Zafarullah Khan

Stress-induced martensitic transformation in metastable austenitic stainless steels: Effect on fatigue crack growth rate

This paper addresses the influence of cyclic stress-induced martensitic transformation on fatigue crack growth rates in metastable austenitic stainless steels. At low applied stress and mean stress values in AISI type 301 stainless steel, fatigue crack growth rate is substantially retarded due to a cyclic stress-induced γ-α′ and γ-ε martensitic transformation occurring at the crack-tip plastic zone. It is suggested that the transformation products produce a compressive residual stress at the tip of the fatigue crack, which essentially lowers the effective stress intensity and hence retards the fatigue crack growth rate. At high applied stress or mean stress values, fatigue crack growth rates in AISI type 301 steels become almost equal to those of stable AISI type 302 alloy. As the amount of transformed products increases (with an increase in applied or mean stress), the strain-hardening effect brought about by the transformed martensite phase appears to accelerate fatigue crack growth, offsetting the contribution from the compressive residual stress produced by the positive volume change of γ → α′ or ε transformation.

Effects of weathering on failure pressure of filament-wound GFRP thermoset pipes:

Burst tests have been performed on the glass fiber-reinforced (GFR) thermoset pipes before and after exposure to natural and accelerated environmental conditions. The main objective of this study was to explore the effects of long-term natural outdoor and artificial accelerated environmental conditions on the burst resistance of the vinyl-based and epoxy-based GFR filament wound thermoset pipes which are targeted for use in crude oil transportation. The pipes were exposed to natural outdoor and accelerated dry heat conditions. The results show that these environments did not produce any noticeable degradation in the hydrostatic burst resistance of the pipes up to 24 months of natural exposure and up to 10,000 h of dry heat exposure.

Effect of High Shear Mixing Parameters and Degassing Temperature on the Morphology of Epoxy-Clay Nanocomposites

Epoxy-clay nanocomposites were prepared by high shear mixing method using Nanomer I.30E nanoclay as nano-reinforcement in diglycidyl ether of bisphenol A (DGEBA). The effect of mixing speed and time on the nature and degree of clay dispersion were investigated by varying the mixing speed in the range of 500-8000 RPM and mixing time in the range of 15-90 minutes. The effect of degassing temperature on the morphology of the resultant nanocomposites was also studied. Scanning and transmission microscopy (SEM & TEM) along with x-ray diffraction (XRD) have been used to characterize the effect of shear mixing speed, mixing time and degassing temperature on the structure of the resultant nanocomposites. The SEM, TEM and XRD examinations demonstrated that the degree of clay dispersion was improved with increasing the high shear mixing speed and mixing time. The results showed that the optimum high shear mixing speed and mixing time were 6000 rpm and 60 min, respectively. It was observed that the structure of the nanocomposites that have been degassed at 65oC was dominated by ordered intercalated morphology while disordered intercalated with some exfoliated morphology was found for the sample degassed at 100oC for the first 2 hours of the degassing process.

The Effects of Clay Content and Sonication Time on Water Uptake in Epoxy-Organoclay Nanocomposites

The effects of clay loading and the extent of sonication time on water uptake in epoxy-organoclay based nanocomposites have been investigated as a function of exposure time in 3.5% NaCl at room temperature. Three clay loadings (2%, 4%, and 5wt.%) were dispersed in the epoxy resin by 10 and 60 minute sonication. The weight gains in the neat epoxy and the nanocomposites with 2 wt.% and 4 wt.% clay loadings showed a common asymptotic saturation value of 0.72 wt.% after 1000 hours of exposure. The saturation value was independent of sonication time. Nanocomposite with 5% clay loading and 60 minutes sonication exhibited a unique behavior and did not show saturation after 1000 hours of exposure. Scanning electron microscopy of tensile fractured nanocomposite specimens revealed the presence of organoclay aggregates, the size and number of which increased with an increase in clay loading. The Glass transition temperatures (Tg) for the nanocomposites decreased by approximately 20-25C° after 1000 hours of exposure in 3.5% NaCl solution.

Nano-second pulse laser treatment of Incoloy 800 HT alloy-corrosion properties

Incoloy alloy 800 HT is widely used material of construction for equipment that must resist corrosion. Moreover, the corrosion properties of the alloy reduce considerably when the alloy is heat treated. However, the short pulse laser treatment of the alloy may offer alternative technique to improve the corrosion properties of the alloy. In the present study, nano-second pulse heating of Incoloy 800 HT alloy is carried out using a Nd-YAG laser. The heating rate and the temperature rise during the laser treatment are predicted theoretically. Electrochemical techniques are applied to determine the corrosion rates of the laser treated and untreated Incoloy 800 HT samples. SEM and EDS are introduced for metallographic examination of the treated alloy surface. It is found that the fine dentritic structures occur at the surface after the laser treatment. The local pitting is observed for the laser melted and re-solidified regions while the scattering of the pits are resulted for the laser heated and unmelted regions. In addition, the corrosion rate reduces for the laser-treated samples.

