PSI - Issue 33

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R.F.N. Brito et al. / Procedia Structural Integrity 33 (2021) 665–672 Brito et al. / Structural Integrity Procedia 00 (2019) 000–000

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Table 1. Orthotropic mechanical properties of the CFRP substrates. The axes y and z are the transverse directions. Based on Ribeiro et al., (2016). Young’s modulus per direction Poisson’s ratios Shear modulus per direction E x = 1.09 x 10 5 MPa  xy = 0.342 G xy = 4315 MPa E y = 8819 MPa  xz = 0.342 G xz = 4315 MPa E z = 8819 MPa  yz = 0.380 G yz = 3200 MPa

2.2. Experimental testing The stepped-lap joint to analyze in this work possesses three equal length steps (Figure 1), with an adhesive thickness ( t A ) of 0.2 mm in all of them, the adhesive thickness in the butt-ends ( t A1 ) was also 0.2 mm. The distance within grips ( L T ) was 180 mm, while the joint width ( b ) was 15 mm. The only parameter that changed was the overlap length ( L O ), considering four different lengths 12.5 mm, 25.0 mm, 37.5 mm, and 50 mm. Consequently, the step length was calculated for each model case. The geometry of the stepped-lap joint is shown in Figure 1. These dimensions were used for the manufacture of the specimens, and later for the creation of the numerical models. Table 2. Mechanical properties of the adhesive Araldite 2015. Adapted from Campilho et al., (2013). Property and symbol Value and units Young’s modulus, E 1.85±0.21 GPa Poisson’s ratio,  0.33 Tensile yield stress,  y 12.63±0.61 MPa Tensile failure strength,  f 21.63±1.61 MPa Tensile failure strain,  f 4.77±0.15 % Shear modulus, G 0.70 GPa Shear yield stress,  y 14.6±1.3 MPa Shear strength,  f 17.9±1.8 MPa Shear strain at failure,  f 43.9±3.4 % Tensile toughness, G IC 0.43±0.02 N/mm Shear toughness, G IIC 4.70±0.34 N/mm Once the substrates were cut to size, the steps were created by milling using a conventional milling machine with a carbide coated end mill, allowing the accuracy of the steps. Then, all the milled steps were prepared for the bonding stage by sanding down the mating surfaces to achieve the desired roughness, and subsequent cleaning and degreasing. The surface preparation process is described in larger detail by da Silva et al., (2012). Furthermore, four specimens for each case were produced and tested, being in total 16 specimens for this work; testing is described later in this section. The bonding of the substrates started with the preparation of an alignment jig using a demolding agent to avoid adhesion of the substrates to the mold. Subsequently, the substrates were placed in the jig, calibrated shims were installed to ensure a uniform adhesive thickness (da Silva et al., 2012). The adhesive was then poured on the mating surfaces using an applicator. Then, the other substrates were placed and aligned, pressure was applied on the jig and kept during the curing process, which was seven days at room temperature. After curing, all specimens were cleaned, all the excess adhesive was removed. Subsequently, the specimens were tested using a Shimadzu AG-X 100 Universal Testing Machine (UTM) (100 kN load-cell); these joints were tested at 1 mm/min. From the experimental data, the maximum load sustained by each specimen was considered as its experimental P m . Afterward, these experimental P m were grouped by L O and averaged. The averages were used for comparison with the numerical models.