This study compares both uniaxial and multiaxial variable amplitude experimental crack growth dat... more This study compares both uniaxial and multiaxial variable amplitude experimental crack growth data for naturally initiated fatigue cracks in tubular specimens of 2024-T3 aluminum alloy to predictions based on two state-of-the-art analysis codes: UniGrow and FASTRAN. For variable amplitude fatigue tests performed under pure axial nominal loading conditions, both UniGrow and FASTRAN analyses were found to produce mostly conservative growth life predictions, despite good agreement with constant amplitude crack growth data. For variable amplitude torsion and combined axial-torsion crack growth analyses, however, the conservatism in growth life predictions was found to reduce. This was attributed to multiaxial nominal stress state effects, such as T-stress and mixed-mode crack growth, which are not accounted for in either UniGrow or FASTRAN, but were found in constant amplitude fatigue tests to increase experimental crack growth rates. Since cracks in this study were initiated naturally,...
An accurate, automated assessment of helicopters’ gross weight (GW) and center-ofgravity (CG) is ... more An accurate, automated assessment of helicopters’ gross weight (GW) and center-ofgravity (CG) is critical for the determination of aircraft fatigue and life estimates since GW/CG greatly affect static and dynamic characteristics of helicopters. Current methods of GW and CG data gathering requires not only meticulous bookkeeping but it also makes several assumptions about fuel burn rate if fuel quantity instrumentation is not available. In addition, if cargos are released/picked-up during a flight, a log of loads and time instants is required to ensure aircraft GW and CG are known at all times during the flight. This paper reviews current state of the art for the GW and CG estimations’, summarizing different techniques and underlining their advantages and limitations. Preliminary results are presented in order to underline the capabilities of the proposed techniques. Then, the general framework of an innovative, hybrid approach that combines Artificial Neural Networks (ANN) and Kalma...
A transparent composite is based on the concept of matching the refractive index of the glass fib... more A transparent composite is based on the concept of matching the refractive index of the glass fiber with that of resin. A novel optically transparent glass-ribbon reinforced polymer matrix composite has been developed by infusing a clear epoxy resin system of matching refractive index into a stacked glass-ribbon preform (micron-size glass fibers with rectangular cross section). Rectangular glass ribbons are used as reinforcement as opposed to the conventional circular glass fibers since ribbons scatter less light than circular fibers for any given condition. The preforms are then laid up, vacuum bagged and infused with the resin. The set-up is then cured in an oven under controlled temperature and vacuum to make transparent composite panels. Various physical and mechanical tests like fiber volume fraction, tensile, flexure and impact have been conducted. Results indicate significant improvements in stiffness, strength and impact resistance. These new glass-ribbon reinforced structural transparent composites will find applications in windshields, windows and other components where a strong, lightweight transparent material is desirable. Transparent composites will also have potential applications as backing material in transparent armor systems
This study investigates the possibility of correlating specimen property to part performance for ... more This study investigates the possibility of correlating specimen property to part performance for laser beam powder bed fusion (LB-PBF) additive manufacturing by altering the process parameters in order to create similar thermal histories experienced during fabrication. In particular, the effects of altering scanning speed on LB-PBF 17-4 precipitation hardening (PH) stainless steel (SS) parts with different geometries on the thermal history, as well as the resultant defect formation, microstructure, and fatigue behavior are studied. It was found that parts with different geometries, all fabricated using the same manufacturer recommended process parameters, exhibited different fatigue strengths, which challenges the specimen property to part performance correlation. Melt pool analysis revealed that altering scanning speed can affect the melt pool characteristics including its depth and overlap depth. Increasing the input energy within the process window, by decreasing the scanning speed during fabrication, was seen to result in deeper melt pools and melt pool overlaps, and consequently, less volumetric defects, specifically lack of fusion, in the material. Therefore, the scanning speed was adjusted for different geometries to result in similar melt pool characteristics, as an indicator of the thermal history experienced during fabrication, which also resulted in these parts having similar porosity. Accordingly, fatigue lives of parts fabricated with adjusted process parameters were observed to be within a similar range. While many other factors may be involved, the findings of this research indicate that maintaining a similar thermal history by altering the process parameters is critical in establishing reliable relationships between specimen property and part performance in additive manufacturing.