Corrosion-fatigue life prediction for notched components based on the local strain and linear elastic fracture mechanics concepts

An initiation-propagation model based on the local strain and linear elastic fracture mechanics concepts has been investigated for application to predict the fatigue life of notched components exposed to a corrosive environment. Estimates of the corrosion-fatigue crack initiation lives were obtained using strain-life relationships. The Paris power law was used to obtain the estimates of corrosion-fatigue crack propagation lives. Estimated corrosion-fatigue lives were compared with the experimentally obtained corrosion-fatigue life data using centre-notched specimens of three types of modified Al-2.5Mg alloy exposed to an Arabian Gulf seawater environment. Good fatigue life estimates were obtained both in air and in Arabian Gulf seawater environments for all three types of alloy. It is shown that good corrosion-fatigue life predictions can be made by determining the relevant fatigue parameters via a few simple constant-amplitude fatigue tests on smooth specimens and a few crack growth rate tests in the environment at the frequency of interest.

Near superhydrophobic-fluorinated THV fiber-like structures and fibers prepared by electrospinning

In the fluoropolymers family, one of the newest members is the terpolymer of tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride (THV). There are scarce data on THV, and therefore it is necessary to study different properties of THV. In the current research, the detailed surface morphology and water wettability of electrospun microfibers prepared from THV/ethyl acetate solutions in the pure state and with the addition of Wyoming-type low-magnesium montmorillonite are discussed. The morphology of the terpolymer microfibers changes as a function of the polymer solution concentration. For the same polymer concentration, changing the applied voltage in microfiber formation does not alter the morphology. The addition of hydrophilic montmorillonite into the THV solution does not modify the rougher hydrophobic nature and morphology of the final electrospun fiber surfaces. Water contact angle measurements show that both in the pure state and in the mixture of montmorillonite, THV electrospun microfibers exhibit “near superhydrophobic” characteristics with contact angles as high as 145°.

Synthesis of graphene based membranes: Effect of substrate surface properties on monolayer graphene transfer

In this work, we report the transfer of graphene onto eight commercial microfiltration substrates having different pore sizes and surface characteristics. Monolayer graphene grown on copper by the chemical vapor deposition (CVD) process was transferred by the pressing method over the target substrates, followed by wet etching of copper to obtain monolayer graphene/polymer membranes. Scanning electron microscopy (SEM), atomic force microscopy (AFM), and contact angle (CA) measurements were carried out to explore the graphene layer transferability. Three factors, namely, the substrate roughness, its pore size, and its surface wetting (degree of hydrophobicity) are found to affect the conformality and coverage of the transferred graphene monolayer on the substrate surface. A good quality graphene transfer is achieved on the substrate with the following characteristics; being hydrophobic (CA > 90°), having small pore size, and low surface roughness, with a CA to RMS (root mean square) ratio higher than 2.7°/nm.

Crude Oil and Outdoor Temperature Effects on the Tensile and Creep Properties of Glass Fiber Reinforced Vinylester Composite Pipes

Composites are overtaking their traditional counterpart materials by prooving themselves to be superior alternatives even for their application in harsh environmental conditions. The technological innovations and developments in manufacturing processes, which lead to a wide range of new composite products, have made them attractive candidates for applications in oil & gas industry, with their application ranging from oil production to transportation. Lack of enough data regarding their long-term environmental and mechanical durability has been a major hinderance in their full-fledge application. This paper investigates the effect on the tensile and creep strength of filament wounded E-glass/vinylester composite pipes (GFRV) exposed to a combination of crude oil and outdoor temperature conditions of Dhahran. The variation in the tensile and creep properties is considered for exposure periods of 6, 12 and 24 months. The combined crude oil filled GFRV pipes exposed to outdoor environmental conditions for a periods of 6, 12 and 24 months have shown a constant increase in the tensile and creep strengths respectively, when compared with the as-received GFRV samples. This increase in the tensile and creep strengths may be attributed to the dual curing of the GFRV pipes i.e both the outer side and inner side of the pipes due to outdoor temperature and crude oil respectively. Microscopic analysis of fractured surfaces using optical microscope and SEM is used to characterize the failure mechanisms responsible.

Statistical modeling of crack growth and reliability assessment of high-density polyethylene

In this work, a statistical evaluation of the crack-growth process in high-density polyethylene (HDPE) was carried out. The specimens were compression molded from virgin, molding-grade HDPE. Edgenotched specimens for replicate fatigue testing were prepared from compression-molded sheets. Fatigue test results were then analyzed, and it is shown that if the crack-growth process can be characterized as a random process following a power-law-type behavior, then the time to reach a critical crack length will be distributed according to an inverted lognormal model.