Ali Nikbakht - Academia.edu (original) (raw)
Papers by Ali Nikbakht
AIP Conference Proceedings, 2008
This paper will present an overview of efforts to investigate the application of Raman spectrosco... more This paper will present an overview of efforts to investigate the application of Raman spectroscopy for the characterization of Kevlar materials. Raman spectroscopy is a laser technique that is sensitive to molecular interactions in materials such as Kevlar, graphite and carbon used in composite materials. The overall goal of this research reported here is to evaluate Raman spectroscopy as a potential nondestructive evaluation (NDE) tool for the detection of stress rupture in Kevlar composite over-wrapped pressure vessels (COPVs). Characterization of the Raman spectra of Kevlar yarn and strands will be presented and compared with analytical models provided in the literature. Results of testing to investigate the effects of creep and hightemperature aging on the Raman spectra will be presented.
Polymers, 2021
Interleaving composite laminates by nanofibers is a well-known method of increasing interlaminar ... more Interleaving composite laminates by nanofibers is a well-known method of increasing interlaminar fracture toughness. Among many possibilities, polycaprolactone (PCL) nanofibers is one of the best choices for toughening composite laminates. The influence of PCL on delamination mode of failure is considered before. However, the effect of PCL on other damage modes, such as fiber breakage and matrix cracking, is yet to be studied. In this study, the acoustic emission (AE) technique is applied to determine the effect of toughening composite laminates by PCL nanofibers on matrix cracking, fiber/matrix debonding, and fiber breakage failure mechanisms. For this purpose, mode I and mode II fracture tests are conducted on modified and non-modified glass/epoxy laminates. Three different methods, i.e., peak frequency, wavelet transform, and sentry function, are utilized for analyzing the recorded AE data from mode I test. The results show that applying PCL nanofibers not only increases the mode...
The geometrical features of nanofibers, such as nanomat thickness and the diameter of nanofibers,... more The geometrical features of nanofibers, such as nanomat thickness and the diameter of nanofibers, have a significant influence on the toughening behavior of composite laminates. In this study, carbon/epoxy laminates were interleaved with polysulfone (PSF) nanofibrous mats and the effect of the PSF nanomat thickness on the fracture toughness was considered for the first time. For this goal, the nanofibers were first produced by the electrospinning method. Then, double cantilever beam (DCB) specimens were manufactured, and mode-I fracture tests were conducted. The results showed that enhancing the mat thickness could increase the fracture toughness considerably (to about 87% with the maximum thickness). The toughening mechanism was also considered by presenting a schematic picture. Micrographs were taken using a scanning electron microscope (SEM).
Volume 4: Advanced Manufacturing Processes; Biomedical Engineering; Multiscale Mechanics of Biological Tissues; Sciences, Engineering and Education; Multiphysics; Emerging Technologies for Inspection, 2012
Graded materials, also known as functionally graded materials (FGMs), are multiphase composites m... more Graded materials, also known as functionally graded materials (FGMs), are multiphase composites mainly composed of a ceramic and a metal; thus, they exploit the heat, oxidation and corrosion resistance typical of ceramics, and the strength, ductility and toughness typical of metals. These materials are mainly used as heat barriers. In addition, many of the present and potential applications of FGMs involve contact problems. On the other hand, the production process of FGMs is somewhat complex and leaves some defects in the produced structure. One of the most important defects in such structures is surface cracks. Here, the combination of the contact and crack problems is investigated in a functionally graded rectangular plate containing a semi–elliptic surface crack indented by a frictionless rigid spherical indenter. The plate is simply supported and the crack is located in the middle of the plate surface in the tension part. The crack surface is parallel to one of the plate edges....
Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, 2012
Functionally graded materials are multiphase composites mainly composed of a ceramic and a metal;... more Functionally graded materials are multiphase composites mainly composed of a ceramic and a metal; thus, they merge the heat, oxidation and corrosion resistance typical of ceramics, and the strength, ductility and toughness typical of metals. Many of the present and possible applications of functionally graded materials involve contact loading. Here, the contact problem of a functionally graded simply supported plate with finite dimensions by a rigid spherical punch is studied by an analytical–numerical method. The contact rule will be derived by solving the equations of equilibrium analytically in terms of the displacement field components and by taking advantage of a numerical method in finding the contact parameters. The stress–strain relation is assumed to be linear and is represented by a refined volume fraction based model originally proposed by Tamura–Tomota–Ozawa model. The results of the analytical–numerical approach are validated by using ABAQUS finite element package. The ...
