Magnetorheological finishing (MRF) is a sub-aperture deterministic process for fabricating high-precision optics by removing material and smoothing the surface. The goal of this work is to study the relative contribut...
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The material removal in magnetorheological finishing (MRF) is known to be controlled by shear stress, τ, which equals drag force, Fd, divided by spot area, As. However, it is unclear how the normal force, Fn, affects...
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Tensile mechanical properties measured for high performance synthetic aramid fibers conditioned in water at 40°C, 60°C, 80°C and 100°C for periods of 17 and 34 days. Two different types of aromatic...
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ISBN:
(纸本)9781615676033
Tensile mechanical properties measured for high performance synthetic aramid fibers conditioned in water at 40°C, 60°C, 80°C and 100°C for periods of 17 and 34 days. Two different types of aromatic fibers were evaluated;homopolymer aramid fibers (Kevlar KM2® and Twaron®) and copolyaramid fibers (Russian Armos®). For both types of fibers, the hygrothermal conditioning did not significantly affect the mechanical properties below 80°C. At 100C, Aramid fibers showed significant degradation in strength (58% for KM2 and 34% for Twaron), while a reduction in tensile strength of 13% was observed by Armos Aramid copolymer fibers. For each material, tested fibers exhibited no significant change in mass as a result of the conditioning procedure and FTIR spectroscopy results did not indicate signs for chemical or thermo oxidative due to hygrothermal conditioning. The significant strength degradation in Aramid Homopolymer fibers is mainly due to the ingress and egress of moisture in the highly ordered core structure, where the crystallites are connected by highly constrained tie molecules in the amorphous grain boundaries. At elevated temperatures and pressures, it has been shown that these tie molecules are vulnerable to chemical attack, while at more moderate temperatures, the presence of water can enable segmental chain motion that can alter crystal orientation and apparent crystallite size. On the other hand, as processed Armos fibers are reported to have a 2D ordered phase with a molecular backbone having significantly less concentrations of amid group and a non systematic network of ordered domains which minimize the degradation growth due to hydrolytic attack.
The magnetic-induced dielectric responses of BiFeO3 (BFO) thin films were measured at the X -band microwave frequency ranged from 7 to 12.5 GHz. The measurement was given initially by a high-precision cavity microwave...
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In this study, fracture toughness of North American elk (Cervus elaphus canadensis) antler and bovine femur were measured using four-point bending tests on single-edge notched compact samples (ASTM C1421). Tests were ...
In this study, fracture toughness of North American elk (Cervus elaphus canadensis) antler and bovine femur were measured using four-point bending tests on single-edge notched compact samples (ASTM C1421). Tests were conducted on crack growth directions longitudinal and transverse to the long axis of antler and bone in both dry and hydrated conditions to study the effects of fiber orientation and hydration. Fracture toughness results in the transverse orientation were much higher than that in the longitudinal orientation and increased with degree of hydration for both antler and bovine femur. The fracture toughness of antler was ∼ 50% higher than that of bovine femur. The highest fracture toughness value was obtained from the hydrated antler in the transverse orientation, which reached 10.31 MPa·m1/2 compared to that measured from bovine femur, which was 6.35 MPa·m1/2. The crack propagation and fracture surface were characterized using scanning electron microscopy. Toughening mechanisms, including crack deflection by osteons, uncracked ligament bridging, and microcracks formation, are observed and discussed. Comparisons between antler and bone are made.
The hardnesses of secondary cell wall laminae (SCWL) and compound corner middle lamellae (CCML) in wood were measured at indentation strain rates between approximately 7×10-4 s-1 and 20 s-1, using a new method ca...
The hardnesses of secondary cell wall laminae (SCWL) and compound corner middle lamellae (CCML) in wood were measured at indentation strain rates between approximately 7×10-4 s-1 and 20 s-1, using a new method called broadband nanoindentation creep. The wood was subsequently modified with ethylene glycol (EG) and the properties were re-measured. The SCWL and CCML responded differently to this modification: in the SCWL, hardness decreased uniformly by a factor of 3.7 ± 0.3 across all strain rates, whereas in CCML, the modification had a similar effect at low strain rates. However, at high strain rates, hardness was only lowered by a factor of 1.8. The EG modification also lowered elastic modulus of the SCWL and CCML, swelled the SCWL and CCML, and caused previously placed indents to disappear (CCML) or partly disappear (SCWL).
Magnetic manipulation, fluorescent tracking, and localized delivery of a drug payload to cancer cells in vitro is demonstrated, using nanostructured porous silicon microparticles as a carrier. The multifunctional micr...
Magnetic manipulation, fluorescent tracking, and localized delivery of a drug payload to cancer cells in vitro is demonstrated, using nanostructured porous silicon microparticles as a carrier. The multifunctional microparticles are prepared by electrochemical porosification of a silicon wafer in a hydrofluoric acid-containing electrolyte, followed by removal and fracture of the porous layer into particles using ultrasound. The intrinsically luminescent particles are loaded with superparamagnetic iron oxide nanoparticles and the anti-cancer drug doxorubicin. The drug-containing particles are delivered to human cervical cancer (HeLa) cells in vitro, under the guidance of a magnetic field. The high concentration of particles in the proximity of the magnetic field results in a high concentration of drug being released in that region of the Petri dish, and localized cell death is confirmed by cellular viability assay (Calcein AM).
The initial stage of polarization switching in ferroelectric thin films depends on phenomena that occur at characteristic time scales of tens to hundreds of nanoseconds, including the nucleation polarization domains a...
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The initial stage of polarization switching in ferroelectric thin films depends on phenomena that occur at characteristic time scales of tens to hundreds of nanoseconds, including the nucleation polarization domains and the propagation of domain walls. These long intrinsic times allow short-duration electric fields with magnitudes far above the low-frequency coercive electric field to be applied across capacitor devices without inducing switching. Using time-resolved x-ray microdiffraction, we have found that a series of 50 ns duration electric field pulses switches the polarization of a 35-nm-thick ferroelectric Pb(Zr,Ti)O3 film only at electric fields greater than 1.5 MV/cm, a factor of three higher than the low-frequency coercive field. There is no switching in response to a large number of short pulses with amplitudes lower than 1.5 MV/cm, even when the total duration reaches several milliseconds. In comparison, a series of microsecond-duration pulses causes cumulative changes in the area of switched polarization and eventually switches the entire capacitor. The difference between long- and short-duration electric field pulses arises from effects linked to domain nucleation and charge transport in the ferroelectric film. A phase-field model shows that the shrinking of the switched domain in the interval between pulses is a less important effect. This opportunity to apply large fields for short times without inducing switching by domain-wall motion raises the possibility that future experiments could reach the intrinsic coercive field of ferroelectric layers and provides a way to study the properties of materials under high electric fields.
A series of demonstrations and laboratory exercises have been developed to teach fundamental concepts in the thermal and fluid sciences of the undergraduate engineering curriculum. This material is part of an educatio...
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