Yazar "Yildiz, Mehmet" seçeneğine göre listele

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    Characterizing damage evolution of CF/PEKK composites under tensile loading through multi-instrument structural health monitoring techniques
    (Elsevier Sci Ltd, 2023) Yildirim, Ceren; Tabrizi, Isa Emami; Al-Nadhari, Abdulrahman; Topal, Serra; Beylergil, Bertan; Yildiz, Mehmet
    This study investigates the damage behavior of autoclave consolidated carbon fiber/Polyetherketoneketone (CF/ PEKK) laminates manufactured by the automated fiber placement (AFP) lay-up process. The damage evaluation of autoclave consolidated samples is studied using a multi-instrument nondestructive monitoring approach. The effect of autoclave consolidation on the microstructure of the laminate is examined via void analysis based on density measurement, thermal analysis, and optical microscopy. The results reveal that the void content is achieved as 0.46% and there is 81.81% increase in the degree of crystallinity following the autoclave consolidation. Moreover, acoustic emission (AE), digital image correlation (DIC), and infrared thermography (IRT) results are cross-correlated to further understand the damage development. The evolution of clustered AE data during mechanical loading is used to divide the failure of the laminate into two stages, each of which signifies a different dominancy in failure modes. Scanning electron microscopy (SEM) is employed to associate damage characteristics with failure monitoring techniques.
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    Coupled Dimensional Energy Balance and Machine Learning Validation for Ballistic Response Prediction of Fiber Composites
    (Korean Fiber Soc, 2025) Beylergil, Bertan; Ulus, Hasan; Yildiz, Mehmet
    In this study, we present a coupled, dimensional energy-balance model enhanced with machine-learning validation to predict residual-velocity curves and ballistic limits of fiber-reinforced composites. Projectile deceleration is described as a three-term balance involving strength-like, drag-like, and inertial effects, mapped to the nondimensional groups Pi(0), Pi(1), and Pi(2); closed-form and RK4 solutions yield residual velocity and regime boundaries (Pi(0) = Pi(1), Pi(1) = Pi(2)). Validation against six literature datasets (CFRP and aramid laminates; Vr-V0 curves) shows high accuracy: median R2 = 0.93-0.96 and typical RMSE = 10-30 ms(-)1, with best case R2 = 0.976 and RMSE = 6.99 ms(-)1 for thin CFRP. Ballistic-limit predictions accurately capture the nonlinear increase with thickness, with errors less than 1 ms(-)1 in brittle CFRP and up to 10 ms(-)1 in Kevlar laminates. A global master curve of wr = Vr/V0 versus parallel to Pi parallel to 2 collapses all data and shows a consistent trend. Energy-budget analysis quantifies the contributions of the three terms: the strength term Pi(0) dominates in about 90% of operational points, while drag-like effects are minimal and inertial effects only appear at thick or high-velocity limits; the dominance fractions and combined contributions support these shifts. The (V-0,h) regime map, derived by setting Pi(0) = Pi(1) and Pi(1) = Pi(2), separates design-relevant domains and aligns with observed transitions in Vr-V0 modes and slopes. An independent machine-learning check using Random Forests achieves R2 = 0.992, RMSE = 17.5 ms(-)1, and MAE = 12.4 ms(-)1 (fivefold cross-validation: R2 = 0.835 +/- 0.145), supporting the mechanistic hierarchy through feature importance. The integrated physics-based model and machine-learning analysis provide traceable parameters (alpha, beta, gamma), uncertainty bounds, and practical screening maps for composite and geometric options under high-velocity impact.
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    Design of Highly Thermally Conductive Hexagonal Boron Nitride- Reinforced PEEK Composites with Tailored Heat Conduction Through-Plane and Rheological Behaviors by a Scalable Extrusion
    (Amer Chemical Soc, 2023) Gul, Saher; Arican, Selin; Cansever, Murat; Beylergil, Bertan; Yildiz, Mehmet; Okan, Burcu Saner
    The challenge of developing highly thermally conductive polymeric composites to meet the growing thermal management demands has recently attracted a lot of attention. To achieve a through-plane thermal conductivity higher than 2 W/mK, a high filler concentration within the poly(ether ether ketone) (PEEK) matrix is required, thus adding to the complexity of polymer processing. In this study, an optimized twin-screw extrusion melt compounding technique was developed by tuning the melt flow of unfilled PEEK, feeding zones, and process cycles for dispersion of hexagonal boron nitride (h-BN) in the PEEK polymer. The prepared composites demonstrated exceptionally high in-plane and through plane thermal conductivity of 12.451 and 2.337 W/mK, respectively, at 60 wt % h-BN loading. This improvement of thermal conduction in both directions can be attributed to two factors: (1) formation of through-thickness surface contacts between h-BN particles due to shear-driven exfoliation during compounding stage and (2) high degree of alignment of h-BN platelets achieved during molding stage. The calorimetric and thermogravimetric analyses indicated that the prepared composites possess enhanced crystallinity compared to unfilled PEEK and are thermally stable in elevated temperature ranges. The rheological characterization exhibited a progressive increase in viscosity and shear-thinning behavior of composite melts proportional to the h-BN concentration. Using the temperature and time-dependent rheological results, viscosity buildup profiles were constructed to outline allowable melt viscosity ranges for each composite composition. These profiles can be utilized to tailor the residence time of a composite melt by varying the filler concentration and process temperature during advanced manufacturing processes such as extrusion-based additive manufacturing and powder bed fusion. Hence, we provide a facile and industrially scalable method for development of h-BNfilled PEEK composites with high thermal dissipation characteristics aimed for thermal management in various harsh environment applications.
