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    An experimental study on low velocity impact performance of bolted composite joints part 1: Influence of halloysite nanotubes on dynamic loading response
    (Elsevier Ltd, 2021) Kaybal, Halil Burak; Ulus, Hasan; Eskizeybek, Volkan; Avcı, Ahmet
    Mechanical joints are a widely utilized to assembly fiber reinforced polymer composites in marine applications. Impact is one of the most encountered unpredictable loading types which significantly diminishes the mechanical properties of structures. The goal of this study is to investigate the dynamic loading response of bolted basalt-epoxy composite laminates under different impact energies. Unlike the existing low velocity impact tests of bolted composite joints, to reveal the effect of localized impact damage, the low-velocity impact tests were conducted on two different regions as the top of bolt (ToB) and the side of washer (SoW). In addition, the effects of HNTs reinforcement on the impact response and the damage propagation were also evaluated. It was obtained that ToB damage was comparatively severe for the composite joints due to the propagation of the damage through the hole center. Moreover, HNTs improved the impact resistance about %15, especially at lower impact energies. However, the nanoreinforcement efficiency diminished with increasing impact energy levels. The obtained results were further supported with macro-size images and scanning electron microscopy (SEM). Together with Part II, this study reports an extensive work of impact tests of bolted composite joints utilized in the marine industry.
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    An experimental study on low velocity impact performance of bolted composite joints-part 2: Influence of long-term seawater aging
    (Elsevier Ltd, 2021) Kaybal, Halil Burak; Ulus, Hasan; Eskizeybek, Volkan; Avcı, Ahmet
    In the first part of this two-part paper (Part 1), the low-velocity impact (LVI) response of bolted fiber-reinforced polymer joints was investigated considering with two scenarios based on the localized impact damage as the impactor hit on the top of the bolt (ToB) and the side of the washer (SoW). Moreover, the influence of halloysite nanotubes (HNTs) reinforcement of the epoxy matrix on the impact performance was also evaluated. As the second part of the research, this paper represents the effects of seawater aging on the LVI response of FRPs. For this, the composite joints were submerged in an artificial seawater environment for six months to accelerate aging. Afterward, as following the systematic experimental path exhibited in Part 1, LVI tests were conducted by dropping the impactor on ToB and SoW regions. The test results showed that the seawater aging impaired almost 30% of the composite joints' impact resistance, where HNTs reinforced multi-scale composite joints exhibited a 13% higher impact loading performance. The ToB impact scenario was considered as visually and quantitatively more detrimental than the SoW tests. The detrimental impact of seawater aging was validated by tracking the elemental evolution in the seawater environment. Based on the mechanical, morphological, and structural analyses, a novel damage mechanism was introduced to address seawater aging's progress, including the role of nanoreinforcements.
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    Effect of long-term stress aging on aluminum-BFRP hybrid adhesive joint's mechanical performance: Static and dynamic loading scenarios
    (Wiley, 2022) Ulus, Hasan; Kaybal, Halil Burak; Cacik, Fatih; Eskizeybek, Volkan; Avci, Ahmet
    Composite-aluminum hybrid adhesive joints represent an ideal solution for designing lightweight structures for the marine industry. However, seawater aging is a serious concern, limiting the safe service life of the joint. Notably, efforts to understand the impact of aging have largely focused on the short-term periods without considering actual operating conditions. Here, we report the mechanical performance of hybrid joints subjected to the long-term stress aging. Besides, we modified the epoxy adhesive with halloysite nanotubes (HNTs) to limit the aging driven adhesive degradation and improve the adhesive's rigidity. We evaluated mechanical performances of hybrid joints by performing tensile, flexural, and drop-weight impact tests. While we increased the load-carrying capacity by over 25% with the HNTs modification before the stress aging process, modified adhesive withstood almost 55% higher tensile load than the neat epoxy adhesive after six-month stress aging. The modified adhesive also absorbed 41% less impact energy, indicating the efficiency of HNTs on limiting the degradation due to the stress aging. Furthermore, the damage mode transformed from adhesion to cohesion, thanks to the improved adhesive-composite interface performance. We envisage that these exciting results will pave the way for designing robust hybrid joints for the marine industry.
