Sites stress

The normal stresses obtained at measurement points El and E2 with displacement boundary conditions are ten times weaker than those obtained with stress boundary conditions. Equivalent results have been obtained for variable ranges of normal and shear fracture stiffnesses. Without on-site stress measurements, choosing between these various boundary conditions is difficult. The simultaneous pressure-deformation measurements at the Coaraze site have however enabled carrying out an analysis on the measurement points as well as determining a threshold for normal stress in the joints (see Table 2). The selected boundary conditions are those that more accurately reproduce the initial stresses at Coaraze (i.e. displacement boundary conditions). 

An important feature of these models, which is particularly relevant for the understanding of their mechanical response, is the very high levels of their atomic site stress tensors associated with the disordered glassy state measured by a technique pioneered by Egami and Vitek (1983) in amorphous metals and discussed in Section 1.4.1. For example, Theodorou and Suter (1986) have calculated these atomic site stresses in polypropylene for the three different atomic environments of C, H, and CH3 by the method of Egami and Vitek. Their results for the average pressure field (p) and deviatoric shear field (r) (= d/VJ) for the three environments are given in Table 2.1 for the structure model of Theodorou and Suter (1985). 

Table 2.1 Characteristic quantities of the invariants p) and (r) [= ff/ /3) of the atomic site stress tensors for atomic environments of carbon (C), hydrogen (H), and methyi groups (CH3 = R) in atactic giassy poiypropyiene...

The system stress over the simulation cube after each distortion increment is obtained as a volume average of all atomic site stress tensors that, in turn, is based on the generalized formulation of Theodorou and Suter (1986) of the classical Born and Huang (1954) operations, which now include both torque and force interactions between atoms, giving the atomic site stress-tensor element for the th atom as... 

Corrosion at discrete sites, stress-corrosion cracking. 

Using flaw visuahzation system data the strength and fracture mechanics estimations are carried out in accordance with defect assessment regulatory procedure M-02-91 [5]. Recently, the additions had been included in the procedure, concerning interpretation of expert flaw visualization sysf em data, computer modelling, residual stresses, in-site properties of metal, methods of fracture analysis. 

The formation of anodic and cathodic sites, necessary to produce corrosion, can occur for any of a number of reasons impurities in the metal, localized stresses, metal grain size or composition differences, discontinuities on the surface, and differences in the local environment (eg, temperature, oxygen, or salt concentration). When these local differences are not large and the anodic and cathodic sites can shift from place to place on the metal surface, corrosion is uniform. With uniform corrosion, fouling is usually a more serious problem than equipment failure. 

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

Typically, ozone cracking initiates at sites of high stress (flaws) on the mbber surface. Thus, in general, mbber articles should be designed to rninirnize potential sites of high elongation such as raised lettering. Similarly, the use of clean molds helps to reduce the incidence of surface flaws. 

Microstructure. Whereas the predominate stmcture of polychloroprene is the head to tail /n7 j -l,4-chloroprene unit (1), other stmctural units (2,3,4) are also present. The effects of these various stmctural units on the chemical and physical properties of the polymer have been determined. The high concentration of stmcture (1) is responsible for crystallization of polychloroprene and for the abiUty of the material to crystallize under stress. Stmcture (3) is quite important in providing a cure site for vulcanization, but on the other hand reduces the thermal stabiUty of the polymer. Stmctures (3),(4), and especially (2) limit crystallization of the polymer. 

The bracket construc tion permits support of the exchanger without fixing the supports to the shell. This provides for thermal movement of the shells within the brackets and prevents the transfer of thermal stresses into the process piping. In special cases the brackets may be welded to the shell. However, this is usually avoided due to the resulting loss of flexibihty in field installation and equipment reuse at other sites and an increase in piping stresses. 

Sites possessing high-residual stresses—such as welded assemblies, rolled-in tube ends—may be susceptible. 

Sites subject to high-service stresses—such as points of physical constraint—may be susceptible. 

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