Fatigue & Fracture of Engineering Materials & Structures, 2019
Although additive manufacturing (AM) has gained significant attention due to the advantages it of... more Although additive manufacturing (AM) has gained significant attention due to the advantages it offers and is currently a focus of much research, design of critical load carrying components utilizing such processes is still at its infancy. This is due to the fact that most of the load carrying components made by AM processes are subjected to cyclic loads, and fatigue behaviour of AM metals is far less understood as compared with those made by conventional methods, such as wrought and cast metals. To better understand the fatigue behaviour of AM metals, a wide range of issues that affect the behaviour in a synergistic manner must be considered. These include the effects of defects, residual stresses, surface finish, geometry and size, layer orientation, and heat treatment. Additionally, due to the multiaxial nature of the loading and/or complex geometries typically manufactured by AM processes, the stress state is often multiaxial including both normal and shear stresses. In this paper, the aforementioned effects influencing the fatigue resistance of AM parts, including torsion and multiaxial fatigue behaviour, are briefly discussed using some recently generated experimental data on Ti-6Al-4V by the authors.
Computer Methods in Applied Mechanics and Engineering, 2018
This study presents the weak form of peridynamic (PD) governing equations which permit the direct... more This study presents the weak form of peridynamic (PD) governing equations which permit the direct imposition of nonlocal essential and natural boundary conditions. It also presents a variational approach to derive the PD form of first-and second-order derivatives of a field variable at a point which is not symmetrically located in its domain of interaction. This capability enables the nonlocal PD representation of the internal force vector and the stress components without any calibration procedure. Furthermore, it removes the concern of truncated domain of interaction for a point near the surface. Thus, the solution is free of nonlocal boundary forces and surface effects. The numerical solution of the resulting equations can be achieved by considering an unstructured nonuniform discretization. The implicit solution to the discrete form of the equations is achieved by employing BiConjugate Gradient Stabilized (BICGSTAB) method which is an iterative technique for solving sparse non-symmetric linear systems. The explicit analysis is performed by constructing a global diagonal mass matrix, and using a hybrid implicit/explicit time integration scheme. The accuracy of this approach is demonstrated by considering an elastic isotropic plate with or without a cutout subjected to a combination of different types of boundary conditions under plane stress conditions.
Fatigue & Fracture of Engineering Materials & Structures, 2017
Fatigue damage characteristics of aluminium alloy under complex biaxial loads such as in-phase an... more Fatigue damage characteristics of aluminium alloy under complex biaxial loads such as in-phase and out-of-phase loading conditions and different biaxiality ratios have been investigated. The effects of microscale phenomena on macroscale crack growth were studied to develop an in-depth understanding of crack nucleation and growth. Material characterization was conducted to study the microstructure variability. Scanning electron microscopy was used to identify the second phase particles, and energy dispersive X-ray spectroscopy was performed to analyse their phases and elements. Extensive quasi-static and fatigue tests were conducted on Al7075-T651 cruciform specimens over a wide range of load ratios and phases. Detailed fractography analysis was conducted to understand the crack growth behaviour observed during the fatigue tests. Significant differences in crack initiation and propagation behaviour were observed when a phase difference was applied. Primarily, crack retardation and splitting were observed because of the constantly varying mode mixity caused by phase difference. The crack growth behaviour and fatigue lives under out-of-phase loading were compared with those under in-phase loading to understand the effect of mixed-mode fracture.
Herein, we report the synthesis, structural characterization, and catalytic properties of a new c... more Herein, we report the synthesis, structural characterization, and catalytic properties of a new cobalt-based metal-organic framework, Co 2 IJBDC) 2 IJDABCO) (where BDC = 1,4-benzenedicarboxylate and DABCO = 1,4-diazabicycloij2.2.2]octane), namely VNU-10 (where VNU = Vietnam National University). Single-crystal X-ray diffraction analysis revealed that VNU-10 crystallizes in the hexagonal space group P6/mmm. The structure of this material is comprised of two-dimensional kgm layers which are interconnected by DABCO pillars to afford an overall three-dimensional porous architecture. VNU-10 exhibited exceptional catalytic activity toward the direct amination of oxazoles via C-H/N-H couplings while a previously reported topological isomer, Co 2 IJBDC) 2 IJDABCO), with the sql topology, displayed poor activity. Leaching tests indicated that homogeneous catalysis via leached active cobalt species is unlikely. Furthermore, the VNU-10 catalyst was facilely isolated from the reaction mixture and reused several times without degradation of the catalytic reactivity.
