The viscoelastic properties of nanocomposites based on epoxy and epoxy-phenolic blend matrix rein... more The viscoelastic properties of nanocomposites based on epoxy and epoxy-phenolic blend matrix reinforced with multi-wall carbon nanotubes were studied. The content of carbon nanotube (0.2 and 0.5 wt.%) and its surface state (pristine vs. oxidized) varied. The characterization of the nanocomposites was done by means of infrared spectroscopy and chemical methods. From the viscoelastic properties of nanocomposites, characteristic properties such as storage modulus ( E0), tan δ and damping peak (tan δ) were obtained. The void content was also determined. Also, Weibull statistics are used to represent the failure of secondary bonds during the relaxation processes that lead to stiffness change over the full range of use temperatures. The incorporation of phenolic resin to an epoxy matrix leads to an increase of the relaxation modulus. The glass transition temperature of the epoxy-phenolic nanocomposites decrease when it is incorporated carbon nanotubes, independently of its state (pristine...
ABSTRACT The experimental program developed in this work aims at investigating the bond behavior ... more ABSTRACT The experimental program developed in this work aims at investigating the bond behavior of glass fiber reinforced plastic embedded in concrete. Different types of surface treatment were applied to the smooth rods in order to enhance the bond with concrete. Firstly, the rods were machined using a lathe and a grinding stone to create lugs with different configurations. The experimental results indicate that the bond strength is highest when the failure mode is a combination of two mechanisms: shearing off of the concrete lugs and that of the FRP lugs. With regard to the variation of rib width, the friction and interlock forces have a greater influence on bond strength. With respect to the height of the rib, the variation of internal diameter does not affect significantly the bond strength. Secondly, the machined rods were coated with sand fixed with a thin layer of epoxy resin. The incorporation of the sand on the surface increases the bond strength compared with the results obtained without sand.
Glass fiber-reinforced polymer bars are currently used to reinforce concrete in an attempt to ove... more Glass fiber-reinforced polymer bars are currently used to reinforce concrete in an attempt to overcome the corrosion issue encountered with ordinary steel. Different types of surface treatment were applied to the smooth rods in order to enhance bonding with concrete. Experimental results show that using bars coated with coarse sand notably improve the bond strength. The influence of granulometry sand, rebar diameter, length embedded, and concrete strength are analyzed. Rebars coated with finer sand lead to a stronger chemical adhesion with concrete. However, the effect of friction and interlocking forces produced by coarse sand prevails over the chemical adhesion in the pull-out test.
Self-reinforced composites based on commercial polypropylene (PP) woven fabrics and a random PP c... more Self-reinforced composites based on commercial polypropylene (PP) woven fabrics and a random PP copolymer modified with quartz were obtained by film stacking. The effect of the incorporation of quartz on the materials fracture and failure behavior was studied through uniaxial tensile tests and quasi-static fracture experiments. Acoustic emission analysis was also performed in situ in the tensile tests. A higher consolidation quality was obtained for the composites containing quartz. In the composite with random PP modified with 5 wt% quartz, the higher consolidation and the better dispersion of quartz particles positively impacted on the materials tensile and fracture behavior. From the results of acoustic emission analysis, fiber fracture appears as the dominant failure mechanism in the investigated composites.
