Initial prestress

By virtue of its basic size and the relative thinness of its component make-up a liner requires continuous structural support in all directions in order to withstand the loading from thermal and pressure conditions in a reactor and still remain Impermeable. Present liners are therefore usually firmly attached to and fully supported by the concrete structure of the PCRV. Due to this positive attachment, the liner is subjected to the stresses and strains Imparted to it by the PCRV during initial prestressing and, later, by movements of the PCRV due to the operating condition of the reactor. To ensure even distribution of these movements across the full size of the liner, attachment of the liner to the concrete is achieved by the embedment of a combination of steel sections and a multitude of studs or hook anchors. See figure 9. 

In following the movements of the vessel during initial prestressing the stress in the liner approaches and, in some cases, reaches yield stress. During operation, when temperature has been applied, and in late life, when concrete creep has taken place, the stress in the liner is certainly yield stress. 

This would Involve blocks like those In figure 11 cast in high quality moulds to very small tolerances and butted together with dry joints. It Is unlikely, however, that tolerances better than + Itnm could be achieved on the dimensions of the blocks. A consideration of strains under Initial prestress has shown that a planes of weakness zone constructed of such blocks would not... 

In order to estimate losses an initial prestress force must be determined. However initial prestress and losses are interdependent. 

Check Average Initial Prestress Force Average initial prestress = 436 kip/ft... 

To obtain avg. final prestress critical section, subtract the losses due to creep shrinkage and tendon relaxation from avg. initial prestress. 

A more important effect of prestressiag is its effect on the mean stress at the bore of the cylinder when an internal pressure is appHed. It may be seen from Figure 6 that when an initially stress-free cylinder is subjected to an internal pressure, the shear stress at the bore of the cylinder increases from O to A. On the other hand, when a prestressed cylinder of the same dimensions is subjected to the same internal pressure, the shear stress at the bore changes from C to E. Although the range of shear stress is the same ia the two cases (distance OA = CE), the mean shear stress ia the prestressed cylinder, represented by point G, is smaller than that for the initially stress-free cylinder represented by point H. This reduction in the mean shear stress increases the fatigue strength of components subjected to repeated internal pressure. 

This effect of pre-stressing is due to physical breakdown of some structure of the filler/rubber composite, its exact nature being unimportant as regards testing procedure. It is self-evident, however, that if a rubber during service is subjected to repeated deformations testing, should be carried out after prestressing rather than in the initial state where the result may be different due to the effect of the unstable structure. 

Initial Investment Can be built with less expensive materials like wood, asbestos-cement board and plastic imaterials. Fan cost is higher. Built with relatively expensive materials such as prestressed, precast and reinforced concrete and asbestos-cement for fill. 

Crack branching is a common feature in failure of ceramic components. Cracks branch at a critical velocity, which is of the order of half the speed of sound in the specific glass under study. The acceleration of crack initiation to the critical velocity depends on the energy dissipation available from the release of stored energy. The energy source can be applied stress, prestressing or residual stress. 

For carbonation-induced corrosion, the service life (ti) is expressed as the sum of the initiation (h) and propagation (tp) periods up to the threshold at which deterioration becomes unacceptable = h + tp (Figure 4.1). The initiation time (h) may be calculated as a function of the properties of concrete, in particular the coefficient K of carbonation, the environment and the thickness of the concrete cover x), for example with models by Tuutti, Bakker, or Parrott (Chapter 5). The propagation time (tp) can be estimated if the corrosion rate is known, once the maximum acceptable penetration of corrosion has been fixed. A maximum penetration for corrosion attack that is often accepted in reinforced (but not prestressed) concrete elements is 100 tm. 

These differences between specimens presumably arise from varying amounts of compressive prestress, which the bronze matrix exerts on the NbaSn reaction layer (because of thermal contraction after the fabrication heat treatment). The compressive strain degrades the initial /c, so that when tension is applied to the composite, the first effect is to relieve the compressive strain on the NbsSn and increase Ic- Eventually, when enough tensile stress is applied to the composite, the... 

For this group of specimens, representing a wide range of prestress, it appears the large disparity of results is removed if the degradation is expressed as a function of the NbsSn s intrinsic strain. Only the starting points on the curve are different, depending on the initial state of prestress in each specimen. 

Specimen curve exhibits an increase in Tc for initial strains, reaches a maximum value, and eventually decreases at larger strains to Tc values less than the unstrained wire. If such behavior were to reflect the removal of compressive prestresses followed by the application of tensile strain, then the observed peak Tc value should be related to that seen when the bronze matrix is etched off an unstrained wire. Figure 4 presents the linear relationship found when the increase in Tc under tensile loading, AT , is plotted against the increase in Tc found when the bronze was removed from an unstrained sample, ATc. The best-fit straight line through the data has nearly a 45 slope, supporting the above interpretation. 

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