Ahmet Avci

Strength of filament wound GRP pipes with surface crack

In this work, the effect of surface cracks on strength has been investigated theoretically and experimentally for glass/epoxy filament wound pipes, which are mainly used for bazooka launchers. The pipes with surface crack which have several notch-aspect ratios a/c and notch-to-thickness ratios a/t in the axial direction were exposed to open-ended internal pressure. Critical stress intensity factors were determined experimentally for several winding angles by tensile tests with center notched ring specimens. The strength values of pipes with surface cracks are compared with internal pressure test results and theoretical results.

Stress corrosion crack growth in glass/polyester composites with surface crack

The propagation characteristics of stress corrosion surface cracks and crack growth rates in a range of unidirectional glass/polyester composites exposed to 0.6 N dilute HCl acid were examined using a fracture mechanics test. Glass/polyester composites were produced from continuous rovings using filament winding method. The shallow surface cracks with various a/c and a/t ratios were machined on the specimens and under uniaxial tension were exposed to one side stress corrosion. The stress intensity factors for glass/polyester composites with surface cracks are obtained from Newman‐Raju, Nishioka, and single edge notch expressions. The material constants, such as A and n, are related to crack growth rate and the stress intensity factors were obtained for the test conditions. q 2001 Elsevier Science Ltd. All rights reserved.

Enhancement of Wear and Friction Characteristics of Epoxy Resin by Multiwalled Carbon Nanotube and Boron Nitride Nanoparticles

Epoxy resins are widely used in engineering applications. However, their low thermal stability limits their usage at high sliding velocities and loads. The mechanical properties and thermal stability of a machine element subjected to friction and wear are very important. In this study, friction and tribology behaviors of multiwalled carbon nanotubes (MWCNTs) and boron nitride (BN)-modified epoxy resin have been investigated. Epoxy resin modified by three different nanoparticle configurations, 0.3% MWCNT, 0.5% BN, and 0.5% MWCNT/0.3% BN, was investigated. The tribological characteristics of nanoparticle-modified epoxy resin were compared with properties of neat resin. The friction and tribological behavior of modified epoxy resin were tested using a ball-on-disc test stand at 1.2 and 1.5 ms−1 sliding velocities under 10 N applied load. The tests were done under dry condition and 1,800 m distance. The friction coefficient, wear loss, and temperature increase during testing were recorded and compared with that obtained for neat epoxy. It is observed that nanoparticle modification resulted in enhanced wear resistance and a reduction in friction coefficient and working temperatures.

The relationship between uric acid and erectile dysfunction in hypertensive subjects

Abstract Background. Endothelial dysfunction plays a major role in erectile dysfunction (ED). Uric acid (UA) is a marker of endothelial dysfunction. We hypothesized that increased UA levels may be associated with ED and aimed to investigate whether there is a relationship between, UA and ED in hypertensive patients. Methods. A total of 200 hypertensive patients who have a normal treadmill exercise test were divided into two groups based on the Sexual Health Inventory for Men (SHIM) test (< 21 defined as ED n = 110, and ≥ 21 defined as normal erectile function n = 90). The differences between the ED and normal erectile function groups were compared and determinants of ED were analyzed. Main results. The prevalence of ED was found to be 55.0%. Office blood pressure level was comparable between groups. UA levels were significantly increased in the ED group (6.20 ± 1.56 vs 5.44 ± 1.32, p = 0.01). In a regression model, age [odds ratio (95% confidence interval): 1.08 (1.04–1.14), p = 0.001], smoking [odds ratio: 2.33 (1.04–5.20), p = 0.04] and UA [odds ratio: 1.76 (1.28–2.41), p = 0.04] were independent determinants of ED. An UA level of > 5.2 mg/dl had 76.2% sensitivity, 43.7% specificity, 62.9% positive and 59.4% negative predictive value for determining ED. Conclusion. UA is an independent determinant of ED irrespective of blood pressure control and questioning erectile function for hypertensive patients with increased UA levels may be recommended.

Fatigue Crack Growth Behavior of Filament Wound Composite Pipes in Corrosive Environment

The stress corrosion crack growth behavior of ±45°, ±55°, and ±75° filament wound composite pipes with surface crack subjected to 0.6 M HCl acid under alternating internal pressure was investigated. E-glass/epoxy pipes consist of six layers with stacking sequences of (+45/-45) 3, (+55/-55)3, and (+75/-75)3. Dilute (0.6M) HCl acid was applied to the surface crack region by a corrosion cell mounted on the outer surface of the pipe. The specimens were tested at room temperature and exposed to open-ended fatigue tests in which the pipes can deform freely in the axial direction. The tests were performed in accordance with the ASTM D-2992 standard. The surface notches with different a/t and a/c forms were cut on the outer surface of the pipe parallel to the pipe axis. The internal pressure was generated by conventional hydraulic oil for fatigue loading. The low cycle tests were applied with 0.42 Hz frequency and R = 0.05 stress ratio. After the corrosion fatigue test, the fracture surfaces were examined and observed damage mechanisms discussed.

