Surface spalling

Notice that neither Chabai nor Baker includes the explosive CJ pressure among the important variables. That is reasonable since the work here is in lifting, PV work. Shattering and shock coupling with resulting surface spalling, which played an important role in the formation of surface blast cratering, are not major effects here. 

MIL-HDBK-304 - Chapter 6 has a simple xocedure in which strips of cellular cushioning materials are wrapped around a cylinder or mandrel under standard conditions through a total angle of 180°. The specimens are then examined for failure, such as cracking, delamination, surface spalling, etc. 

The use of coatings on concrete has shown a marked increase in recent years in order to resist carbonation or to control the ingress of chlorides. These factors have been responsible for much of the reinforcement corrosion and surface spalling of concrete which is now evident. 

Surface spalling (fatigue and crack formation through tribological fluctuating loads)... 

The pore solution is effectively neutralized by this reaction. Carbonation damage usually appears as a well-defined front parallel to the outside surface. Behind the front, where all the calcium hydroxide has reacted, the pH is reduced to around 8, whereas ahead of the front, the pH remains above 12.6. When the carbonation front reaches the reinforcement, the passive film is no longer stable, and active corrosion is initiated. Figure 1.14 shows that active corrosion is possible at the reduced pH level. Damage to the concrete from carbonation-induced corrosion is manifested in the form of surface spalling, resulting from the buildup of voluminous corrosion products at the concrete-rebar interface (Fig. 1.15). 

Industrial materials without sufficient scaling resistance frequendy fail after a short period of time as a result of rapid oxidation or hot corrosion, in conjunction with severe spalling owing to poor adherence of the scale to the metallic component. As a result, the permissible limits of metal loss often are exceeded and expensive, and premature replacement of parts is requited. Extensive efforts are made to develop alloys which are not simply strong at elevated temperatures but which also possess the adequate surface stabiUty. 

A problem in nitriding is that a layer of nitrides may form on the surface which does not have desirable properties. For example, if the layer is too thick, it is brittle and may spall off. This is called the white layer or compound layer, and may have to be machined or ground off after nitriding. 

The result is a hard, abrasion-resistant surface, important in many appHcations of cast kon. The depth of the chill may be controlled by regulating the amount of tellurium added. The casting shows a sharp demarcation line between the chilled and unchilled regions there is no intermediate or motded zone. Yet, the chilled portion shows excellent resistance to spalling from thermal or mechanical shock. Tellurium-treated kon is more resistant to sulfuric and hydrochloric acids than is untreated, unchilled gray kon. The amount added ranges from 0.005 to 0.1% ca 60% is lost by volatilization. Excessive addition causes porosity in the castings. 

When alloy steels do not give adequate corrosion protection— particularly from sulfidic attack—steel with an aluminized surface coating can be used. A spray coating of aluminum on a steel is not likely to spall or flake, but the coating is usually not continuous and... 

Figure 13.64 Red, dezincified gasketed surface of a cast valve throat. Note the partial spalling of the copper corrosion product.

Figure 4.16. Free-surface velocity profiles measured on 1400° C molybdenum. The free-surface velocity profile is characterized by an 0.05 km/s amplitude elastic precursor, a plastic wave front, and a spall signal (characteristic dip) upon unloading. The dashed lines represent the expected free surface velocity based on impedance-match calculation [Duffy and Ahrens, unpublished].

Spall is the process of internal failure or rupture of condensed media through a mechanism of cavitation due to stresses in excess of the tensile strength of the material. Usually, a dynamic failure is implied where transient states of tensile stress within the body are brought about by the interaction of stress waves. Free surfaces are assumed to be well removed from the material point of interest and play no role in the spall process. 

More general dynamic loading conditions can lead to more complex domains of tensile stress and spall. For example, in a Taylor impact experiment (Kipp and Davison, 1981), where a short cylinder of material is caused to undergo symmetric normal impact on the flat surface of a large block of material, a roughly spherical region within the cylinder is carried into dynamic tension and can undergo spall. 

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