Papers by Chandrasekaran Margam
Wear, 1998
Much of the work on ontologies in AI has focused on describing some aspect of reality: objects, r... more Much of the work on ontologies in AI has focused on describing some aspect of reality: objects, relations, states of affairs, events, and processes in the world. A goal is to make knowledge sharable, by encoding domain knowledge using a standard vocabulary based on the ontology. A parallel attempt at identifying the ontology of problem-solving knowledge has a goal of sharable problem-solving methods. For example, when one is dealing with abductive inference problems, the following are some of the terms that occur in the representation of problem-solving methods: hypotheses, explanatory coverage, evidence, likelihood, plausibility, composite hypothesis, etc. Method ontology is, in good part, goal-and method-specific.
Wear, 1998
Much of the work on ontologies in AI has focused on describing some aspect of reality: objects, r... more Much of the work on ontologies in AI has focused on describing some aspect of reality: objects, relations, states of affairs, events, and processes in the world. A goal is to make knowledge sharable, by encoding domain knowledge using a standard vocabulary based on the ontology. A parallel attempt at identifying the ontology of problem-solving knowledge has a goal of sharable problem-solving methods. For example, when one is dealing with abductive inference problems, the following are some of the terms that occur in the representation of problem-solving methods: hypotheses, explanatory coverage, evidence, likelihood, plausibility, composite hypothesis, etc. Method ontology is, in good part, goal-and method-specific.
Rapid Prototyping Journal, 2006
Home > Rapid Prototyping Journal > Volume 12 issue 4 > Indirect fabrication of collagen ... more Home > Rapid Prototyping Journal > Volume 12 issue 4 > Indirect fabrication of collagen scaffold based... ... Icon: Abstract. Icon: Backfiles. Icon: Print. Icon: Reprints & permissions. ... Purpose This paper presents a new indirect scaffold fabrication method for soft tissue based on ...
Tribology International, 2002
Friction and wear of non-ferrous metals and coatings are of considerable interest in the light of... more Friction and wear of non-ferrous metals and coatings are of considerable interest in the light of use of the above in tribo-contacts. As most of the mechanisms governing the seizure of materials are concentrated at the center of the contact and are hidden, it is difficult to observe any precipitate changes in the behavior of such materials using conventional tools. In this work, we have used an X-ray microscope for in-situ observation of frictional seizure, wear and interfacial features during the testing of aluminium (AA1100) specimens sliding against Al 6061 disk. This technique enables the observation of interfacial features of the hidden contact despite the small density differences that exist between the sliding pair of materials. Seizure tests were conducted at different sliding speeds of 2 and 4 m/s. Aluminium was found to undergo incipient melting during sliding at a sliding speed of 2 m/s whereas the specimen tested at 4 m/s was found to undergo a severe plastic distortion thus restricting further testing of the same. Frictional heating of concentrated spots resulted in melting wear and diffusion assisted bonding of surface layers. This was later found to develop into a conformal contact following a lumpy transfer of material. The mechanisms of seizure and wear were affected by the sliding speeds during testing. At lower sliding speed (2 m/s) the transfer and bonding of deposits occurred due to direct contact of nascent sliding surfaces.
Journal of Biomedical Materials Research Part A, 2007
Hydroxyapatite (HA) was incorporated as filler into polycaprolactone (PCL) matrix to improve the ... more Hydroxyapatite (HA) was incorporated as filler into polycaprolactone (PCL) matrix to improve the bioactivity as well as the compressive properties of the polymer composites that can be typically used in tissue engineering scaffolds. The compressive properties of five PCL/HA composites of different compositions were investigated in conjunction with the study of their rate of degradation. As PCL has a slow degradation rate, the experiment was conducted in a concentrated 5M sodium hydroxide medium to accelerate the degradation process. The compressive strength and modulus of all PCL/HA compositions were observed to decrease as the degradation experiment progressed, with samples having high HA content degraded most significantly as compared with samples with lower HA content. Pure PCL samples, however, were found to retain their mechanical properties comparatively well in the same degradation experiments. Although the addition of HA as filler into the PCL matrix was shown to have improved mechanical properties and bioactivity initially, these results do raise concerns of material properties being compromise during hydrolytic degradation. © 2006 Wiley Periodicals, Inc. J Biomed Mater Res, 2007
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2000
ABSTRACT
Wear, 1996
The lubrication of stellite in dry sliding by a graphite rod loaded against the wear track on the... more The lubrication of stellite in dry sliding by a graphite rod loaded against the wear track on the stellite surface was investigated. Stellite sliding against stellite was studied with a pin-on-disc machine which was also fitted with a spring loaded graphite pin. Tests were performed in air from ambient temperature to 400 °C for varying sliding speed and load. It was found that the wear of the stellite pin was reduced by approximately a factor of 10 compared to unlubricated sliding when the graphite pin was present. The wear coefficient of the graphite pin was similar in magnitude to the wear coefficient of the lubricated stellite. The friction coefficient of the stellite was also lowered significantly, but the overall frictional energy dissipation of both the steilile and graphite specimens was only marginally less than dry sliding under the same conditions. Surface analysis and microscopy of worn specimens revealed extensive deposition of graphite films on both the stellite pin and disc.