The structural integrity of aircraft has become a critical issue in ensuring the combat readiness... more The structural integrity of aircraft has become a critical issue in ensuring the combat readiness of aging aircraft and in the need for more efficient use and maintenance of vital assets. In order to address this need, a science-based approach was taken to develop the Structural Integrity Prognosis System (SIPS). Based on collaboration between detailed models of fatigue damage, sensor systems, and a unique reasoning system, SIPS provides probabilistic predictions of structural viability for individual assets under user-specified future use scenarios. After an extensive development program involving hundreds of laboratory experiments and three full-scale tests of retired aircraft wing panels, the SIPS Program has entered the transition stage where its utility is being evaluated on active vehicles in the Navy fleet. Under joint funding with the Navy, an operational P-3 Orion based at the Naval Air Station in Jacksonville, Florida, was designated to be the first aircraft to demonstrate...
Most engineering components and structures contain stress concentrations, such as notches. The st... more Most engineering components and structures contain stress concentrations, such as notches. The state of stress at such concentrations is typically multiaxial due to the notch geometry, and/or multiaxiality of the loading. Significant portions of the fatigue life of notched members are usually spent in crack initiation (crack formation and microscopic growth) and macroscopic crack growth. Synergistic complexity of combined stress and stress concentration has been evaluated in a limited number of studies. Available experimental evidence suggests the current life estimation and fatigue damage analysis techniques commonly used may not be capable of accurate predictions for such complex and yet highly practical conditions. This paper investigates notched fatigue behavior under multiaxial loads using aluminum alloys. Many effects involved in such loading conditions are included. These include the effects of stress state (axial, torsion, combined axial-torsion), geometry condition (smooth ...
We discovered a general and comprehensive approach for the regioselective hydroboration of termin... more We discovered a general and comprehensive approach for the regioselective hydroboration of terminal and internal alkynes to synthesize vinylboronates using inexpensive and magnetically separable copper ferrite nanoparticles at low catalyst loading using Bis(pinacolato)diboron in the absence of ligand and additives, under mild and greener conditions. A diverse range of functional groups was tolerated in the reaction, including allene and enones, and the corresponding boronates were obtained in high yields under air. Moreover, the assynthesized alkenylboronates were used as precursors to prepare wide variety of vinylorgano chalcogenides regioselectively, in high yields. The present protocol enable the conversion of C sp H bonds to make C sp 2 B bonds via activation of B B bond, followed by formation of C sp 2 Se (Te or S) bonds via activation of Se (Te or S)-Se (Te or S) bonds in a regioselective manner. Deuterium isotope labeling studies showed that the proton source of vinyl boronate stem from the solvent employed.
A zeolite imidazolate framework, ZIF-8, was synthesized and characterized by dynamic laser light ... more A zeolite imidazolate framework, ZIF-8, was synthesized and characterized by dynamic laser light scattering, X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, Fourier transform infrared, atomic absorption spectrophotometry, and nitrogen adsorption measurements. The ZIF-8 was highly crystalline and porous with a surface area of over 1600 m 2 /g. Friedel-Crafts acylation of anisole and benzoyl chloride proceeded well in the presence of ZIF-8 (2-6 mol%) without the need for an inert atmosphere. The reaction afforded a selectivity of 93%-95% to the p-isomer. The solid catalyst can be separated from the reaction mixture by simple centrifugation and reused without significant degradation in catalytic activity. There was no leaching of active acid species into the reaction solution.
ABSTRACT we have described a method for oxidative cross coupling reactions between N-alkyl anilin... more ABSTRACT we have described a method for oxidative cross coupling reactions between N-alkyl anilines and terminal alkynes forming N-aryl-N-methylpropargylamines. Superparamagnetic CuFe2O4 nanoparticles were employed as efficient and robust catalyst. The optimal conditions involved the use of tert-butyl hydroperoxide (TBHP) as oxidant and methylating reagent in dimethylacetamide (DMA) solvent at 140oC. The two-step reaction, methylation and C-C cross coupling, proceeds efficiently and high selectivity as well as good conversions were achieved in short reaction time. Preliminary mechanistic investigation was conducted. The CuFe2O4 nanoparticles could be facilely separated from the reaction mixture by magnetic decantation and could be reused several times with only a slight decrease in catalytic activity.
A crystalline porous metal-organic framework Cu 2 (BDC) 2 (DABCO) was synthesized and characteriz... more A crystalline porous metal-organic framework Cu 2 (BDC) 2 (DABCO) was synthesized and characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), atomic absorption spectrophotometry (AAS), hydrogen temperature-programmed reduction (H 2-TPR), and nitrogen physisorption measurements. The Cu 2 (BDC) 2 (DABCO) was used as a heterogeneous catalyst for the direct CC coupling reaction via cascade methylation and C-H functionalization of N-methylaniline and terminal alkynes. tert-Butyl hydroperoxide also served as the methylating reagent in the transformation, and N-methyl-N-(3-phenylprop-2-ynyl)benzenamine but not N-(3-phenylprop-2-ynyl)benzenamine was produced as the principal product. The Cu-MOF could be recovered and reused several times without a significant degradation in catalytic activity. To the best of our knowledge, the direct oxidative CC coupling between N-methylaniline and alkynes with methylation transformation was not previously mentioned in the literature.
