Helicopter Group

Ankara, Turkey

Helicopter Group

Ankara, Turkey

Time filter

Source Type

Gozluklu B.,Helicopter Group | Coker D.,METU - MEMS Center
Journal of Physics: Conference Series | Year: 2014

In the wind energy industry, new advances in composite manufacturing technology and high demand for lightweight structures are fostering the use of composite laminates in a wide variety of shapes as primary load carrying elements. However, once a moderately thick laminate takes highly curved shape, such as an L-shape, Interlaminar Normal Stresses (ILNS) are induced together with typical Interlaminar Shear Stresses (ILSS) on the interfaces between the laminas. The development of ILNS promotes mode-I type of delamination propagation in the curved part of the L-shaped structure, which is a problem that has recently raised to the forefront in in-service new composite wind turbines. Delamination propagation in L-shaped laminates can be highly dynamic even though the loading is quasistatic. An experimental study to investigate dynamic delamination under quasi-static loading is carried out using a million fps high speed camera. Simulations of the experiments are conducted with a bilinear cohesive zone model implemented in user subroutine of the commercial FEA code ABAQUS/explicit. The experiments were conducted on a 12-layered woven L-shaped CFRP laminates subjected to shear loading perpendicular to the arm of the specimen with a free-sliding fixture to match the boundary conditions used in the FEA. A single delamination is found to initiate at the 5th interface during a single drop in the load. The delamination is then observed to propagate to the arms at intersonic speed of 2200m/s. The results obtained using cohesive zone models in the numerical simulations were found to be in good agreement with experimental results in terms of load displacement behavior and delamination history. © Published under licence by IOP Publishing Ltd.


Gozluklu B.,Helicopter Group | Gozluklu B.,Middle East Technical University | Uyar I.,Middle East Technical University | Coker D.,Middle East Technical University
11th World Congress on Computational Mechanics, WCCM 2014, 5th European Conference on Computational Mechanics, ECCM 2014 and 6th European Conference on Computational Fluid Dynamics, ECFD 2014 | Year: 2014

In wind energy and aerospace industries, new advances in composite manufacturing technology enable to produce primary load carrying elements as composite materials in wide variety of shapes large such as an L-shape. However, due to the geometry, Interlaminar Normal Stresses (ILNS) are induced once a moderately thick laminate takes highly curved shape. In the curved part of the L-shaped structure, the development of ILNS promotes mode-I type of delamination propagation which is the weakest fracture mode. This is a problem that has recently risen to the forefront in in-service new composite civil aircrafts. This study focuses on experimental and computational investigation of dynamic delamination in a 12-layered woven L-shaped CFRP laminates subjected to quasi-static shear loading. Delamination initiation and propagation processes were captured with a million fps high speed camera. A single delamination is found to initiate in the curved region at the 5th interface during a single drop in the load. The delamination is then observed to propagate at intersonic speed of 2200m/s. The experiments are simulated using cohesive elements by implementing bilinear cohesive model into ABAQUS/Explicit. The experiments and computations are found to be in good agreement, at the macroscale in terms of load-displacement behavior and the failure load, and at the mesoscale in terms of the location of delamination nucleation and delamination crack tip speeds. Shear Mach waves emanating from the crack tips are also observed in the simulations during intersonic crack propagation.


Uyar I.,Middle East Technical University | Arca M.A.,Middle East Technical University | Gozluklu B.,Middle East Technical University | Gozluklu B.,Helicopter Group | Coker D.,Middle East Technical University
Conference Proceedings of the Society for Experimental Mechanics Series | Year: 2015

Curved composite parts are increasingly replacing metal ribs and box structures in recent civil aerospace structures and wind turbine blades. Delamination of L-shaped composite laminates occurs by interlaminar opening stresses in addition to the interlaminar shear stresses at the curved region. An experimental setup is designed to investigate dynamic delamination in L-shaped composite brackets under quasi static shear loading. The materials are unidirectional [0]17and cross-ply [0/90]17 epoxy/graphite composite laminates. The load displacement curves are recorded and subsequent dynamic delamination is captured with a million fps high speed camera. The failed specimens are analyzed under a microscope. It is seen that layup differences change the failure mechanism in composites. Multiple delaminations in one load drop are observed in failure of unidirectional laminate whereas sequential delamination at each discrete load drop is seen in cross ply laminates. In the [0] laminate single delamination in the center ply followed by symmetric delamination nucleations around the two crack tips are observed. In the 0/90 cross-ply laminate, multiple load drops are recorded and delaminations start near the inner radius by peeling of 0/90 plies sequentially at each load drop. In both layups, first time observation of intersonic delamination speeds up to 2,200 m/s are made. © The Society for Experimental Mechanics, Inc. 2015.


