Cevdet Kaynak

Effects of liquid rubber modification on the behaviour of epoxy resin

Abstract The aim of this study was to investigate the behaviour of brittle epoxy resin when modified by liquid rubber for toughening purposes. The changes in mechanical and thermal properties of diglycidyl ether of bisphenol-A based epoxy resin with hydroxyl terminated polybutadiene (HTPB) (1% and 1.5%) have been studied. A silane coupling agent (SCA) was also utilized to improve the compatibility of HTPB and epoxy matrix. Four groups of specimens were prepared by using different orders of mixing of HTPB with SCA and hardener. Tension and impact tests were performed in order to observe the effect of rubber modification, SCA and mixing processes on the mechanical properties. Tensile strength and impact toughness values were increased in the specimens containing 1% HTPB and 2% SCA. Furthermore, it was found that rubber modification markedly increased the plastic strain at failure due to the possible decrease in crosslinking density with the change in path of the reactions. The changes in storage modulus and glass transition temperatures of all specimens were also evaluated by dynamic mechanical analysis. Fracture surfaces of all specimens were examined by scanning electron microscope. Use of SCA and different mixing orders improved the interaction between HTPB and epoxy matrix. The deformed rubber domains and higher amounts of deformation lines in some specimens indicated the improvement in toughness.

Uniaxial fatigue behavior of filament-wound glass-fiber/epoxy composite tubes

Abstract Filament-wound polymer-matrix composite tubes find uses as pressure vessels, rocket launchers, etc., where internal pressure is the primary constraint. Increasing use of these structural parts has given rise to the need for tests to identify characteristics of their fatigue behavior. The aim of this study was to determine fatigue data, and observe macroscopic and microscopic damage mechanisms of filament-wound composite tubes under axial cyclic loading. For this purpose, fatigue lives of the epoxy matrix, ±55° glass fiber-wound specimens are determined for stress levels of 60, 70, and 80% of their tensile strength. Three different frequencies; 0.1, 1, and 10 Hz, were applied at each stress level for the constant amplitude sinusoidal loading with a stress ratio of R= 0.1. Results were evaluated in the form of S-N curves to reveal the effects of stress level and loading frequency on the three stages of fatigue damage. These stages observed were; first craze initiation in the matrix, then craze propagation and densification along the fiber winding direction, and finally breakage in the fibers leading to disintegration. Fatigue lives of the specimens decreased with an increase in the stress level, while they were generally increased by increasing the frequency.

Use of silane coupling agents to improve epoxy-rubber interface

Abstract Scrap car tyre particles can be used as filler or toughening agent in rigid epoxy matrices if the resultant interface is adequate. The objective of this work was to investigate the effects of use of different silane coupling agents (SCAs) to improve the interface between the epoxy resin and recycled rubber particles. For this purpose, seven different SCA were used to modify surfaces of the rubber particles. After the preparation of epoxy–rubber specimens, tensile Charpy impact and plane-strain fracture toughness tests were conducted. Mechanical tests and fractographic studies revealed that some of the SCA can improve of the interface between the epoxy matrix and the rubber particles leading to increases in strength while slight decreases in toughness of the samples.

Flexibility improvement of short glass fiber reinforced epoxy by using a liquid elastomer

In certain applications of fiber reinforced polymer composites flexibility is required. The aim of this study was to improve flexibility of short glass fiber reinforced epoxy composites by using a liquid elastomer. For this purpose, diglycidyl ether of bisphenol-A (DGEBA) based epoxy matrix was modified with hydroxyl terminated polybutadiene (HTPB). A silane coupling agent (SCA) was also used to improve the interfacial adhesion between glass fibers and epoxy matrix. During specimen preparation, hardener and HTPB were premixed and left at room temperature for an hour before mixing with epoxy resin to allow possible reactions to occur. In order to compare flexibility of the specimens flexural tests were conducted and the data were evaluated numerically by using a derived relation. Test data and scanning electron microscope analysis indicated that surface treatment of glass fibers with SCA, and HTPB modification of epoxy matrix improved flexural properties especially due to the strong interaction between fibers, epoxy, and rubber. It was also observed that HTPB modification resulted in formation of relatively round rubber domains in the epoxy matrix leading to increased flexibility of the specimens.

Effects of short cracks on fatigue life calculations

In this study, the potential impact of the short crack problem in En7A steel with a high content of elongated MnS inclusions has been observed. Experimental short crack data were compared first with the predicted S-N curves determined using long crack growth parameters and second with the calculated lives using only long crack data and using both short crack and long crack data. All these life predictions revealed that long crack data alone cannot be used in fatigue lifetime analyses that treat short crack growth, because they can all lead to dangerous over-predicted lives.