ABSTRACT A crystalline porous metal-organic framework [Cu2(EDB)2(BPY)] was synthesized and used a... more ABSTRACT A crystalline porous metal-organic framework [Cu2(EDB)2(BPY)] was synthesized and used as a heterogeneous catalyst for the direct oxidative amidation between N,N-dimethylanilines and anhydrides to form tertiary amides as the principal products. The [Cu2(EDB)2(BPY)] exhibited similar activity as compared to that of [Cu2(BDC)2(BPY)], [Cu2(BDC)2(DABCO)], MOF-143, and other common homogeneous salt catalysts. The optimal reaction conditions employed were [Cu2(EDB)2(BPY)] (10 % mol), TBHP (2 equiv), pyridine (1 equiv) in CH3CN at 80 °C over 2 h. The Cu2(EDB)2(BPY) could be separated from the reaction mixture by filtration, and could be recovered and reused several times without a significant degradation in catalytic activity and selectivity. Furthermore, generality of the optimal conditions was confirmed by employing various N,N-dimethylaniline and anhydride derivatives.
Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2013, 2013
ABSTRACT An extrinsic Fabry-Perot interferometric (EFPI) fiber optic sensor is presented for meas... more ABSTRACT An extrinsic Fabry-Perot interferometric (EFPI) fiber optic sensor is presented for measurement of strain at high ambient temperatures. The sensor is fabricated using a femto-second (fs) laser. The EFPI sensor is fabricated by micromachining a cavity on the tip of a standard single-mode fiber and is then self-enclosed by fusion splicing another piece of singlemode fiber. The fs-laser based fabrication makes the sensor thermally stable to sustain temperatures as high as 800 °C. The sensor is relatively insensitive towards the temperature as compared to its response towards the applied strain. The sensor can be embedded in Carbon fiber/Bismaleimide (BMI) composite laminates for strain monitoring at high ambient temperatures.
Studies on amyloid beta (Ab|), the peptide thought to play a crucial role in the pathogenesis of ... more Studies on amyloid beta (Ab|), the peptide thought to play a crucial role in the pathogenesis of Alzheimer's disease, have implicated mitochondria in Ab-mediated neurotoxicity. We used differentiated PC12 cells stably transfected with an inducible green fluorescent protein (GFP) fusion protein containing an N¢-terminal mitochondrial targeting sequence (mtGFP), to examine the effects of sub-lethal Ab on the import of nuclear-encoded proteins to mitochondria. Exposure to sub-lethal Ab 25-35 (10 lmol/L) for 48 h inhibited mtGFP import to mitochondria; average rates decreased by 20 ± 4%. Concomitant with the decline in mtGFP, cytoplasmic mtGFP increased significantly while mtGFP expression and intramitochondrial mtGFP turnover were unchanged. Sub-lethal Ab 1-42 inhibited mtGFP import and increased cytoplasmic mtGFP but only after 96 h. The import of two endogenous nuclear-encoded mitochondrial proteins, mortalin/mtHsp70 and Tom20 also declined. Prior to the decline in import, mitochondrial membrane potential (mmp), and reactive oxygen species levels were unchanged in Ab-treated cells versus reverse phase controls. Sustained periods of decreased import were associated with decreased mmp, increased reactive oxygen species, increased vulnerability to oxygen-glucose deprivation and altered mitochondrial morphology. These findings suggest that an Ab-mediated inhibition of mitochondrial protein import, and the consequent mitochondrial impairment, may contribute to Alzheimer's disease.
ABSTRACT A highly porous metal–organic framework (IRMOF-3) was synthesized from the reaction of z... more ABSTRACT A highly porous metal–organic framework (IRMOF-3) was synthesized from the reaction of zinc nitrate hexahydrate and 2-amino-1,4-benzenedicarboxylic acid by solvothermal method. Physical characterizations of the material were achieved by using a variety of different techniques, including X-ray powder diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), atomic absorption spectrophotometry (AAS), and nitrogen physisorption measurements. The IRMOF-3 was used as an efficient heterogeneous catalyst for the Paal–Knorr reaction of benzyl amine with 2,5-hexanedione. Excellent conversions were obtained under mild conditions in the presence of 3 mol% catalyst. The IRMOF-3 catalyst could be reused several times without a significant degradation in catalytic activity.
