Stress Intensity Limits

the designer. If the program is a well known and well used one such

justification may simply be a detailed description of the model geometry,

input loads and boundary conditions applied. In other cases, the

justification may require full documentation of the program and examples of

analyses carried out by the use of the program that can be easily checked by

"classical" analysis, "exact" analysis, or data existing in the literature.

b. Stress Intensity Limits. There are two mandatory appendices of

Section VIII, Division 2, which form the entire basis of the concept of

design by analysis. These are Appendix 4, Design Based on Stress Analysis

and Appendix 5, Design Based on Fatigue Analysis. A detailed understanding

of these two appendices is necessary to understand Section VIII, Division 2.

Both appendices deal with the establishment of "stress intensity limits" and

the interpretation of stresses calculated by the designer to meet these

limits. Appendix 4 gives such information as pertains to vessels that are to

be loaded either statically or cyclically, while Appendix 5 gives further

information to be applied to vessels that are to be loaded cyclically. Note

carefully that the data presented in Appendix 4 must be applied in all cases

of loading.

(1) Appendix 4, Article 4-1 Design Based on Stress Analysis.

This

article presents the methods that a designer should use in:

(a)

Classifying the type of stresses that he has calculated;

(b)

Converting these classes of stress into "stress

intensities"; and

(c) Comparing the calculated "stress intensities" to the

Code-allowable "stress intensity limits" for the material from which the

vessel is fabricated.

A brief description is given below of part of the information found in

Article 4-1. The designer is urged to read and fully understand the complete

article.

c. Types of Stresses.

In Article 4-1 stresses are broadly categorized

into three groups:

(1) Primary stress. Such a stress is imposed by loading which is

necessary to satisfy equilibrium and which is not self-limiting. That is, as

the loading increases, the distortion of the area over which this stress acts

continues to increase until failure.

(a) Primary general membrane stress. This is a membrane

stress existing in a shell at a position removed from any gross geometric

discontinuity.

(b) Primary local membrane stress. This is a membrane

stress existing close to a geometric discontinuity which decreases rapidly

away from that location.