Stress categories

Appendix 4 gives definitions and rules for stress analysis for shells, flat and formed heads, and tube sheets, layered vessels, and nozzles including discontinuity stresses. Of particular importance are Table 4-120.1, Classification of Stresses for Some Typical Cases, and Fig. 4-130.1, Stress Categories and Limits of Stress Intensity. These are veiy useful in that they clarify a number of paragraphs and simphfy stress analysis. 

The dimensioning of vessel walls is generally controlled by the static failure mode "gross plastic deformation". Elastic stresses have to be kept below the limits according to the applicable stress category. 

The Code establishes allowable stresses by stating in Para. UG-23(c) that the maximum general primary membrane stress must be less than allowable stresses outlined in material sections. Further, it states that the maximum primary membrane stress plus primary bending stress may not exceed 1.5 times the allowable stress of the material sections. In other sections, specifically Paras. l-5(e) and 2-8, higher allowable stresses are permitted if appropriate analysis is made. These higher allowable stresses clearly indicate that different stress levels for different stress categories are acceptable. 

This should make it obvious that the type and category of loading will determine the type and category of stress. This will be expanded upon later, but basically each combination of stresses (stress categories) will have different allowables, i.e. ... 

Tlie shell thickness as computeil by Ciode formulas for internal or external pressure alone is often not sufficient to withstand the combined effects ol all other loadings. Detailed calculations consider the effects of each loading separately and then must be combined to give the total state of. stress in that part. The stres.ses that are present in pressure vessels are separated into various classes in accordance with the types of loads that produced them, and the hazard they represent to the vessel. Each class of stress must be maintained at an acceptable level and the combined total stress must be kept at another acceptable level. The combined stresses due to a combination of loads acting simultaneously are called stress categories. Plea.se note that this terininolo differs from that given in Division 2, but is clearer for the pui poses intended here. 

Primary plus secondary stresses are allowed to be higher and so on. Before considering the combination of stresses (categories), we must first define the various types and classes of stress. 

Local primary membrane stress, Pl. Local primary membrane stress is not technically a classification of stress but a stress category, since it is a combination of two stresses. The combination it represents is primary membrane stress, P, , plus secondary membrane stress, Qn produced from sustained loads. These have been grouped together in order to limit the allowable stress for this particular combination to a level lower than allowed for other primary and secondary stress apphcations. It was felt that local stress from sustained (unrelenting) loads presented a great enough hazard for the combination to be classified as a primaiy stress. 

Whisker formation is a typical feature of metallic tin. There can be two kinds of whisker phenomena at the component itself and also at the tin surface finish. Various factors can cause whisker formation. One key factor is internal stress. There are two stress categories. 

The designer must be familiar with the various types of loadings and their stresses in order to accurately understand the results of the analysis. The designer must also consider the stress categories to determine the allowable stress limits. 

The foregoing list provides the categories and subcategories. It is, however, too general to provide a basis with which to combine stresses or apply allowable stresses. Stress categories are defined by the type of loading which produces them and the hazard they represent to the vessel. 

Typically, the adhesive and/or the matrix in FRP retrofitting applications transfers three different stress categories. These are shear, peel and thermal residual stresses. The latter occur in FRP composite joints either upon fabrication due to mismatch in the hygrothermal and elastic properties of the fibres, matrices/adhesives and adherends or due to the difference between curing and operating temperatures of the FRP material. These three stress categories can be referred to as the good, the bad and the unavoidable, respectively. 

Usually the above-mentioned stresses occur simultaneously. However, in most cases, it will not always be possible to find a positive allocation of the stresses to one of the above-mentioned stress categories. Thus, e.g., corrosion can be attributed to the influence of a medium however, environmental conditions like high humidity in the air or corrosive atmosphere may also lead to corrosion. In specific cases, an evaluation of the operational stress of a machine may constitute an extremely difficult undertaking. 

Figure 8.3 Primary Stress Categories (Courtesy of the American Society of Mednnieal En neers.)...

Table 8.2 shows the allowable stress for various stress categories. Application of Table 8.2 to various stress categories is given in the following example,... 

Table 8.2 Stress Categories and Limits of Stress Intensity (Ref. 1)... 

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... 

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