This study compares both uniaxial and multiaxial variable amplitude experimental crack growth dat... more This study compares both uniaxial and multiaxial variable amplitude experimental crack growth data for naturally initiated fatigue cracks in tubular specimens of 2024-T3 aluminum alloy to predictions based on two state-of-the-art analysis codes: UniGrow and FASTRAN. For variable amplitude fatigue tests performed under pure axial nominal loading conditions, both UniGrow and FASTRAN analyses were found to produce mostly conservative growth life predictions, despite good agreement with constant amplitude crack growth data. For variable amplitude torsion and combined axial-torsion crack growth analyses, however, the conservatism in growth life predictions was found to reduce. This was attributed to multiaxial nominal stress state effects, such as T-stress and mixed-mode crack growth, which are not accounted for in either UniGrow or FASTRAN, but were found in constant amplitude fatigue tests to increase experimental crack growth rates. Since cracks in this study were initiated naturally,...
An accurate, automated assessment of helicopters’ gross weight (GW) and center-ofgravity (CG) is ... more An accurate, automated assessment of helicopters’ gross weight (GW) and center-ofgravity (CG) is critical for the determination of aircraft fatigue and life estimates since GW/CG greatly affect static and dynamic characteristics of helicopters. Current methods of GW and CG data gathering requires not only meticulous bookkeeping but it also makes several assumptions about fuel burn rate if fuel quantity instrumentation is not available. In addition, if cargos are released/picked-up during a flight, a log of loads and time instants is required to ensure aircraft GW and CG are known at all times during the flight. This paper reviews current state of the art for the GW and CG estimations’, summarizing different techniques and underlining their advantages and limitations. Preliminary results are presented in order to underline the capabilities of the proposed techniques. Then, the general framework of an innovative, hybrid approach that combines Artificial Neural Networks (ANN) and Kalma...
A transparent composite is based on the concept of matching the refractive index of the glass fib... more A transparent composite is based on the concept of matching the refractive index of the glass fiber with that of resin. A novel optically transparent glass-ribbon reinforced polymer matrix composite has been developed by infusing a clear epoxy resin system of matching refractive index into a stacked glass-ribbon preform (micron-size glass fibers with rectangular cross section). Rectangular glass ribbons are used as reinforcement as opposed to the conventional circular glass fibers since ribbons scatter less light than circular fibers for any given condition. The preforms are then laid up, vacuum bagged and infused with the resin. The set-up is then cured in an oven under controlled temperature and vacuum to make transparent composite panels. Various physical and mechanical tests like fiber volume fraction, tensile, flexure and impact have been conducted. Results indicate significant improvements in stiffness, strength and impact resistance. These new glass-ribbon reinforced structural transparent composites will find applications in windshields, windows and other components where a strong, lightweight transparent material is desirable. Transparent composites will also have potential applications as backing material in transparent armor systems
This study investigates the possibility of correlating specimen property to part performance for ... more This study investigates the possibility of correlating specimen property to part performance for laser beam powder bed fusion (LB-PBF) additive manufacturing by altering the process parameters in order to create similar thermal histories experienced during fabrication. In particular, the effects of altering scanning speed on LB-PBF 17-4 precipitation hardening (PH) stainless steel (SS) parts with different geometries on the thermal history, as well as the resultant defect formation, microstructure, and fatigue behavior are studied. It was found that parts with different geometries, all fabricated using the same manufacturer recommended process parameters, exhibited different fatigue strengths, which challenges the specimen property to part performance correlation. Melt pool analysis revealed that altering scanning speed can affect the melt pool characteristics including its depth and overlap depth. Increasing the input energy within the process window, by decreasing the scanning speed during fabrication, was seen to result in deeper melt pools and melt pool overlaps, and consequently, less volumetric defects, specifically lack of fusion, in the material. Therefore, the scanning speed was adjusted for different geometries to result in similar melt pool characteristics, as an indicator of the thermal history experienced during fabrication, which also resulted in these parts having similar porosity. Accordingly, fatigue lives of parts fabricated with adjusted process parameters were observed to be within a similar range. While many other factors may be involved, the findings of this research indicate that maintaining a similar thermal history by altering the process parameters is critical in establishing reliable relationships between specimen property and part performance in additive manufacturing.