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    Enhanced Salty Water Durability of Halloysite Nanotube Reinforced Epoxy/Basalt Fiber Hybrid Composites
    (Korean Fiber Soc, 2019) Ulus, Hasan; Kaybal, Halil Burak; Eskizeybek, Volkan; Avci, Ahmet
    In this study, we report the effect of halloysite nanotube (HNT) modification on salty water aging durability of epoxy (Ep)/basalt fiber (BF) hybrid composites. For this, various amounts of HNTs were introduced into the Ep matrix, and the HNTs/Ep mixture was used to impregnate basalt fabrics to fabricate hybrid laminated composites. The hybrid composites were exposed substantial increases in the tensile strength and the fracture toughness. Besides, after salty water aging for 6 months, the hybrid composites exhibited remarkably improved aging performance with almost 10 % less reduction in both tensile and flexural strengths and fracture toughness compared to the neat basalt-epoxy composites. SEM analysis showed relatively less number of cracks, micro-voids and better interfacial bonding for the 2 wt% HNTs reinforced hybrid composite specimens in comparison to the neat counterpart, similarly conditioned in all cases. The consequences of salty water aging on micro-scale morphology were discussed based on the fracture morphologies to reveal degradation mechanisms in the existence of HNTs reinforcement.
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    Fracture and dynamic mechanical analysis of seawater aged aluminum-BFRP hybrid adhesive joints
    (Pergamon-Elsevier Science Ltd, 2022) Ulus, Hasan; Kaybal, Halil Burak; Cacik, Fatih; Eskizeybek, Volkan; Avci, Ahmet
    Adhesively bonded hybrid FRP-aluminium structures have recently become an efficient solution for marine engineering applications. However, polymer adhesives' bond performance is sensitive to the marine environment due to polymer and interfacial degradation. This study aims to develop mode I, mode II delamination toughness, and Tg data as a comprehensive design guideline for hybrid BFRP-aluminum modified-adhesively bonded joints subjected to seawater aging. The hybrid joints were exposed to long-term seawater aging (for 6 months) to reveal their fracture and thermomechanical performances. Besides, the adhesive was reinforced with HNTs to increase fracture resistance with additional nano-scale toughening mechanisms and to delay the water absorption. After the long-term aging, reinforced adhesively bonded joints exhibited -36% higher fracture toughness than neat adhesively bonded joints. Moreover, DMA was conducted on miniaturized SLJ samples, which revealed that HNT modified adhesive joints showed -11.5 degrees C higher Tg. The calculated aging rates also proved the effectiveness of HNTs modification on the epoxy adhesive's aging performance since the HNT reinforced adhesive represented 43% lower aging rates in terms of storage modulus. It is considered that experimental results will help comprehend long-term aging influences on the composite-aluminum hybrid designs' fracture and thermomechanical performances. These exciting findings will pave the way for the safe use of high stiffness and cost-effective aluminum-BFRP hybrid structures for the marine industry.
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    Halloysite nanotube reinforcement endows ameliorated fracture resistance of seawater aged basalt/epoxy composites
    (Sage Publications Ltd, 2020) Ulus, Hasan; Kaybal, Halil Burak; Eskizeybek, Volkan; Avci, Ahmet
    Seawater aging-dominated delamination failure is a critical design parameter for marine composites. Modification of matrix with nanosized reinforcements of fiber-reinforced polymer composites comes forward as an effective way to improve the delamination resistance of marine composites. In this study, we aimed to investigate experimentally the effect of halloysite nanotube nanoreinforcements on the fracture performance of artificial seawater aged basalt-epoxy composites. For this, we introduced various amounts of halloysite nanotubes into the epoxy and the halloysite nanotube-epoxy mixtures were used to impregnate to basalt fabrics via vacuum-assisted resin transfer molding, subsequently. Fracture performances of the halloysite nanotubes modified epoxy and basalt/epoxy composite laminated were evaluated separately. Single edge notched tensile tests were conducted on halloysite nanotube modified epoxy nanocomposites and the average stress intensity factor (K-IC) was increased from 1.65 to 2.36 MPa.m(1/2) (by 43%) with the incorporation of 2 wt % halloysite nanotubes. The interlaminar shear strength and Mode-I interlaminar fracture toughness (G(IC)) of basalt-epoxy hybrid composites were enhanced from 36.1 to 42.9 MPa and from 1.22 to 1.44 kJ/m(2), respectively. Moreover, the hybrid composites exhibited improved seawater aging performance by almost 52% and 34% in interlaminar shear strength and G(IC) values compared to the neat basalt-epoxy composites after conditioning in seawater for six months, respectively. We proposed a model to represent fracture behavior of the seawater aged hybrid composite based on scanning electron microscopy and infrared spectroscopy analyses.
