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

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ITER G 74 MA 8 01-05-28 W0.2 Note: A conservative estimate for KI can be obtained by using the following expression for a single-edge notched plate subjected to a uniform tensile stress s at infinity K I 1. 122 = æ ao 1 - ö è h ø 3/ 2 s p a B 3214.2 Elasto-plastic analysis (Fast fracture) o If the postulated crack for fast fracture analysis is embedded in a yielded region, linear elastic fracture mechanics methodology may give highly unconservative results. In such cases, elasto-plastic fracture mechanics methodology should be used. Any procedure based on elasto-plastic fracture mechanics (such as section XI of ASME, Appendix A16 of RCC-MR, R6 rule) may be considered but its application shall be justified. Special attention shall be given to the effect of thermal loads, complex loading modes, and spreading of plasticity. The J-integral is an acceptable criterion for such cases. Most commercially available finite element programs (e.g., ANSYS, ABAQUS, etc.) have the capability to compute the Jintegral either by a contour integral method or by an energy method. B 3300 RULES FOR THE PREVENTION OF C TYPE DAMAGE (LEVELS A AND C) B 3310 Progressive deformation or ratcheting B 3311 Elastic analysis (Progressive deformation or ratcheting) B 3311.1 3Sm rule The 3Sm limit for the PL+Pb+D(P+Q) stresses ensures that the membrane and bending stresses will shakedown after the first cycles of loading and unloading, thus ensuring that there will be no overall (large scale) progressive deformation. Only peak stresses (F), which are local in nature, may continue to cycle plastically. Peak stresses are considered in the performance of fatigue evaluation and also for fracture in embrittled materials. Indeed, the requirement that there be no overall progressive deformation is necessary to ensure that the fatigue tests, which are conducted without progressive deformation, represent the behaviour of the structure. All cyclic stress ranges including the stress range due to disruption-induced electromagnetic loading have to be added to the secondary stress range (see appendix C 3311.3). For annealed austenitic stainless steels, if 0.9Sy controls the Sm value, some ratcheting strain may occur until strain hardening increases the cyclic yield stress sufficiently to achieve shakedown. B 3311.2 Bree-diagram rule Since the Bree diagram approach is mathematically equivalent to the Efficiency diagram approach of RCC-MR (see appendix C 3311), the Bree diagram rule was adopted for the SDC-IC. However, the expression for the Bree diagram used in the SDC-IC is slightly SDC-IC, Appendix B. Guidelines for Analysis page 42
ITER G 74 MA 8 01-05-28 W0.2 different from that in the ASME Code, Section III. In the SDC-IC, the normalizing factor for the stresses is the average of the Sy values at the maximum and the minimum wall-averaged temperature and neutron fluence during the cycle being evaluated for all values of Sm, In the ASME Code, it is permissible to use 1.5ÊSm , instead of Sy , whenever it is greater than Sy. The reason for choosing the Sy value in this way is to minimize the ratcheting strain accumulated prior to shakedown and to assure that the allowable stresses determined by the Bree diagram rule fall within (i.e., are conservative with respect to) those determined by the Efficiency diagram rule of RCC-MR (see Appendix C 3311). The Bree diagram approach is less conservative than the 3Sm rule. However, since the yield stress rather than the proportional limit is used in the analysis, a small amount of ratcheting strain (~ 0.2%) may occur until strain hardening raises the proportional limit to the yield stress. If this is unacceptable, the 3Sm rule should be used instead. B 3312 Elasto-plastic analysis (Progressive deformation or ratcheting) Rules for preventing progressive deformation based on elastic analysis are contained in ICÊ3311. They require the partition of stresses into primary and secondary components. In certain cases, this partition can lead to overly conservative results with regard to the ratcheting resistance of the component. Elasto-plastic analysis could provide a less conservative evaluation. In order to check the limits on these strains, an elasto-plastic analysis, giving either the exact value or an upper bound to the strains resulting from all the cycles envisaged, should be carried out. This can be obtained: - either by a cyclic elasto-plastic calculation until stabilization of stress-strain response, or - by extrapolated cyclic elasto-plastic calculation, using a validated method. If a certain block of cycles (containing one or more types of cycles) is repeated periodically, the block can be represented by one type of cycle (called the envelope cycle) which sequentially adds all cycles in the block. (The individual cycles for which the linear stress does not induce plastic strain may be excluded). The analysis can then be simplified as follows: - analyze in detail all the cycles inside one envelope cycle, - compute a ratcheting strain increment in a single application of the envelope cycle. - the usage fraction for each type of envelope cycle is this ratcheting strain increment multiplied by the number of times the envelope cycle is repeated, divided by the allowable ductility term. For the envelope cycle elasto-plastic analysis, and in the absence of explicit instructions, an initial zero stress and strain condition may be assumed. However, the stress and strain conditions at the beginning of each cycle contained within the envelope cycle are those remaining at the end of the calculation of the previous cycle. The permissible limits for the accumulated membrane and peak strains are given in IC 3312. SDC-IC, Appendix B. Guidelines for Analysis page 43
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