Membrane stress or σp
Average stress value across the thickness of a solid section, excluding the effects of discontinuities and stress concentrations. EXAMPLE The general primary membrane stress in a pipe loaded in pure tension is the tension divided by the cross-sectional area. σp may include global bending as in the case of a simple pipe loaded by a bending moment. NOTE Definition taken from API 2RD.
Source: API RP 17L2, Recommended Practice for Flexible Pipe Ancillary Equipment, First Edition, March 2013. Global Standards
Membrane stress in a section is the average stress induced by a force normal to the section. It is calculated using the classical equation for normal stress (S = F/A).
Source: API SPEC 16R, Specification for Marine Drilling Riser Couplings, Exploration and Production Department, First Edition, January 1997. Global Standards