PSI - Issue 54

Jaroslav Václavík et al. / Procedia Structural Integrity 54 (2024) 294–299 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

295

2

parts with parts, stiffened with CFRP layers) which enable to achieve measurable benefits in the cost-effect relationship. Thanks to relatively simple operation which does not require modification of the production process it is possible to gain the opportunity to strengthen steel structures which is the first step on the way to the lighter supporting structure. In the construction of components in the automotive industry, the anisotropy of the composite materials could create new design concepts allowing a reduction of the number of parts and weight, so that, overall, it is a competitive advantage and that their use is increasing. There are some ways, how to use composites in bus structures. In several solutions the whole bus body is made from composite. Larodé (1915) proposed to make many parts of the bus including frames from composite profiles and sandwich structures which also supported by calculations. It has been shown here, that the weight of the bus structure can be reduced by 50% using composite materials. The second way, how to use the composites in the bus structure is to change some parts for composite nodes and/or adding some parts to stiffening the structure and make it more resistant also to roll-over. A review of strategies for composite-metal joining technologies was given by Jahn (2016). The bus body is typically made of beams with hollow rectangular cross sections joined together by means of welding technique. A new concept of substitution the welded structure node joint (side wall near rear door) with the CFRP one is proposed by Galvez (2017). Here, SikaTack Drive® polyurethane adhesive (currently, used for bonding windshields of cars) is proposed for connection between the CFRP SIKA Carbodur node (fictively designed by computations) and steel hollow beams. Relatively new way for straightening the structure with CFRP without the necessity substantially change the structure is used in this project. It is based on bonding a thin layer of CFRP plates to the steel element. This technique is widely used in the civil engineering However adhesive joints used in automobiles are subjected to very adverse operating conditions. Under these conditions, deterioration of the adhesive bond can be produced due to degradation both environmental and temperature, which leads to a decrease of the mechanical properties of the assembly over time. Results of testing of this behavior were done by Galvez (2017). 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 %. The 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. The safety of the driver and passengers according to frontal crashworthiness and roll-over accident has become an important issue for bus manufactures and customers. Metal hybrid structures have shown great potential in crash load cases especially during bending. Krieschner (2011) has tested a foam filled steel beam to have a 3 times higher weight specific energy absorption, compared to a hollow beam. 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 large deformation tests with bus structural nodes. The hybrid nodes are 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. Theory 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