Stress measurement subsurface

The existence of asperity contacts in mixed lubrication causes great many local events and significant consequences. For example, the parameters describing lubrication and contact conditions, such as film thickness, pressure, subsurface stress, and surface temperature, fluctuate violently and frequently over time and space domain. It is expected that these local events would have significant effects on the service life of machine elements, but experimental measurements are difficult because of the highly random and time-dependent nature of the signals. Only a few successes were reported so far in experimental studies of mixed lubrication, mostly limited to the artificially manufactured... 

Zhu, D. and Hu, Y. Z., A Computer Program Package for the Prediction of EHL and Mixed Lubrication Characteristics, Friction, Subsurface Stresses and Flash Temperatures Based on Measured 3-D Surface Roughness," Tribol. Trans., Vol. 44, No. 3,2001, pp. 383-390. 

Our experimental estimates of growth rate as a function of iron concentration yielded a half-saturation constant for growth of 0.45 nM dissolved Fe for colonial P. antarctica at an irradiance of -20 m 2 s 1 (Sedwick et al., this issue). These results imply that at this relatively low irradiance, colonial P. antarctica was iron-limited (sensu Morel et al. 1991) at ambient dissolved Fe concentrations < 0.45 nM (Fuco -Hex < 0.035), and was still significantly iron-stressed (sensu Morel et al. 1991) at dissolved iron concentrations of 1-2 nM (Fuco Hex -0.075-0.15). Thus, the mean Fuco Hex ratio of 0.076 measured in subsurface, P. antarctica-dominated... 

Corrosion is an electrochemical process and corrosion processes follow the basic laws of thermodynamics. Under controlled conditions, corrosion can be measured, repeated, and predicted. However, because corrosion takes place on an atomic level, corrosion can take place in an accelerated localized fashion, appear as uniform visible attack, or result in subsurface microscopical damage. Normal service environments can rapidly complicate these processes and mechanisms with such variables as pH, temperature, stress, surface finish, flow rates, etc. With the wide range of variables that can come into play, it should not be surprising that corrosion appears to be unpredictable at times. 

The accumulation of substances such as silica, iron, aluminum, carbonate, and other salts can result in cemented layers, which change the physical, chemical, and biological behavior of the soil. For example, a cemented layer retards percolation and restricts root activity. Furthermore, the availability of nutrients for plant growth is reduced, i.e., the cation exchange capacity is reduced. There are accumulations in the soil which show the enrichment of one substance and/or the depletion of another substance. This can be expressed by diagnostic subsurface horizons, which are listed in alphabetically order in Table 14.5. It should be stressed that some characteristics can be measured only in the laboratory and not in the field. 

Hardness measurements such as Rockwell or Vicker s indentation properties are time-dependent as a result of the viscoelastic flow and relaxation processes (236) (see Hardness). Microhardness measurements have been used to correlate with other properties such as Yoimg s modulus and compressive yield stress in polyethylenes (237) and glass-transition temperature of amorphous plastics (238). Scratch resistance in polyproplyene studies was found to have shear yielding as the main cause of plastic flow scratch pattern with tensile tear effects on the surface and shear-induced fracture on the subsurface (239). 

This is an intriguing development because so far cracked indents have been seen to be a problem which must be avoided in precise hardness determinations, not because the energy used to propagate cracks is significant compared to the energy used to produce the plastic yield in the volume of the indent and beyond, but because cracks profoundly alter the stress state in surface and subsurface regions. Perhaps the most important reason why cracks are considered to be unfortunate in ceramic studies is that substantial surface disruption makes it impossible in many cases to define and measure the indent diagonals. In this chapter, however, we see a requirement for cracks around the indented area. 

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