PSI - Issue 54

Rahul Iyer Kumar et al. / Procedia Structural Integrity 54 (2024) 164–171 Iyer Kumar, De Waele / Structural Integrity Procedia 00 (2023) 000–000

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the joint is post-cured in a temperature-controlled environment. On the contrary, for marine applications, the building docks are more often than not located near the coasts, river banks or quays and exposed to the elements (Eyres and Bruce, 2012b,a). The use of a thick adhesive bondline between adherends is needed as close tolerances cannot be adequately achieved. Due to the lack of guidelines and standards on adhesive bonding of primary structures in marine applications, it is crucial to investigate the bonded structure to provide the industry with data that helps predict the bond behaviour over the operating lifetime of a ship. Adhesive bonds in marine structures are exposed to the harsh saline environment over their service lifetime; this necessitates an in-depth understanding of the long-term behaviour of thick structural adhesives under representative loading. The knowledge of fatigue properties and crack resistance of the adhesive and adhesive-adherend interface under cyclic loading allows one to make better designs and manufacture structural parts with adhesive joints. How ever, no standards are available that elucidate a method to determine fatigue crack resistance and crack growth of thick adhesive joints under mode I loading conditions (Sekiguchi and Sato, 2021; Eklind et al., 2014). Nonetheless, researchers have performed experiments demonstrating a relationship between crack growth per cycle and crack driv ing force under mode I loading conditions. Kinloch and Osiyemi (1993) and Jethwa and Kinloch (1997) performed fatigue experiments on double cantilever beam (DCB) specimens and tapered double cantilever beam (TDCB) spec imens respectively which are bonded with an epoxy-based adhesive having a bondline thickness of 0 . 4 mm. Pirondi and Nicoletto (2004), Eklind et al. (2014) and Sekiguchi and Sato (2021) performed experiments on DCB specimens bonded using structural adhesives having a bondline thickness in the range of 0 . 15 mm to 1 mm. The above-mentioned researchers evaluated the fatigue properties of adhesives relating the fatigue crack growth per cycle to a fracture mechanics-based parameter. The linear region of this relation (in a double logarithmic diagram) is described by Eqn: 1 Here, a is the crack length, N is the number of cycles, G is energy release rate and a measure of crack driving force and C and m are empirical constants. The e ff ect of bondline thickness on fatigue crack growth behaviour was studied by Mall and Ramamurthy (1989), Azari et al. (2010) and Sekiguchi and Sato (2021). They reported a positive correlation between fatigue toughness and bondline thickness. In contrast, Pascoe et al. (2020) did not find any relationship between bondline thickness and fatigue crack growth rate. However, the reader should note that the bondline thickness that was investigated did not exceed 1 mm. The existing methods for testing DCB specimens with thin adhesive bondlines may not accurately reflect the behaviour and failure mechanisms of adhesive joints with thicker bondlines. The current work focuses on developing a test method to evaluate the mode I steel-adhesive interface characteristics of adhesively bonded DCB specimens with thick bondline under fatigue loading conditions. A brief description of the specimen configuration, testing method and preliminary results are presented. da dN = C ( G ) m (1)

2. Materials and Methods

2.1. Specimen Configuration

Tests are performed on DCB specimens which are manufactured under shipyard conditions by Damen Naval. The specimens (see Fig. 1) consist of two 250 × 250mm (length × width) AH36 steel plates of thickness 6mm bonded together with the two-component methyl-methacrylate (MMA) adhesive having a nominal thickness of 8mm. The steel surfaces are first grit blasted up to SA 2.5 standard, degreased and cleaned with isopropyl alcohol. The adhesive is injected using a pneumatic gun. A non-adhering Teflon film of length 80 mm is placed at the interface between the adhesive and one of the steel plates to obtain a discontinuity in the bond which represents the initial crack. The specimens are allowed to cure at room temperature for 24h. Following the guidelines of the ISO 25217 standard, the initial crack length between the loading pin and the crack tip is equal to a 0 = 50mm as shown in Fig. 2.