Microcracking

Specimen 1 contained a crack of 0,91 mm depth and a width between 0,001 mm and 0,011 mm. The tightness ring contained microcracks. 

Water loss in operating an HDR faciUty may result from either increased storage within the body of the reservoir or diffusion into the rock body beyond the periphery of the reservoir (38). When a reservoir is created, the joints which are opened immediately fill with water. Micropores or microcracks may fill much more slowly, however. Figure 11 shows water consumption during an extended pressurization experiment at the HDR faciUty operated by the Los Alamos National Laboratory at Fenton Hill, New Mexico. As the microcracks within the reservoir become saturated, the water consumption at a set pressure declines. It does not go to zero because diffusion at the reservoir boundary can never be completely elirninated. Of course, if a reservoir joint should intersect a natural open fault, water losses may be high under any conditions. 

Fig. 11. Water consumption during extended pressurization of an HDR reservoir. The amount of water required to maintain a constant pressure declines with the logarithm of time as the microcracks in the reservoir rock are slowly filled with the pressurized fluid.

Polyamides, like other macromolecules, degrade as a result of mechanical stress either in the melt phase, in solution, or in the soHd state (124). Degradation in the fluid state is usually detected via a change in viscosity or molecular weight distribution (125). However, in the soHd state it is possible to observe the free radicals formed as a result of polymer chains breaking under the appHed stress. If the polymer is protected from oxygen, then alkyl radicals can be observed (126). However, if the sample is exposed to air then the radicals react with oxygen in a manner similar to thermo- and photooxidation. These reactions lead to the formation of microcracks, embrittlement, and fracture, which can eventually result in failure of the fiber, film, or plastic article. 

Strength. The fracture strength of vitreous sihca depends on its surface quaUty, which can be affected by thermal treatment and handling conditions. Microcracks, surface contamination, and crystallisa tion can reduce the strength from the value of pristine vitreous sihca by several orders of magnitude. 

Most recent studies (69) on elevated temperature performance of carbon fiber-based composites show that the oxidation resistance and elevated temperature mechanical properties of carbon fiber reinforced composites are complex and not always direcdy related to the oxidation resistance of the fiber. To some extent, the matrix acts as a protective barrier limiting the diffusion of oxygen to the encased fibers. It is therefore critical to maintain interfacial bonding between the fiber and the matrix, and limit any microcracking that may serve as a diffusion path for oxygen intmsion. Since interfacial performance typically deteriorates with higher modulus carbon fibers it is important to balance fiber oxidative stabiHty with interfacial performance. 

Hard plating is noted for its excellent hardness, wear resistance, and low coefficient of friction. Decorative plating retains its brilliance because air exposure immediately forms a thin, invisible protective oxide film. The chromium is not appHed directiy to the surface of the base metal but rather over a nickel (see Nickel and nickel alloys) plate, which in turn is laid over a copper (qv) plate. Because the chromium plate is not free of cracks, pores, and similar imperfections, the intermediate nickel layer must provide the basic protection. Indeed, optimum performance is obtained when a controlled but high density (40—80 microcrack intersections per linear millimeter) of microcracks is achieved in the chromium lea ding to reduced local galvanic current density at the imperfections and increased cathode polarization. A duplex nickel layer containing small amounts of sulfur is generally used. In addition to... 

Decorative chromium plating, 0.2—0.5 ]lni deposit thickness, is widely used for automobile body parts, appHances, plumbing fixtures, and many other products. It is customarily appHed over a nonferrous base in the plating of steel plates. To obtain the necessary corrosion resistance, the nature of the undercoat and the porosity and stresses of the chromium are all carefliUy controlled. Thus microcracked, microporous, crack-free, or conventional chromium may be plated over duplex and triplex nickel undercoats. 

Mechanical and Chemical Properties. Colorants, especially pigments, can affect the tensile, compressive, elongation, stress, and impact properties of a polymer (5). The colorants can act as an interstitial medium and cause microcracks to form in the polymer colorant matrix. This then leads to degradation of the physical properties of the system. Certain chemicals can attack colorants and there can be a loss of physical properties as well as a loss of the chromatic attributes of the colorant. Colorants should always be evaluated in the resin in which they will be used to check for loss of properties that ate needed for the particular appHcations. 

Fig. 8. Process-2one shielding mechanisms (a) Microcrack cloud (b) phase transformation (c) yielding of ductile reinforcements.

Triple Nickel and More. As an extension to the dual nickel, a thin, higher sulfur-containing nickel strike is deposited between the sulfur-free and the bright nickel. The sulfur content of this minimumally 2.5 p.m-sttike is 0.15—0.20 mass %. Ttiple nickel and dual nickel are covered by ASTM specification B456 (89). A fourth nickel deposit has shown improved protection by the effects it has on subsequent chromium deposits. Highly stressed, these nickel strikes have been used to aid in producing microcracked chromium. 

The process zone is a measure of the yield stress or plasticity of the material in comparison to its brittleness. Yielding within the process zone may take place either plastically or by dimise microcracking, depending on the brittleness of the material. For plastic yielding, / is also referred to as the plastic zone size. 

The evolution of spall in a body subject to transient tensile stresses is complex. A state of homogeneous tensile stress is intrinsically unstable and small perturbations in the material microstructure (microcracks, inclusions, etc.) can lead to the opening of voids and initiation of the spall process. 

In many materials, the inherent flaws are easily recognized. Brittle polycrystalline materials, for example, contain microcracks, voids, and other imperfections that can be identified in micrographs, and are expected to provide sites for internal fracture activation. Artificial flaws introduced into a hollow metal shell by uniform scoring can be expected, under rapid expansion, to fracture the shell along the paths of scoring. 

Fig. 16.7. Microstructural features of a crystalline ceramic grains, grain boundaries, pores, microcracks and second phases.

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