Yeliz Pekbey

A Numerical and Experimental Investigation of Critical Buckling Load of Rectangular Laminated Composite Plates with Strip Delamination

In the present study, experimental measurements and numerical solutions on the buckling of single-delaminated glass-fiber composite laminates are carried out on rectangular plates. During the fabrication process, rectangular teflon films of 13 mm thickness are introduced between plies of different orientation in order to form a macro defect. In addition, the variation in structural configurations, such as ply stacking sequence, width of the delamination, and specimen geometry (width to unsupported length), are considered. In all cases, the delamination is centrally placed through-the-thickness of the laminate. Compression tests are carried out on EP GC 203 glass/epoxy woven composites with built-in single embedded delamination in order to evaluate the critical buckling load. Finite element modeling is used to gain further understanding of the critical buckling load. ANSYS (version 6.1) are used to analyze the critical buckling load of various laminated plates in order to see how changes in the rectangular composite laminated plates would affect the buckling load. A good agreement between finite element predictions and experimental measurements are found for the delamination geometries that were tested.

Buckling Optimization of Composite Columns with Variable Thickness

A novel approach to the optimization of flexible columns against buckling is presented. The problem of determining the shape of the column that has possibly the largest critical buckling load of columns of a given length and volume is solved for composite materials. The objective of this study was to develop and design an optimized composite column against buckling. Determining what shape of column has the largest possible buckling load of composite column of a given length and volume was considered. Clamped—clamped supported column is an important limit case because it caused debate in many publications. Moreover, in this study, the optimization problem of the clamped—clamped column under buckling load, which was previously dealt with by Tadjbakhsh and Keller [5], Olhoff and Rasmussen [6] and Masur [7] is reinvestigated. It is also proved that the solutions of Tadjbakhsh and Keller, Olhoff and Rasmussen and Masur are not optimum for columns with clamped ends. The present model formulation considers columns for which crush is taken into account in the formulation of the column optimization problem, allowing for bimodal optimum solution. This leads to the necessity for both stability and crush criterion formulation of the optimization problem. The new proposed optimum model solution has been verified with numerical analysis using ANSYS and experimental data for columns with clamped—clamped ends. It was shown that the new proposed model was given the optimum solution for the clamped—clamped case. Detailed results are presented and discussed for clamped—clamped columns having circular cross-sectional configuration. As a result of this study, it was shown that results obtained in previous studies of variation optimum cross-sectional area for columns under compressive forces clamped—clamped ends were erroneous. The corrected optimum form was obtained and results checked by numerical calculations and experimental tests of composite columns.

Experimental analysis of single-lap composite joints with two different adhesives at various conditions

Composite materials have been used in the transport industry in the recent years. The assembly of composite materials has been performed with either adhesive joints or other mechanical fasteners. Adhesive joints are more preferred because of their easy usages, lightness and the stress concentration, which is very important in the design, occurs less than the other type joints. In this study, load-carrying capacity of single-lap joints bonded with different adhesives was found, experimentally. Woven glass fibre–epoxy composites manufactured by using vacuum assisted resin infusion method (VARIM) were used as adherends. FM73 epoxy film adhesive and DP-460 paste adhesive were used as adhesive materials. Several loading conditions, 5, 10 and 15 J, that possibly happen in the operation life were considered. The tests were carried out room temperature (25℃), 50℃ and 80℃. From the results, it is observed that the load-carrying capacity of single-lap joints significantly varies with adhesive types and temperature.

Progressive Failure Analysis of Reinforced-Adhesively Single-Lap Joint

The failure behavior of reinforced-adhesively single-lap joints was investigated experimentally and numerically. The reinforced adhesive was produced by mixing waste composite particles and an epoxy-based commercial adhesive. The single-lap joint was prepared with an adhesive and unidirectional fiber glass/epoxy composite plates with a (0°/90°)3 stacking sequence. Three types of adhesive were used: an un-reinforced adhesive (ADH), an adhesive mixed with glass fiber-reinforced epoxy resin composite plate particles (GFRC), and an adhesive mixed with carbon fiber-reinforced epoxy resin composite plate particles (CFRC). The adhesive thickness (ta) and overlap length (lap) were 0.4, 0.8, 1.2, and 1.6 mm and 10, 20, 30, and 40 mm, respectively. Progressive failure analysis was performed with the ANSYS™ 11.0 finite element program using ANSYS™ parametric design language (APDL) code. In the numerical study, the failure loads of the composite and the adhesive were determined with the Hashin failure criteria and the Tresca failure criteria, respectively. The difference between the experimental and numerical studies ranged from 2% to 10%. The failure load of reinforced-adhesively single-lap joints was 1.3–22.8% higher than that of the un-reinforced adhesive.

Elastic-Plastic Stress Analysis of a Thermoplastic Composite Disc under Linear Temperature Distribution

In the present study, an elastic-plastic stress analysis is carried out on a composite thermoplastic disc reinforced by steel fibers, curvilinearly. Radial and tangential stresses are obtained under a liner temperature distribution. The magnitude of the tangential stress component for elastic and elastic-plastic cases is higher than that of the radial stress component. The tangential stress component is compressive and tensile on the inner and outer surfaces, respectively and is the highest on the inner surface. The elastic-plastic solution is performed for the plastic region expanded around the inner surface by an analytical formulation and a numerical solution. The solution is also carried out by the finite element method (ANSYS solution). These two solutions give very similar results. The intensities of the residual stress component of the tangential stress and plastic flow are the highest at the inner surface.

