The degradation of mechanical properties of a SiC fiber reinforced SiNC ceramic matrix composite ... more The degradation of mechanical properties of a SiC fiber reinforced SiNC ceramic matrix composite due to thermal shock by water quenching have been investigated. Post thermal shock tensile tests were performed to determine the degradation of mechanical properties of this composite. In situ acoustic emission (AE) tests were also conducted. The tensile tests data and acoustic emission data were correlated. The AE signal indicated a sudden increase in AE events at critical points in the stress-strain relationship. The effects of thermal shock temperature and the number of thermal shock cycles on the mechanical properties, and on the AE responses were also evaluated. It was observed that an increase in either factor resulted in more AE responses. Fracture damage in the tensile test specimens was examined by Scanning Electron Microscopy. It was observed that the failure mechanism changed as the thermal shock temperature increased. The fracture surfaces of the specimens tested without thermal shock indicated an extensive fiber pullout while the thermally shocked specimens showed reduced fiber pullout.
It is well known that in a sandwich structure, the core plays an important role in enhancing the ... more It is well known that in a sandwich structure, the core plays an important role in enhancing the flexural rigidity and by controlling the failure mechanisms. If the core is made from foam, the strength of the core material and the debond strength at the core -skin interface almost entirely dictate the performance of structural sandwich composites especially under flexure. In this investigation attempts have been made to improve the performance of the sandwich by strengthening the core but partially sacrificing the debond fracture toughness of the sandwich construction. Strengthening of the core has been accomplished by infusing nanoparticles into the parent polymer of the core material when it was in the liquid stage. The core material is polyurethane foam made from polymeric isocyanate (Part A) and reacting with polyol (Part B). Spherical nanoparticles such as TiO 2 of about 29 nm in diameters were dispersed in Part A of liquid polyurethane through an ultrasonic cavitation process. The amount of nanoparticles infused into liquid foam varied from 1 to 3% by weight. Once Part A was doped with nanoparticles, it was mixed with Part B, and was cast in a rectangular mold to produce the nanophased polyurethane foam. The nanophased foam was then used with regular S-2 Glass fiber preforms and SC-15 epoxy to manufacture sandwich composites in a VARTM set up. Test coupons were then extracted from foam as well as from sandwich panels to conduct flexural and various other chemical tests. A parallel set of control panels were also made with neat polyurethane core materials. Thermogravimetric and SEM analyses have indicated that the decomposition temperature of the nanophased foams increases by about 27 8C and the cell size almost doubles with nanoparticle infusion. A significant improvement in flexural strength and stiffness has also been observed with 3% loading of TiO 2 nanoparticles. Debond fracture toughness parameters ðG c Þ were also determined for both categories of sandwich constructions, and it was seen that nanoparticle infusion reduces the value of G c by almost a factor of three. Despite this reduction, strength of nanophased sandwich increased by about 53% over the neat system. Details of manufacturing and analyses of test results are included in the paper. q
The mechanical properties of ceramic composites change due to variations in the size and geometry... more The mechanical properties of ceramic composites change due to variations in the size and geometry of the reinforcement. In the present work the effects of Sic whisker (SIC,) and Sic platelets (Sic,) on the mechanical properties of SiC/Al,O, composites have been investigated. The flexural properties of both whisker-and platelet-reinforced SiC/Al,O, composites were measured using the four-point bend test. Fracture toughness was determined for both straight-and chevronnotched specimens. A scanning electron microscopy examination of the fractured surfaces was conducted to perform failure analyses and understand toughening mechanisms. The results of the investigation indicate that SiC,/Al,O, composites provides higher flexural strength and fracture toughness compared to SiC,/Al,Os composites. During hot pressing, the control of the orientation of the Sic, is extremely difficult and it appears to form a porous matrix structure which substantially lowers the strength. Moreover, it has been observed that the low I/d ratio of SIC, has a major influence on the fracture toughness.
