A low-dimensional model for delamination in a laterally loaded composite beam: Theory and experiment

Delamination is an important failure mechanism in certain types of composite structures. As layers of the composite become separated from one another, the composite loses some of its structural integrity and may not be capable of supporting the intended loads. Detecting this type of damage is currently a problem of interest to the structural health monitoring community. However, in order to design an appropriate detection strategy, knowledge of the underlying physics of delaminated structures is essential. Here, a low-dimensional model of a delamination in a laminated beam is developed. The model consists of only five elements yet is capable of capturing much of the behavior observed in an accompanying experiment. Both experimental and theoretical results are presented for the static response of a thin, delaminated beam.     

Effect of ozone oxidation on single-walled carbon nanotubes

Exposing single-walled carbon nanotubes to room-temperature UV-generated ozone leads to an irreversible increase in their electrical resistance. We demonstrate that the increased resistance is due to ozone oxidation on the sidewalls of the nanotubes rather than at the end caps. Raman and X-ray photoelectron spectroscopies show an increase in the defect density due to the oxidation of the nanotubes. Using ultraviolet photoelectron spectroscopy, we show that these defects represent the removal of pi-conjugated electron states near the Fermi level, leading to the observed increase in electrical resistance. Oxidation of carbon nanotubes is an important first step in many chemical functionalization processes. Because the oxidation rate can be controlled with short exposures, UV-generated ozone offers the potential for use as a low-thermal-budget processing tool.     

A Bayesian approach to identifying structural nonlinearity using free-decay response: Application to damage detection in composites

This work discusses a Bayesian approach to approximating the distribution of parameters governing nonlinear structural systems. Specifically, we use a Markov Chain Monte Carlo method for sampling the posterior parameter distributions thus producing both point and interval estimates for parameters. The method is first used to identify both linear and nonlinear parameters in a multiple degree-of-freedom structural systems using free-decay vibrations. The approach is then applied to the problem of identifying the location, size, and depth of delamination in a model composite beam. The influence of additive Gaussian noise on the response data is explored with respect to the quality of the resulting parameter estimates.     

Directionally oriented LB films of an OPE derivative: assembly, characterization, and electrical properties

Langmuir films have been fabricated from 4-[4'-(4'-thioacetyl-phenyleneethynylene)-phenyleneethynylene]-aniline (NOPES) after cleavage of the thioacetyl protecting group. Characterization by surface pressure vs area per molecule isotherms and Brewster angle microscopy reveal the formation of a high quality monolayer at the air-water interface. One layer Langmuir-Blodgett (LB) films were readily fabricated by the transfer of the NOPES Langmuir film onto solid substrates. X-ray photoelectron spectroscopy (XPS), surface polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS), and quartz crystal microbalance (QCM) experiments conclusively demonstrate the formation of one layer LB films in which the functional group associated with binding to the substrate can be tailored by the film transfer conditions. Using LB methods this molecule could be transferred to gold samples with either the amine or thiol group attached to the gold surface. The amine group is directly attached to the gold substrate (Au-NH(2)-OPE-SH) when the substrate is initially immersed in the subphase and withdrawn during the transfer process; in contrast, monomolecular films in which the thiolate group is attached to the gold substrate (Au-S-OPE-NH(2)) are obtained when the substrate is initially out of the subphase and immersed during the transfer process. The morphology of these films was analyzed by atomic force microscopy (AFM), showing the formation of homogeneous layers. Film homogeneity was confirmed by cyclic voltammetry, which revealed a large passivation of gold electrodes covered by NOPES monolayers. Electrical properties for both polar orientated junctions have been investigated by scanning tunnelling microscopy (STM), with both orientations featuring a nonrectifying behavior.     

Synthesis of substituted 2-amino-4-quinazolinones via ortho-fluorobenzoyl guanidines

A novel route to 12 substituted 2-amino-4-quinazolinones is described. Starting from 2,6-difluoro-4-methoxybenzonitrile, substitution of one of the fluorine atoms either directly or indirectly with heterocycles (e.g., pyridyl, thiazolyl, pyrazolyl) followed by hydrolysis of the nitrile gave a series of o-fluorobenzoic acid derivatives. Condensation with a set of six N,N-disubstituted guanidines followed by base-promoted ring closure afforded 2-amino-4-quinazolinone derivatives.     

Photobleaching of reduced nicotinamide adenine dinucleotide and the development of highly fluorescent lesions in rat basophilic leukemia cells during multiphoton microscopy

Endogenous reduced nicotinamide adenine dinucleotide (NADH) fluorescence provides an intrinsic indicator of the cellular metabolic state, but prolonged monitoring is limited by photobleaching and/or phototoxicity. Multiphoton excitation of NADH by ultrashort, 740-nm laser pulses provides a significant improvement over UV excitation by eliminating peripheral photobleaching; however, molecules within the subfemtoliter excitation volume remain susceptible. We have investigated the photophysical mechanisms responsible for multiphoton photobleaching of NADH in living cells to permit the imaging technique to be optimized. The loss of fluorescence because of multiphoton photobleaching was measured by repetitively imaging individual planes within rat basophilic leukemia cells. The photobleaching rate was proportional to the fourth power of the laser intensity. Based on these measurements, we propose a double-biphotonic, four-photon photobleaching mechanism and estimate the quantum yield of photobleaching of intracellular NADH to be 0.0073 +/- 0.0002 by this mechanism. In addition to photobleaching, the development of bright, punctate fluorescent lesions can also be observed. The frequency of lesion formation also increased approximately as the fourth power of the laser intensity after an intensity-dependent threshold number of images had been exceeded. The consequences for two-photon metabolic imaging are discussed.     

The multiconfigurational spin tensor electron propagator method (MCSTEP): Comparison with extended Koopmans' theorem results

Summary We applied the multiconfigurational spin tensor electron propagator method (MCSTEP) for determining the lowest few (in energy) vertical ionization potentials (IPs) of HF, H₂O, NH₃, CH₄, N₂, CO, HNC, HCN, C₂H₂, H₂CO, and B₂H₆. We chose these molecules so that we could compare MCSTEP IPs with recently reported extended Koopmans' theorem (EKT) IPs on the same molecules. Using standard Dunning core-valence basis sets with relatively small complete active spaces, MCSTEP results are in very good to excellent agreement with experiment. These MCSTEP IPs are obtained using matrices no larger than 400 × 400. EKT matrices are even smaller; however, to obtain similar but generally slightly worse agreement with experiment, fairly large active spaces are required with EKT.