MECHANICAL PROPERTIES OF REPAIRED CARBON FIBER REINFORCED POLYMER COMPOSITES

2021-12-10
Sonat, Emine Evren
Carbon fiber reinforced polymer (CFRP) composites are increasingly used in the aerospace industry due to their high specific strength compared to conventional metallic materials. However, a significant shortcoming of these composites is their increased susceptibility to damage. Structural repair is a common method to restore the load-carrying capacity of a damaged part when the damage size exceeds the pre-defined tolerances. Scarf and stepped bonded repair methods are the primary choice for cases that require high strength recovery and aerodynamic smoothness. So, there is an increasing need to understand the mechanical performance of the repaired zone. This thesis study investigated the mechanical performance of flush repairs, emphasizing industrial practices and applications. The experimental part of the study quantified the effect of scarf angle on the strength of 2D joints and also determined the strength recovery rates for prepreg and wet lay-up repairs. In addition, the temperature and moisture effect on the repairs have been investigated through the testing of conditioned specimens. The results show that a small scarf angle has a strong positive influence on the strength of the repaired zone. When the scarf angle is 1.9, the recovery rate reaches 88.6 % of the intact specimens. In addition, temperature and moisture can adversely affect the strength of the repair, especially in the case of a wet lay-up. An evaluation of the porosity content in the repair area through microscopy and ultrasonic inspection has been performed. Numerical modeling of the 2D scarf joints and 3D repairs through finite element modeling complemented the experimental results and provided further insight into the failure modes. The model employed the built-in Cohesive Zone Method (CZM) of ABAQUS, and the Hashin failure theorem was implemented through a specially developed script. Fracture toughness tests performed on the adhesives provided accurate modeling of the adhesive response under loading. The simulation results closely predict the experimental behavior both in terms of failure strength and failure modes. Then the verified model was used to predict the effect of porosity on the joint strength of 2D scarf joints as well as the impact of strength and toughness of the adhesive on the repair strength. The thesis developed a systematic approach to the experimental and numerical analysis of composite repairs. The results provided a deeper understanding of repair zones’ mechanical behavior and offered a framework for future study of different CFRP composites and their structure-property relationships.

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Citation Formats
E. E. Sonat, “MECHANICAL PROPERTIES OF REPAIRED CARBON FIBER REINFORCED POLYMER COMPOSITES,” Ph.D. - Doctoral Program, Middle East Technical University, 2021.