of double-shear pull test specimen was observed. The length of this local damage zone was found to be less than 1% of the bond length. Presented work deals with strengthening of steel elements by carbon fibre reinforced plastic (CFRP) plates. Our effort is utilize the results from previous research works and spread it for new suitable hybrid nodes in the bus structure and verify it with fatigue tests with bus structural nodes. The hybrid nodes will be based on Sika CarboDur® plates and hollow profiles filed with foam. This solution can be applied not only as far as for newly produced but also for already operated vehicles. Thanks to this technology service repairs of the supporting structure are going to exclude the vehicle from operation for shorter time periods which is more economical than currently conducted repairs. 2. Calculations 2.1. Analytical stress based approach The dominant failure mode which can prevent the full load bearing capacity of the hybrid bus structure is the debonding of the CFRP plate. An analytical formulation for prediction the failure load of steel/CFRP joints was derived by Bocciarelli (2014). Using stress based approach; the shear stress distribution close to the reinforcement ends for one-side reinforcement (OSR) is given by   =−         = , ∙   , where (1) =         , and =     =             +       (2) Where P = σ ·A s is the loading force, σ corresponding nominal stress, b a is steel and CFRP width and x is the distance of reinforced end, E s , A s , E f , A f are steel plate end CFRP plate Young’s modulus and section area, G a is adhesive sheer modulus. The sheer stress level is the indicator of unbonding the CFRP layer. The debonding failure load can be estimated as follows   =1−      , ∙ ∙ (3) An estimation of the required length l of the adhesive joint in order to achieve a 99% of the maximum strength was derived to be l min ≥ 5/ λ . The calculation of stress distribution and limit nominal stress was made for specimen used for experimental tests. The width of the steel specimen was 30 mm, thickness 3 mm and the length 450 mm. The adhesive layer thickness was 0.3 mm. The CFRP plate has the length 140 mm, the thickness 1.2 mm and 25 mm width. The required minimum length of CFRP plate was calculated for our sample of investigation l min = 39 mm. The specimens were manufactured from the ferritic stainless steel 1.4003 with a nominal Young’s modulus E s = 210,000 MPa, a tensile strength equal to 480 MPa and a yield stress equal to 350 MPa. The reinforcement plate