Fatigue & Fracture of Engineering Materials & Structures, 2019
Although additive manufacturing (AM) has gained significant attention due to the advantages it of... more Although additive manufacturing (AM) has gained significant attention due to the advantages it offers and is currently a focus of much research, design of critical load carrying components utilizing such processes is still at its infancy. This is due to the fact that most of the load carrying components made by AM processes are subjected to cyclic loads, and fatigue behaviour of AM metals is far less understood as compared with those made by conventional methods, such as wrought and cast metals. To better understand the fatigue behaviour of AM metals, a wide range of issues that affect the behaviour in a synergistic manner must be considered. These include the effects of defects, residual stresses, surface finish, geometry and size, layer orientation, and heat treatment. Additionally, due to the multiaxial nature of the loading and/or complex geometries typically manufactured by AM processes, the stress state is often multiaxial including both normal and shear stresses. In this paper, the aforementioned effects influencing the fatigue resistance of AM parts, including torsion and multiaxial fatigue behaviour, are briefly discussed using some recently generated experimental data on Ti-6Al-4V by the authors.
Computer Methods in Applied Mechanics and Engineering, 2018
This study presents the weak form of peridynamic (PD) governing equations which permit the direct... more This study presents the weak form of peridynamic (PD) governing equations which permit the direct imposition of nonlocal essential and natural boundary conditions. It also presents a variational approach to derive the PD form of first-and second-order derivatives of a field variable at a point which is not symmetrically located in its domain of interaction. This capability enables the nonlocal PD representation of the internal force vector and the stress components without any calibration procedure. Furthermore, it removes the concern of truncated domain of interaction for a point near the surface. Thus, the solution is free of nonlocal boundary forces and surface effects. The numerical solution of the resulting equations can be achieved by considering an unstructured nonuniform discretization. The implicit solution to the discrete form of the equations is achieved by employing BiConjugate Gradient Stabilized (BICGSTAB) method which is an iterative technique for solving sparse non-symmetric linear systems. The explicit analysis is performed by constructing a global diagonal mass matrix, and using a hybrid implicit/explicit time integration scheme. The accuracy of this approach is demonstrated by considering an elastic isotropic plate with or without a cutout subjected to a combination of different types of boundary conditions under plane stress conditions.
Fatigue & Fracture of Engineering Materials & Structures, 2017
Fatigue damage characteristics of aluminium alloy under complex biaxial loads such as in-phase an... more Fatigue damage characteristics of aluminium alloy under complex biaxial loads such as in-phase and out-of-phase loading conditions and different biaxiality ratios have been investigated. The effects of microscale phenomena on macroscale crack growth were studied to develop an in-depth understanding of crack nucleation and growth. Material characterization was conducted to study the microstructure variability. Scanning electron microscopy was used to identify the second phase particles, and energy dispersive X-ray spectroscopy was performed to analyse their phases and elements. Extensive quasi-static and fatigue tests were conducted on Al7075-T651 cruciform specimens over a wide range of load ratios and phases. Detailed fractography analysis was conducted to understand the crack growth behaviour observed during the fatigue tests. Significant differences in crack initiation and propagation behaviour were observed when a phase difference was applied. Primarily, crack retardation and splitting were observed because of the constantly varying mode mixity caused by phase difference. The crack growth behaviour and fatigue lives under out-of-phase loading were compared with those under in-phase loading to understand the effect of mixed-mode fracture.
Herein, we report the synthesis, structural characterization, and catalytic properties of a new c... more Herein, we report the synthesis, structural characterization, and catalytic properties of a new cobalt-based metal-organic framework, Co 2 IJBDC) 2 IJDABCO) (where BDC = 1,4-benzenedicarboxylate and DABCO = 1,4-diazabicycloij2.2.2]octane), namely VNU-10 (where VNU = Vietnam National University). Single-crystal X-ray diffraction analysis revealed that VNU-10 crystallizes in the hexagonal space group P6/mmm. The structure of this material is comprised of two-dimensional kgm layers which are interconnected by DABCO pillars to afford an overall three-dimensional porous architecture. VNU-10 exhibited exceptional catalytic activity toward the direct amination of oxazoles via C-H/N-H couplings while a previously reported topological isomer, Co 2 IJBDC) 2 IJDABCO), with the sql topology, displayed poor activity. Leaching tests indicated that homogeneous catalysis via leached active cobalt species is unlikely. Furthermore, the VNU-10 catalyst was facilely isolated from the reaction mixture and reused several times without degradation of the catalytic reactivity.
