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PASS/NOZZLE-FEM 3.5. Program Manual | |
To evaluate component strength, the results from an elastic stress analysis of the component subject to defined loading conditions are categorized and compared to an associated limiting value.
There are three basic equivalent stress categories and associated limits used for elastic analysis: primary stress (general primary membrane stress, local primary membrane stress, primary bending stress), secondary stress, and peak stress. The terms are more detail described in the following codes [12] or [17], [3-7] and are defined below as per [25].
Primary Stress is produced by steady mechanical loads, excluding discontinuity stresses or stress concentrations. Its main characteristic is that it is not self limiting. Primary stress is divided into two subcategories; general and local.
General primary stress is imposed on the vessel by the equilibration of external and internal mechanical forces. Any yielding through the entire shell thickness will not distribute the stress, but will result in gross distortions, often carried to failure. General primary stress is divided into primary membrane stress and primary bending stress; the limit design method shows that a higher stress limit can be applied to the primary bending stress than to the primary membrane stress. Typical examples of general primary membrane stress in the vessel wall are: stress due to internal or external pressure and stress due to vessel weight or external moments caused by wind or seismic forces. Typical example of primary bending stress is the bending stress due to pressure in flat heads.
Local primary stress is produced by the design pressure alone or by other mechanical loads. It has some self-limiting characteristics. If the local primary stress exceeds the yield point of the material, the load is distributed and carried by other parts of the vessel. However, such yielding could lead to excessive and unacceptable deformations, so it is necessary to assign a lower allowable stress limit to this type of stress than to secondary stresses. An important property of local primary stress is that the maximum stress remains localized and diminishes rapidly with distance from the point of load application. Local primary stress can be divided into direct membrane stress and bending stress. Both, however, have the same stress intensity limits. Typical examples of local primary stress are stresses at supports and local membrane stresses due to internal pressure at structural discontinuities.
Secondary stress is that it is self-limiting. Minor yielding will reduce the forces causing excessive stresses. Secondary stress can be divided into membrane stress and bending stress, but both are controlled by the same limit stress intensities. Typical examples of secondary stress are thermal stresses and local bending stresses due to internal pressure at shell discontinuities.
Peak stress is the highest stress at some local point under consideration. In case of failure, peak stress does not generate any noticeable distortion, but it can be a source of fatigue cracks, stress-corrosion, and delayed fractures. Generally, the computation of the peak stresses is required only for vessels in cyclic service (fatigue assessment). Typical examples of peak stress are thermal stress in carbon steel plate with stainless steel integral cladding and stress concentrations due to local structural discontinuities such as a notch, a small-radius fillet, a hole, or an incomplete penetration weld.
The assessment is provided more details in follows codes:
PASS/NOZZLE-FEM 3.5. Program Manual
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