Centration mechanism of soft contact lenses

A theoretical model is developed in which the force that restores a soft hydrogel lens to its centered position is ascribed to flexure of the lens. It is shown that flexure makes the dominant contribution to the strain energy in the displaced position. The restoring force is calculated explicitly. The return speed is estimated explicitly, assuming that the dominant resisting force is the viscous drag offered by the postlens tear film. Satisfactory agreement with observations is found.     

Vulnerability-Based Risk Assessment for Stream Instability at Bridges

The Federal Highway Administration recommends that stream stability analyses at bridges over water begin with a Level 1 assessment. Following this assessment of the physical condition of the stream in the vicinity of the bridge, the user must determine whether or not the relative risk is low. If it is low, then no action is needed. If the risk is greater than low, then a Level 2 analysis is recommended; however, no method is given for determining the risk. In this paper, the relative risk of unsatisfactory conditions at bridge foundations is assessed as a simple function of vulnerability and criticality. Vulnerability is based on a stream stability assessment and data from the National Bridge Inventory (NBI). Criticality is determined indirectly as a function of the bridge serviceability and cost by using data extracted from the NBI. Relative risk is then qualitatively determined by combining vulnerability and criticality. Three examples in central Pennsylvania are provided in which the relative level of risk is used to determine the need for a Level 2 analysis.     

Peripheral curve design and the tear pump mechanism of soft contact lenses

The role of the peripheral curve of a soft lens in the supply of oxygen to the cornea via the tear pump mechanism is considered. A group of 18 patients was optimally fitted with spherical DuraSoft lenses with and without peripheral curves; the absence of the peripheral curve was not found to affect adversely the physiological response of the cornea as determined by pachometric changes in central corneal thickness.     

Instability and Static Liquefaction on Proportional Strain Paths for Sand at Low Stresses

The behavior of Hostun RF sand on proportional strain paths at low confining pressures (20 to 100 kPa) is considered in this paper. In such paths, a constant dilation rate is imposed during shear. The usual features of pore pressure increase (contracting material) or decrease (dilating material) are here observed depending upon whether the imposed dilation rate is respectively greater or smaller than the “natural” dilation rate at failure (as measured in a drained test). Particular attention is given to the static liquefaction phenomenon, which is seen to occur for loose as well as dense sand provided the imposed dilation rate is large enough to lead to a continuous pore pressure increase during shear. Instability tests performed at low confining pressures on proportional strain paths show that the instability line is strain path dependent. It does not coincide with the peak deviator stress line in proportional strain paths tests, in general, but does coincide with the line d2W = 0 (nil second increment of total work).     

Macroscopic mechanism of formation of interparticle contact in electric current sintering of powders

Conclusions We proposed the mechanism of electric discharge sintering which consists of three stages: the first stage is the formation of the initial contact frame, the second stage is the preferential destruction of a specific volume of the contact frame, and the third stage is the formation of a short-circuited core or of interparticle discrete bridges which close up the interparticle gaps, the increase of the perfection of the contact bridges as a result of consolidation of the central part of the contact bridge, and filling of the resultant voids by the finest particles of electroerosion origin.Translated from Poroshkovaya Metallurgiya, No. 3(315), pp. 33–39, March, 1989.