The structural integrity of aircraft has become a critical issue in ensuring the combat readiness... more The structural integrity of aircraft has become a critical issue in ensuring the combat readiness of aging aircraft and in the need for more efficient use and maintenance of vital assets. In order to address this need, a science-based approach was taken to develop the Structural Integrity Prognosis System (SIPS). Based on collaboration between detailed models of fatigue damage, sensor systems, and a unique reasoning system, SIPS provides probabilistic predictions of structural viability for individual assets under user-specified future use scenarios. After an extensive development program involving hundreds of laboratory experiments and three full-scale tests of retired aircraft wing panels, the SIPS Program has entered the transition stage where its utility is being evaluated on active vehicles in the Navy fleet. Under joint funding with the Navy, an operational P-3 Orion based at the Naval Air Station in Jacksonville, Florida, was designated to be the first aircraft to demonstrate...
Most engineering components and structures contain stress concentrations, such as notches. The st... more Most engineering components and structures contain stress concentrations, such as notches. The state of stress at such concentrations is typically multiaxial due to the notch geometry, and/or multiaxiality of the loading. Significant portions of the fatigue life of notched members are usually spent in crack initiation (crack formation and microscopic growth) and macroscopic crack growth. Synergistic complexity of combined stress and stress concentration has been evaluated in a limited number of studies. Available experimental evidence suggests the current life estimation and fatigue damage analysis techniques commonly used may not be capable of accurate predictions for such complex and yet highly practical conditions. This paper investigates notched fatigue behavior under multiaxial loads using aluminum alloys. Many effects involved in such loading conditions are included. These include the effects of stress state (axial, torsion, combined axial-torsion), geometry condition (smooth ...
We discovered a general and comprehensive approach for the regioselective hydroboration of termin... more We discovered a general and comprehensive approach for the regioselective hydroboration of terminal and internal alkynes to synthesize vinylboronates using inexpensive and magnetically separable copper ferrite nanoparticles at low catalyst loading using Bis(pinacolato)diboron in the absence of ligand and additives, under mild and greener conditions. A diverse range of functional groups was tolerated in the reaction, including allene and enones, and the corresponding boronates were obtained in high yields under air. Moreover, the assynthesized alkenylboronates were used as precursors to prepare wide variety of vinylorgano chalcogenides regioselectively, in high yields. The present protocol enable the conversion of C sp H bonds to make C sp 2 B bonds via activation of B B bond, followed by formation of C sp 2 Se (Te or S) bonds via activation of Se (Te or S)-Se (Te or S) bonds in a regioselective manner. Deuterium isotope labeling studies showed that the proton source of vinyl boronate stem from the solvent employed.
A zeolite imidazolate framework, ZIF-8, was synthesized and characterized by dynamic laser light ... more A zeolite imidazolate framework, ZIF-8, was synthesized and characterized by dynamic laser light scattering, X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, Fourier transform infrared, atomic absorption spectrophotometry, and nitrogen adsorption measurements. The ZIF-8 was highly crystalline and porous with a surface area of over 1600 m 2 /g. Friedel-Crafts acylation of anisole and benzoyl chloride proceeded well in the presence of ZIF-8 (2-6 mol%) without the need for an inert atmosphere. The reaction afforded a selectivity of 93%-95% to the p-isomer. The solid catalyst can be separated from the reaction mixture by simple centrifugation and reused without significant degradation in catalytic activity. There was no leaching of active acid species into the reaction solution.
ABSTRACT we have described a method for oxidative cross coupling reactions between N-alkyl anilin... more ABSTRACT we have described a method for oxidative cross coupling reactions between N-alkyl anilines and terminal alkynes forming N-aryl-N-methylpropargylamines. Superparamagnetic CuFe2O4 nanoparticles were employed as efficient and robust catalyst. The optimal conditions involved the use of tert-butyl hydroperoxide (TBHP) as oxidant and methylating reagent in dimethylacetamide (DMA) solvent at 140oC. The two-step reaction, methylation and C-C cross coupling, proceeds efficiently and high selectivity as well as good conversions were achieved in short reaction time. Preliminary mechanistic investigation was conducted. The CuFe2O4 nanoparticles could be facilely separated from the reaction mixture by magnetic decantation and could be reused several times with only a slight decrease in catalytic activity.
A crystalline porous metal-organic framework Cu 2 (BDC) 2 (DABCO) was synthesized and characteriz... more A crystalline porous metal-organic framework Cu 2 (BDC) 2 (DABCO) was synthesized and characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), atomic absorption spectrophotometry (AAS), hydrogen temperature-programmed reduction (H 2-TPR), and nitrogen physisorption measurements. The Cu 2 (BDC) 2 (DABCO) was used as a heterogeneous catalyst for the direct CC coupling reaction via cascade methylation and C-H functionalization of N-methylaniline and terminal alkynes. tert-Butyl hydroperoxide also served as the methylating reagent in the transformation, and N-methyl-N-(3-phenylprop-2-ynyl)benzenamine but not N-(3-phenylprop-2-ynyl)benzenamine was produced as the principal product. The Cu-MOF could be recovered and reused several times without a significant degradation in catalytic activity. To the best of our knowledge, the direct oxidative CC coupling between N-methylaniline and alkynes with methylation transformation was not previously mentioned in the literature.
