Browsing by Author "Sahin, Resul"

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The Effect of Different Chemical Surface-Modified Carbon Fiber Structure and Ratio on Bond Strength in Adhesive Joints
(Elsevier Sci Ltd, 2025) Akpinar, Salih; Sahin, Resul; Gurses, Ahmet; Akpinar, Iclal Avinc
The adhesive bonding techniques are widely used for assembling composite structures with complex geometries due to their several advantages compared to conventional bonding techniques. The present study is concerned with improving the interfacial interactions between fiber and epoxy matrix by reinforcing the adhesive with fiber structures and treating these fiber structures with different chemical surface modifications to improve the performance of the bonded joints. In this study, two different adhesives (rigid and flexible) were reinforced with carbon fibers at 0.5 %, 1 %, 2 %, and 3 % by weight, and the performance of single-lap joints obtained with these adhesives was investigated. Furthermore, a variety of surface modifications were applied to the carbon fibers, including acetone (C3H6O), sulfuric acid (H2SO4), nitric acid (HNO3), hydrochloric acid (HCl), and a mixture of nitric acid and hydrochloric acid (HNO3/HCl). Consequently, the direct incorporation of acetone-treated fibers into the epoxy matrix significantly increased the standard deviation in bonding tests, with values ranging between approximately 12 % and 21 %. However, when acetone was used to reduce the epoxy matrix viscosity before adding the fibers, the standard deviation among test samples decreased to as low as 1.5 %. Moreover, the addition of chemically modified fibers to the adhesive improved the performance of the joints by approximately 4 %-126 %, depending on the type of chemical surface modifier and the type of adhesive used. Based on the bonding test results, the optimal fiber content for the adhesive was determined to be 1 %. To further interpret these findings, scanning electron microscopy (SEM), Fourier-Transform Infrared Spectroscopy (FTIR), and fracture surface analysis were conducted.
The Effects of Adherend Thickness on the Fatigue Strength of Adhesively Bonded Single-Lap Joints
(Elsevier Sci Ltd, 2021) Sahin, Resul; Akpinar, Salih
Different adhesive bonding methods have been prominently used in aerospace applications since the 1980s because adhesively bonded joints are regarded as an alternative to traditional bonding methods. However, the effects of fatigue on adhesively bonded joints - one of the most significant issues in the aerospace industry - are still currently being investigated. This study experimentally investigated the lifetimes of single-lap joints (SLJs) of five different adherend thicknesses (2, 3, 4, 5 and 6 mm) - obtained using AA2024-T3 aluminum alloy and DP460 structural adhesive - under varying tensile fatigue loads. Accordingly, before performing fatigue tests, static tensile strength tests were carried out on the SLJs to determine the maximum loads to be used for tensile fatigue tests and also to evaluate their quasi-static performances. The fatigue tests were performed at a loading ratio (R) of 0.1 and a frequency of 20 Hz. The static tensile strength of the joints was found to increase by approximately 22% with increasing adherend thickness. However, this increase was not the same as the increase in adherend thickness. In addition, increased adherend thickness was observed to change the maximum load applied to the joints over infinite cycles (taken to be 10(6) cycles). This was because the change in adherend thickness changed the bending moment formed in the joint and the flexural rigidity of the adherend.
The Fracture Load Analysis of Different Material Thickness in Adhesively Bonded Joints Subjected to Fully Reversed Bending Fatigue Load
(Elsevier, 2021) Akpinar, Salih; Sahin, Resul
The adhesive joints used in airframe and wing sections of aircrafts are generally subject to dynamic (fatigue) loading rather than static loading due to environmental conditions. The dynamic loading formed on wing sections of aircraft is generally in the form of fully reversed fatigue. This study experimentally and numerically investigated the lifetimes of single-lap joints (SLJs) of five different adherend thicknesses (2, 3, 4, 5 and 6 mm) - obtained using AA2024-T3 aluminum alloy and Araldite-2015 structural adhesive - under fully reversed bending fatigue loads. Accordingly, before performing fatigue tests, static bending strength tests were carried out on the SLJs to determine the maximum loads to be used for bending fatigue tests and also to evaluate their static performances. Fatigue tests were performed at a loading ratio (R) of -1 and a frequency of 4 Hz. It was determined that the static bending strength of the joints increased by approximately between 43% and 74% with increasing adherend thickness. However, this increase was not the same as the increase in material thickness. In addition, the increased adherend material thickness changed (decreased or increased) the maximum load applied to the joints over infinite cycles (taken to be 2 x 105 cycles). This was because the change in material thickness changed the bending moment formed in the joint and the flexural rigidity of the material.