Journal of Materials Science-materials in Medicine, 2008
In scaffold guided tissue engineering (TE), temporary three-dimensional scaffolds are essential t... more In scaffold guided tissue engineering (TE), temporary three-dimensional scaffolds are essential to guide and support cell proliferation. Selective Laser Sintering (SLS) is studied for the development of such scaffolds by eliminating pore spatial control problems faced in conventional scaffolds fabrication methods. SLS offers good user control over the scaffold’s microstructures by adjusting its main processing parameters, namely the laser power, scan speed and part bed temperature. This research focuses on the improvements in the fabrication of TE scaffolds using SLS with powder biomaterials, namely hydroxyapatite (HA) and poly(vinyl alcohol) (PVA). Grinding of as-received PVA powder to varying particle sizes and two methods of mixing are investigated as the preparation process to determine a better mixing method that would enhance the mixture homogeneity. Suitable sintering conditions for the improved biocomposite are then achieved by varying the important process parameters such as laser power, scan speed and part bed temperature. SLS fabricated samples are characterized using Fourier Transform Infrared Spectrometer (FTIR) and Scanning Electron Microscope (SEM). FTIR results show that the grinding and sintering processes neither compromise the chemical composition of the PVA nor cause undue degradation. Visual analysis of the grinding, powder mixing and sintering effect are carried out with SEM. The SEM observations show improvements in the sintering effects. The favorable outcome ascertains PVA/HA biocomposite as a suitable material to be processed by SLS for TE scaffolds.
Rapid Prototyping Journal, 2006
An important requirement for a bone tissue engineering scaffold is a stiffness gradient that mimi... more An important requirement for a bone tissue engineering scaffold is a stiffness gradient that mimics that of native bone. Such scaffolds can be achieved by controlling their structure and porosity and are termed functionally graded scaffolds (FGS). Currently, the main challenges in FGS fabrication include the iterative and tedious design process as well as a heavy reliance on the user's CAD/CAM skills. This work aims to bring automated FGS production a step closer by providing a database that correlates scaffold porosity values and the corresponding compressive stiffness and integrating it into the design process. To achieve this goal, scaffolds with different structural configurations were designed using CASTS (Computer Aided System for Tissue Scaffolds), an in-house developed library system consisting of 13 different polyhedral units that can be assembled into scaffold structures. Polycaprolactone (PCL) was chosen as the scaffold material, while selective laser sintering, a powder-based rapid prototyping or additive manufacturing system was employed to fabricate the scaffolds. Mathematical relations correlating scaffold porosity and compressive stiffness readings were formulated and compiled. In addition, cytotoxicity assessment was conducted to evaluate the toxicity of the fabricated PCL scaffolds. Lastly, a brief demonstration of how the formulated relations are used in the FGS design process is presented.
Journal of Biomedical Materials Research Part B-applied Biomaterials, 2007
Porous collagen scaffolds with predefined 3-dimesional (3-D) networks of internal channels are fa... more Porous collagen scaffolds with predefined 3-dimesional (3-D) networks of internal channels are fabricated via an indirect rapid prototyping technique. To obtain the scaffolds, two drying methods, namely critical point drying and freeze-drying were investigated. The latter was found to be a more suitable process as it induced less shrinkage and reproduced the design morphology accurately. The resulting scaffold contained internal hollow channels with porous foam-like structure occupying the surrounding volume. The mean diameter of the pores was 180 ± 60 μm and the channels diameter obtained was measured as 437 ± 100 μm. The channels were defined and strengthened by a layer of skin due to the process of freeze-drying. These networks of internal channels serve to enhance the mass transport rate through the scaffold and help to increase the depth of cells penetration in the scaffold. © 2006 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 2006.
Wear, 1997
The friction and wear of butyl rubber sliding on a hard abrasive in unidirectional sliding was in... more The friction and wear of butyl rubber sliding on a hard abrasive in unidirectional sliding was investigated as a function of temperature and load. Experiments were conducted in an X-ray microscope chamber using apin on disk apparatus. Dry tests and tests under lubricated conditions with water ...
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2001
Aluminum alloy composites have evoked keen interest in recent times for potential applications in... more Aluminum alloy composites have evoked keen interest in recent times for potential applications in aerospace and automotive industries owing to their superior strength-to-weight ratio and high-temperature strength. In the automotive industry, these materials have been ...
