A.G. Odeshi

A cost effective route for the densification of carbon-carbon composites

Abstract Carbon–carbon (C/C) composites are candidate materials for many high temperature structural applications, but the high costs of manufacture prohibit their application in many cases, where the properties are desired but economically not affordable. This study focuses on the catalytic effects of metal carbonyl additions on the cross-linking efficiency of pre-ceramic silicon polymers, which are used for a rapid and cost effective densification of porous C/C composites. Whereas, the open pores in the C/C matrix, obtained via the polymer pyrolysis, are effectively closed by a one shot infiltration/pyrolysis step applying a dicobaltoctacarbonyl [Co 2 (CO) 8 ] modified polysilane, a similar level of densification of porous C/C composites by a one time infiltration/pyrolysis of polycarbosilane could not be achieved, although catalytic additions of Co 2 (CO) 8 were used as well. Besides the successful densification of C/C composites using the modified polysilane, improvement in the oxidation behaviour of the composites at elevated temperatures was also recorded. The influence of certain manufacturing process variables, such as curing temperature of the carbon precursor before pyrolysis and heat-treatment temperatures of the manufactured C/C composites before densification, on the structures and properties of the obtained composite materials were investigated.

Characterisation of coir fibre hybrid composites reinforced with clay particles and glass spheres

Hybrid composite materials were fabricated using an optimum coir fibre with functionalised clay and glass spheres using a vacuum-assisted resin transfer moulding. The coir fibres were treated with 10% sodium hydroxide (NaOH) to improve their bonding properties by removing the cellulose and lignin found on the fibre surface. Clay functionalisation was done using 3-aminopropyltriethoxysilane at different ratios and obtained an optimum ratio of 1 g clay: 2 g silane. Composite specimens were fabricated using epoxy and coir fibre at 7%, 10%, 15% and 20% volume fractions, respectively. Based on the mechanical properties, an optimum volume fraction of 15% coir fibre was selected to fabricate the hybrid composites with functionalised clay and glass spheres reinforcements, respectively. These composite specimens were then characterised to obtain their tensile, flexural and impact properties. From the results, it was realised that hybrid composites containing 4% functionalised clay particles and 8% glass spheres have superior mechanical properties. The reason behind the improved properties might be due to the reinforcing effect of the particles, which improved the load transfer between the fibre and the matrix.

The Effects of Micro- and Nano-Fillers’ Additions on the Dynamic Impact Response of Hybrid Composite Armors Made of HDPE Reinforced with Kevlar Short Fibers

ABSTRACT Hybrid composite armors consisting of Kevlar short fibers reinforced high-density polyethylene were prepared and the effects of the addition of micro and nano-fillers on the dynamic impact response and the energy absorption under ballistic impact were investigated. Five groups of specimens were manufactured using compression molding of pellets containing mixtures of high-density polyethylene and the reinforcing materials. The first group consist of high-density polyethylene reinforced with 10 wt% Kevlar pulp (KN-1). The rest are hybrid composites created by the addition of 20 wt% of micro and nano-fillers. The natural micro-fillers used are particles of chonta palm wood (KN-2) and potato flour (KN-3). The synthetic nanofillers are colloidal silica (KN-4) and gamma alumina (KN-5). Microstructure (scanning electronic microscope) and compositional (energy-dispersive spectroscopy) analysis of the hybrid composites were carried out to evaluate matrix-reinforcements-interface. The fabricated composites plates were subjected to high velocity impact using split Hopkinson pressure bar system and ballistic impact, according to NIJ standard–0101.06 for ballistic resistance. Significant stiffness improvements of up to 43.5% were achieved as a result of the addition of synthetic nano-particles to Kevlar fiber reinforced high-density polyethylene. X-ray diffractometer analysis revealed that the crystalline structure of the Kevlar reinforced high-density polyethylene is unaffected by addition of the nano-particles as fillers. However the intensity of the crystalline peaks decreased depending on the type of the added fillers. The results of dynamic impact test using split Hopkinson pressure bar revealed improved impact resistance by addition of synthetic nanofillers (silica and alumina). The results of the ballistic impact test showed the gamma alumina nano-particles (KN-5) exhibited the highest energy absorption capability. The results of these investigations indicate that hybridization Kevlar short fibers reinforced high-density polyethylene by micro and nano-fillers addition enhances the stiffness, impact resistance and ballistic energy absorption capability of the composites. GRAPHICAL ABSTRACT

Effects of high-strain-rate deformation on magnetic hysteresis in high-tensile steels

We have studied a relationship between magnetic hysteresis and microstructures on high-tensile AISI 4340 steels after impact loading with a strain rate up to 2100 s−1 We find that coercivity, and minor-loop coefficient which is deduced from a power-law scaling between minor-loop parameters increase with strain rate, show a maximum at around a strain rate of 1400 s−1, followed by a decrease at a higher strain rate, associated with magnetic anisotropy with respect to impact direction. These observations are explained from the viewpoints of heat generation and heterogeneous microstructures characteristic to steels subjected to high strain rate deformation.

