| Περίληψη: | During normal operation, structures rarely experience static loads. Structures spend the majority of their lifespan submitted to dynamic loads which lead to fatigue. According to ASTM “Fatigue is the progressive, localized, permanent structural change that occurs in materials, subjected to fluctuating stresses and strains that may result in cracks or fracture after a sufficient number of fluctuations”. The structural change mentioned refers to microscopic cracks which gradually propagate and combine to form larger cracks that lead to fatal failure of the material. These cracks can develop even under loads lower than the yield stress of the material. In composite materials, these cracks usually appear in the bondline of co-consolidated or adhesively bonded materials or interlaminarly between adjacent plies. In order to improve the behavior of composite materials under such conditions and lengthen their joints’ lifespan, a variety of Disbond Arrest Features (DAF) are employed in critical areas.
In the current diploma thesis, the optimization of the design of disbond arrest features of two adherends was investigated using finite element analysis. The adherends were made of TORAY Cetex TC1225 which is a thermoplastic material fabricated with PAEK resin reinforced with carbon fibers in pre-impregnated fabric form. The disbond arrest feature which was tested derived from the Refill Friction Stir Spot Welding (RFSSW) process. Firstly, a comparison of the RFSSW method and other commonly used joining methods such as the use of adhesives or bolts on SLS specimens was presented followed by an optimization of three parameters concerning the placement and size of the RFSSW. Crack initiation and propagation were simulated using the Cohesive Zone Model (CZM) method in numerical models for two different types of specimens. The first was End Notched Flexure (ENF) specimens and the second was Cracked Lap Shear (CLS) specimens, for which two different diameters of sleeves (6 mm and 9 mm), three different plunge depths (70%, 75% and 80% of the co-consolidated plates’ depth), as well as three distances from the beginning of the bonded area (10 mm, 20 mm and 30 mm) for the CLS specimens, were examined. From these analyses, Force-Displacement, Failed Surface Area–Displacement diagrams as well as shear stress and effective plastic deformation contour plots were exported.
It became apparent from the collected results that these features contributed to the reduction of the disbonding rate while increasing the static load-bearing capacity of the adherent system.
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