Local membrane stress intensity

Local membrane stress intensity. The maximum allowable stress intensity for S derived from <7l stresses is 1.5<7n, times the factor k when applicable. 

Specifically for the ASME Code, the primary membrane stress intensity. Pm, and the combined membrane plus bending stress intensity. Pm + Pb, (also the local membrane plus bending stress intensity, Pl + Pb in some cases) for the various loading conditions are shown below. 

Primary plus secondary stress intensity. The maximum stress intensity S as based on the primary or local membrane stresses plus the primary bending stress plus the secondary stress (cr m or ctl + (7b -f X) 2) cannot exceed the... 

During cell/stack operation, water content in the membrane is affected by the local intensive variables, such as local temperature, water vapor concentration in the gas phase, gas temperature and velocity in the channel, and the properties of the electrode and gas diffusion media. The power fluctuation can result in temperature variation inside the cell/stack, which will subsequently change the local membrane water content. As the water content in the membrane tends to be non-uniform and unsteady, this results in operation stresses. When the membrane uptakes water from a dry state, it tends to expand as there is no space for it to extend in plane and it can wrinkle up as schematically shown in Fig. 4 when the membrane dries out, the wrinkled part may not flatten out, and this ratcheting effect can cause the pile up of wrinkles at regions where membrane can find space to fold. The operation stress is typically cyclic in nature due to startup-shutdown cycles, freeze-thaw cycles, and power output cycles. 

The basic characteristic of 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. 

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