Researcher:
Yenilmez, Bekir

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PhD Student

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Bekir

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Yenilmez

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Yenilmez, Bekir

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2009 - 200932010 - 20175

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Now showing 1 - 8 of 8
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    Minimizing thickness variation in the vacuum infusion (VI) process
    (Adcotec Ltd., 2011) N/A; Department of Mechanical Engineering; N/A; N/A; Yenilmez, Bekir; Sözer, Murat; Akyol, Talha; Çağlar, Barış; PhD Student; Faculty Member; Master Student; PhD Student; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; N/A; 110357; N/A; N/A
    In the Vacuum Infusion (VI) process, the thickness of a composite part changes as the compaction pressure on the vacuum bag and reinforcing fibre preform changes. Pressure and thickness were monitored along a 1D resin fl ow using pressure transducers and non-contact laser displacement sensors. To decrease the thickness variation, control actions were taken by adjusting the injection conditions, such as opening/closing gates/vents, changing pressure of them in the post-mold filling stage and bleeding out the excess resin. The control actions were taken based on an available compaction/decompaction database for the fabric type used. Compared to the case study with no control action other than bleeding, a better job was done in the controlled case study by decreasing the maximum thickness variation from 5.44% to 0.39%. A coupled fl ow and compaction model qualitatively verified the pressure and thickness distributions for both filling and post-filling stages.
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    Pressure-controlled compaction characterization of fiber preforms suitable for viscoelastic modeling in the vacuum infusion process
    (Sage Publications Ltd, 2017) N/A; N/A; Department of Mechanical Engineering; Yenilmez, Bekir; Çağlar, Barış; Sözer, Murat; PhD Student; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 110357
    A woven fabric's compaction in the vacuum infusion process is characterized by applying an initial settling under a minor load, compaction, settling under a major load, decompaction and relaxation. The effects of compaction rate, relaxation pressure, wetting and debulking cycles are all investigated. Although wetting helps by increasing fiber volume fraction insignificantly, its contribution is more significant during debulking cycles by increasing the fiber volume fraction to 57.4% as compared to 55.4% for the debulked dry specimens. Recovery during decompaction is much less than the deformation during compaction, and thinning/thickening of the specimens with time under constant pressure, so called settling/relaxation pressures, indicates that fabric specimens are not elastic materials, but viscoelastic. The experimental data of this study will be valuable to compare different viscoelastic and elastic compaction models in our next study.
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    Compaction of e-glass fabric preforms in the vacuum infusion process, A: characterization experiments
    (Elsevier Sci Ltd, 2009) N/A; Department of Mechanical Engineering; Yenilmez, Bekir; Sözer, Murat; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 110357
    An experimental procedure was designed to realistically characterize the compaction behavior of e-glass fabric preforms during initial application of vacuum and mold filling stages of Vacuum Infusion (VI). To mimic VI, the loading (compaction) was done on a dry preform, and the unloading (decompaction) was done after the preform was saturated with resin. When fabrics were wetted at constant full compaction pressure, a significant decrease in thickness was observed for the random fabric, but not for woven and biaxial fabrics. The rate of change of thickness, ∂h/∂t had different signs and order of magnitudes when various constant compaction pressures were applied during fiber relaxation stage. Thus, previous compaction-mold filling models based on static relationship between thickness and compaction pressure do not appropriately simulate the compaction physics of VI. Time-dependent database of this study is a useful and straightforward tool to model VI, as demonstrated in Part B of this study.
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    A grid of dielectric sensors to monitor mold filling and resin cure in resin transfer molding
    (Elsevier Sci Ltd, 2009) N/A; Department of Mechanical Engineering; Yenilmez, Bekir; Sözer, Murat; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 10357
    A grid of 50 dielectric sensors has been embedded in the walls of a mold to monitor resin transfer molding (RTM). The capacitance of each sensor increased as resin occupied the space between sensor plates, and it decreased with curing. Monitoring data can be used for process control to prevent dry spots and to determine when to de-mold the part. In previous studies, Skordos et al. [Skordos AA, Karkanas PI, Partridge IK. A dielectric sensor for measuring flow in resin transfer molding. Meas Sci Technol 2000; 11:25-31] used a lineal sensor, Hegg et al. [Hegg MC, Ogale A, Mescher A, Mamishev AV, Minaie B. Remote monitoring of resin transfer molding processes by distributed dielectric sensors. J Compos Mater 2005;39(17)] used three large sensors. As experimentally shown in this study, these lineal or large-plate dielectric sensors may mislead since a sensor measures total fraction of the sensor's plate area occupied by resin but not the resin's whereabouts. To avoid ambiguity and yet maintain detailed monitoring, a sensor grid was made at the projections of embedded orthogonal electrodes. The developed sensor operation system eliminated tedious and costly manufacturing of conventionally shielded separate sensors. The success of the developed sensor system was demonstrated in RTM experiments.
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    Viscoelastic modeling of fiber preform compaction in vacuum infusion process
    (Sage, 2017) Department of Mechanical Engineering; Department of Mechanical Engineering; Department of Mechanical Engineering; Yenilmez, Bekir; Çağlar, Barış; Sözer, Murat; PhD Student; PhD Student; Faculty Member; Department of Mechanical Engineering; College of Engineering; College of Engineering; College of Engineering; N/A; N/A; 110357