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    Effect of atmospheric plasma treatment on Mode-I and Mode-II fracture toughness properties of adhesively bonded carbon fiber/PEKK composite joints
    (Pergamon-Elsevier Science Ltd, 2023) Yildirim, Ceren; Ulus, Hasan; Beylergil, Bertan; Al-Nadhari, Abdulrahman; Topal, Serra; Yildiz, Mehmet
    This study aims to assess the influence of peel-ply (PP), mechanical abrasion (MA), and atmospheric plasma activation (APA) treatments on Mode-I and Mode-II fracture toughness of carbon fiber/ poly-ether-ketone-ketone (CF/PEKK) composite joints. A comprehensive examination of the topography and wettability of the adherend surfaces is conducted using various methods. The CF/PEKK adherends are produced through an automated fiber placement (AFP) process, and the CF/PEKK bonded joints are prepared using two different structural adhesive films, one of which has a lower strength, while the other has a higher strength. To evaluate their fracture toughness properties, double cantilever beam (DCB) and end-notched flexure (ENF) tests are carried out in accordance with ASTM standards. Acoustic emission sensors are used to monitor the test specimens during DCB tests, allowing for an in-depth evaluation of the failure modes and damage propagations in the joints. The results show that the GIC and GIIC values of the APA-treated CF/ PEKK bonded joints are remarkably higher than those of the untreated ones, with a range of improvement of 34.0-84.8 times and 7.5-17.4 times, respectively. Adhesive failure is the dominant failure mode on the surfaces of non-treated (NT) and PP samples, while cohesive failure is more prominent in those treated with MA and APA. The failure modes of the treated samples varied depending on the adhesive used, with APA-treated samples always exhibiting a cohesive failure. It is observed that the AE counts increase more slowly in APA-treated samples compared to MA-treated joints as delamination progresses more slowly with cohesive failure dominant, which leads to a lower release of AE energy.
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    Engineering interfacial thermal transport through comparative analysis of electrospraying and dip coating of silanized h-BN for thermo-mechanical enhancement of CF/Epoxy composites
    (Elsevier Sci Ltd, 2025) Mehdipour, Mostafa; Dogan, Semih; Tabrizi, Arvin Taghizadeh; Bafqi, Mohammad Sajad Sorayani; Beylergil, Bertan; Yildiz, Mehmet; Okan, Burcu Saner
    The inherently low thermal conductivity of carbon fiber (CF) reinforced epoxy composites is mainly due to porosity and fabrication defects that interrupt thermal pathways. This study demonstrated a pathway to control heat in both out-of-plane and in-plane directions by incorporating hexagonal boron nitride (h-BN) as a thermally conductive agent and by configuring interface interactions on the CF and within the epoxy resin while evaluating physical and chemical interactions. Two integration techniques of dip coating and electrospraying were employed to apply h-BN, effectively creating robust h-BN layers on CF and dispersing neat or silane-modified hBN within the epoxy matrix by combining vacuum bag and hot compression processes to reduce void content. Electrospraying silane-modified h-BN onto carbon fiber, together with incorporating 20 wt% silane-modified hBN into the matrix, resulting in a total loading of 11 wt% in the composite-led to the highest out-of-plane thermal conductivity of 1.3 W/mK, representing a 166 % increase compared to CF reinforced into epoxy composite (CF+/ EP) with the out-of-plane thermal conductivity of 0.49 W/mK. Mechanically, the configuration using neat h-BN in both the matrix and dip-coated CF achieved a 127 % increase in flexural modulus and a 49 % improvement in Charpy impact strength versus unfilled CF/epoxy composites. Resizing the CF improved directional thermal conductivity in CF/epoxy composites by controlling porosity, achieving approximately an 81 % reduction in porosity when using silanized h-BN.
