Contact-damage processes

Ceramics can often be damaged under low contact loads and, thus, it is important to understand these damage mechanisms. These processes not only give rise to strength degradation but are linked to wear and erosion. Indentation fracture mechanics has been found to be a very useful approach in understanding these contact-damage processes (see Section 8.8). 

The elastic problem for an elastic half-space contacted by a normal point force P was solved by Boussinesq in 1885. The stress field is axisymmetric around the force direction and has the general form, in spherical coordinates. 

The elastic problem of contact between spherical particles was solved by Hertz in 1881 Of interest here is the limiting case of the contact of a sphere, radius / , with a flat surface (blunt contact). The spherical indenter forms a circular contact, radius a, with the surface. The contact area increases with size as the load increases. It can be shown that 

For this loading geometry, the maximum tensile stress occurs on the surface, just outside the contact area and is given by 

Outside the contact area, the tensile stresses fall off as r where r is the radial distance from the center of the contact area. Inside the contact area, the stresses are compressive and form a hemispherical distribution. It is also found that the region below the indenter is compressive to a depth 2a, in contrast to the tensile stress field found for a point indentation. At larger distances, the stress field (necessarily) converges to that for the point-force problem. From Eq. (8.90), it is clear, even for elastic contacts, that the load eflection behavior is non-linear. This is a reflection of the growth in the contact area as the load increases. If the contact area can be measured, it is sometimes useful to rearrange Eq. (8.88) into the (linear) form 

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DNA waters of solvation play a significant role in the DNA damage process from ionizing radiation. In fiilly hydrated DNA, approximately 8-10 water molecules per nucleotide (T = 8-10 H2O/ nucleotide) form a primary solvation shell of molecules directly in contact with the DNA and its coimtetion. When these molecules are ionized, hole transfer to the DNA molecule is faster than deprotonation... 

Damaging processes. The organism is injured by the hemotoxic processes - hemoglobinaemia and severe hemolysis with shock fits, neural, liver and kidney cells metabolism disorders occur. A mild irritative effect on the contact mucosa is also observed. 

Apart from the pressure of the reaction, other process parameters of importance are the temperature and the earth/oil contact time. Process temperatures between 80 and 180 °C have been advocated for the process. Adsorption effects are usually improved by the use of high temperatures but in the treatment of low-quality oils the reverse can be true (Zschau, 1982). Because of damage to the oil caused by the ingress of air in industrial plant the temperatures employed are normally between 90 and 120 °C. 

Z. Chen, J. C. Cuneo, J. J. Mecholsky and S. Hu, Damage Processes in Si3N4 Bearing Material Under Contact Loading, Wear, 198,197-207 (1996). 

Coatings must be considered for aU applications of steel. Cathodic protection should be considered for steel pipe where soil or groundwater resistivity is less than 10,000 t2-cm, and where steel win be in contact with process streams. Cathodic protection of steel is strongly recommended where resistivity is less than 5000 Q-cm. For aU exposures, steel should be electrically isolated from dissimilar metals to prevent the formation of unfavorable galvanic corrosion ceUs. In areas where abrasive materials are hkely to damage coatings, cathodic protection by impressed current or galvanic anodes may be desirable. 

Chen, Z., Cuneo, J., Mecholsky, J.J., Damage processes in Si3N4 bearing material under contact loading. Wear, 198 (19%) 197-207... 

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