Compressibilities residual properties

Figure 4 shows a typical hardness distribution (7). The case depth is considerably less than that for flame and induction hardening. The case has a high compressive residual stress, which improves the fatigue properties (8). 

The surface may gain a very high (eg, 1000 Vickers) hardness from this process. Surface deformation also produces a desired high compressive residual stress. Figure 9 illustrates the improvement in fatigue properties of a carburized surface that has been peened (18). 

Hydrogen sulfide at 450 K and 15 atm is to be compressed to 66 atm. The isentropic final temperature and the isentropic enthalpy change will be found with the aid of Figure 7.28 for the residual properties. 

The successful application of hard coatings on cutting tool substrates is due to the combination of physical and mechanical properties of the coating. From a functional standpoint, chemical stability, hot hardness, and good adhesion to the substrate are essential optimum coating thickness, fine microstructures and compressive residual stresses can further enhance their performance. CVD A1203 and PVD TiAIN provide the necessary chemical inertness required to machine irons and steels. 

However, during long exposures to medium-temperature operating conditions, e.g. 1000°C, spinel formation is certainly expected. Wang etal.60 demonstrated this for the Ni-alumina system, showing the diffusion of Ni atoms to the free surface of the nanocomposite, followed by the formation of a nickel spinel surface coating which then limits the kinetics of subsequent oxidation. In this case the formation of a spinel surface layer may be beneficial to mechanical properties, since the reaction results in a volume increase, and the formation of compressive residual stresses. An analogous behavior was reported for ceramic particle nanocomposites, where oxidation of SiC particles results in an increase in volume and compressive residual stresses.61... 

The gradient composition in FGMs not only results in a spatial variation in properties but will also generate residual stresses, which will affect the mechanical properties. One of the potential advantages of FG components is the positive influence of compressive residual surface stresses on the strength and wear resistance. A correct design of the gradient for an optimal distribution of the residual stresses is therefore important, as discussed in this chapter. 

The compressibility factor is by definition Z = PV/RT values of Z and of (dZ/BT)P are calculated directly from experimental PVT data, and the two integrals in Eqs. (6.40) through (6.42) are evaluated by numerical or graphical methods. Alternatively, the two integrals are evaluated analytically when Z is expressed by an equation of state. Thus, given PVT data or an appropriate equation of state, we can evaluate HR and SR and hence all other residual properties. It is this direct connection with experiment that makes residual properties essential to the practical application of thermodynamics. 

Of the two kinds of data needed for evaluation of thermodynamic properties, heat capacities and PVT data, the latter are most frequently missing. Fortunately, the generalized methods developed in Sec. 3.6 for the compressibility factor are also applicable to residual properties. 

Figure 3.16, drawn specifically for the compressibility-factor correlation, is also used as a guide to the reliability of the correlations of residual properties based on generalized second virial coefficients. However, all residual-property correlations are less precise than the compressibility-factor correlations on which they are based and are, of course, least reliable for strongly polar and associating ... 

As with the generalized compressibility-factor correlation, the complexity ol the functions (H f/RZ. H Y/RZ. (S /R, and S Y/R preclude then general representation by simple equations. However, the correlation for Z basec on generalized virial coefficients and valid at low pressures can be extended U the residual properties. The equation relating Z to the functions and ia derived in Sec. 3.6 from Eqs. (3.46) and (3.47) ... 

As with the generalized compressibility-factor correlation, tire complexity of tire fuirc-tioirs (H f/RTc, (H f/RTc, S f/R, and (Sy/R precludes tlreir geireral representation by simple equations. However, tire generalized secoird-virial-coefficientcorrelationvalid at low pressures fonrrs the basis for airalytical correlations of tire residual properties. The equation relating B to the functions aird 5 is derived iir Sec. 3.6 ... 

The residual properties of gases and vapors depend on their PVT behavior. This is often expressed through correlations for the compressibility factor Z, defined by Eq. (4-36). Analytical expressions for Z as functions of T and P or T and V are known as equations of state. They may also be reformulated to give P as a function of T and V or V as a function of T and P. 

Although residual properties have formal reality for liquids as well as for gases, their advantageous use as small corrections to ideal gas state properties is lost. Calculation of property changes for the liquid state are usually based on alternative forms of Eqs. (4-32) through (4-35), shown in Table 4-1. Useful here are the definitons of two liquid-phase properties—the volume expansivity 3 and the isothermal compressibility K ... 

In this chapter we have developed ways for computing conceptual thermodynamic properties relative to well-defined states provided by the ideal gas. We identified two ways for measuring deviations from ideal-gas behavior differences and ratios. Relative to the ideal gas, the difference measures are the isobaric and isometric residual properties, while the ratio measures are the compressibility factor and fugacity coefficient. These differences and ratios all apply to the properties of any single homogeneous phase (liquid or gas) composed of any number of components. 

In the previous section we discussed the calculation of residual properties from cubic equations of state. The calculations are straightforward, though somewhat time consuming. A quicker alternative is to use generalized graphs. In Chapter 2 we discussed the Pitzer method for calculating the compressibility factor in terms of reduced temperature, reduced pressure, and acentric factor. Analogous equations can be obtained for the residual enthalpy and entropy. In this approach, the residual enthalpy, made dimensionless by the product RTc, is computed as... 

Using the residual property estimations from the generalized correlations estimate the change of enthalpy and entropy of 1 mol of carbon dioxide gas compressed from 1 atm to 400 K to 20 atm and 550 K. 

Mechanical properties of SiC-AlN-Y203 composites (SiC 50%wt-AlN 50%wt), pressureless-sintered with an innovative and cost-effective method, were determined before and after oxidation performed at 1300°C for 1 h. As a consequence of the oxidative treatment, fracture toughness increased from 4.6 MPa m to 6.6 MPa m, flexural strength from 420 MPa to 488 MPa, Weibull modulus from 4.5 to 5.3 and thermal shock resistance (expressed as critical temperature difference) from 3I0°C to 380°C. First of all, these results demonstrated that a pre-oxidation treatment is needed to increase the mechanical resistance and reliability of SiC-AlN-Y203 components. Secondarily, the beneficial effects of the oxidation on the mechanical properties could be explained in terms of compressive residual stresses and crack healing ability. 

One of the reasons for the improved strengfli properties of the functionally-graded ceramics is due to the surface formation of compressive-residual stresses, which counteract some of the tensile stress generated during the thermal shock process. 

Some mechanical properties for both the ME and MEH series are given in Table 3.3. As already discussed, ME series specimens were identified to be toughened dominantly by cavitation, whereas those of MEH were claimed to be toughened dominantly by compressive residual stresses in the absence of cavitation. 

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