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    An experimental investigation of free vibration response of curved sandwich beam with face/core debond
    (Sage Publications Ltd, 2010) Baba, Buket Okutan; Thoppul, Srinivasa
    The aim of this research was to investigate the curvature and face/core debond influence on the vibration behavior of curved composite sandwich beams made up of carbon/epoxy laminate skins over a polyurethane foam core. Flat and curved sandwich beams with debond were prepared by keeping the arc length of the sandwich beams equal to the length of the flat sandwich beams. Sandwich specimens with three different curvatures and debond between the top and bottom face/core were manufactured. Systematic vibration and static experiments were carried to determine the response and to study the effect of curvature and debond. Flexural stiffness and strength of the sandwich beams were determined by four-point bending tests. The natural frequencies and damping loss factors of the sandwich beams were determined using impulse frequency response technique under free-free boundary conditions. Four-point bending test results indicate that stiffness of the beams decrease with presence of debond. Vibration test results show that face/core debond causes reduction of the natural frequencies whereas damping loss factor values increase with presence of debond. The effects of debond and curvature on damping loss factor is higher than that on natural frequency.
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    Buckling behavior of laminated composite plates
    (Sage Publications Ltd, 2007) Baba, Buket Okutan
    In this study, the influence of boundary conditions on the buckling load for rectangular plates of various cutout shape, length/thickness ratio, and ply orientation is examined. Boundary conditions consisting of clamped, pinned, and their combinations are considered. Numerical and experimental studies are conducted to investigate the effect of boundary conditions, cutout shape, length/thickness ratio, and ply orientation on the buckling behavior of E-glass/epoxy composite plates under in-plane compression load. Buckling analysis of the laminated composites is performed by using finite element analysis software ANSYS. Tests have been carried out on laminated composites with circular and semicircular cutouts under various boundary conditions. Comparisons are made between the test results and predictions based on finite element analysis. The results show a complex interaction between plate orthotropy and boundary conditions.
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    Buckling characteristics of symmetrically and antisymmetrically laminated composite plates with central cutout
    (Springer, 2007) Baba, Buket Okutan; Baltaci, Aysun
    A numerical and experimental study was carried out to determine the effects of anti-symmetric laminate configuration, cutout and length/thickness ratio on the buckling behavior of E/glass-epoxy composite plates. The buckling loads were presented for symmetrically and anti-symmetrically laminated plates subjected to axial compression load. The study included two different laminate configurations ([90/45/-45/0](as) and [90/45/-45/0](s)), two different cutout shapes (circular and semi-circular), two different length/thickness ratios (L/t=75 and 37.5) and three boundary conditions (clamped-clamped [CC], clamped-pinned [CP] and pinned-pinned [PP]). Firstly, the buckling loads of eight-ply E/glass-epoxy rectangular plates were determined experimentally. Then, the buckling loads of the laminated composites were calculated by ANSYS finite-element computer code. The changing in buckling load of the composites due to the presence of cutout and changing of length/thickness ratio was calculated. Finally, the experimental test results were compared to the buckling loads of plates obtained from the finite element analysis.
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    Buckling response of rectangular laminated composite plates with cutouts
    (Walter De Gruyter Gmbh, 2007) Baba, Buket Okutan
    Results from numerical and experimental study of the buckling response of compression-loaded E/glass-epoxy composite plates with centrally located circular and semicircular cutouts are presented. The effects of ply orientation angle and cutout shape on the buckling response of laminates subjected to uniaxial static loads are investigated. The laminated composites are analyzed using the finite element analysis software ANSYS. The change in critical buckling response of thin rectangular symmetric and antisymmetric laminates with respect to the ply orientation angle and cutout shape has been studied. In addition, results are presented from buckling analysis and these results are compared to experimentally measured results.
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    Experimental evaluation of the vibration behavior of flat and curved sandwich composite beams with face/core debond
    (Elsevier Sci Ltd, 2009) Baba, Buket Okutan; Thoppul, Srinivasa
    A systematic experimental study to determine the effect of curvatures and debond on the flexural stiffness and strength of composite sandwich beam structures made from fiber-glass/epoxy laminate skins and polyurethane foam core has been carried out. Flat and curved sandwich beams (with 15 degrees, 30 degrees, 45 degrees and 60 degrees open angle) with debond located at the top (upper) and bottom (lower) face/core interface were manufactured. Flexural stiffness and strength were determined from impulse frequency response technique and four-point bending. Vibration test results indicate that the natural frequencies and apparent stiffness are less affected by the presence of curvature and debond, whereas, the loss factors are significantly affected. Four-point test results indicate a reduction in stiffness due the presence of debonds and curvature, but more predominantly, the results show that the peak load reduces sharply with the presence of upper debond. (C) 2009 Elsevier Ltd. All rights reserved.
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    Free Vibration Analysis of Curved Sandwich Beams with Face/Core Debond Using Theory and Experiment
    (Taylor & Francis Inc, 2012) Baba, Buket Okutan
    This work deals with vibration behavior of sandwich beams made of fiber-glass laminate skins wrapped over polyurethane foam core. Experimental and numerical investigations on the natural frequencies of both flat and curved beams with and without debond were carried out. The vibration tests were performed for clamped-clamped end conditions using an impulse-frequency response method. Finite element analysis was carried out using ABAQUS. Results indicate that fundamental natural frequencies are seriously affected by curvature angle and debond, whereas the higher order natural frequencies show relatively small changes. Face/core debond causes reduction of the natural frequencies.
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    The vibration response of composite sandwich beam with delamination
    (Adcotec Ltd, 2007) Baba, Buket Okutan; Gibson, Ronald F.
    The aim of this study is to report the effect of delamination on the vibration characteristics of composite sandwich beams. The natural frequencies and corresponding vibration modes of a free-free sandwich beam with delamination of various sizes and locations are predicted using a two-dimensional finite element analysis (FEA). The presence of delamination affects the stiffness of the delaminated beam and results in differences on the natural frequencies of the beam. Assessment of the differences light the way for the existence, size and location of the delaminated region and can be used for a non-destructive evaluation of the damage characteristics of the delaminated beams. Vibration tests are conducted on fully bonded sandwich beams with carbon/epoxy laminated composite faces and foam core to verify the finite element results. Agreement between predictions of the model and experimental observations is good.