Numerical elastoplastic analysis of the shear stress distribution in the adhesive layer for single-lap joints

Abstract The purpose of this paper was to investigate the stress distribution of adhesive single-lap joint loaded in the elastic and elastoplastic range. The stress variation across the adhesive length was achieved both analytically and numerically. A two-dimensional finite element solution presented a detailed picture of the stress behavior in a single-lap joint loaded in the elastic and elastoplastic range. After the finite element results matched very well with results obtained from analytic solution in the elastic range, it was decided that adhesive plasticity needed to be added to the finite element models in order to make them as accurate as possible. The residual stresses after elastic-plastic loading were calculated. The results obtained from numerical analysis showed that the residual stress was important as the joint strength can be increased by residual stresses. In addition, the effect of the adhesive length, thickness of adhesive, and thickness of adherends on the shear distribution of the adhesive was examined numerically by using elastoplastic finite element method. The target was to change the geometry of the joint and study its effects on the stress distribution in the adhesive.

Flexural-torsional buckling of FRP thin-walled composite with various sections

Abstract The flexural-torsional buckling of thin-walled pultruded fiber-reinforced plastic (FRP) members composed of unstiffened, stiffened cruciform- and I-shaped sections under uniform compressive loads was investigated using finite element methods (FEM). As the basic method, an eigenvalue solution using the minimum potential energy method was utilized to obtain the critical buckling stress and buckling mode shapes. FEM results were compared with the closed-form solutions and literature results. Furthermore, a parametric study was carried out to investigate the different cross-section geometries and span lengths on the critical buckling stresses and buckling mode shapes, that is, flexural, torsional, or mixed buckling.

Enhancement of flexural performance of wood beams using textile fabrics

Abstract The aim of this study was to investigate the flexural performance of wood beams (beech — Fagus orientalis Lipsky) reinforced with woven and selected weft knitted glass fabrics, namely, Milano and plain knit. Some physical and mechanical properties of the beech wood and the textile fabrics were determined in accordance with relevant ASTM standards. Twenty-four wood beams which have two different cross sections (I-shaped and square hollow) were manufactured and tested under a three-point load. They were divided into two groups: Group A specimens were not reinforced to serve as a reference, whereas Group B specimens were reinforced with textile fabrics combined with adhesion. The flexural behavior of the specimens was studied through their load-deflection characteristics. The modes of failure were identified and categorized. The experimental results showed that the load-bearing capacity of reinforced beams increased significantly compared to the beam without reinforcement. This method can be used to repair and strengthen damaged wood beams.

Karbon Nano-Tüp Katkılı Düz Örgü Cam-Epoksi Kompozitlerin Düşük Hızlı Darbe Deneylerinin Deneysel Olarak İncelenmesi

Bu calismada, Cesitli oranlarda karbon nanotup katkili duz orgu cam epoksi tabakali kompozitlerin Fractovis Plus darbe cihazi ile dusuk hizli darbeye gosterdikleri tepki arastirilmistir. Nanokompozit plaklar el yatirma yontemi ile uretilmistir. Epoksi recine icerisine katilan karbon nanotup etkisi incelenmistir. Agirlikca %1 ve %1,25 oranlari kullanilarak nanokarbon iceriginin etkisi darbe uzerindeki etkisi elde edilmistir. Nanokompozit plaklarin dusuk hizli darbe tepkileri karsilastirildi ve darbe deneyi sonrasi hasar alani uzerine CNT etkisi incelenmistir. Eklenen dolgu maddeleri duz orgu cam/epoksi kompozitlerin dusuk hizli darbe tepkilerini degistirdi. Sonuclar cesitli oranlarda CNT katkili olarak uretilen numunelerin katkisiz numuneye gore hem daha buyuk hasar alanina hem de daha buyuk penetrasyon esik enerjisine sahip oldugunu gostermistir. Ayrica, agirlikca %1 CNT oranli plaklar en yuksek reaksiyon kuvvetine sahiptir.

Determination of the critical load and energy release rate in mode II delamination using a meshfree method

Abstract Simulation of fracture by using numerical methods is important to treat geometries that change in time. In this study, both numerical and experimental investigations are presented for the delamination under mode II loading, detailing the derivation of the formulations in numerical simulations of fracture. The simulation of the delamination under mode II loading based on the cohesive segments model was investigated by using a meshfree method. Then, an experimental investigation was used to verify the meshfree method’s results. For tests under mode II loading, three-point end-notched flexure specimens, which are made of carbon/epoxy laminate (AS4/3501-6) which consists of 10 plies in [0]10 and [0/90/0/90/0]s lay-up with delamination inserted in the middle of the laminate, were used for the interlaminar fracture toughness tests. The problem was solved for [0]10, [0/45/-45/90/0]s, [0/90/0/90/0]s, [0/90/0/90/30]s, [0/90/0/90/45]s and [0/90/0/90/60]s laminates with mid-plane delaminations, and the results were verified for different composite materials. The critical fracture force, which can be experimentally measured, was used to calculate the mode II delamination fracture toughness of the carbon/epoxy laminate. In addition, values of the integral for 209 (11×19) and 253 (11×23) background meshes with equivalent interval sizes were compared. For a relatively fine background mesh, the critical load was converged. Results obtained from the meshfree element-free Galerkin method showed very good agreement with experimental data for single-mode delamination under mode II loading. The results presented will help in the implementation of mesh design techniques that protect numerical accuracy while minimizing computational expense.