Issue 31

J. Lopes et alii, Frattura ed Integrità Strutturale, 31 (2015) 67-79; DOI: 10.3221/IGF-ESIS.31.06 68 of parts to manufacture and to assemble. A composite component, however complex, still needs to be assembled to other components. Bolted joints are the most common joining technique between composite components. Bolted joints offer the advantage of being capable of carrying large loads, are simple to install and to inspect. This type of joint is more challenging in composite structures than in metallic structures due to their low bearing strength, high notch sensitivity, brittle and anisotropic nature, and high dependence on composite configuration. The typical solution to joint composite parts is to do a thickness build-up in the vicinity of the bolted area. This solution leads to an increase in weight, therefore cost, and additional stresses caused by eccentricities. The research work of the German Aerospace Centre with other partners has demonstrated that a significant improvement in bearing load can be achieved by replacing some CFRP plies by metallic plies in the vicinity of the bolted area. Fink and Kolesnikov [2] demonstrated the suitability of hybrid CFRP/Metal composites in experimental tests using CFRP and Ti6Al4V alloy. The inter-laminar shear stress was measured using the short beam test method in several conditions of temperature and humidity. Bearing ultimate strength tests were also performed with different ratios of Ti alloy contents. It was concluded that hybrid CFRP/Ti has a high bearing strength, high shear strength, and high compression strength. Also, the hybrid specimens have a low sensitivity to temperature and humidity. Kolesnikov and Fink [3] performed impact tests, bearing tests and dynamic tests in CFRP/Ti6AL-4V. The research concluded that compressive strength of hybrid material is higher than monolithic CFRP; the bearing tests using a three row bolted joint showed that CFRP/TI has a joint efficiency over 65% in the case of a 0º-on axis loading, and a satisfactory fatigue behaviour. Camanho et al. [4] investigated the experimental and numerical response of bolted joints using a hybrid CFRP and Ti– 15V–3Cr–3Sn–3Al alloy with several percentages of Ti content. The experimental results showed that the bearing strength increases with the increase of Ti content, a hybrid joint with 50% titanium has a higher specific stiffness than the reference monolithic CFRP; the critical region in a hybrid joint is the bolt-bearing region and not the transition zone from hybrid to CFRP; The numerical models yielded good results in predicting the bearing strength of hybrid and monolithic composites. Fink et al. [5] investigated the mechanical response and the manufacturability of hybrid CFRP/Ti alloy in a spacecraft payload adaptor. Using CFRP and Ti–15V–3Cr–3Sn–3Al the research concluded that the CFRP/metal hybridization had, among others, the advantages of high bearing, shear and pull-out strength, high specific bearing strength, and considerable weight savings. Fink and Camanho [6] performed inter-laminar shear stress by the use of the short beam method and experimental and numerical response of the bearing strength of a hybrid CFRP/Ti alloy. This research concluded that hybrid bearing strength increase to a factor of 2.5 when compared with typical CFRP laminates. Also it concluded for the feasibility and mechanical effectiveness of hybrid joints. All the previous works were done using titanium alloys as reinforcement material. However the use of steel was already considered as a potential alternative to titanium [5, 6]. Titanium has the advantage of high specific strength, galvanic compatibility, and lower coefficient of thermal expansion (CTE) than corrosion resisting steel. Corrosion resisting steel has the advantage of higher stiffness, higher ultimate strength, excellent fatigue properties and much lower cost when compared with Ti alloys. It has the disadvantage of having a higher CTE producing a significant residual thermal stress. Although these stresses are not addressed in the present work, Stefaniak et. al could show that due to mechanical interaction, residual stresses in multi-layered FMLs are lower than often assumed when only thermo-mechanical behavior is considered [7]. Additionally, by modifications in the curing process, the “smart curing” [8], residual stresses can be lowered significantly. In this research the objective is to measure the inter-laminar shear stress in a hybrid CFRP/Steel composite. This research comprises an experimental test program and a numerical simulation in a finite element commercial package. The test program is exposed in detail as well as the numerical simulations. The results of both the experiments and the simulations are presented and compared. E XPERIMENTAL TESTING n experimental program on apparent inter-laminar shear stress (ILSS) was determined by short beam three-point bending tests with two main objectives: i) to identify the most suitable surface treatment method for the metal foil in this particular fibre metal laminate; ii) to determine the required cohesive element parameters to simulate delamination failure. A