Initiation and early growth of short fatigue cracks at inclusions

In this study, the initiation and early growth behaviour of short fatigue cracks in En 7A steel with a high content of elongated MnS inclusions was investigated, by generating and evaluating data on the growth of short fatigue cracks under various stress levels and stress ratios for the six principal specimen orientations. Short cracks usually initiated at the debonded interfaces between the matrix and the inclusions. If there was no debonding, cracking sometimes occurred in the inclusions. In the early stages, short cracks propagated by a mechanism of inclusion influenced growth. Under low stress levels, usually one short crack was initiated which dominated most of the fatigue life, while under high stress levels there was multicrack interaction.MST/3249A

Using various techniques to characterize oxidative functionalized and aminosilanized carbon nanotubes for polyamide matrix

The main purpose of this study was to reveal usability of various characterization techniques for certain aspects of surface functionalized multi-walled carbon nanotubes. Surfaces were first oxidative functionalized by sulphuric acid/nitric acid mixture, then aminosilanized by γ-aminopropyltriethoxysilane. Chemical groups formed on carbon nanotubes due to these surface treatments were characterized by X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy and also energy dispersive spectroscopy. Morphological changes and crystal structure of surface-treated carbon nanotubes were analyzed by scanning electron microscopy and X-ray diffraction, respectively. Thermogravimetric analysis was also used to observe thermal degradation of the chemical groups formed on the nanotube surfaces. In the second part of the study, Polyamide-6 nanocomposites were produced by using unmodified and surface functionalized carbon nanotubes. Transmission electron microscopy indicated that surface functionalization improves distribution of carbon nanotubes in the matrix, while flexural tests revealed that strength and modulus values could be increased as much as 30% and 40%, respectively, due to enhanced interfacial bonding between the matrix and nanotubes.

Short fatigue crack growth in AL 2024-T3 and AL 7075-T6

Abstract Short fatigue crack growth behavior was investigated. The crack growth was monitored using a replica technique for cracks propagating on the specimen notch from an initial length of 40–50 μm to the specimen thickness of 2.3 mm. As part of the AGARD round robin test program Al 2024-T3 and Al 7075-T6 specimens with semi-circular notches on one edge were tested under R-ratios of 0.5, 0, −1 and −2. For each R-ratio several stress levels were used. In order to compare the behavior of short and long cracks, their crack growth rate data were drawn on the same graph. The general conclusion for both aluminum alloys is that the growth rates of short cracks were faster than the rates of long cracks for R-ratios of −1 and −2, no significant differences were observed for R = 0, and the short cracks actually grew more slowly at R = 0.5.

Interfacial Interactions and Flammability of Flame-Retarded and Short Fiber-Reinforced Polyamides

Interfacial properties, crystallinity and flammability of short fiber reinforced and flame retarded polyamide 6 and polyamide 66 compounds are investigated, emphasizing the influence of flame retardant fillers on the resistance of fiber/matrix interface to shear. Interfacial shear strengths are derived through a micromechanical approach by determining the tensile properties and residual fiber length distributions. Validated by fracture morphologies, interfacial strengths are found to be governed by filler – induced apparent crystallinities and fractional occurrence of polyamide polymorphs, obtained via peak deconvolution of X-Ray diffraction patterns. Although flame retardant additives based on Br/Sb synergism are found to impart excellent flammability reductions regarding oxygen index and UL94 classifications (V-0 rating), degree of crystallinity; thus, interfacial properties are deteriorated due to lowered thermal expansion and increased cooling rates. Red phosphorus as a flame retardant also induces a UL94 V-0 and significant reduction in flammability together with the facts that crystallinity is not altered and a strong fiber/matrix interface is maintained. Use of melamine cyanurate in an unreinforced polyamide improves the limiting oxygen index considerably; however, the UL94 rating remains unchanged as V-2 as a consequence of increased level of melt dripping. Melamine cyanurate additionally increases the degree of crystallinity through promotion of heterogeneous nucleation.

A comparison of short and long fatigue crack growth in steel

In this study, the growth behaviour of short fatigue cracks in En7A steel with a high content of elongated MnS inclusions has been investigated by generating and evaluating short crack growth data for the six principal specimen orientations. The anomalous initiation and growth behaviour of short cracks was also compared with that of long crack data for the same material and loading conditions determined before. This comparison revealed the sub-threshold initiation and faster growth rate of short cracks compared with long cracks under the same stress intensity factor range. In order to simulate this discrepancy between short and long cracks, a crack closure model for long crack data was used.