Issue 49

F.J.P. Moreira et alii, Frattura ed Integrità Strutturale, 49 (2019) 435-449; DOI: 10.3221/IGF-ESIS.49.42 437 including at the interface. However, this technique is limited by the existence of a potential discontinuity in the crack at the XFEM-CZM transition. Stuparu et al. [22] conducted a study on the combined use of CZM and XFEM for the strength prediction of bonded joints. The SLJ configuration was tested, with aluminium adherends and the adhesive Araldite ® AV138. The following parameters were used: adherends’ thickness ( t P ) of 5 mm, adhesive thickness ( t A ) of 1, 3 and 5 mm, overlap length ( L O ) of 20 mm and sample width ( B ) of 25 mm. The numerical analysis was done in Abaqus ® . The XFEM was used to simulate failure within the adhesive, considering a strain criterion (less mesh dependent than stress criteria) for crack onset prediction. Thus, crack initiation/propagation will always take place orthogonally to the maximum principal strains. On the other hand, CZM was equated to simulate an interfacial failure between the adhesive and adherends. Different t A and the positions of initial bonding flaws were tested, resulting in modifications of the XFEM crack trajectories, eventually attaining the interface. It was shown that the use of XFEM is well complemented by CZM to promote crack growth after the XFEM crack attained the interface. This work consists of an experimental and XFEM analysis of aluminium alloy T-joints, adhesively-bonded with three adhesive types. A parametric study is undertaken regarding t P2 , with values between 1 and 4 mm. The adhesives Araldite ® AV138 (strong but brittle), Araldite ® 2015 (less strong but moderately ductile) and the Sikaforce ® 7752 (with the smallest strength but highly ductile) were tested. A comparative analysis between the different joints conditions was undertaken by plotting  y and  xy stresses, and analysing the damage variable. The XFEM predictive capabilities were tested with different damage initiation and propagation criteria. E XPERIMENTAL WORK Adherends and adhesives he T-joints are made of three AW6082 T651 aluminium alloy aluminium adherends bond together. This is a high- strength alloy, characterized in a previous work [23]. Fig. 1 shows typical stress-strain (  -  ) curves of this aluminium alloy, whose relevant properties in bulk tensile testing are: Young’s modulus ( E ) of 70.1±0.8 GPa, tensile yield stress (  y ) of 261.7±7.7 MPa, tensile strength (  f ) of 324.0±0.2 MPa and tensile failure strain (  f ) of 21.7±4.2%. Figure 1 : Experimental and numerical  curves of the aluminium. The following adhesives were tested in the T-joint configuration: Araldite ® AV138 (brittle epoxy), Araldite ® 2015 (ductile epoxy) and the Sikaforce ® 7752 (high-elongation polyurethane). All adhesives were formerly tested and the respective properties detailed in references [23-25]. The tensile mechanical properties ( E ,  y ,  f and  f ) resulted from bulk tests to dogbone specimens. The fabrication process for these specimens followed the indications stipulated in the NF T 76-142 French standard. The shear mechanical properties (Shear modulus – G , shear yield stress –  y , shear strength –  f and shear failure strain –  f ) were estimated by Thick Adherend Shear Tests (TAST). In this case, the 11003-2:1999 ISO standard was considered for fabrication and testing protocols. The TAST specimens were made with DIN Ck 45 steel adherends, and curing was undertaken in a rigid mould to guarantee that the cured specimens are aligned [26]. The toughness properties of the adhesives were estimated with the Double-Cantilever Beam (DCB) test (tensile fracture toughness or G IC ) and the End- Notched Flexure (ENF) test (shear fracture toughness or G IIC ). Tab. 1 gives an overview of the obtained data, which will 0 50 100 150 200 250 300 350 0 0.05 0.1 0.15 0.2 0.25  [MPa]  Experimental Numerical approximation T