Stress that satisfies the laws of equilibrium of pressure, external force and moment (i.e. load effects). NOTE 1 Regarding the mechanical behaviour of a structure, the basic characteristic of this type of stress is that in case of (non-admissible) increment of external load, the deformations upon full plasticification of the section considerably increases without being self-limiting. NOTE 2 Regarding primary stress, distinction is made between membrane stress and bending stress with respect to their distribution across the cross-section governing the load-carrying behaviour. Primary membrane stress is defined as the average value of the respective stress components distributed over the section governing the load-carrying behaviour. Primary bending stress is defined as primary stress distributed linearly across the considered section and proportional to the distance from the neutral axis. NOTE 3 Regarding the distribution of membrane stress along the wall, distinction is made between general primary membrane stress and local primary membrane stress. Membrane stress due to gross structural discontinuities (e.g. integral attachments) is considered as local.
Source: API RP 17G, Recommended Practice for Completion/Workover Risers, Second Edition, July 2006 (Reaffirmed April 2011). Global Standards
A primary stress is one that is induced by the external loads or preload and is necessary to satisfy the laws of static equilibrium. Examples of primary stress are the membrane stress in a rod loaded by an axial force and the bending stress in a simple beam.
Source: API SPEC 16R, Specification for Marine Drilling Riser Couplings, Exploration and Production Department, First Edition, January 1997. Global Standards
Normal or shear stress developed by the imposed loading which is necessary to satisfy the laws of equilibrium of external and internal forces and moments (ASME BPVC, Section VIII, Division 2, Paragraph 5.12). NOTE The basic characteristic of a primary stress is that it is not self-limiting. Primary stresses that considerably exceed the yield strength will result in failure or at least in gross distortion. A general primary membrane stress is one that is distributed in the structure such that no redistribution of load occurs as a result of yielding.
Source: API TR 1PER15K-1, Protocol for Verification and Validation of High-pressure High-temperature Equipment, First Edition, March 2013. Global Standards