ABSTRACT A crystalline porous metal-organic framework [Cu2(EDB)2(BPY)] was synthesized and used a... more ABSTRACT A crystalline porous metal-organic framework [Cu2(EDB)2(BPY)] was synthesized and used as a heterogeneous catalyst for the direct oxidative amidation between N,N-dimethylanilines and anhydrides to form tertiary amides as the principal products. The [Cu2(EDB)2(BPY)] exhibited similar activity as compared to that of [Cu2(BDC)2(BPY)], [Cu2(BDC)2(DABCO)], MOF-143, and other common homogeneous salt catalysts. The optimal reaction conditions employed were [Cu2(EDB)2(BPY)] (10 % mol), TBHP (2 equiv), pyridine (1 equiv) in CH3CN at 80 °C over 2 h. The Cu2(EDB)2(BPY) could be separated from the reaction mixture by filtration, and could be recovered and reused several times without a significant degradation in catalytic activity and selectivity. Furthermore, generality of the optimal conditions was confirmed by employing various N,N-dimethylaniline and anhydride derivatives.
Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2013, 2013
ABSTRACT An extrinsic Fabry-Perot interferometric (EFPI) fiber optic sensor is presented for meas... more ABSTRACT An extrinsic Fabry-Perot interferometric (EFPI) fiber optic sensor is presented for measurement of strain at high ambient temperatures. The sensor is fabricated using a femto-second (fs) laser. The EFPI sensor is fabricated by micromachining a cavity on the tip of a standard single-mode fiber and is then self-enclosed by fusion splicing another piece of singlemode fiber. The fs-laser based fabrication makes the sensor thermally stable to sustain temperatures as high as 800 °C. The sensor is relatively insensitive towards the temperature as compared to its response towards the applied strain. The sensor can be embedded in Carbon fiber/Bismaleimide (BMI) composite laminates for strain monitoring at high ambient temperatures.
Studies on amyloid beta (Ab|), the peptide thought to play a crucial role in the pathogenesis of ... more Studies on amyloid beta (Ab|), the peptide thought to play a crucial role in the pathogenesis of Alzheimer's disease, have implicated mitochondria in Ab-mediated neurotoxicity. We used differentiated PC12 cells stably transfected with an inducible green fluorescent protein (GFP) fusion protein containing an N¢-terminal mitochondrial targeting sequence (mtGFP), to examine the effects of sub-lethal Ab on the import of nuclear-encoded proteins to mitochondria. Exposure to sub-lethal Ab 25-35 (10 lmol/L) for 48 h inhibited mtGFP import to mitochondria; average rates decreased by 20 ± 4%. Concomitant with the decline in mtGFP, cytoplasmic mtGFP increased significantly while mtGFP expression and intramitochondrial mtGFP turnover were unchanged. Sub-lethal Ab 1-42 inhibited mtGFP import and increased cytoplasmic mtGFP but only after 96 h. The import of two endogenous nuclear-encoded mitochondrial proteins, mortalin/mtHsp70 and Tom20 also declined. Prior to the decline in import, mitochondrial membrane potential (mmp), and reactive oxygen species levels were unchanged in Ab-treated cells versus reverse phase controls. Sustained periods of decreased import were associated with decreased mmp, increased reactive oxygen species, increased vulnerability to oxygen-glucose deprivation and altered mitochondrial morphology. These findings suggest that an Ab-mediated inhibition of mitochondrial protein import, and the consequent mitochondrial impairment, may contribute to Alzheimer's disease.
ABSTRACT A highly porous metal–organic framework (IRMOF-3) was synthesized from the reaction of z... more ABSTRACT A highly porous metal–organic framework (IRMOF-3) was synthesized from the reaction of zinc nitrate hexahydrate and 2-amino-1,4-benzenedicarboxylic acid by solvothermal method. Physical characterizations of the material were achieved by using a variety of different techniques, including X-ray powder diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), atomic absorption spectrophotometry (AAS), and nitrogen physisorption measurements. The IRMOF-3 was used as an efficient heterogeneous catalyst for the Paal–Knorr reaction of benzyl amine with 2,5-hexanedione. Excellent conversions were obtained under mild conditions in the presence of 3 mol% catalyst. The IRMOF-3 catalyst could be reused several times without a significant degradation in catalytic activity.
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