Shear band formation in AISI 4340 steel under dynamic impact loads: Modeling and experiment

In this study, the occurrence of the adiabatic shear bands in AISI 4340 steel under high velocity impact loading was investigated using finite element analysis and experimental tests. The cylindrical specimen subjected to the impact load was divided into different regions separated by nodes using finite element method in ABAQUS environment with boundary conditions specified. The material properties were assumed to be lower in the region where the probability of strain localization is high based on prior experimental results in order to initialize the formation of the adiabatic shear bands. The finite element model was used to determine the maximum flow stress, the strain hardening, the thermal softening, and the time to reach the critical strain for the formation of adiabatic shear bands. Experimental results show that deformed bands were formed at low strain rates and there was a minimum strain rate required for the formation of the transformed band in the alloy and the cracks were initiated and propagated along the transformed bands leading to fragmentation under the impact loading. The susceptibility of the adiabatic shear bands to cracking was markedly influenced by the strain-rates and the initial material microstructure. The simulation results obtained were compared with the experimental results obtained from the AISI 4340 steel under high strain-rate loading in compression using split impact Hopkinson bars. A good agreement between the experimental and simulation results was obtained.

X-ray photoemission electron microscopic study of transformed shear bands in AISI 4340 steel

Abstract Failures of structural materials under dynamic mechanical loading at high strain rates are commonly initiated by shear strain localisation along adiabatic shear bands, which act as preferential sites for crack initiation and propagation. We have used synchrotron based X-ray photoemission electron microscopy (XPEEM) to identify chemical elements and measure compositional contrast in the shear band in AISI 4340 steel. The high spatial resolution of XPEEM combined with near edge X-ray absorption fine structure (NEXAFS) spectroscopy is used to study the microstructural evolution in transformed shear bands that formed in quench hardened and tempered AISI 4340 steel under dynamic impact loading. We compared our XPEEM findings with other complementary techniques such as energy dispersive X-ray spectroscopy and scanning electron microscopy to get a complete picture. As in the case of optical and scanning electron microscopy, XPEEM images show a featureless transformed band devoid of the martensite laths found in the parent metal. Interestingly, XPEEM images and corresponding Cr 2p→3d and Ni 2p→3d NEXAFS spectra confirms a compositional contrast in the transformed shear band that resulted in more nickel inside the shear band than in the adjacent region. Les défaillances des matériaux structuraux sous charge mécanique dynamique à des vitesses élevées de déformation sont communément initiées par la localisation de la déformation de cisaillement le long des bandes de cisaillement adiabatique, lesquelles agissent comme sites préférentiels pour l’amorçage et la propagation de fissure. Nous avons utilisé le synchrotron basé sur la spectromicroscopie d’électrons photo-excités par rayons X (XPEEM) pour identifier les éléments chimiques et pour mesurer le contraste de composition dans la bande de cisaillement de l’acier AISI 4340. La haute résolution spatiale par XPEEM, combinée avec la spectroscopie des structures fines d’absorption X proches du seuil (NEXAFS), est utilisée pour étudier l’évolution de microstructure dans les bandes de cisaillement transformées qui se forment dans l’acier AISI 4340 durci par trempe et revenu sous charge dynamique par impact. Nous avons comparé nos trouvailles par XPEEM à d’autres techniques complémentaires comme la spectroscopie aux rayons X à dispersion d’énergie et la microscopie électronique à balayage afin d’obtenir une image complète. Comme dans le cas de la microscopie optique et électronique à balayage, les images XPEEM montrent une bande transformée sans particularité et sans martensite massive trouvée dans le métal parent. Ce qui est intéressant, c’est que les images XPEEM et les spectres correspondants de NEXAFS Cr 2p →3d et Ni 2p →3d confirment un contraste de composition dans la bande de cisaillement transformée qui a pour résultat la présence de plus de nickel à l’intérieur de la bande de cisaillement que dans la région adjacente.

HIGH STRAIN RATE STUDY OF CERAMICS USING HOPKINSON BAR SYSTEM

There are at present several applications where high strength ceramics have replaced metals that are subjected to high speed impact from projectiles. This requires an evaluation of behavior of ceramics under impact at high strain rates. This current study provides information on high strain-rate behavior of alumina tested in shear using torsional Hopkinson bar. Dynamic stress-strain curves were generated to investigate deformation behavior prior to fracture while fractography of the broken specimens was carried out to establish the mode of failure. The results of this investigation are similar to what is obtainable in metallic materials in which mechanism of damage is controlled by strain localization and formation of adiabatic shear bands.