The influence of oxidized multi‐walled carbon nanotubes (o‐MWCNTs) on the curing kinetics of a no... more The influence of oxidized multi‐walled carbon nanotubes (o‐MWCNTs) on the curing kinetics of a novolac resin was studied by means of non‐isothermal differential scanning calorimetry. Regarding the kinetics issues, the high concentration of hydroxyl groups on the o‐MWCNTs slightly modified the curing reaction of the novolac resin, shifting the differential scanning calorimetry exothermic peak to higher temperatures. The effective activation energy of the curing reaction was calculated by the isoconversional Kissinger‐Akahira‐Sunose method and increased by the presence of o‐MWCNTs with respect to neat novolac. This change was attributed to the increase of the material viscosity. In addition, thermogravimetric analysis revealed that nanocomposites samples containing 0.4 and 1.0 wt% o‐MWCNTs presented increased char yield values, indicating an improvement of flame retardancy. KEYWORDS differential scanning calorimetry (DSC), isoconversional Kissinger‐Akahira‐Sunose method, oxidized multi‐walled carbon nanotubes (o‐MWCNTs), thermogravimetric analysis (TGA)
The change in stiffness with temperature in the presence of different media has been discussed fo... more The change in stiffness with temperature in the presence of different media has been discussed for a long time because the ability to predict this behavior becomes fundamental to the design of new materials and their applications. That is why, in this work, the application of a mathematical model, which is able to predict the elastic properties of two polymers, is presented. The study takes into account the relationship between the viscoelastic and absorption properties of these materials in alkaline solution (Ca (OH) 2 , pH 12.5) and in distilled water (H 2 O, pH 7) as the immersion media. Diffusion coefficient values were higher when the resins were immersed in water than in the alkaline solution. In addition, the effect of the alkaline medium was higher for the unsaturated polyester resin (UPR). The highest decrease in modulus at the glassy state of the polymer network was observed for the UPR immersed in the alkaline medium. The greatest reduction of the T g value due to network plasticizing was found for the epoxy resin (ER) in the alkaline medium. Therefore, the ER exhibited a more stable behavior after aging at moderate temperature than the UPR. C
—Starting from commercial MgB 2 powder of 325 mesh, we further reduced the agglomerates by attrit... more —Starting from commercial MgB 2 powder of 325 mesh, we further reduced the agglomerates by attrition milling in nitrogen atmosphere to minimize oxidation. Using different milling media (tungsten carbide, yttria stabilized zirconium oxide and stainless steel balls), we obtained different milling energies and, consequently, powders with different strain states. Crystallite size and micro-strain within the grains were estimated by x-ray diffraction. We investigated the changes in the superconducting properties with the use of different milling media by measuring the magnetic moment as a function of temperature in a SQUID magnetometer, after zero field cooling. Single-filament stainless steel-sheathed wires and tapes were prepared with the obtained powders. The flexural properties of the tapes were determined by 4-point bending micro-tests. The use of a precursor powder with a higher strain resulted in more flexible wires and tapes, with lower flexural strength. Three different regions were determined for the maximum normal stress-strain curves. Some in-situ monel-sheathed hollow wires and ex-situ titanium-sheathed tapes and monel-sheathed 7-filament tapes were measured for comparative purposes. We discuss the correlation between sheath yielding and superconducting core cracking with the possible critical current degradation due to mechanical failure of the wires and tapes.
The viscoelastic properties of nanocomposites based on epoxy and epoxy-phenolic blend matrix rein... more The viscoelastic properties of nanocomposites based on epoxy and epoxy-phenolic blend matrix reinforced with multi-wall carbon nanotubes were studied. The content of carbon nanotube (0.2 and 0.5 wt.%) and its surface state (pristine vs. oxidized) varied. The characterization of the nanocomposites was done by means of infrared spectroscopy and chemical methods. From the viscoelastic properties of nanocomposites, characteristic properties such as storage modulus ( E0), tan δ and damping peak (tan δ) were obtained. The void content was also determined. Also, Weibull statistics are used to represent the failure of secondary bonds during the relaxation processes that lead to stiffness change over the full range of use temperatures. The incorporation of phenolic resin to an epoxy matrix leads to an increase of the relaxation modulus. The glass transition temperature of the epoxy-phenolic nanocomposites decrease when it is incorporated carbon nanotubes, independently of its state (pristine...
ABSTRACT The experimental program developed in this work aims at investigating the bond behavior ... more ABSTRACT The experimental program developed in this work aims at investigating the bond behavior of glass fiber reinforced plastic embedded in concrete. Different types of surface treatment were applied to the smooth rods in order to enhance the bond with concrete. Firstly, the rods were machined using a lathe and a grinding stone to create lugs with different configurations. The experimental results indicate that the bond strength is highest when the failure mode is a combination of two mechanisms: shearing off of the concrete lugs and that of the FRP lugs. With regard to the variation of rib width, the friction and interlock forces have a greater influence on bond strength. With respect to the height of the rib, the variation of internal diameter does not affect significantly the bond strength. Secondly, the machined rods were coated with sand fixed with a thin layer of epoxy resin. The incorporation of the sand on the surface increases the bond strength compared with the results obtained without sand.
Glass fiber-reinforced polymer bars are currently used to reinforce concrete in an attempt to ove... more Glass fiber-reinforced polymer bars are currently used to reinforce concrete in an attempt to overcome the corrosion issue encountered with ordinary steel. Different types of surface treatment were applied to the smooth rods in order to enhance bonding with concrete. Experimental results show that using bars coated with coarse sand notably improve the bond strength. The influence of granulometry sand, rebar diameter, length embedded, and concrete strength are analyzed. Rebars coated with finer sand lead to a stronger chemical adhesion with concrete. However, the effect of friction and interlocking forces produced by coarse sand prevails over the chemical adhesion in the pull-out test.
