If there are only bending stresses, the safety factor against collapse is again 1.5. The stress intensity from the sum of membrane and bending stress should not exceed the yield stress.This gives a safety factor of 1.5 against plastic collapse when only membrane stresses are present. The stress intensity (Tresca equivalent stress) from the primary membrane stresses should not exceed two-thirds of the yield stress.Without going into detail, the basic requirements of the code are: The choice of SCL is not unique, so here we must use our engineering judgment to find the critical locations.Īlthough not fully correct (but conservative), the linearized stresses are sometimes viewed as equivalent to the primary stresses. Secondary stresses do not lead to a collapse when they exceed elastic limits, since they are just redistributed.ĭuring the analysis, the stress is studied along a number of lines through the section, referred to as stress classification lines (SCLs). Typically, secondary stresses are local effects caused by either geometric discontinuities or displacement-controlled loading. Secondary stresses are caused by other effects.
Here, we are required to classify stresses as either primary or secondary.Ī primary stress is a stress that is required for maintaining force and moment equilibrium. The concept of stress linearization is an important part of the qualification of pressure vessels, as described in ASME Boiler & Pressure Vessel Code, Section III, Division 1, Subsection NB. If the structure is subjected to cyclic loading, the peak stresses are of utmost importance, as they determine the risk of fatigue crack initiation at the surface. The safety factor, which is implicit in the bending collapse, must also be taken into account. Using the true peak stress gives an overly conservative design. The conclusion is that for determining safety conditions within plastic collapse, the linearized stress is the relevant parameter, since it is proportional to the axial force and bending moment. The difference can be explained by the fact that a small plastic hardening is used in the model to stabilize the analysis. This value matches the final parameter value of 0.76 rather well.