Deformation and Strengthening Mechanisms in AISI 321 Austenitic Stainless Steel Under Both Dynamic and Quasi-Static Loading Conditions

The mechanical response of AISI 321 austenitic stainless steel under compressive loads at strain rates of 6600 s−1 and 4.2 × 10−3 s−1 were studied using the split Hopkinson pressure bar and Instron R5500 mechanical testing system respectively. Specimens subjected to quasi-static compression showed lower yield strength and higher strain hardening capacity than the dynamically impacted specimen. High-resolution electron backscattered diffraction (HR-EBSD) study revealed that precipitation of nano-sized carbide and evolution of strain-induced martensite contributed to strengthening while plastic deformation mechanisms occurred in the specimens by slip and mechanical twinning during deformation under both quasi-static and dynamic loading conditions. The strain-induced phase transformation follows the FCC ɣ-austenite → BCC ά-martensite kinetic path with both phases maintaining the Kurdjumov-Sachs’ {(111)ɣ||(110)ά and ɣ|| ά} orientation relationship. A transformed shear band consisting of nano-grains with an average size of 0.28 µm was one of the microstructural features of the dynamically impacted specimen. HR-EBSD analysis revealed that the equiaxed ultra-fine grain structure in the TSB developed by rotational dynamic recrystallization mechanism while dynamic recovery occurred at the interface between the inside and outside of the band. During the deformation under both loading conditions, volume fraction of compression direction (CD)//{110} and CD//{111} increases substantially and slightly, respectively at the expense of CD//{100} fibre texture for the austenitic phase.

Dynamic Deformation Behavior of AA2099-T8 Under Compression and Torsion Loads

The suitability of aluminum alloys in a vast majority of engineering applications forms the basis for the need to understand the mechanisms responsible for their deformation and failure under various loading conditions. Aluminum AA2099 alloy finds application in fuselage structures that are statically and dynamically loaded, stiffness dominated designs, and in lower wing structures. The fuselage structures and wings of aircraft experience huge damage due to foreign object impacts. AA2099 aluminum alloy has an advantage of high specific strength compared with other alloys in the AA2000, 6000, and 7000 series; this characteristic makes it the material of choice in high performance aerospace structures. In this paper, the dynamic high strain rate impact deformation of AA2099 aluminum alloy under compression and torsion loading conditions using the split Hopkinson pressure and Kolsky torsion bars was performed. Digital image photogrammetric evolution of localized strain in aluminum samples during deformation process using high speed digital camera is reported. Microstructural analysis of deformed aluminum samples was performed using high resolution electron microscopes in order to determine the influence of impact strain rate on localized strain along narrow adiabatic shear bands in the AA2099 aluminum alloys. Results obtained indicate that peak flow stress in the deformed aluminum sample depends on the strain rate at which the deformation test was performed. An increase in impact strain rate results into an increase in the peak flow stress observed in the impacted aluminum sample. The type of adiabatic shear band localized in the aluminum sample also depends on the strain rate at which material was impacted.

Modeling and Simulation of Adiabatic Shear Bands in AISI 4340 Steel Under Impact Loads

In this study, the effects of microstructure and strain rate on the occurrence and failure of adiabatic shear bands in AISI 4340 steel under high velocity impact loads are investigated using finite element analysis and experimental tests. The shear band generated due to impact load was divided into some set of elements separated by nodes using finite element method in ABAQUS environment with initial and boundary conditions specified. The material properties were assumed to be lower at the second element set in order to initialize the adiabatic shear bands. The strain energy density for each successive node was calculated successively starting from the first element where initial boundary condition, initial strain hardening constant, and stress resistance had been specified. As the load time is increased, its corresponding effect on the localized shear deformation and width of the adiabatic shear band was also determined. The finite element model was used to determine the maximum stress, the strain hardening, the thermal softening, and the time to reach critical strain for formation of adiabatic shear bands. Experimental results show that deformed bands were formed at low strain rates and there was a minimum strain rate required for formation of transformed band in the alloy. The experimental results also show that cracks were initiated and propagated along transformed bands leading to fragmentation under the impact loading. The susceptibility of the adiabatic shear bands to cracking was markedly influenced by strain-rates and the initial material microstructures. The numerical results obtained were compared with the experimental results obtained for the AISI 4340 steel under high strain-rate loading in compression using split impact Hopkinson bars. A good agreement between the experimental and simulation results are also obtained.Copyright