Thermal buckling of ballasted tangent track

Euler type analysis usually used for compression members in structural engineering does not work for railroad track. Euler type analytical formulas for horizontal and vertical buckling endorsed in a recent literature is reviewed to demonstrate its weakness. Using definition of moment and curvature as well as principle of equilibrium, the author suggests formula for horizontal buckling load of railroad track and demonstrates validation in context with currently accepted values, published results, and past field tests. The buckling load from suggested formula agrees with the recent buckling load formula based on total energy theorem. A formula is suggested to study the effect of misalignment on critical temperature differential or critical load. A vertical buckling load formula is derived from horizontal buckling load formula. A buckling process is narrated through step by step computation. Formulas are suggested to compute the effect of track misalignment on critical buckling load and threshold radius of a vertical curve.

Accretion of eroding pebbles and planetesimals in planetary envelopes

Wind erosion is a destructive mechanism that completely dissolves a weakly bound object like a planetesimal into its constituent particles, if the velocity relative to the ambient gas and the local gas pressure are sufficiently high. In numerical simulations we study the influence of such wind erosion on pebble and planetesimal accretion by a planetary body up to 10 REarth. Due to the rapid size reduction of an in-falling small body, the accretion outcome changes significantly. Erosion leads to a strong decrease in the accretion efficiency below a threshold size of the small body on the order of 10 m. This slows down pebble accretion significantly for a given size distribution of small bodies. The threshold radius of the small body increases with increasing planet radius and decreases with increasing semi-major axis. Within the parameters studied, an additional planetary atmosphere (up to 1 bar) is of minor importance.

Super-massive objects in Yarman–Arik–Kholmetskii (YARK) gravitation theory

We continue to analyze the implications of the gravitational framework of our theoretical approach, christened YARK (abbreviated from Yarman–Arik–Kholmetskii), with respect to super-massive celestial bodies. We emphasize in particular that a gravitating test particle in the presence of a ponderable mass must adhere to the law of energy conservation, which remarkably does not yield any singularity according to YARK. Even so, for a given spherically shaped extremely compact super-massive body, one can achieve a theoretical radius below which “light” of, say, the visible frequency range can indeed be trapped. Yet, such a radius comes out to be tens of times shorter than the threshold radius for black hole formation as established by the general theory of relativity (GTR). In accordance with our derivations, the minimal mass for a celestial object capable of recapturing emitted light in its environs — similar to textbook “intermediate class black holes” — is found to be about 103MS, where MS stands for the mass of the Sun. For less massive celestial objects, the crucial radius that produces a “YARK black hole” (i.e., without singularity) corresponds to a higher density than the density of a baryon; and hence, such entities cannot apparently exist in nature. Black holes allowed therefore in our approach are not related, in any case, to the singularity conceptualization of GTR. As a consequence, we are able to present a resolution to the “black hole information paradox”. The findings of YARK will be discussed hereinafter with regards to the foundations of observational cosmology.

Semiclassical dynamics of horizons in spherically symmetric collapse

In this paper, we consider a semiclassical description of the spherically symmetric gravitational collapse with a massless scalar field. In particular, we employ an effective scenario provided by holonomy corrections from loop quantum gravity (LQG), to the homogeneous interior spacetime. The singularity that would arise at the final stage of the corresponding classical collapse, is resolved in this context and is replaced by a bounce. Our main purpose is to investigate the evolution of trapped surfaces during this semiclassical collapse. Within this setting, we obtain a threshold radius for the collapsing shells in order to have horizons formation. In addition, we study the final state of the collapse by employing a suitable matching at the boundary shell from which quantum gravity effects are carried to the exterior geometry.

Investigation of Pore Structure for Manufactured Sand Mortar

This paper focuses on the pore structure parameters of mortars produced with manufactured sand and natural sand via water saturation and MIP methods. Test results show that, total porosity, as well as compressive strength, of manufactured sand mortar, is higher than that of natural sand mortar at fixed w/c and s/c ratio. Furthermore, considerable volume of large pores present in specimens of manufactured sand at higher w/c ratio rather not at the lower w/c ratio, which caused by the larger binder-aggregate interface. Manufactured fine aggregate in mortar probably accelerate hydrated reaction of cement, which result in the most probable pore size is finer than that of natural sand mortar. It can be concluded that the threshold region becomes flatten and threshold radius increases due to the aggregate volume concentration rises. Finally, a new theoretical model with a double-lognormal distribution function is demonstrated to be reasonable to fit pore size distribution in mortars.

Influence of Pore Structure on Compressive Strength of Cement Mortar

This paper describes an experimental investigation into the pore structure of cement mortar using mercury porosimeter. Ordinary Portland cement, manufactured sand, and natural sand were used. The porosity of the manufactured sand mortar is higher than that of natural sand at the same mix proportion; on the contrary, the probable pore size and threshold radius of manufactured sand mortar are finer. Besides, the probable pore size and threshold radius increased with increasing water to cement ratio and sand to cement ratio. In addition, the existing models of pore size distribution of cement-based materials have been reviewed and compared with test results in this paper. Finally, the extended Bhattacharjee model was built to examine the relationship between compressive strength and pore structure.

Star Formation and Threshold in Nearby Galaxies Observed with GALEX

Star formation on galactic scales is the process driving the evolution of galaxies. It is important to understand its various aspects in the nearby universe to properly interpret high redshift observations, and to construct correct models of the evolution of galaxies. Combining UV (GALEX) and infrared (IRAS) data for 43 nearby spatially resolved galaxies with corollary data (HI,CO), we study the star formation law (“Schmidt” law). The absence of a “threshold radius” in the UV supports the idea that star formation proceed at low gas densities and that the usual threshold observed in H-alpha profiles is a small-number statistic effect.