Experimental study of adhesive layer effect on Mode-II interlaminar fracture toughness in unidirectional Glass Fiber/Epoxy composite and Mode-I fracture toughness at the 0/90-degree interface
Topic(s) : Material and Structural Behavior - Simulation & Testing
Co-authors :
Sasan KARIMI (TURKEY), Ali BARZEGAR (TURKEY), Mehmet YILDIZ , Hatice HATICE S. SAS (TURKEY)Abstract :
Nowadays fiber-reinforced polymer composites have found a wide range of applications across diverse industries. Directional dependence of strength and stiffness of composites can be tailored to match the loading boundary conditions of the structural elements. Laminating composite material in different ply orientations, however, induces interlaminar shear and normal stresses due to the mismatch in properties of laminas. These stresses result in interlaminar delamination which is known as the most predominant life-limiting failure mechanism in composites. Improving the interlaminar fracture toughness with adhesive film in high stress points is one of promising methods known as local toughening method [1-3].
In this study, the effect of an adhesive layer is investigated experimentally on fracture toughness of unidirectional Glass fiber/Epoxy composite in Mode II, shear mode, based on ASTM D7905 standard. The applied adhesive film has embedded knitted fibers which interact with the primary prepreg matrix and adhesive film epoxy itself during crack propagation, both aspects are studied. Glass Fiber/Epoxy prepreg and adhesive film are hand-laminated and subsequently processed in a hot press to produce unidirectional composite sheets. Samples are then cut using a waterjet based on standardized dimensions. The ASTM D7905 standard only considers the crack initiation in Mode II without any crack propagation. Hence, DIN EN 6034 standard procedure with 25 mm loading roller diameter is adopted for tested samples to create crack propagation for previously initiated cracks in samples. Force-head displacement curves are generated, and, notably, the induced crack propagation results in a sufficiently extended crack surface for scanning electron microscopy (SEM) analysis. Fracture mechanisms are discussed based on SEM imaging, with a specific focus on understanding the adhesive layer's influence on the 0/0° interface. The findings indicate that, in both the initiation and propagation phases, the adhesive layer enhances fracture toughness.
In the subsequent stage of experiments, following same fabrication procedure but a mid-plane 0-90° interface was incorporated, with an adhesive layer inserted between the 0° and 90° layers. This part focused on the impact of the adhesive layer on the fracture mechanism between two laminates with a 90-degree orientation difference, employing the ASTM D5528 Mode-I test. Microscopic examination of crack propagation was conducted to elucidate the fracture mechanism during both the initiation and propagation phases. Due to the asymmetry in the lamination of samples, crack propagation predominantly avoids advancing in the 0° and adhesive layer interface, consistently choosing to progress in the non-reinforced 0-90° interface. Consequently, the effect of the adhesive layer in these experiments is distinctly observable only in the initiation phase, with no discernible influence during the propagation process.
In this study, the effect of an adhesive layer is investigated experimentally on fracture toughness of unidirectional Glass fiber/Epoxy composite in Mode II, shear mode, based on ASTM D7905 standard. The applied adhesive film has embedded knitted fibers which interact with the primary prepreg matrix and adhesive film epoxy itself during crack propagation, both aspects are studied. Glass Fiber/Epoxy prepreg and adhesive film are hand-laminated and subsequently processed in a hot press to produce unidirectional composite sheets. Samples are then cut using a waterjet based on standardized dimensions. The ASTM D7905 standard only considers the crack initiation in Mode II without any crack propagation. Hence, DIN EN 6034 standard procedure with 25 mm loading roller diameter is adopted for tested samples to create crack propagation for previously initiated cracks in samples. Force-head displacement curves are generated, and, notably, the induced crack propagation results in a sufficiently extended crack surface for scanning electron microscopy (SEM) analysis. Fracture mechanisms are discussed based on SEM imaging, with a specific focus on understanding the adhesive layer's influence on the 0/0° interface. The findings indicate that, in both the initiation and propagation phases, the adhesive layer enhances fracture toughness.
In the subsequent stage of experiments, following same fabrication procedure but a mid-plane 0-90° interface was incorporated, with an adhesive layer inserted between the 0° and 90° layers. This part focused on the impact of the adhesive layer on the fracture mechanism between two laminates with a 90-degree orientation difference, employing the ASTM D5528 Mode-I test. Microscopic examination of crack propagation was conducted to elucidate the fracture mechanism during both the initiation and propagation phases. Due to the asymmetry in the lamination of samples, crack propagation predominantly avoids advancing in the 0° and adhesive layer interface, consistently choosing to progress in the non-reinforced 0-90° interface. Consequently, the effect of the adhesive layer in these experiments is distinctly observable only in the initiation phase, with no discernible influence during the propagation process.