Investigation of the Effects of Incorporating Modified Carbon Fibers with Different Chemical Surface Treatments and Cationic Surfactant Adsorption on Adhesive Joints
(Elsevier Sci Ltd, 2026) Akpinar, Iclal Avinc; Sahin, Resul; Gurses, Ahmet; Akpinar, Salih
Adhesively bonded joints play a critical role in joining different types of materials and forming structural elements with complex geometries. This study investigates the chemical surface treatments applied to carbon fibers added to adhesives, along with the adsorption of cationic surfactant, cetyltrimethylammonium bromide (CTAB), at various concentrations. In the study, carbon fibers were first cleaned with acetone (C3H6O) and subjected to surface modifications using sulfuric acid (H2SO4), nitric acid (HNO3), hydrochloric acid (HCl), and a mixture of nitric and hydrochloric acids (HNO3/HCl) before being added to the adhesive at a weight fraction of 1 %. Subsequently, these chemically treated fibers underwent CTAB adsorption at concentrations of 15, 30, 60, and 90 mg/L single-lap joints were then produced using the adhesive containing these carbon fibers, and the joints were subjected to bending and impact tests to evaluate their performance. The results revealed that the application of CTAB adsorption at varying concentrations to the carbon fibers added to the adhesive enhanced the performance of the joints under bending loads by approximately 23 %-30 %. Moreover, it was determined that applying CTAB adsorption to fibers that had undergone cleaning with acetone and chemical surface treatments using sulfuric acid, nitric acid, hydrochloric acid, and a nitric/hydrochloric acid mixture further improved joint performance by up to 70 %. The findings from the bending tests also indicated an increase in fracture energy ranging from approximately 10 %-48 %. According to the joint test results, CTAB adsorption applied at concentrations of 30 mg/L and 60 mg/L produced the most optimal performance improvements. The changes in surface functional groups and surface morphology of the samples were examined through FTIR spectra and SEM images.
An Investigation of the Fatigue Performance of Adhesively Bonded Step-Lap Joints: An Experimental and Numerical Analysis
(Elsevier Sci Ltd, 2021) Gavgali, Esma; Sahin, Resul; Akpinar, Salih
Adhesively bonded joints are used in many engineering applications and are manufactured by joining various materials together. Static and dynamic analysis of adhesively bonded joints is very important in both scientific and sector studies on these joints. In this study, the static and fatigue strengths of single-lap joints (SLJs) and three-step-lap joints (TSLJs) that were subjected to tensile and four-point bending tests were examined experimentally and numerically. Adhesively bonded joints were produced using DP460 toughened type adhesive and AA2024-T3 aluminum alloy was used as the adherent. Fatigue tests were performed at a loading ratio (R) of 0.1 and a frequency of 10 Hz. As a result, applying a step-lap to the overlap area of adhesively bonded joints considerably increased the tensile static and fatigue strength of the joint. For the single-lap joint, both the static tensile and the tensile fatigue lifetime limit value of the three-step-lap joint increased approximately 100%. One of the reasons for that increase was that the moment effect formed in single lap joints due to the eccentric loading is minimized and the other is that a step-lap applied to the overlap area retards the formation of damage. However, static bending and fatigue strength values of TSLJs significantly decreased compared to those of SLJs. In addition, the results of the experimental and numerical analyses were found to be highly compatible with each other.