A Definition of Plasticity
THE word ‘plasticity’, although one which has attained a state almost of ubiquity in several branches of applied physics and industrial chemistry, has, according to Scott Blair1, never been given a quantitative and dimensional definition. In the technical literature a group of quantitative ‘plasticity numbers’ have taken their place. Such expressions as the Williams plasticity number, or the Goodrich plasticity number (both used in rubber technology), have no quantitative meaning other than that of the reading of a particular type of testing equipment when used in a particular manner, and as such cannot be expressed in C.G.S. units.
Rheological Evaluation of Petroleum Jelly as a Base Material in Ointment and Cream Formulations : Linear Viscoelastic Behavior
ABSTRACT − The objective of the present study is to systematically characterize a linear viscoelastic behavior of petro-leum jelly in small amplitude oscillatory shear flow fields correspondent to the rheological ground state. With this aim, usinga strain-controlled rheometer, the dynamic viscoelastic properties of commercially available petroleum jelly have been mea-sured at 37 o C (body temperature) over a wide range of angular frequencies at an extremely small strain amplitude of 0.1%. In this article, the linear viscoelastic behavior was reported in detail and then explained from a structural view-point ofpetroleum jelly and discussed in depth with respect to the consumer’s requirements. Main findings obtained from this studycan be summarized as follows : (1) The storage modulus is always greater than the loss modulus over an entire range ofangular frequencies studied, meaning that the linear viscoelastic behavior of petroleum jelly is dominated by an elastic naturerather than a viscous nature. (2) Petroleum jelly shows a desirable linear viscoelastic behavior with respect to the consumer’srequirements because it is undesirable for the product to flow down from the skin at an initial stage upon contact with thehuman skin. (3) A fractional derivative model shows an excellent applicability to describe a linear viscoelastic behavior ofpetroleum jelly. However, this model should be used with a special caution because there exists no physical meaning forthe model parameters. (4) A modified form of the Cox-Merz rule gives a good ability to predict the relationship betweensteady shear flow properties (nonlinear behavior) and dynamic viscoelastic properties (linear behavior) for petroleum jelly.Key words
Neutron Residual Stress Measurements on Rail Sections for Different Production Conditions
Rail sectioning with subsequent neutron diffraction experiments has been used to assess residual stresses in the rails. In this study we present the results of neutron stress measurements performed at the NIST Center for Neutron Research (NCNR) on rail sections from rails that were produced under various conditions. Specifically, these are air-cooled, air-cooled and roller-straightened, head-hardened and head-hardened and roller-straightened. More significantly, a head-hardened and roller-straightened rail was also studied after service to elucidate evolution of the service-induced residual stresses. In the latter case both a transverse-cut slice and the central region of a 0.53 m long piece were studied. Measurements on this piece are the first in which triaxial stresses have been determined for an intact rail. Neutron strain measurements with 3×3×3 mm3 spatial resolution were successfully employed for transversally cut slices to verify the difference in the stress state depending on the production process. Although examination of slices allows determination of only two-dimensional stresses in the plane of the slice, additional measurements on obliquely-cut slices, which were also carried out, and utilization of FEM gives the possibility of reconstructing the full triaxial stress distribution. Together, these approaches provide a better understanding of rail fabrication and the possibility of improving the durability and safety of rails in the future.Copyright © 2004 by ASME