Self-reinforced composites based on commercial polypropylene (PP) woven fabrics and a random PP c... more Self-reinforced composites based on commercial polypropylene (PP) woven fabrics and a random PP copolymer modified with quartz were obtained by film stacking. The effect of the incorporation of quartz on the materials fracture and failure behavior was studied through uniaxial tensile tests and quasi-static fracture experiments. Acoustic emission analysis was also performed in situ in the tensile tests. A higher consolidation quality was obtained for the composites containing quartz. In the composite with random PP modified with 5 wt% quartz, the higher consolidation and the better dispersion of quartz particles positively impacted on the materials tensile and fracture behavior. From the results of acoustic emission analysis, fiber fracture appears as the dominant failure mechanism in the investigated composites.
The influence of oxidized multi‐walled carbon nanotubes (o‐MWCNTs) on the curing kinetics of a no... more The influence of oxidized multi‐walled carbon nanotubes (o‐MWCNTs) on the curing kinetics of a novolac resin was studied by means of non‐isothermal differential scanning calorimetry. Regarding the kinetics issues, the high concentration of hydroxyl groups on the o‐MWCNTs slightly modified the curing reaction of the novolac resin, shifting the differential scanning calorimetry exothermic peak to higher temperatures. The effective activation energy of the curing reaction was calculated by the isoconversional Kissinger‐Akahira‐Sunose method and increased by the presence of o‐MWCNTs with respect to neat novolac. This change was attributed to the increase of the material viscosity. In addition, thermogravimetric analysis revealed that nanocomposites samples containing 0.4 and 1.0 wt% o‐MWCNTs presented increased char yield values, indicating an improvement of flame retardancy. KEYWORDS differential scanning calorimetry (DSC), isoconversional Kissinger‐Akahira‐Sunose method, oxidized multi‐walled carbon nanotubes (o‐MWCNTs), thermogravimetric analysis (TGA)
The change in stiffness with temperature in the presence of different media has been discussed fo... more The change in stiffness with temperature in the presence of different media has been discussed for a long time because the ability to predict this behavior becomes fundamental to the design of new materials and their applications. That is why, in this work, the application of a mathematical model, which is able to predict the elastic properties of two polymers, is presented. The study takes into account the relationship between the viscoelastic and absorption properties of these materials in alkaline solution (Ca (OH) 2 , pH 12.5) and in distilled water (H 2 O, pH 7) as the immersion media. Diffusion coefficient values were higher when the resins were immersed in water than in the alkaline solution. In addition, the effect of the alkaline medium was higher for the unsaturated polyester resin (UPR). The highest decrease in modulus at the glassy state of the polymer network was observed for the UPR immersed in the alkaline medium. The greatest reduction of the T g value due to network plasticizing was found for the epoxy resin (ER) in the alkaline medium. Therefore, the ER exhibited a more stable behavior after aging at moderate temperature than the UPR. C
—Starting from commercial MgB 2 powder of 325 mesh, we further reduced the agglomerates by attrit... more —Starting from commercial MgB 2 powder of 325 mesh, we further reduced the agglomerates by attrition milling in nitrogen atmosphere to minimize oxidation. Using different milling media (tungsten carbide, yttria stabilized zirconium oxide and stainless steel balls), we obtained different milling energies and, consequently, powders with different strain states. Crystallite size and micro-strain within the grains were estimated by x-ray diffraction. We investigated the changes in the superconducting properties with the use of different milling media by measuring the magnetic moment as a function of temperature in a SQUID magnetometer, after zero field cooling. Single-filament stainless steel-sheathed wires and tapes were prepared with the obtained powders. The flexural properties of the tapes were determined by 4-point bending micro-tests. The use of a precursor powder with a higher strain resulted in more flexible wires and tapes, with lower flexural strength. Three different regions were determined for the maximum normal stress-strain curves. Some in-situ monel-sheathed hollow wires and ex-situ titanium-sheathed tapes and monel-sheathed 7-filament tapes were measured for comparative purposes. We discuss the correlation between sheath yielding and superconducting core cracking with the possible critical current degradation due to mechanical failure of the wires and tapes.
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Papers by Juan Pablo Morales Arias