Cyclic debonding of adhesively bonded composites

The fatigue behavior of a simple composite to composite bonded joint was analyzed. The cracked lap shear specimen subjected to constant amplitude cyclic loading was studied. Two specimen geometries were tested for each bonded system: (1) a strap adherend of 16 plies bonded to a lap adherend of 8 plies; and (2) a strap adherend of 8 plies bonded to a lap adherend of 16 plies. In all specimens the fatigue failure was in the form of cyclic debonding with some 0 deg fiber pull off from the strap adherend. The debond always grew in the region of adhesive that had the highest mode (peel) loading and that region was close to the adhesive strap interface

Geometrically nonlinear analysis of adhesively bonded joints

A geometrically nonlinear finite element analysis of cohesive failure in typical joints is presented. Cracked-lap-shear joints were chosen for analysis. Results obtained from linear and nonlinear analysis show that nonlinear effects, due to large rotations, significantly affect the calculated mode 1, crack opening, and mode 2, inplane shear, strain-energy-release rates. The ratio of the mode 1 to mode 2 strain-energy-relase rates (G1/G2) was found to be strongly affected by he adhesive modulus and the adherend thickness. The ratios between 0.2 and 0.8 can be obtained by varying adherend thickness and using either a single or double cracked-lap-shear specimen configuration. Debond growth rate data, together with the analysis, indicate that mode 1 strain-energy-release rate governs debond growth. Results from the present analysis agree well with experimentally measured joint opening displacements

Repeatability of mixed-mode adhesive debonding

The repeatability of debond growth rates in adhesively bonded subjected to constant-amplitude cyclic loading was studied. Debond growth rates were compared from two sets of cracked-lap-shear specimens that were fabricated by two different manufacturers and tested in different laboratories. The fabrication method and testing procedures were identical for both sets of specimens. The specimens consisted of aluminum adherends bonded with FM-73 adhesive. Critical values of strain-energy-release rate were also determined from specimens that were monotonically loaded to failure. The test results showed that the debond growth rates for the two sets of specimens were within a scatter band which is similar to that observed in fatigue crack growth in metals. Cyclic debonding occurred at strain-energy-release rates that were more than an order of magnitude less than the critical strain-energy-release rate in static tests

Can a circulating light beam produce a time machine?

In a recent paper, Mallett found a solution of the Einstein equations in which closed timelike curves (CTC's) are present in the empty space outside an infinitely long cylinder of light moving in circular paths around an axis. Here we show that, for physically realistic energy densities, the CTC's occur at distances from the axis greater than the radius of the visible universe by an immense factor. We then show that Mallett's solution has a curvature singularity on the axis, even in the case where the intensity of the light vanishes. Thus it is not the solution one would get by starting with Minkowski space and establishing a cylinder of light.Comment: 5 pages, RevTe

Limits on Stellar Companions to Exoplanet Host Stars With Eccentric Planets

Though there are now many hundreds of confirmed exoplanets known, the binarity of exoplanet host stars is not well understood. This is particularly true of host stars which harbor a giant planet in a highly eccentric orbit since these are more likely to have had a dramatic dynamical history which transferred angular momentum to the planet. Here we present observations of four exoplanet host stars which utilize the excellent resolving power of the Differential Speckle Survey Instrument (DSSI) on the Gemini North telescope. Two of the stars are giants and two are dwarfs. Each star is host to a giant planet with an orbital eccentricity > 0.5 and whose radial velocity data contain a trend in the residuals to the Keplerian orbit fit. These observations rule out stellar companions 4-8 magnitudes fainter than the host star at passbands of 692nm and 880nm. The resolution and field-of-view of the instrument result in exclusion radii of 0.05-1.4 arcsecs which excludes stellar companions within several AU of the host star in most cases. We further provide new radial velocities for the HD 4203 system which confirm that the linear trend previously observed in the residuals is due to an additional planet. These results place dynamical constraints on the source of the planet's eccentricities, constraints on additional planetary companions, and informs the known distribution of multiplicity amongst exoplanet host stars.Comment: 10 pages, 7 figures, 2 tables, accepted to Ap

A Method to Find Community Structures Based on Information Centrality

Community structures are an important feature of many social, biological and technological networks. Here we study a variation on the method for detecting such communities proposed by Girvan and Newman and based on the idea of using centrality measures to define the community boundaries (M. Girvan and M. E. J. Newman, Community structure in social and biological networks Proc. Natl. Acad. Sci. USA 99, 7821-7826 (2002)). We develop an algorithm of hierarchical clustering that consists in finding and removing iteratively the edge with the highest information centrality. We test the algorithm on computer generated and real-world networks whose community structure is already known or has been studied by means of other methods. We show that our algorithm, although it runs to completion in a time O(n^4), is very effective especially when the communities are very mixed and hardly detectable by the other methods.Comment: 13 pages, 13 figures. Final version accepted for publication in Physical Review

Time travel paradoxes, path integrals, and the many worlds interpretation of quantum mechanics

We consider two approaches to evading paradoxes in quantum mechanics with closed timelike curves (CTCs). In a model similar to Politzer's, assuming pure states and using path integrals, we show that the problems of paradoxes and of unitarity violation are related; preserving unitarity avoids paradoxes by modifying the time evolution so that improbable events bewcome certain. Deutsch has argued, using the density matrix, that paradoxes do not occur in the "many worlds interpretation". We find that in this approach account must be taken of the resolution time of the device that detects objects emerging from a wormhole or other time machine. When this is done one finds that this approach is viable only if macroscopic objects traversing a wormhole interact with it so strongly that they are broken into microscopic fragments.Comment: no figure

Assessing the Effect of Stellar Companions from High-Resolution Imaging of Kepler Objects of Interest

We report on 176 close (<2") stellar companions detected with high-resolution imaging near 170 hosts of Kepler Objects of Interest. These Kepler targets were prioritized for imaging follow-up based on the presence of small planets, so most of the KOIs in these systems (176 out of 204) have nominal radii <6 R_E . Each KOI in our sample was observed in at least 2 filters with adaptive optics, speckle imaging, lucky imaging, or HST. Multi-filter photometry provides color information on the companions, allowing us to constrain their stellar properties and assess the probability that the companions are physically bound. We find that 60 -- 80% of companions within 1" are bound, and the bound fraction is >90% for companions within 0.5"; the bound fraction decreases with increasing angular separation. This picture is consistent with simulations of the binary and background stellar populations in the Kepler field. We also reassess the planet radii in these systems, converting the observed differential magnitudes to a contamination in the Kepler bandpass and calculating the planet radius correction factor, $X_R = R_p (true) / R_p (single)$. Under the assumption that planets in bound binaries are equally likely to orbit the primary or secondary, we find a mean radius correction factor for planets in stellar multiples of $X_R = 1.65$. If stellar multiplicity in the Kepler field is similar to the solar neighborhood, then nearly half of all Kepler planets may have radii underestimated by an average of 65%, unless vetted using high resolution imaging or spectroscopy.Comment: 23 pages, 12 figures. Accepted for publication in The Astronomical Journa

MTC Meeting of September 9, 1952

Present: H. Anderson, J. Crane, R. R. Everett, W. A. Hosier, R. Hughes, K. H. Olsen, R. Pfaff, N. H. Taylor, R. von Buelow.Decisions and progress relevant to the proposed Memory Test Computer (MTC) are summarized for distribution to those interested, and to trace development of the computer