Issue 48

C. E. Cruz Gonzalez et alii, Frattura ed Integrità Strutturale, 48 (2019) 530-544; DOI: 10.3221/IGF-ESIS.48.51 531 testing, Basquin and Wholer graphs were built for each adhesive at 12.7 and 50.0 mm of overlap length. The results suggested that at higher overlapping, the cyclic maximum load increased. Additionally, the maximum fatigue loading at 10 6 cycles for MP55420 adhesive was 1.3 kN for an overlapping of 12.7 mm and 2.9 kN for 50 mm. For DC80 adhesive was 1.75 kN for overlapping 12.7 mm and 4.8 kN for 50 mm. Finally, for the Betamate 120 adhesive was 1.8 kN for 12.7 mm of overlapping and 6 kN for 50 mm. The post-fracture visual inspection revealed that MP55420 and Betamate 120 adhesives had a cohesive failure, while the DC-80 showed cohesive-adhesive failure. Additionally, the scanning electron microscopy evaluation on the spew fillet revealed resolved striations and a network of small micro-dimples for the Betamate 120 and MP55420 adhesives. On the other hand, DC-80 adhesive exhibited notable facet fragile failure that was confirmed by the shape of stress-strain plot with straight line from the origin to the point of fracture. K EYWORDS . Fatigue; Overlap length; Adhesive joint strength. Published: 01.04.2019 Copyright: © 2019 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION he application of adhesively bonded joints in automotive and aeronautical industries has increased significantly in recent years, having several procedures to join metals, polymers and composites by using adhesives [1]. An adhesive distributes stresses more evenly than riveted and screwed joints, thus avoiding areas of high stress concentration. Additionally, adhesive bonding allows to generate joints without microstructural changes on the parts joined, distortion, design flexibility, joining of dissimilar or new materials and good fatigue strength [2]. Joining dissimilar materials such as fiber-reinforced composites to metals or novel materials, has a goal of generating lightweight structures for decreasing fuel consumption, CO 2 emissions, among others, in transportation sector. In this sense, materials like polymer matrix composites, aluminum, magnesium or high strength steels (HSS) (< 0.5 mm thick sheets) become an alternative. A single type material in structure is rarely realistic, because its employment depends basically on the maximum stress (and its orientation) that the structure can withstand. Even if metals cannot be excluded as fundamental base material from many applications, plastic materials are used because their low weight (2.7 or 7.8 times lighter than aluminum or steel, respectively), high corrosion resistance, excellent formability and greater design flexibility [3]. Fatigue is undoubtedly a very important type of loading for many structural components that contain adhesive bonding systems. Under fatigue loading stress, a structure may fail far bellow of its static strength [4]. The time that the material remains in service before the failure occurs is determined by the level of stresses and the frequency of the load cycles. The analysis is carried out by obtaining the stress-number of cycles curves (S-N) until the failure of the component occurs. The above allow an estimation of the fatigue resistance of a given material or structure [5]. Fatigue is important for reliability and durability of materials and structures, such as car or plane frames. The effect of joint geometry has been studied in order to determine its effect in the fatigue strength. An example of that is the work published by Altan et al [6], where the results suggest that fatigue strengths of adhesively-bonded butterfly joints have a longer life span than those of the bonded butt joints under the same circumstances. Fessel et al [7] have investigated the effect of a reverse-bent joint geometry for the improvement of fatigue performance of adhesively bonded joints. The results suggest, that the improvements obtained under static tests conditions can be translated to even higher benefits in fatigue. The effect of a highly-toughened epoxy adhesive bond-line thickness (130 to 790 μm), on the fatigue strength of 12.7 mm thick AA6061-T651 adherends has been investigated by Azari et al [8]. The experiments consisted in a double cantilever beam (DCB) and asymmetric double cantilever beam (ADCB) specimens with a surface finishing (R a = 1.33 ± 0.16  m) prepared by using a silicon carbide nylon mesh. The results suggest, that the critical strain energy release rate for quasi- static fracture increased linearly with aforementioned bond-line thickness. T