Thick-walled components

The autofrettage treatment (Fig. 1.4-9, B) is certainly one of the oldest, but still very useful methods to create beneficial residual stresses in thick-walled components (e.g., pipes). The autofrettage pressure must be adjusted to a level so that the material in the thick wall is plastically strained within a certain percentage (e.g., 50 %), the rest staying only elastically strained. 

During autofrettage treatment the thick-walled components must be pressurized to such a level as to generate plastic strains through a certain fraction of the wall thickness, the outer region staying elastically strained only. 

A component made of an aluminum wrought alloy by thixoforging is illustrated in Figure 27. The forging has a very complex geometry but it was reproducible very accurately. The thick-walled component regions as well as parts with essential differences in wall thickness are characterized by a homogeneous structure free of voids. 

The blank is typically formed in a single roller pass. Blank shape can be either a flat sheet or a preform produced by other means. Generally, a single roller is used to form the sheet however, in case of high-strength and thick-walled components, a multiple roller configuration may be used. 

A medium reactive, low viscosity, orthophthalic resin with very good wet-out properties and offering a low peak exotherm, suitable for the Injection moulding of thick-walled components, b. 58% c. 240-360... 

The duration of cross-linking depends on the volume and wall thickness of the elastomer component and the used mixture. It can take several hours for thick-walled components. Upon cooling, the blank shrinks by a few percent. This shrinkage is to be considered when designing the mold. In this case, simulations on the basis of empirical values are used [1]. 

In order to shield particular substrates against light, UV absorbers require a corresponding absorption depth consequently, the protective effect is limited in films and fibers and on the surface of thick-walled components [529]. Effectiveness depends on the absorption characteristic (A ), the stabilizer concentration, and the thickness of the object. According to the Lambert-Beer law, for a given concentration of UV absorbers, UV radiation intensity decreases exponentially with the distance from the surface. For the concentrations used in engineering (between 0.1 and 0.6%), this value drops almost to zero at a distance of approx. 0.1 mm from the surface [86]. 

Autofrettage The autofrettage treatment is one of the oldest, but very useful methods to create beneficial residual stresses in thick-walled components (e.g.,... 

Vetter, G., Lambrecht, D., and Mischorr, G. (1990) The fatigue of thick-walled components with soft martensitic and semi-austenitic chrome-nickel steels under pulsating pressure. Abstract Handbook 2nd International Symposium on High Pressure Chemical Engineering, September 24—26, 1990, Erlangen, Germany, pp. 499-506. 

For thick-walled components, the plastic analysis methods (limit load analysis method and the elastic-plastic stress analysis method) are more adequate. Part 5 also gives a guideline to assess the protection against failure from cyclic loading. 

In EN 13445 Section 3 [8], there is the possibility to evaluate the results of an FE analysis based on Appendix C Procedure of stresses categories for the dimension based on analysis methods. Appendix C describes essentially the elastic stress analysis method. Stresses are determined using an elastic analysis, classified into categories, and limited to allowable values that have been conservatively estabUshed so that a plastic collapse will not occur. For thick-walled components, the plastic analysis methods in Appendix B Directly dimension with analysis methods are... 

The loss of material reduces the available load-bearing cross-section, so that as oxidation proceeds, the stresses acting on the component increase. The magnitude of the effect will, of course, depend on the initial wall thickness for thick-walled components, such as steam headers (30-50 mm wall thickness), a 1 mm loss will not be very significant, but a similar material loss in a thin-walled tube of say 5 mm wall thickness would have grave repercussions for the service life [10]. Figure 11.11 illustrates this for a P92 pipe (300 mm outer diameter and 40 mm wall... 

Compound temperature inside the cylinder 200-250 °C To avoid sink marks on very thick-walled components, it may be necessary to work at 170 °C. This gives a very high melt viscosity which requires a high screw torque (risk of overload). 

The sizes of the components required for the startup schemes are assessed. The sliding pressure startup with a steam separator in a bypass line is the best from the viewpoint of weight of the components. A study of the times needed for the startup schemes remains as future work. There is a limitation on the rate due to thermal stresses on thick-walled components such as the RPV. In BWRs, the temperature rise rate of the RPV wall is limited to below per hour. 

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