ABSTRACT Hybrid ultrahigh molecular weight polyethylene–nylon 6–single-wall carbon nanotube fiber... more ABSTRACT Hybrid ultrahigh molecular weight polyethylene–nylon 6–single-wall carbon nanotube fibers were processed using solution spinning method. Elastic properties and normalized velocity () of the hybrid fibers were measured before and after strain hardening through repeated loading–unloading cycles. Phenomenal improvement in the properties was found: strength, modulus, and normalizing velocity increased by almost one order of magnitude after strain hardening. Neat and reinforced filaments were characterized through differential scanning calorimetry, Raman spectroscopy, and scanning electron microscope before and after strain hardening. It has been revealed that nylon 6 contributed to the deformation ability of the composite fiber, while carbon nanotubes contributed to the sharing of load as they aligned during extrusion and strain hardening processes. Important morphological features determining the fiber properties were the change in crystallinity and rate of crystallization, formation of microdroplets, interfacial sliding, polymer coating of nanotubes, alignment of polymer fibrils and nanotubes.
ABSTRACT When a sandwich structure is subjected to transverse loads, the face sheets carry bendin... more ABSTRACT When a sandwich structure is subjected to transverse loads, the face sheets carry bending moments as tensile and compressive stresses and the core carries transverse forces as shear stresses. The core is typically the weakest component of the structure and is the first to fail in shear. In this study the shear fatigue behavior of two closed-cell cellular PVC foams, Divinycell HD130 (linear) and H130 (cross linked), with the same nominal density of 130 kg/m3, were investigated. Static shear tests reveal that HD130 foams are more ductile, have almost twice the energy absorption capability, and an extraordinary crack propagation resistance when compared to the H130 foams. Shear fatigue tests were conducted at room temperature, at a frequency of 3 Hz and at a stress ratio, R = 0.1 on the HD130 and H130 foams. S–N curves were generated and shear fatigue characteristics were determined. The number of cycles to failure for the linear foams was substantially higher than that of the cross-linked PVC foams. HD foams have smaller cells with thicker faces and edges. This microstructure supports absorption of larger amounts of liquid resin forming a resin rich sub interface zone just below the actual core skin interface. The high intrinsic toughness of the sub interface delays the initiation of fatigue cracks and thereby increases the fatigue life of the HD foams. For both foams, shear deformation occurs without volume change and the materials fail by shearing in the vicinity of the centerline of the specimen along the longitudinal axis. In both cases numerous 45° shear cracks form across the width of the specimen and are equidistantly spaced along the length of the specimen. The occurrence of these through the thickness shear cracks signals the final failure event during fatigue. Details of the experimental investigation and the evaluation of the fatigue performance are presented.
The degradation of mechanical properties of a SiC fiber reinforced SiNC ceramic matrix composite ... more The degradation of mechanical properties of a SiC fiber reinforced SiNC ceramic matrix composite due to thermal shock by water quenching have been investigated. Post thermal shock tensile tests were performed to determine the degradation of mechanical properties of this composite. In situ acoustic emission (AE) tests were also conducted. The tensile tests data and acoustic emission data were correlated. The AE signal indicated a sudden increase in AE events at critical points in the stress-strain relationship. The effects of thermal shock temperature and the number of thermal shock cycles on the mechanical properties, and on the AE responses were also evaluated. It was observed that an increase in either factor resulted in more AE responses. Fracture damage in the tensile test specimens was examined by Scanning Electron Microscopy. It was observed that the failure mechanism changed as the thermal shock temperature increased. The fracture surfaces of the specimens tested without thermal shock indicated an extensive fiber pullout while the thermally shocked specimens showed reduced fiber pullout.