Gozluklu B.,Helicopter Group | Gozluklu B.,Middle East Technical University | Uyar I.,Middle East Technical University | Coker D.,Middle East Technical University
Mechanics of Materials | Year: 2015

Dynamic delamination in curved composite laminates is investigated experimentally and numerically. The laminate is 12-ply graphite/epoxy woven fabric L-shaped laminate subject to quasi-static loading perpendicular to one arm. Delamination initiation and propagation are observed using high speed camera and load-displacement data is recorded. The quasi-static shear loading initiates delamination at the curved region which propagates faster than the shear wave speed of the material, leading to intersonic delamination in the arms. In the numerical part, the experiments are simulated with finite element analysis and a bilinear cohesive zone model. Cohesive interface elements are used between all plies with the interface properties obtained from tests. The simulations predict a single delamination initiating at the corner under pure mode-I stress field propagating to the arms under pure mode-II stress field. The crack tip speeds transition from sub-Rayleigh to intersonic in conjunction with mode change. In addition to intersonic mode-II delamination, shear Mach waves emanating from the crack tips in the arms are observed. The simulations and experiments are found to be in good agreement at the macro-scale, in terms of load-displacement behavior and failure load, and at the meso-scale, in terms of delamination initiation location and crack propagation speeds. Finally, a mode dependent crack tip definition is proposed and observation of vibrationsduringdelaminationispresented. This paper presents the first conclusive evidence of intersonic delamination in composite laminates triggered under quasi-static loading. © 2014 Elsevier Ltd. All rights reserved.


Gunduz A.,Helicopter Group | Saribay Z.B.,Helicopter Group | Yilmaz S.,Helicopter Group | Kaynar E.,Helicopter Group
Annual Forum Proceedings - AHS International | Year: 2015

In this work, a comprehensive analytical model is established to study the performance of the bearings of a helicopter high speed input gear shaft. The high speed shaft, rotating above 15,000 rpm, transmits power to the second stage through a bevel gear that is straddle mounted by a duplex ball bearing and a cylindrical roller bearing. The effects of centrifugal forces, the change of bearing internal clearances due to interference fits and uneven thermal expansions are considered in the outlined approach. Then the influence of temperature elevations (that may arise due to inadequate cooling) on the performance of high speed bearings is explored by employing the proposed model. This study points out that bearing internal clearances are very critical in assessing the performance of the high speed system; and optimal manufactured bearing clearances are greatly dependent on the operating conditions. Thus, a bearing internal clearance optimization study is carried out considering different operating temperatures. This optimization study provides "operation condition maps" for high speed bearings and guides the designer for the optimal selections of bearing clearances by considering different operating conditions. © 2015 by the American Helicopter Society International, Inc.


Isci H.,Helicopter Group | Donmez Y.,Helicopter Group | Mersin K.,Helicopter Group
Annual Forum Proceedings - AHS International | Year: 2013

The main objective of "low loss" gears is to reduce friction between contacting teeth during the meshing cycle and there are two paths to attain such objective: modify the tooth geometry in order to reduce the specific sliding, select a oil that generates lower friction, or a combination of both strategies. There are also load-independent (spin) power losses of gear pairs due to oil churning and windage. The oil churning power loss model is intended to simulate spin losses in dip - lubricated conditions. It is difficult to isolate and model load dependent and independent power losses; further to that, differentiation of load dependent power losses require reliable models. In order to reach optimum gearbox geometry and gear properties, power losses need to be modeled and validated through tests. To model gear mesh losses tooth contact analysis and mixed elastohydrodynamic lubrication (EHL) software were developed. For load independent and bearing mechanical loss calculations, available standards were used and adapted for spiral bevel gears. In this study, 300 HP/1600 rpm 1:2 ratio Tail Gear Box (TGB) was designed and manufactured to include certain flows in geometry. Through tests, different load and lubrication conditions are tested and related models will be correlated. With this study, the effects of the load dependent and the load independent power losses to total power loss of the system can be isolated. Thus at the end, spiral bevel gears are better understood and important aspects of the power losses are investigated. Gear sets can be optimized for better efficiency with the same load carrying capacity and fatigue life. Furthermore an in-house tool is developed to calculate dynamic transmission error, which will be verified and correlated with the test results. Copyright© (2013) by the American Helicopter Society International.

Loading Helicopter Group collaborators
Loading Helicopter Group collaborators