iter structural design criteria for in-vessel components (sdc-ic)

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ITER G 74 MA 8 01-05-28 W0.2 Neutron irradiation effects (e.g. embrittlement, swelling, irradiation creep) must be included in analysis. B2500 STRESS DEFINITION AND CLASSIFICATION BÊ2501 Mechanical stress Mechanical stresses are stresses which result from the application of mechanical loads such as internal pressure, weight, earthquakes and, where applicable, reactions of supports and other components. BÊ2502 General and local thermal stress in a shell Thermal stresses are self-equilibrated stresses resulting from a non-uniform spatial distribution of temperature or from the presence of materials with different thermal expansion coefficients. For the purposes of applying stress criteria, two types of thermal stresses are recognized: general thermal stresses and local thermal stresses, as defined in the following. If a small portion of the shell is considered, that is, a portion whose dimensions normal to the median surface does not exceed a few thicknesses, general thermal stresses entail a general deformation (or strain) of this portion; local thermal stresses do not induce such general deformation (or strain). Examples of general thermal stress are - stress produced by axial temperature distribution in a cylindrical shell, - stress produced by temperature difference between a nozzle and the shell to which it is attached, - equivalent linear stress (IC 2515) distribution produced by a radial temperature distribution in a cylindrical shell, - equivalent linear stress (IC 2515) distribution produced by a through-thickness temperature distribution in a constrained flat plate. Examples of local thermal stress are - the stress in a small hot spot, - the non-linearly distributed (IC 2516) thermal stress, i.e., the difference between actual stress and equivalent linear stress, - thermal stress in a cladding material which has a different thermal expansion coefficient than that of the base metal. BÊ 2503 Swelling induced stress These are self-equilibrated stresses resulting from a non-uniform spatial distribution of fluence or from the presence of materials with different swelling laws. As with thermal stresses (B2512), general and local swelling stresses can be distinguished. Swelling-induced stresses are limited by relaxation due to irradiation-induced creep (IC 2151). Swelling can SDC-IC, Appendix B. Guidelines for Analysis page 2
ITER G 74 MA 8 01-05-28 W0.2 also lead to large deformations of the structure which might require that all analysis include the effects of finite deformation on stresses and strains (B 3021). On the other hand, under certain circumstances (B 3022) if the temperature and neutron damages are sufficiently low, the effects of irradiation-induced swelling may be neglected. If the effects of irradiationinduced swelling is not negligible, the swelling-induced stress, including the relaxing effects of irradiation-induced creep, has to be taken into consideration either by detailed inelastic (elasto-visco-plastic) analysis or by elastic-irradiation-induced creep analysis (B 3024.1.1.1). B 2510 Breakdown of stresses The decomposition of stresses into membrane, bending, and peak categories is used for elastic analysis of shell and beam-like structures because, for these types of structures, different allowable stresses may be determined (by a simple limit analysis) for the different stress categories. Primary membrane stress is limited by Sm. For primary bending stresses, a bending shape factor K accounts for redistribution of stress due to plasticity. For peak stresses, which are strain controlled, there is no limit for a ductile material apart from fatigue. The line integration method used in IC 2513 - 2514 to decompose the stress components is strictly applicable only to homogeneous shells. With some modification (see below) the concept may be applied to beam-like structures. However, the concept of stress decomposition cannot be generalized for a three-dimensional structure. The treatment of these structure types is discussed below. For a 3-D structure, depending on the structure and the loading, it is possible that the analyst can use judgement to decompose the stresses and apply the rules for elastic analysis directly. When this is not possible, the recommended approach is to compare the applied loadings to the limit loadings that would cause failure, and to ensure that the safety factor on the applied loading is consistent with the safety factors implied in the SDC-IC rules. An elastic stress analysis is generally insufficient to determine the limit load. Therefore, for a three-dimentional structure, it may be approporate to use the rules for elasto-plastic analysis (ICÊ3211.2, ICÊ3212.2, IC 3213.2, and ICÊ3214.2). Alternatively, the assessment may be based either on experiment or a nonlinear analysis of a representative structure. The remainder of this section addresses stress decomposition in structures other than three dimensional. For single layer, homogeneous shells, each separate component of a stress tensor defined along the supporting line segment can be decomposed into its membrane and bending components. The rules for the decomposition of stresses in a single-layer homogeneous shell are given in IC 2513 - 2514. For shell or beam-like structures other than a single-layer homogeneous shell, the decomposition of a stress component into its membrane and bending components is not always straightforward, and a general rule for decomposition of stresses cannot be given. The decomposition would depend on how the structure is modeled and the type of stress analysis conducted, to derive the distribution of stresses through the thickness. In some cases, the decomposition can be better implemented by an integration over an area rather than along a supporting line segment. As a simple example, consider a thick walled tube running along the x2 direction. If the tube is analyzed as a shell, then the membrane and bending stress vary with position on the shell, SDC-IC, Appendix B. Guidelines for Analysis page 3
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