Stick-slip behaviour of a viscoelastic flat sliding along a rigid indenter

The sliding contact of soft material surface due to a rigid indenter is different from metal and some other polymers. A stick-slip motion is more frequently obtained than a smooth motion. By modeling the soft material as low damping viscoelastic material, this study proposes an analytical model to identify the stick-slip behavior of sliding system. The sliding system is a fixed rigid indenter that slides against on a moving soft material surface. A stick-slip model is developed and the motion of the sliding system is assumed to be in a solely tangential direction. By implementing the simple coulomb friction law, an exact solution is presented in the case of no damping of the sliding system. Results show that the periodic displacement of the stick-slip model is strongly depending on the friction force, sliding velocity and material stiffness. Furthermore, the effect of a viscous damping and velocity-dependent friction on the behaviour of the sliding system are discussed

The Axis-Ratio Distribution of Galaxy Clusters in the SDSS-C4 Catalog as a New Cosmological Probe

We analyze the C4 catalog of galaxy clusters from the Sloan Digital Sky Survey (SDSS) to investigate the axis-ratio distribution of the projected two dimensional cluster profiles. We consider only those objects in the catalog whose virial mass is close to 10^{14}h^{-1}M_{sun}, with member galaxies within the scale radius 1000 kpc. The total number of such objects turns out to be 336. We also derive a theoretical distribution by incorporating the effect of projection onto the sky into the analytic formalism proposed recently by Lee, Jing, & Suto. The theoretical distribution of the cluster axis-ratios is shown to depend on the amplitude of the linear power spectrum (sigma_8) as well as the density parameter (Omega_{m}). Finally, fitting the observational data to the analytic distribution with Omega_{m} and sigma_{8} as two adjustable free parameters, we find the best-fitting value of sigma_{8}=(1.01 +/- 0.09)(Omega_{m}/0.6)^{(0.07 +/- 0.02) +0.1 Omega_{m}}$. It is a new sigma_{8}-Omega_{m} relation, different from the previous one derived from the local abundance of X-ray clusters. We expect that the axis-ratio distribution of galaxy clusters, if combined with the local abundance of clusters, may put simultaneous constraints on sigma_{8} and Omega_{m}.Comment: accepted for publication in ApJ, 17 pages, 3 figures, improved analysis, more discussion on the validity and the caveats of the mode

Friction and Contact Temperatures in the Cleaving of Bone and Wood Using Stone Tools – A Case Study in Palaeolithic Tribology

During archaeological fieldwork wedge-shaped quartz stones that show clearly visible “glossy patches” composed of high quartz have been found. It is generally accepted that these tools have been used to cleave or punch wood and bone materials. For the transformation from quartz to high-quartz to occur, the temperature should exceed 574 °C. The hypothesis tested in this manuscript is that the phase change in the stone tool results from frictional heating during the cleaving action. Dry sliding friction measurements were carried out on a reciprocating tribometer using four types of stone, representing the punch tool, and pine, oak and bovine bone, representing the work piece. Measured coefficients of friction were approximately 0.1 on oak, 0.2 on pine and up to 0.35 on bovine bone, with some minor fluctuations for the different types of stone. These coefficients of friction were inserted into a computational model describing the flash temperatures in a moving contact, from which it was shown that the hypothesis might hold in the case of lydite-bone contact. This means that the glossy patches on the stone tools may have been caused by frictional heating during the cleaving of bone.</p

CFD analysis of texture depth effect on the performance of hydrodynamic lubricated bearing

Surface modification of the lubricated bearing such texturing has proven to improve the hydrodynamic performance. The present paper examined the effect of texture depth as well as the texture length on the tribological performance using computational fluid dynamic (CFD) approach. The cavitation model was also considered to obtain more realistic characteristic of bearing. It was shown that by increasing texture depth of the surface of bearing, the enhanced hydrodynamic pressure was achieved. Moreover, other interesting result is that the length of cavitation region decreases with the increase in texture depth and the decrease in texture length

The impact of hunting on tropical mammal and bird populations

Hunting is a major driver of biodiversity loss, but a systematic large-scale estimate of hunting-induced defaunation is lacking. We synthesized 176 studies to quantify hunting-induced declines of mammal and bird populations across the tropics. Bird and mammal abundances declined by 58% (25 to 76%) and by 83% (72 to 90%) in hunted compared with unhunted areas. Bird and mammal populations were depleted within 7 and 40 kilometers from hunters’ access points (roads and settlements). Additionally, hunting pressure was higher in areas with better accessibility to major towns where wild meat could be traded. Mammal population densities were lower outside protected areas, particularly because of commercial hunting. Strategies to sustainably manage wild meat hunting in both protected and unprotected tropical ecosystems are urgently needed to avoid further defaunation

Response to Dr Greenwood’s Comments on “Extending the Double-Hertz Model to Allow Modeling of an Adhesive Elliptical Contact”

© 2018, The Author(s). An adhesive elliptical contact is normally found in microscale applications that involve cylindrical solids, crossing at an angle between 0° and 90°. Currently, only one model is available to describe the elliptical contact’s surface interaction: the approximate Johnson–Kendall–Roberts (JKR) model which is limited to soft materials. In this paper, a new adhesive elliptical model is developed for a wide range of adhesive contacts by extending the double-Hertz theory, where adhesion is modeled by the difference between two Hertzian pressure distributions. Both Hertzian pressures are assumed to have an equivalent shape of contact areas, the only difference being in size. Assuming that the annular adhesive region is obtained by the area difference between the two Hertzian contact areas, the pull-off force curves can be calculated. In the limiting case of an adhesive circular contact, the results are very close to results from the existing models. However, for an adhesive elliptical contact in the JKR domain, lower pull-off forces are predicted when compared to the JKR values. Unlike the developed model, the shape of the JKR contact area varies throughout contact. Results show, particularly for conditions close to the JKR domain, that it is important to take into account that the adhesive region is the result of the two Hertzian contact areas having a non-equivalent shape

The impact of constructive operating lease capitalisation on key accounting ratios

Current UK lease accounting regulation does not require operating leases to be capitalised in the accounts of lessees, although this is likely to change with the publication of FRS 5. This study conducts a prospective analysis of the effects of such a change. The potential magnitude of the impact of lease capitalisation upon individual users' decisions, market valuations, company cash flows, and managers' behaviour can be indicated by the effect on key accounting ratios, which are employed in decision-making and in financial contracts. The capitalised value of operating leases is estimated using a method similar to that suggested by Imhoff, Lipe and Wright (1991), adapted for the UK accounting and tax environment, and developed to incorporate company-specific assumptions. Results for 1994 for a random sample of 300 listed UK companies show that, on average, the unrecorded long-term liability represented 39% of reported long-term debt, while the unrecorded asset represented 6% of total assets. Capitalisation had a significant impact (at the 1% level) on six of the nine selected ratios (profit margin, return on assets, asset turnover, and three measures of gearing). Moreover, the Spearman rank correlation between each ratio before and after capitalisation revealed that the ranking of companies changed markedly for gearing measures in particular. There were significant inter-industry variations, with the services sector experiencing the greatest impact. An analysis of the impact of capitalisation over the five-year period from 1990 to 1994 showed that capitalisation had the greatest impact during the trough of the recession. Results were shown to be robust with respect to key assumptions of the capitalisation method. These findings contribute to the assessment of the economic consequences of a policy change requiring operating lease capitalisation. Significant changes in the magnitude of key accounting ratios and a major shift in company performance rankings suggest that interested parties' decisions and company cash flows are likely to be affected