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    Enhancement of mechanical properties of carbon fiber epoxy composites using methylmethacrylate-butadiene-styrene (MBS) core-shell nanoparticles
    (Sage Publications Ltd, 2025) Beylergil, Bertan; Ozturkmen, Mahide Betul; Al-Nadhari, Abdulrahman; Yildiz, Sema; Aydogan, Berkay; Yildiz, Mehmet
    This work investigates the use of readily dispersed methylmethacrylate-butadiene-styrene (MBS) core-shell nanoparticles to improve the mechanical properties of carbon fiber epoxy (CF/EP) composites. Through the vacuum-assisted resin transfer molding (VARTM) process, CF/EP composites were manufactured with varying MBS particle loadings from 1 wt. to 7 wt. %. The mechanical properties of the composites were determined via three-point bending, Charpy impact, short-beam shear, and Mode-I fracture toughness tests, adhering to the relevant ASTM standards. The results show that the addition of MBS particles significantly increased Mode-I interlaminar fracture toughness (GIc), with the highest increase observed at 7 wt. % particle loading, demonstrating a nearly 177% improvement over the reference composite. The flexural modulus of composites slightly decreased with 1 wt. % MBS nanoparticles, indicating increased flexibility, while a synergistic effect at 7 wt. % MBS enhanced stiffness and structural reinforcement. The incorporation of MBS nanoparticles in CF/EP composites also enhanced Charpy impact strength and damping properties, with the highest impact strength observed at 7 wt. % MBS. Higher MBS content reduced the storage modulus, while the glass transition temperature remained relatively unchanged.
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    Experimental validation and numerical modeling of interfacial effects in silanized hexagonal boron nitride (h-BN) reinforced epoxy composites by tailoring silane concentration
    (Taylor & Francis Ltd, 2025) Mehdipour, Mostafa; Beylergil, Bertan; Yildiz, Mehmet; Okan, Burcu Saner
    This study investigates the use of h-BN particles as fillers, focusing on tailoring surface chemistry to enhance the thermal conductivity of epoxy composites. By enriching the interface between h-BN particles with amino-silane groups and the epoxy matrix through controlled surface modification, thermal performance, and interfacial bonding were improved. To achieve a high degree of functionalization, h-BN particles were oxygenated to 13.6 atomic percent (at. %) through thermal treatment, followed by reduction using 3-amino-propyl-3-ethoxy-silane (APTES), which increased the amino content by 3.5% at. % under optimized reaction conditions. During composite manufacturing, 10 wt.% functionalized h-BN particles were reinforced into the epoxy matrix, increasing bulk thermal conductivity by 53%, from 0.2 W/mK to 0.34 W/mK. Heat flux simulations with ANSYS confirmed the interface interactions and thermal performance, with silanized h-BN achieving the highest heat flux of 70 W/mm2, aligning well with experimental results. While silanization improved thermal conductivity by strengthening interfacial bonding between h-BN and the epoxy matrix, it introduced brittleness, making the composites stiffer and more fragile. However, the silanized h-BN composite showed a 57.14% increase in toughness compared to neat h-BN, while the highest flexural modulus of 4126 MPa was achieved with neat h-BN.
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    Influence of functionalized h-BN particle interphase and interface regulation with structural design on the directional thermal conductivity and mechanical performance of carbon fiber/epoxy composites
    (Elsevier Sci Ltd, 2025) Mehdipour, Mostafa; Dogan, Semih; Al-Nadhari, Abdulrahman; Bafqi, Mohammad Sajad Sorayani; Beylergil, Bertan; Yildiz, Mehmet; Okan, Burcu Saner
    This study highlights the importance of interfacial adhesion between carbon fiber (CF) and the epoxy matrix by adopting a novel approach that combines untreated and silane-treated h-BN in a multilayered structure. The interface was engineered by electrospraying h-BN particles, while the interphase was modified by incorporating up to 20 % h-BN into the epoxy matrix. The highest out-of-plane thermal conductivity of 2.31 W/mK, a 116 % increase compared to the reference value of 1.07 W/mK, was achieved by sizing CF with silanized h-BN through electrospraying, in conjunction with the 20 % h-BN-loaded epoxy matrix. Conversely, the incorporation of h-BN in the epoxy alone resulted in the best mechanical performance, with approximately a 46.4 % increase in elastic modulus, a 105 % improvement in flexural modulus, and a nearly 5 % increase in Charpy impact strength. Based on CT scan results, the resizing of CF fabrics improved directional thermal conductivity in CF/epoxy composites with controlled porosity.
