Energy radiation of moving cracks

The energy radiated by moving cracks in a discrete background is analyzed. The energy flow through a given surface is expressed in terms of a generalized Poynting vector. The velocity of the crack is determined by the radiation by the crack tip. The radiation becomes more isotropic as the crack velocity approaches the instability threshold.Comment: 7 pages, embedded figure

Shear, compressive and dilatational response of rubberlike solids subject to cavitation damage

In this paper we examine the change in material response, in particular the dilatational response, due to cavitation damage arising from tensile hydrostatic stresses of sufficient magnitude. A general discussion of stress softening and cavitation is followed by a description of some new experimental results concerning the change in response in hydrostatic tension or compression or in shear due to cavitation damage. In hydrostatic tension there is a progressive reduction in the value of the tensile bulk modulus of the material during loading and significant stress softening on unloading. As a result of the cavitation damage the tensile bulk modulus in the natural configuration is reduced. Ultimately, failure of the material occurs at sufficiently large hydrostatic tension, typically when the volume increase locally exceeds a critical value, of the order of 2–3%. However, the compressive bulk modulus is unaffected by the cavitation damage. Moreover, it is also found that the shear modulus is likewise unchanged by cavitation. The experimental data are used to develop a theoretical model, based on the concept of pseudo-elasticity, to describe these phenomena. Specifically, the dilatational part of the strain-energy function of an elastic material depends on a damage parameter which provides a means for switching the form of the strain-energy function, thereby reflecting the stress softening associated with unloading. A good correspondence between the theory and the data is obtained

Preliminary X-ray crystallographic analysis of ornithine acetyltransferase (Rv1653) from Mycobacterium tuberculosis

Rv1653, an ornithine acetyltransferase from M. tuberculosis, has been crystallized and diffraction data have been collected to 1.7 Å resolution

Experimental investigation on the aging of the base isolator elastomeric component

Base isolation is a well-established technique of vibration mitigation. Among the many devices available on the market, elastomeric base isolators are widely adopted. Their drawback is the aging of the elastomeric component, which makes a strict maintenance plan mandatory. This manuscript summarizes an experimental study where the performance of new base isolators is compared with that of 10-years old devices

On the Sensitivity of Adhesion between Rough Surfaces to Asperity Height Distribution

There has been a long debate about the validity of asperity models in the contact between rough surfaces, much of it concentrated on relatively minor aspects of the solution for the special case of Gaussian random processes for roughness, like the exact value of the area–load slope or the extent of the linear regime. It is shown here that in the case of adhesion, the behavior is extremely sensitive to the shape of the height distribution. We show for example results for Weibull distributions, which has been suggested in a number of practical cases from macroscopic to nanoscopic roughness. Pull-off force is found to vary by several orders of magnitude both lower and higher than in the Gaussian case, whereas the “stickiness” criterion on the adhesion parameter changes by an order of magnitude. Additionally, in some operations like chemical-mechanical polishing, tails are almost completely removed and a sharp peak develops instead of a tail: modeling this with contact on the bounded side of the Weibull distribution, stickiness seems to occur for any level of roughness. Some qualitative comparison with recent numerical experiments is attempted