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.


Experimental determination of tool wear in routing and trimming of CFRP composites
Peker, Zühal; Akkök, Metin; Kılıç, S. Engin; Department of Mechanical Engineering (2015)
In order to fulfill growing demand for lighter aircraft, Carbon fiber reinforced polymers (CFRP) have been developed, which differ from metals with regards to their machinability characteristics. The main problem in machining process is the damage caused by delamination of CFRP materials which need to be both cut and shorn of fibers at the same time, since they are remarkably abrasive in nature. This research aims at accessing the most appropriate cutting condition with less tool wear in routing and trimmin...
Mechanical properties of epoxy matrix composite reinforced with multi-walled carbon nanotubes
Yüceer, Kevser.; Çöker, Demirkan; Department of Aerospace Engineering (2019)
Improving usage preference of composite material in the aerospace industry brings the requirement of improving mechanical properties of the material. In this thesis, mechanical improvement of epoxy composite materials is analyzed with contribution of functionalized multi-walled CNT with carboxyl group (-COOH) and non-functionalized MWCNT with epoxy for CNT weight fractions of 0.8, 1.0, 1.2, 1.5 and 2.0 wt%. The nanomaterial is dispersed in epoxy resin by calendering mixing method. Functionalization of CNT p...
Mechanical, electrical and thermal properties of carbon fiber reinforced poly(dimethylsiloxane)/polypyrrole composites
Cakmak, G; Kucukyavuz, Z; Kucukyavuz, S; Cakmak, H (2004-01-01)
Conductive and flexible carbon fiber (CF) reinforced polydimethylsiloxane (PDMS)/polypyrrole (PPy) composites were synthesized electrochemically. Electrochemical synthesis was performed at + 1.1 V using p-toluenesulfonic acid as supporting electrolyte and water as solvent. Composites were characterized by thermal gravimetric analysis, scanning electron microscopy (SEM), conductivity measurements and mechanical tests. Conductivities of composites were observed in the range of 2.2-4 S/cm. SEM studies show tha...
Meso-scale finite element modelling of carbon nanotube reinforced polymer composites
Haydar, Altay; Esat, Volkan; Mechanical Engineering (2021-12)
Carbon nanotube (CNT) reinforced polymer composites (CNTRPs) are promising materials which can be utilized in a variety of industries. Several experimental research studies have been conducted to determine the mechanical properties of CNTRPs, however results have not been conclusive. In this study, meso-scale representative volume elements (RVEs) of straight and coiled CNT (CCNT) reinforced epoxy composites were analysed by using commercial finite element analysis software MSC Marc-Mentat. CNTs were randoml...
Experimental investigation of the effect of CNT addition on the strength of CFRP curved composite beams
Arca, M.A.; Uyar, I.; Çöker, Demirkan (2015-01-01)
Carbon nanotubes (CNT) have been attracting attention as a toughening material in composite matrix due to their excellent mechanical properties. However, superior properties of CNTs have not yet been realized in the strengthening of composites against fracture. This study focuses on investigating the effect of CNT variation in the epoxy resin on the strength of curved composite beams. Specimens are [0/90] fabric carbon/epoxy composite laminates manufactured by hand layup technique 3 % wt CNT fractions in th...
Citation Formats
E. E. Sonat, “MECHANICAL PROPERTIES OF REPAIRED CARBON FIBER REINFORCED POLYMER COMPOSITES,” Ph.D. - Doctoral Program, Middle East Technical University, 2021.