Thermal Buckling of Orthotropic Plates with Angle Crack

In this study, thermal buckling behaviors of the composite square plates consisting of antisymmetric four laminates with angle crack are examined. Finite element method is used for the solution. First critical buckling loads of an orthotropic plate without crack subjected to unidirectional compression are found. Constraint conditions have been taken two edge clamped and the other edges are free. The thickness of plate is 2mm (each layer has 0.5mm thickness). Thermal buckling behaviors of this plate are examined and critical temperatures are determined. Values have been given in the graphs and discussions are presented in the conclusion.

Electrospun TiO2/ZnO/PAN hybrid nanofiber membranes with efficient photocatalytic activity

Electrospun polyacrylonitrile (PAN) nanofibers were decorated with TiO2, ZnO and TiO2/ZnO nanoparticles for the first time to prepare flexible multifunctional nanofibrous membranes. First, the arc-discharge process was utilized to prepare TiO2, ZnO and TiO2/ZnO nanoparticles and then the hybrid electrospun nanofibers were spun from PAN/nanoparticle colloids. X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized to characterize the resulting nanoparticles and nanofiber loaded nanoparticles. The microscopic investigations revealed that the specifically TiO2 nanoparticles tend to agglomerate within the PAN nanofiber resulting increased surface roughness; however, ZnO nanorods with 1D morphology are aligned as parallel to the fiber axis. Photocatalytic activity of the hybrid nanofibers was performed by pursuing the degradation of malachite green (MG) dye under UV light irradiation. The fabricated TiO2/ZnO/PAN hybrid nanofibers showed excellent photocatalytic efficiency with at least two times higher reaction rates compared to the bare PAN nanofibers. The results suggest that the photocatalytically active TiO2/ZnO/PAN hybrid nanofibers can be considered as filtering materials for a variety of applications in the fields of wastewater systems without the need of post processing stages for separating catalysts from the liquid medium.

Preparation and mechanical properties of carbon nanotube grafted glass fabric/epoxy multi-scale composites

In the present paper, carbon nanotubes (CNTs) were chemically grafted onto surfaces of the amino silane treated glass fabric by a novel chemical route for the first time to create 3D network on the glass fibers. The chemical bonding process was confirmed by Fourier transform infrared spectroscopy and scanning electron microscopy. The glass fabric/CNT/epoxy multi-scale composite laminates were fabricated with the CNT grafted fabrics using vacuum assisted resin infusion molding. Tensile tests were conducted on fabricated multi-scale composites, indicating the grafting CNTs on glass fabric resulted a decrease (11%) in ultimate tensile strength while toughness of the multi-scale composite laminates were increased up to 57%. Flexural tests revealed that the multi-scale composite laminates prepared with CNT grafted glass fabric represent recovering after first load fall. The interfacial reinforcing mechanisms were discussed based on fracture morphologies of the multi-scale composites.

Plastic zones in a transversely isotropic solid cylinder containing a ring-shaped crack

In this study, a problem of a ring shaped-crack contained in a infinitely long solid cylinder of elastic perfectly-plastic material is considered.The problem is formulated for a tranversely isotropic matrial by using integral transform technique under uniform load. Due to the geometry of the configuration, Hankel and Fourier integral transform techniques are chosen and the problem is reduced to a singular integral equation. This integral equation is solved numerically by using Gaussian Quadrature Formulae and the values are evaluated for various for discrete points. The plastic zone widths are obtained by using the plastic strip model. They are plotted for various ring-shaped crack sizes and transversely isotropic matrials. It is found that the width of the plastic zone at the inner tip of the crack is greater than the outer one.

Low-velocity impact behavior of carbon fiber/epoxy multiscale hybrid nanocomposites reinforced with multiwalled carbon nanotubes and boron nitride nanoplates

In this article, the mechanical properties and dynamic response of hybrid filler-modified epoxy/carbon fiber multiscale composites were investigated. The hybrid fillers composed of multiwalled carbon nanotubes and boron nitride nanoplates were dispersed in epoxy resin and used as matrix material. The multiscale hybrid laminated composites were stacked symmetrically consisting of 10 plies of woven carbon fibers and fabricated by vacuum infusion technique. The mechanical properties of the hybrid composites were investigated by tensile tests. Impact response and energy absorption capacity were investigated by using weight drop test method and the tests were performed according to ASTM-D-7136 standard with impact energies of 5, 10, and 15 J. The impact force and displacement versus interaction time were measured. The impulsive force, energy absorption capability, and damage formation were also investigated. It is observed that when the resin is modified by nanoparticles, both strength and the % strain at fracture increase considerably. However, it is shown in the subject manuscript that the enhancement of mechanical has not fully transferred to dynamic response and energy absorption capacities of nanocomposites.