DBA-Design by analysis

ASME Section VIII, Division 2, Part 5

There are four failure modes that need to be evaluated:

1) Plastic Collapse

2) Local Failure

3) Buckling Collapse

4) Fatigue

5) Ratcheting

For the Plastic Collapse failure mode, it is permissible to use any of the three methods presented: elastic, limit load, and elastic-plastic.

For Local Failure (unless you can exempt yourself by applying 5.3.1.1), it is permissible to use either of the two methods presented: elastic, and elastic-plastic.

For Buckling Collapse, it is permissible to use one of the three types: bifurcation analysis (elastic buckling), bifurcation analysis (elastic buckling) with pre-stress from an elastic-plastic analysis, or elastic-plastic analysis.

For Fatigue, it is permissible to use one of the two methods presented: elastic or elastic-plastic (except for checking welds, in which case you must use the elastic analysis method applying either the FSRF method or the structural stress method).

For Ratcheting, it is permissible to use one of the two method presented: elastic or elastic-plastic. However, the elastic-plastic requires the use of an elastic-perfectly-plastic stress-strain curve.

Safety Margin in ASME Section VIII Division 2

Stress Categorization

The three basic equivalent stress categories and associated limits that are to be satisfied for plastic collapse are defined below. The terms general primary membrane stress, local primary membrane stress, primary bending stress, secondary stress, and peak stress used for elastic analysis are defined.

• General Primary Membrane Stress (Pm) The general primary membrane equivalent stress is the equivalent stress, derived from the average value across the thickness of a section, of the general primary stresses produced by the design internal pressure and other specified mechanical loads but excluding all secondary and peak stresses.
• Local Primary Membrane Stress (P_L) The local primary membrane equivalent stress is the equivalent stress, derived from the average value across the thickness of a section, of the local primary stresses produced by the design pressure and specified mechanical loads but excluding all secondary and peak stresses.
• Primary Membrane (General or Local) Plus Primary Bending Equivalent Stress (P_L +Pb) The Primary Membrane (General or Local) Plus Primary Bending Equivalent Stress is the equivalent stress, derived from the highest value across the thickness of a section, of the linearized general or local primary membrane stresses plus primary bending stresses produced by design pressure and other specified mechanical loads but excluding all secondary and peak stresses.
• Secondary Stress (P_L +Pb +Q) A normal stress or a shear stress developed by the constraint of adjacent parts or by self constraint of a structure. The basic characteristic of a secondary stress is that it is self-limiting. Local yielding and minor distortions can cause satisfy the conditions which cause the stress to occur and failure from one application of the stress is not expected. The examples of secondary stress are general thermal stress and bending stress at a gross structural discontinuity.
• Peak Stress (P_L +Pb +Q +F) The basic characteristic of a peak stress is that it does not cause any noticeable distortion and is objectionable only as a possible source of a fatigue crack or brittle fracture. The examples of secondary stress are the stress at a local structural discontinuity.

In ASME, the safety margin for the stress categories is about 1.5 to 2 on stress level. In addition, the actual material property has strain hardening and it is concluded that the safety margin is 2.0 for all stress categories.