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    Multiscale nano-integration in the scarf-bonded patches for enhancing the performance of the repaired secondary load-bearing aircraft composite structures
    (Pergamon-Elsevier Science Ltd, 2023) Serra, Topal; Al-Nadhari, Abdulrahman; Yildirim, Ceren; Beylergil, Bertan; Kan, Cihan; Unal, Serkan; Yildiz, Mehmet
    This study investigates a novel approach of improving the mechanical performance of scarf-repaired carbon fiber reinforced composites by integrating nanomaterials in the patch constituents. Two distinct types of carbon-based nanomaterials, thermally exfoliated graphene oxide grade-2 (TEGO) and Epocyl T 128-06 (MWCNT) are integrated into the patch resin matrix and fiber/matrix interface in an upscaled manner. Another group of composites are repaired with pristine patches in accordance with the existing and prevalent composite repair method. Compared to the reference panel, 109.9%, 99.7% and 99.3% stiffness recoveries are achieved for the patches with CNT and TEGO-incorporated resin matrices and TEGO-electrosprayed fibers, respectively. Location-wise analyses of the test data show that the stiffness, strength, Poisson's ratio, and strain values depend on the number of patch plies in each specimen. Fractographic inspections show that the failure sites shift towards the outer areas of the scarf region demonstrating an enhanced stress redistribution due to the nanomaterials. SEM observations show that nanoparticles affect toughening mechanisms based on the type, location, and alignment of the nano-reinforcement, which in turn limits the shear-dominated failures (SDF) in the baseline and pristine patch repair system. In CNT- and TEGO-reinforced resin patches, efficient crack bridging and fracture plane tilting/twisting or crack bifurcations are observed whereas the electrosprayed TEGO particles positioned at the perimeter of fibers operate as a shield for the fibers to prevent SDFs. These findings demonstrate that failure behavior of repair systems and therefore their mechanical performance are governed by the type of nanoreinforcement and its integration process.
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    Optimization of Charpy-impact strength of 3D-printed carbon fiber/polyamide composites by Taguchi method
    (Wiley, 2023) Beylergil, Bertan; Al-Nadhari, Abdulrahman; Yildiz, Mehmet
    This study utilizes the Taguchi optimization technique to investigate the effects of FDM processing parameters on the Charpy impact strength of 3D printed CF/PA composites experimentally and statistically. The four 3D printing parameters employed in the experiment are the infill density, raster angle, extruder temperature, and printing speed, which were used to create the experimental plan with the L18 orthogonal array. Signal to noise (S/N) ratios and analysis of variance (ANOVA) were utilized to identify the optimum values and the interactions between the process parameters. SEM and thermography techniques were employed to assess the microstructural and damage status of the CF/PA composite specimens. ANOVA results determined that only three factors-infill density, raster angle, and extruder temperature-had a statistical significance, while printing speed did not. The outcomes demonstrated that the optimal 3D printing parameters are infill density (100%), raster angle (60 degrees), extruder temperature (260 degrees C), infill density (100%), and printing speed (30 mm/s), with the maximum contribution of 54.19% belonging to infill density, and the minimum contribution of 2.84% belonging to printing speed. The optimal combination of these 3D printing parameters yielded a Charpy impact strength of 10.54 kJ/m2, resulting in an increase of almost 150% compared to the worst-case situation. The Taguchi approach proves to be a proficient technique to boost the Charpy impact strength of 3D-printed CF/PA composites.
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    Tailoring adherend surfaces for enhanced bonding in CF/PEKK composites: Comparative analysis of atmospheric plasma activation and conventional treatments
    (Elsevier Sci Ltd, 2024) Yildirim, Ceren; Ulus, Hasan; Beylergil, Bertan; Al-Nadhari, Abdulrahman; Topal, Serra; Yildiz, Mehmet
    Here, we propose the utilization of atmospheric plasma activation (APA), which outperforms peel-ply (PP) treatment and mechanical abrasion (MA) in achieving high-performance adhesively bonded carbon fiber/polyetherketoneketone (CF/PEKK) composites. This study covers several key aspects, including the chemical and morphological characterization of treated surfaces and mechanical performance assessments of single lap-joints (SLJs) under tensile and flexural loading conditions. In addition, in-situ acoustic emission (AE) monitoring is employed during tensile tests to determine dominant damage types and failure modes in the SLJs. Surface analysis shows that MA increases roughness, PP treatment decreases wettability, while APA enhances wettability by modifying the surface chemistry. Tensile and flexural tests reveal that APA-treated joints surpassed nontreated (NT) ones, with up to 5- and 7-times higher load-carrying performance, respectively, while fracture analysis suggests a shift from adhesive to cohesive failure. AE results show that increased AE events related to cohesive failure align with improved interface interactions.