    A woven fabric's compaction was modeled by using five viscoelastic models - Maxwell, Kelvin-Voigt, Zener, Burgers, and Generalized Maxwell - to reveal the capabilities and limitations of the models. The model parameters were optimized by minimizing the deviation between the model results and experimental data collected in our previous material characterization study mimicking different compaction stages (loading, fiber settling, wetting, unloading, and fiber relaxation) that a fiber structure undergoes during vacuum infusion process. Although Burgers and Generalized Maxwell models have the highest performance due to their almost equal coefficient of determination values, they have diverse characteristics in terms of modeling different stages of compaction. Burgers model allowed modeling the permanent deformation in relaxation stage, but failed in modeling permanent deformation in settling stage. Generalized Maxwell model could do the opposite, i.e. failed in the former and could handle the latter. This study's major contribution is a holistic numerical approach and its conclusions by modeling all stages of the vacuum infusion process instead of one stage at a time, and thus optimizing only one set of model parameters (constants of springs and dampers) since they do not change with time. The numerical results of different models were fit to the results of a specially designed compaction characterization experiments conducted in our complementary study.
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    Variation of part thickness and compaction pressure in vacuum infusion process
    (Elsevier Sci Ltd, 2009) N/A; N/A; Department of Mechanical Engineering; Yenilmez, Bekir; Senan, Murat; Sözer, Murat; PhD Student; Master Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 110357
    In vacuum infusion (VI), it is difficult to manufacture a composite part with small dimensional tolerances, since the thickness of the part changes during resin injection. This change of thickness is due to the effect of varying compaction pressure on the upper mold part, a vacuum bag. In this study, random fabric layers with an embedded core distribution medium is used. The thickness of the composite part and resin pressure are monitored using multiple dial gages and pressure transducers; the results are compared with the model developed by Correia et al. [Correia NC, Robitaille F, Long AC, Rudd CID, Simacek P, Advani SC. Analysis of the vacuum infusion molding process: 1. Analytical formulation. Composites Part A: Applied Science and Manufacturing 26, 2005. p. 1645-1656]. To use this model, two material characteristics databases are constructed based on the process parameters: (i) the thickness of a dry/wet fabric preform at different compaction pressures, and (ii) the permeability of the preform at different thicknesses. The dry-compacted preform under vacuum is further compacted due to fiber settling in wet form after resin reaches there; the part thickens afterwards as the resin pressure increases locally. The realistic model solution can be achieved only if the compaction characterization experiments are performed in such a way that the fabric is dry during loading, and wet during unloading, as in the actual resin infusion process. The model results can be used to design the process parameters such as vacuum pressure and locations of injection and ventilation tubes so that the dimensional tolerances can be kept small.
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    Modeling of post-filling stage in vacuum infusion using compaction characterization
    (Sage Publications Ltd, 2015) N/A; N/A; Department of Mechanical Engineering; Çağlar, Barış; Yenilmez, Bekir; Sözer, Murat; PhD Student; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 110357
    Two-dimensional finite-element method solution of the post-filling stage of vacuum infusion was studied based on mass conservation in an infinitesimal control volume. First, resin pressure distribution at the instant of mold filling was calculated and then used as the initial condition for the transient post-filling stage. Explicit time-marching algorithm was used for the evolution of resin pressure and part thickness, and its stability was ensured by selecting the time step adaptively. Finite-element method solution was verified analytically for one-dimensional case and numerically for two-dimensional cases using global mass conservation. The time that it took for the settlement of pressure and thickness was investigated to compare the effectiveness of different resin-bleeding scenarios where different number and locations of gates were used. It was shown that the settlement time increased exponentially as the dimensions of the mold increased, which proved that process simulation fed with correctly designed material characterization can replace tedious trial-and-error search of control actions to reduce the settlement time and variation in part thickness.
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    Compaction of e-glass fabric preforms in the vacuum infusion process: (a) use of characterization database in a model and (b) experiments
    (Sage Publications Ltd, 2013) N/A; Department of Mechanical Engineering; Yenilmez, Bekir; Sözer, Murat; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 110357
    Compaction of e-glass fabric preforms (random, woven and biaxial) embedded with a distribution medium (polypropylene) is coupled with 1D resin (polyester) flow during initial application of vacuum, mold filling and fiber relaxation stages of vacuum infusion. In our previous study,(1) the compaction characterization procedure had been designed and conducted to realistically model the compaction behavior of fiber preforms in vacuum infusion such that the loading was done on a dry specimen; fiber settling was allowed under constant compaction pressure; unloading was done after the specimen was wetted and the fiber relaxation was characterized at constant pressure. To investigate the effects of characterization components on the part thickness evolution, two compaction models (unloading only and unloading and time-dependent relaxation) were coupled with two models of flow (uncoupled and coupled pressure-thickness-permeability). The results of the coupled model of unloading and time-dependent relaxation and coupled pressure-thickness-permeability was the closest to the vacuum infusion experiments.