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    Multi-Scale Mechanical Behavior of Liquid Elium® Based Thermoplastic Matrix Composites Reinforced with Different Fiber Types: Insights from Fiber-Matrix Adhesion Interactions
    (Korean Fiber Soc, 2024) Kaybal, Halil Burak; Ulus, Hasan; Cacik, Fatih; Eskizeybek, Volkan; Avci, Ahmet
    Elium (R) liquid thermoplastic resin, with room-temperature curing and recyclability, enables large-scale production. However, limited research exists on the fiber-matrix interface, and understanding micro-scale interactions is key to influencing the composite's macro-scale mechanical properties. This study investigates the interfacial adhesion of glass, carbon, basalt, and aramid fibers-reinforced liquid Elium (R) thermoplastic matrix composites at micro-, meso-, and macro-scales. Contact angle measurements show 53-56 degrees for glass fibers, indicating superior wettability with the Elium (R) matrix, while carbon, aramid, and basalt fibers exhibit 58-62 degrees, 73-74 degrees, and 79-86 degrees, respectively. Micro-bond tests demonstrate the highest load-carrying capacity in the interface between glass fibers and the matrix, with glass fibers carrying 11.4% more load than carbon fibers and 25.8% more than basalt fibers. Fiber bundle tests, including transverse and 45 degrees fiber bundle tests, highlight the superior load-carrying performance of glass fibers, with all fiber types showing increased load-carrying capacities in the 45 degrees tests. The micro-scale and meso-scale data obtained from micro-bond and fiber bundle tests corroborated the results of the macro-scale interlaminar shear stress (ILSS) tests, confirming the significant influence of the fiber-matrix interface on the mechanical integrity of the composites. The shear strength at the glass/Elium (R) interface was 47.54 MPa, which was 8.5% higher than carbon, 20.3% higher than aramid, and 25.9% higher than basalt interfaces. These findings advance our understanding of the mechanical behavior and interfacial adhesion in thermoplastic matrix composites. They underscore the crucial role of the fiber/matrix interface in determining the mechanical properties of composites and offer insights into the compatibility of diverse fiber reinforcements with the innovative Elium (R) matrix.
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    Significantly improved shear, dynamic-mechanical, and mode II fracture performance of seawater aged basalt/epoxy composites: The impact of halloysite nanotube reinforcement
    (Elsevier B.V., 2021) Ulus, Hasan; Kaybal, Halil Burak; Eskizeybek, Volkan; Avcı, Ahmet
    The primary concern of fiber-reinforced polymers (FRPs) subjected to seawater environment is losing their initial mechanical performance since water can diffuse into the composite and deteriorates the fiber-matrix interface. Recent studies related to aging performance in the seawater environment have shown that introducing halloysite nanotubes (HNTs) into the polymer matrix offers a combination of an efficient barrier effect and an improved fiber-matrix interface. Hereupon, the principal objective of this study was to experimentally investigate the impact of HNTs on shear and mode II fracture performances of the seawater aged basalt fiber (BF) reinforced epoxy (EP) composites. After six months of aging in seawater, the findings indicated that HNTs reinforced multi-scale composites exhibited 34 and 46% higher shear strength and mode II delamination toughness compared to the neat specimens. Moreover, according to the dynamic-mechanical analysis, higher glass transition temperatures (8%) were obtained for the multi-scale composites. The reduction in mechanical performances induced by fiber-matrix interfacial degradation was also confirmed by scanning electron microscopy analysis. Chemical deterioration of the polymer matrix was explored by Raman spectroscopy to reveal the efficiency of HNTs induced barrier effect. As a result of these investigations, HNT modified BF/EP multi-scale composites were offered for future advanced engineering applications.