European Journal of Mechanics - A/Solids, 2014
Abstract Elastic contact of a functionally graded plate of finite dimensions with continuous vari... more Abstract Elastic contact of a functionally graded plate of finite dimensions with continuous variation of material properties and a rigid spherical indenter is studied. The plate is consisted of a ductile (metal) phase at the lower and a brittle (ceramic) phase at the upper surface. The punch acts on the upper surface which is the ceramic richer section of the plate. The contact problem in functionally graded (FG) structures has been studied widely; in such problems the main focus has been on FG structures with infinite dimensions where Hertzian or modified Hertzian contact laws can properly predict the contact parameters such as the size of the contact region and the pressure distribution under the punch. However, due to the finite dimensions of the considered plate in this study, the contact problem needs to be reconsidered. While Hertz's contact law predicts a power equal to 1.5 for the force indentation relation, the results of this study show that for an FG plate the exponent of the contact law depends on the brittle to ductile phases ratio of moduli of elasticity and material properties distribution. In cases in which the brittle phase has a lower modulus of elasticity compared to the ductile phase, the contact law exponent is independent of material properties distribution. In addition, in such cases the maximum compressive contact stress is located directly on the upper surface of the plate. On the other hand, in cases in which the brittle phase is stiffer than the metal phase, the exponent of the contact law is a function of material properties distribution and the location of the maximum compressive contact stress is beneath the upper surface. In addition, in general the contact parameters are independent from the microstructural interactions of the constituting phases. Since several numerical examples are examined here, these findings can be interpreted as the most general rules in the contact problem between an FG plate and a rigid sphere.
Composite Structures, 2013
ABSTRACT a b s t r a c t Frictionless elastic contact of a functionally graded vitreous enameled ... more ABSTRACT a b s t r a c t Frictionless elastic contact of a functionally graded vitreous enameled low carbon steel plate and a rigid spherical punch is studied in this paper. The graded modulus of elasticity of the plate is assumed to vary through the thickness and is found experimentally by means of nano-indentation technique and also esti-mated mathematically by a mathematical model originally proposed by Tamura, Tomota and Ozawa (TTO model). The contact problem of the plate and the rigid sphere is studied both analytically and numerically by means of ABAQUS finite element package. In the analytical approach, a higher order displacement field plate theory along with an exact Green's function is used to find the contact parameters. The acquired results are then used to investigate the effect of material distribution, punch radius and plate span on the contact parameters. The results show that increasing the metal phase volume fraction of graded med-ium increases both the contact force and stress in the graded plate. In addition, in the vitreous enameled graded plates, if a polynomial function is fitted to the contact force–indentation relation, the power is equal to 2.0 regardless of the material distribution and plate geometry.
AIP Conference Proceedings, 2008
This paper will present an overview of efforts to investigate the application of Raman spectrosco... more This paper will present an overview of efforts to investigate the application of Raman spectroscopy for the characterization of Kevlar materials. Raman spectroscopy is a laser technique that is sensitive to molecular interactions in materials such as Kevlar, graphite and carbon used in composite materials. The overall goal of this research reported here is to evaluate Raman spectroscopy as a potential nondestructive evaluation (NDE) tool for the detection of stress rupture in Kevlar composite over-wrapped pressure vessels (COPVs). Characterization of the Raman spectra of Kevlar yarn and strands will be presented and compared with analytical models provided in the literature. Results of testing to investigate the effects of creep and hightemperature aging on the Raman spectra will be presented.
Polymers, 2021
Interleaving composite laminates by nanofibers is a well-known method of increasing interlaminar ... more Interleaving composite laminates by nanofibers is a well-known method of increasing interlaminar fracture toughness. Among many possibilities, polycaprolactone (PCL) nanofibers is one of the best choices for toughening composite laminates. The influence of PCL on delamination mode of failure is considered before. However, the effect of PCL on other damage modes, such as fiber breakage and matrix cracking, is yet to be studied. In this study, the acoustic emission (AE) technique is applied to determine the effect of toughening composite laminates by PCL nanofibers on matrix cracking, fiber/matrix debonding, and fiber breakage failure mechanisms. For this purpose, mode I and mode II fracture tests are conducted on modified and non-modified glass/epoxy laminates. Three different methods, i.e., peak frequency, wavelet transform, and sentry function, are utilized for analyzing the recorded AE data from mode I test. The results show that applying PCL nanofibers not only increases the mode...
The geometrical features of nanofibers, such as nanomat thickness and the diameter of nanofibers,... more The geometrical features of nanofibers, such as nanomat thickness and the diameter of nanofibers, have a significant influence on the toughening behavior of composite laminates. In this study, carbon/epoxy laminates were interleaved with polysulfone (PSF) nanofibrous mats and the effect of the PSF nanomat thickness on the fracture toughness was considered for the first time. For this goal, the nanofibers were first produced by the electrospinning method. Then, double cantilever beam (DCB) specimens were manufactured, and mode-I fracture tests were conducted. The results showed that enhancing the mat thickness could increase the fracture toughness considerably (to about 87% with the maximum thickness). The toughening mechanism was also considered by presenting a schematic picture. Micrographs were taken using a scanning electron microscope (SEM).