Wear, 2000
Lubricated sliding and seizure of stainless steel is of considerable interest in light of the bet... more Lubricated sliding and seizure of stainless steel is of considerable interest in light of the better properties as it provides improved corrosion resistance, compared to conventional steels. The effect of stainless steel on oil stability during sliding is of importance as it contributes to the wear behaviour of the system. We have used an X-ray microscope for in situ observation of frictional seizure, wear and interfacial features during the testing of stainless steel specimens sliding against Al 6061 disk. This technique enables the observation of interfacial features at the hidden surfaces. Seizure tests were conducted at sliding speeds of 2, 4 and 5 m/s under lubricated conditions with plain mineral oil. The images obtained during the tests indicated a possible degradation of the lubricating oil, resulting in darkening at isolated locations as sliding continued. An increase in the shear rate (sliding speed) resulted in faster degradation of the oil and hence greater susceptibility to early seizure.
Wear, 2001
Artificial joints in orthopedics occupy a principal position owing to the increase in number of c... more Artificial joints in orthopedics occupy a principal position owing to the increase in number of cases suffering from arthritis and associated diseases in addition to impairment caused by accidents. In this work, one of the most commonly used joint material, i.e. ultrahigh molecular weight polyethylene (UHMWPE), was tested against the duplex stainless steels instead of the conventional 316 L stainless steel. The UHMWPE was found to exhibit the lowest friction coefficient and wear rates when lubricated with water followed by globulin and glucose. The friction coefficient in the presence of egg albumen was higher along with high wear rates recorded. Post-test evaluation of surface roughness and wear scar/track analysis was performed to identify the wear mechanisms. Worn surfaces were analyzed using a differential scanning calorimeter for changes in crystallinity with sliding. The specimens tested under lubricated conditions with glucose, egg albumen and globulin indicated the presence of reaction products on the worn surface. Adhesive and corrosive wear mechanisms were the predominant modes of wear identified on the polymer samples. The wear tracks indicated that the proteins did react with the counterface material forming a thin deposit on them. Low temperature nitriding of the duplex stainless steel counterfaces were performed and the UHMWPE specimens were tested under similar conditions against the nitrided surfaces. Low temperature nitriding of the counterface did result in improved tribological behavior of UHMWPE and the corrosive effects were minimal.
Journal of Materials Science Letters, 1998
Plasma sprayed zirconia coating is formed by multilayer deposition of rapidly solidi®ed ceramic p... more Plasma sprayed zirconia coating is formed by multilayer deposition of rapidly solidi®ed ceramic particles, and it is increasingly chosen for the surface modi®cation of engineering materials to impart high hardness, abrasion resistance and corrosion resistance . However, there are two main shortcomings with plasma sprayed zirconia coatings. One is poor bonding strength between coating and substrate; the other is high porosity density in the coating, which is caused by trapped air or protection gas during the deposition process. These are generally detrimental to the application of plasma sprayed coatings. The coating may peel off during service because of the poor bond strength. The porosity permits contaminants from the environment to penetrate the coating, which can lead to destabilization of the coating [2]. Laser processing makes use of the high energy in a single wavelength narrow laser beam to melt part or all of the coating and at the same time rapidly quenches the thin layer of metal surface to produce a ®ne-grained micro-crystalline structure . Laser remelting of plasma sprayed coating promises to reduce the porosity density and enhances the coating adhesion strength by developing metallurgical bonding between the coating and the interface. The compositional homogeneity of the coating is also improved [4±7].
Journal of Materials Science, 1998
Friction surfacing was carried out with tool steel (AISI 01) and inconel 600 consumables on mild ... more Friction surfacing was carried out with tool steel (AISI 01) and inconel 600 consumables on mild steel 1020 substrate in an argon atmosphere. Inconel bonded strongly with the substrate and there was evidence of interfacial compound formation between the substrate and coating. For tool steel coatings, a sharp boundary between the substrate and coating was observed by scanning electron microscopy. X-ray fluoroscopic imaging also revealed this boundary. Mechanical interlocking between the coating and the substrate appears to be insignificant so adhesion between the coatings and the substrate may be caused by solid-phase bonding. For friction surfacing of both tool steel and inconel, a nominal contact pressure as high as 21.8 MPa was required to obtain an adherent coating of uniform quality. © 1998 Chapman & Hall
Journal of Materials Processing Technology, 1997
ABSTRACT
Wear, 1998
Much of the work on ontologies in AI has focused on describing some aspect of reality: objects, r... more Much of the work on ontologies in AI has focused on describing some aspect of reality: objects, relations, states of affairs, events, and processes in the world. A goal is to make knowledge sharable, by encoding domain knowledge using a standard vocabulary based on the ontology. A parallel attempt at identifying the ontology of problem-solving knowledge has a goal of sharable problem-solving methods. For example, when one is dealing with abductive inference problems, the following are some of the terms that occur in the representation of problem-solving methods: hypotheses, explanatory coverage, evidence, likelihood, plausibility, composite hypothesis, etc. Method ontology is, in good part, goal-and method-specific.
Uploads
Papers by Chandrasekaran Margam