An Energy-Based Torsional-Shear Fatigue Lifing Method

An energy-based fatigue-life prediction framework for the determination of full-life, remaining-life, and critical-life of in-service structures subjected to torsional-shear loading has been developed. This framework is developed upon the existing foundation of energy-based fatigue models crafted for the axial, uniaxial bending, and transverse-shear loading cases, which state: the total strain energy density accumulated during both a monotonic event and a cumulative cyclic process is the same material property. The modified energy-based torsional-shear fatigue-life prediction framework is composed of the following entities: (1) the development of a torsional-shear fatigue testing procedure capable of assessing strain energy density per cycle in a pure shear stress state and (2) the determination of the remaining-life and critical-life of in-service aluminum (Al) 6061-T6 structures subjected to shear fatigue through the application of the energy-based prediction method. Experimental data was shown to be affected by load-frame misalignment which was estimated and successfully incorporated into the validation results. Close correlation between adjusted experimental results and the full-life and critical-life predictions stemmed from a 3-to-2 shear-to-axial biaxial loading assumption, which was supported by crack path comparisons. Results of the study effectively demonstrated the versatility of the energy-based lifing method.     

Transmutation of fullerenes

Fullerenes were pyrolyzed by subliming them into a stream of flowing argon gas and then passing them through an oven heated to approximately 1000 degrees C. C(76), C(78), and C(84) all readily lost carbons to form smaller fullerenes. In the case of C(78), some isomerization was seen. Pyrolysis of (3)He@C(76) showed that all or most of the (3)He was lost during the decomposition. C(60) passes through the apparatus with no decomposition and no loss of helium.     

Observation of a deuteron nuclear magnetic resonance Knight shift in conductive polyaniline

Solid state deuteron magic angle spinning nuclear magnetic resonance spectra of conductive ring-deuterated polyaniline consist of two peaks, one at the same chemical shift as the insulating form of the polymer and the second shifted by 5.8+/-1 ppm. The magnitude of the shift is field and temperature independent and is identified as a Knight shift. The deuterons undergoing a Knight shift originate from both the crystalline and amorphous regions of the sample, implying that conduction is mediated by delocalized polarons in both these regions. Spin count experiments demonstrate that in highly conductive samples, signal is lost not only by dephasing due to the proximity of localized unpaired electrons but also to high rf reflectance.     

MHD “switch-on” shock structure

An experimental study of the structure of an MHD switch-on shock propagating into an almost fully ionized helium plasma is described. It is found that electrons are heated in preference to ions.     

An experimental investigation of electromechanical response in a dielectric acrylic elastomer

Electric-field-induced transverse-strain response was investigated in a dielectric acrylic elastomer at 1 Hz. The strain was observed to be proportional to square of the electric field strength within the measurement range (0 to 2 MV/m). Elastic compliance of the elastomer was measured as a function of frequency and was found to exhibit strong frequency dependence between 1 and 10 Hz. It was indicated that the field-induced-strain response in the acrylic elastomer originates primarily from the Maxwell stress effect. The experimental data obtained at low field and frequency were used to estimate the electromechanical behavior at higher field and frequency. PACS 77.65.-j; 77.84.Jd; 81.05.Lg     

Towards quantitative measurements in solid-state CPMAS NMR: A Lee-Goldburg frequency modulated cross-polarization scheme

A new scheme combining a Lee-Goldburg (LG) sequence with frequency modulation is proposed for cross-polarization (LG-FMCP) in solid-state magic-angle-spinning nuclear magnetic resonance. During the CP contact time, the (1)H magnetization is spin-locked along the magic angle by the LG sequence and the irradiation offset of the S spins (e.g., (15)N) is modulated sinusoidally with a constant RF amplitude. It is shown experimentally that the LG sequence significantly lengthens the proton spin-lattice relaxation time in the tilted rotating frame and that the frequency modulation shortens the cross-polarization time for non-protonated S spins. As a result of substantially increasing the difference in these relaxation rates, the non-protonated and protonated S spins can be more efficiently and more uniformly polarized with a relatively long CP contact time, making quantitative CP measurements possible. A sample of (15)N-delta 1-L-histidine lyophilized from a solution of pH 6.3 and a (15)N-delta 1-L-His labeled transmembrane helical peptide in hydrated lipid bilayers were used to illustrate the advantages of this scheme.     

Compression of frequency-modulated pulses using helically corrugated waveguides and its potential for generating multigigawatt rf radiation

A new method to generate ultrahigh-power microwave pulses compatible with mildly relativistic electron sources is proposed. This method involves a novel microwave compressor in the form of a metal helically corrugated waveguide, which can enhance the power of frequency-modulated nanosecond pulses up to the multigigawatt level. The results of the proof-of-principle experiments at kilowatt power levels are in good agreement with theory.     

Short-pulse, high-brightness X-ray production with the PLEIADES Thomson-scattering source

PLEIADES is a compact, tunable, high-brightness, ultra-short-pulse, Thomson-scattering X-ray source. Picosecond pulses of hard X-rays (10–200 keV) are created by colliding an ultra-relativistic (20–100 MeV), picosecond-duration electron beam with a high-intensity, sub-picosecond, 800-nm laser pulse. Initial operation of this source has produced 78-keV X-rays with 10⁶ photons per pulse using a 57-MeV, 0.3-nC, 50-m rms width electron beam and a 180-mJ, 15-m rms width laser pulse. The angular distribution, energy, and energy spectrum of the source are found to agree well with theory and simulations. Source optimization is expected to increase X-ray output to between 10⁷ and 10⁸ photons per pulse with a peak brightness approaching 10²⁰ photons/s/0.1% bandwidth/mm²/mrad². PACS 41.50.+h; 07.85.Fv; 41.75.Ht; 42.62.-b