Issue 48

C. E. Cruz Gonzalez et alii, Frattura ed Integrità Strutturale, 48 (2019) 530-544; DOI: 10.3221/IGF-ESIS.48.51 532 Adhesives exhibit a large amount of plasticity that has an important impact on the fatigue strength of adhesive joints. In that sense, Markolefas and Papathanassiou [9] proposed a shear-lag model to evaluate stress redistributions in double-lap joints under axial (tensile) lap shear cyclic loading. In this work, a double lap shear specimen model was exposed to develop stress-strain equations, then equations that governs adhesive shear stress redistributions under cyclic loading was developed. Example solutions that considers metal to metal and composite to metal were exposed. The materials, like 7076-T6 aluminum alloy (Young Modulus=71 GPa, Poisson ratio=0.33, Maximum strength=482.50 MPa and 2 mm thick), high strength graphite-epoxy composites (Young Modulus=55.15 GPa, Inter-laminar strength=55.15MPa, Maximum strength=475.69 MPa and 1.5 mm thick) and Hysol Shell EA951 adhesive (Shear Modulus=415.67 MPa, Young Modulus=3.447 GPa, Shear strength=41.36 MPa and 0.127 mm thick) were used in diverse configurations considering thermal load by mismatch ((  2 -  1 )  T= ‒ 0.001) in the case of dissimilar joints. According to the results, the adherent materials exhibit linear elastic behavior, whereas the material of the adhesive layer satisfies the elastic–perfectly plastic shear stress–strain constitutive relation. The fatigue behavior of adhesive joints made with commercially coil-coated thin aluminum sheets (2 mm thick AA5754- O, Young modulus=68.9 GPa, Poisson ratio=0.33 and yield strength=165 MPa bonded with single-part, heat-cured rubber toughened adhesive (Young modulus=1.96 GPa, Poisson ratio=0.45 and yield strength=40 MPa was investigated by Datla et al [10]. In this work, double cantilever beam (DCB) and cracked lap shear (CLS) at 0° and 90° of rolling direction (different roughness) were employed. The results suggest, that the fatigue behavior was sensitive to the surface roughness introduced by the rolling lines (for example the CLS-Longitudinal specimens has an average value of fracture toughness of 181 J.m -1/2 until CLS-Transversal has 282 J.m -1/2 ). Additionally, the presence of moisture in the test environment greatly reduced the fatigue threshold (25°C @ dry air: 145 J.m -1/2 , 40°C @ 100% RH: 57 J.m -1/2 ). The employment of hybrid (adhesive-bolt or weld) joints were investigated by several researchers [11, 12, 13, 14, 15]. However, according to Wahab [4], the benefit to the fatigue performance of an adhesively bonded joint by additional bolting, riveting, or welding is questionable. Recently, the behavior of multi-material structural adhesives has been studied for civil, automotive and aerospace applications. In that sense, Galvez et al [16], presented a study of the behavior of adhesive joints of steel with carbon fiber reinforced polymers (CFRP) for its application in bus structures. Such study presents a finite element model (FEM) of a bus steel, in order to obtain the forces that work on the reference node. Additionally, lap shear testing specimens were carried out; those specimens were made of CFRP-steel bonded with SikaTack Drive adhesive (polyurethane based). The results suggest, that the raised adhesive joint shows values far superior to the stresses that it must resist, greater elasticity at the reference node, decreasing the relative rigidity of the surroundings and minimizing mechanical fatigue. The objective of the present paper is to analyze the influence of the adhesive and overlap length in the quasi-static fracture behavior and the dynamic loading response. The adhesive joints are composed of 6 mm thick anodized 6061-T6 aluminum alloy bonded to a 3 mm thick Dual Phase (DP) steel. Three different adhesives, with significant differences in mechanical properties were employed like Lord DC-80, Betamate 120 and MP55420. Extensometers were adhered in the joint center-line in order to measure strains generated during fatigue testing and validate if adherents are within the linear range. Overlap lengths of 12.7 and 50 mm were used in the experiment and 30, 50 and 70% of the maximum load were applied to the joint at R=0.1 and 30 Hz of frequency for fatigue tests. In order to predict fatigue life, maximum load- number of cycles Basquin and Wholer curves were built. Finally, scanning electron microscopy fractographs were taken upon the fracture surfaces, thus determine fractographic differences between adhesives. M ATERIALS AND M ETHODS wo substrates have been used to prepare the single lap shear specimens: Anodized 6061-T6 aluminum plate (with dimensions of 1500×1500×6.4 mm), and DUAL-TEN® 590/600 (1500×1500×2.9 mm), a dual phase steel (ferrite matrix and approx. 5-15% martensite).Three different adhesives were used: one-component structural epoxy (Betamate 120), a two-component epoxy system (DC-80) and, a two component methyl methacrylate (MP55420. The Betamate 120 adhesive, is a one component epoxy paste adhesive with aluminum as filler, density of 1.14 g cm -3 and viscosity of 50 Pa s. The DC ‒ 80 adhesive, is a two components adhesive that consist of resin and hardener. The resin (density of 1.21 g cm -3 at 25 °C) is a paste that consists of 90 to 95% epoxy resin, 1 to 5% titanium dioxide, and 0.1 to 0.9% glass oxide. Moreover, the hardener (density of 1.01 g cm -3 at 25 °C) consists of 85 to 90% polyamide resin, 1 to 5% amine compound, 0.1 to 0.9% glass oxide and 0.1 to 0.9% epoxy resin. Finally, the MP55420 is a two-component methacrylate adhesive, consisting of resin and activator, with viscosities of 130 and 50 Pa s at 25 °C, respectively. T