Volume 4: Advanced Manufacturing Processes; Biomedical Engineering; Multiscale Mechanics of Biological Tissues; Sciences, Engineering and Education; Multiphysics; Emerging Technologies for Inspection, 2012
Graded materials, also known as functionally graded materials (FGMs), are multiphase composites m... more Graded materials, also known as functionally graded materials (FGMs), are multiphase composites mainly composed of a ceramic and a metal; thus, they exploit the heat, oxidation and corrosion resistance typical of ceramics, and the strength, ductility and toughness typical of metals. These materials are mainly used as heat barriers. In addition, many of the present and potential applications of FGMs involve contact problems. On the other hand, the production process of FGMs is somewhat complex and leaves some defects in the produced structure. One of the most important defects in such structures is surface cracks. Here, the combination of the contact and crack problems is investigated in a functionally graded rectangular plate containing a semi–elliptic surface crack indented by a frictionless rigid spherical indenter. The plate is simply supported and the crack is located in the middle of the plate surface in the tension part. The crack surface is parallel to one of the plate edges....
Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, 2012
Functionally graded materials are multiphase composites mainly composed of a ceramic and a metal;... more Functionally graded materials are multiphase composites mainly composed of a ceramic and a metal; thus, they merge the heat, oxidation and corrosion resistance typical of ceramics, and the strength, ductility and toughness typical of metals. Many of the present and possible applications of functionally graded materials involve contact loading. Here, the contact problem of a functionally graded simply supported plate with finite dimensions by a rigid spherical punch is studied by an analytical–numerical method. The contact rule will be derived by solving the equations of equilibrium analytically in terms of the displacement field components and by taking advantage of a numerical method in finding the contact parameters. The stress–strain relation is assumed to be linear and is represented by a refined volume fraction based model originally proposed by Tamura–Tomota–Ozawa model. The results of the analytical–numerical approach are validated by using ABAQUS finite element package. The ...
European Journal of Mechanics - A/Solids, 2014
Abstract Elastic contact of a functionally graded plate of finite dimensions with continuous vari... more Abstract Elastic contact of a functionally graded plate of finite dimensions with continuous variation of material properties and a rigid spherical indenter is studied. The plate is consisted of a ductile (metal) phase at the lower and a brittle (ceramic) phase at the upper surface. The punch acts on the upper surface which is the ceramic richer section of the plate. The contact problem in functionally graded (FG) structures has been studied widely; in such problems the main focus has been on FG structures with infinite dimensions where Hertzian or modified Hertzian contact laws can properly predict the contact parameters such as the size of the contact region and the pressure distribution under the punch. However, due to the finite dimensions of the considered plate in this study, the contact problem needs to be reconsidered. While Hertz's contact law predicts a power equal to 1.5 for the force indentation relation, the results of this study show that for an FG plate the exponent of the contact law depends on the brittle to ductile phases ratio of moduli of elasticity and material properties distribution. In cases in which the brittle phase has a lower modulus of elasticity compared to the ductile phase, the contact law exponent is independent of material properties distribution. In addition, in such cases the maximum compressive contact stress is located directly on the upper surface of the plate. On the other hand, in cases in which the brittle phase is stiffer than the metal phase, the exponent of the contact law is a function of material properties distribution and the location of the maximum compressive contact stress is beneath the upper surface. In addition, in general the contact parameters are independent from the microstructural interactions of the constituting phases. Since several numerical examples are examined here, these findings can be interpreted as the most general rules in the contact problem between an FG plate and a rigid sphere.
Composite Structures, 2013
ABSTRACT a b s t r a c t Frictionless elastic contact of a functionally graded vitreous enameled ... more ABSTRACT a b s t r a c t Frictionless elastic contact of a functionally graded vitreous enameled low carbon steel plate and a rigid spherical punch is studied in this paper. The graded modulus of elasticity of the plate is assumed to vary through the thickness and is found experimentally by means of nano-indentation technique and also esti-mated mathematically by a mathematical model originally proposed by Tamura, Tomota and Ozawa (TTO model). The contact problem of the plate and the rigid sphere is studied both analytically and numerically by means of ABAQUS finite element package. In the analytical approach, a higher order displacement field plate theory along with an exact Green's function is used to find the contact parameters. The acquired results are then used to investigate the effect of material distribution, punch radius and plate span on the contact parameters. The results show that increasing the metal phase volume fraction of graded med-ium increases both the contact force and stress in the graded plate. In addition, in the vitreous enameled graded plates, if a polynomial function is fitted to the contact force–indentation relation, the power is equal to 2.0 regardless of the material distribution and plate geometry.