It is well known that in a sandwich structure, the core plays an important role in enhancing the ... more It is well known that in a sandwich structure, the core plays an important role in enhancing the flexural rigidity and by controlling the failure mechanisms. If the core is made from foam, the strength of the core material and the debond strength at the core -skin interface almost entirely dictate the performance of structural sandwich composites especially under flexure. In this investigation attempts have been made to improve the performance of the sandwich by strengthening the core but partially sacrificing the debond fracture toughness of the sandwich construction. Strengthening of the core has been accomplished by infusing nanoparticles into the parent polymer of the core material when it was in the liquid stage. The core material is polyurethane foam made from polymeric isocyanate (Part A) and reacting with polyol (Part B). Spherical nanoparticles such as TiO 2 of about 29 nm in diameters were dispersed in Part A of liquid polyurethane through an ultrasonic cavitation process. The amount of nanoparticles infused into liquid foam varied from 1 to 3% by weight. Once Part A was doped with nanoparticles, it was mixed with Part B, and was cast in a rectangular mold to produce the nanophased polyurethane foam. The nanophased foam was then used with regular S-2 Glass fiber preforms and SC-15 epoxy to manufacture sandwich composites in a VARTM set up. Test coupons were then extracted from foam as well as from sandwich panels to conduct flexural and various other chemical tests. A parallel set of control panels were also made with neat polyurethane core materials. Thermogravimetric and SEM analyses have indicated that the decomposition temperature of the nanophased foams increases by about 27 8C and the cell size almost doubles with nanoparticle infusion. A significant improvement in flexural strength and stiffness has also been observed with 3% loading of TiO 2 nanoparticles. Debond fracture toughness parameters ðG c Þ were also determined for both categories of sandwich constructions, and it was seen that nanoparticle infusion reduces the value of G c by almost a factor of three. Despite this reduction, strength of nanophased sandwich increased by about 53% over the neat system. Details of manufacturing and analyses of test results are included in the paper. q
The mechanical properties of ceramic composites change due to variations in the size and geometry... more The mechanical properties of ceramic composites change due to variations in the size and geometry of the reinforcement. In the present work the effects of Sic whisker (SIC,) and Sic platelets (Sic,) on the mechanical properties of SiC/Al,O, composites have been investigated. The flexural properties of both whisker-and platelet-reinforced SiC/Al,O, composites were measured using the four-point bend test. Fracture toughness was determined for both straight-and chevronnotched specimens. A scanning electron microscopy examination of the fractured surfaces was conducted to perform failure analyses and understand toughening mechanisms. The results of the investigation indicate that SiC,/Al,O, composites provides higher flexural strength and fracture toughness compared to SiC,/Al,Os composites. During hot pressing, the control of the orientation of the Sic, is extremely difficult and it appears to form a porous matrix structure which substantially lowers the strength. Moreover, it has been observed that the low I/d ratio of SIC, has a major influence on the fracture toughness.
ABSTRACT Hybrid ultrahigh molecular weight polyethylene–nylon 6–single-wall carbon nanotube fiber... more ABSTRACT Hybrid ultrahigh molecular weight polyethylene–nylon 6–single-wall carbon nanotube fibers were processed using solution spinning method. Elastic properties and normalized velocity () of the hybrid fibers were measured before and after strain hardening through repeated loading–unloading cycles. Phenomenal improvement in the properties was found: strength, modulus, and normalizing velocity increased by almost one order of magnitude after strain hardening. Neat and reinforced filaments were characterized through differential scanning calorimetry, Raman spectroscopy, and scanning electron microscope before and after strain hardening. It has been revealed that nylon 6 contributed to the deformation ability of the composite fiber, while carbon nanotubes contributed to the sharing of load as they aligned during extrusion and strain hardening processes. Important morphological features determining the fiber properties were the change in crystallinity and rate of crystallization, formation of microdroplets, interfacial sliding, polymer coating of nanotubes, alignment of polymer fibrils and nanotubes.
ABSTRACT When a sandwich structure is subjected to transverse loads, the face sheets carry bendin... more ABSTRACT When a sandwich structure is subjected to transverse loads, the face sheets carry bending moments as tensile and compressive stresses and the core carries transverse forces as shear stresses. The core is typically the weakest component of the structure and is the first to fail in shear. In this study the shear fatigue behavior of two closed-cell cellular PVC foams, Divinycell HD130 (linear) and H130 (cross linked), with the same nominal density of 130 kg/m3, were investigated. Static shear tests reveal that HD130 foams are more ductile, have almost twice the energy absorption capability, and an extraordinary crack propagation resistance when compared to the H130 foams. Shear fatigue tests were conducted at room temperature, at a frequency of 3 Hz and at a stress ratio, R = 0.1 on the HD130 and H130 foams. S–N curves were generated and shear fatigue characteristics were determined. The number of cycles to failure for the linear foams was substantially higher than that of the cross-linked PVC foams. HD foams have smaller cells with thicker faces and edges. This microstructure supports absorption of larger amounts of liquid resin forming a resin rich sub interface zone just below the actual core skin interface. The high intrinsic toughness of the sub interface delays the initiation of fatigue cracks and thereby increases the fatigue life of the HD foams. For both foams, shear deformation occurs without volume change and the materials fail by shearing in the vicinity of the centerline of the specimen along the longitudinal axis. In both cases numerous 45° shear cracks form across the width of the specimen and are equidistantly spaced along the length of the specimen. The occurrence of these through the thickness shear cracks signals the final failure event during fatigue. Details of the experimental investigation and the evaluation of the fatigue performance are presented.
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