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

ITER G 74 MA 8 01-05-28 W0.2
and with maximum nominal stresses (P+Q) not exceeding the 3Sm limit (IC 3311.1), a
conservative value of r = 3. The design guide for the Monju reactor of Japan recommends a
default value of r=3 for creep-fatigue design. However, the r-factor methodology has not
been used in the fission reactor codes to design against fracture, because loss of ductility is
not a damage considered in these codes. Since, in the SDC-IC, we propose to use the r-factor
methodology to design against fracture, two factors have to be considered, both of which tend
to increase the value of r. They are (1) reduced or zero strain hardening capability of
materials under neutron irradiation (see Figure C 3024-6 of Appendix C) and (2) the elastic
stress that corresponds to fracture far exceeds the 3Sm limit.
Analysis of displacement-controlled three-point bend tests (see C 3024.1.3 of Appendix C) of
a beam made of a material with a bilinear stress-strain law has shown that the value of r as a
function of the peak plastic strain is bounded (£ 4) as long as the ratio of the tangent modulus
to Young's modulus (ET/E) is ³ 0.01. At lower values of ET/E ratio, the r value increases
with increasing peak plastic strain and, in particular, for an elastic-perfectly plastic material
(ET/E = 0), it increases indefinitely with increasing peak plastic strain.
Experimental results on three-point bend tests on irradiated type 304 stainless steel have
shown that the value of r can be large if the uniform elongation of the material is £ 1% (see C
3024.1.4 of Appendix C). When extrapolated from this set of data, the value of r should be £
4 when the uniform elongation ³ 2%. These results are in good agreement with the
analytically determined r as discussed in the previous paragraph.
In contrast to the three-point bend tests, single-edge notched tensile tests on irradiated type
304 stainless steel have shown that the value of r is relatively constant (» KT) even when the
uniform elongation is < 1% (see C 3024.1.3 of Appendix C). These results are in agreement
with r-values predicted analytically on the assumption that strain localization at the notch
cannot occur and that plastic strains remain distributed homogeneously at the notch root (see
C 3024.1.4 of Appendix C). However, because of the stiffness of the testing machine, these
specimens did not have the freedom to fail by sliding off at an angle starting at the notch. In
a load controlled test, depending on the notch geometry, strain localization could occur. This
must either be prevented by restricting the range of uniform elongation or accounted for by
increasing the r factor.
It is clear that the value of r can be quite sensitive to the component geometry and loading
mode. Therefore, in the design rules (IC 3212.1 and 3213.1), which use the r-factor
methodology for setting the limits on elastically calculated maximum stresses, the value of r
has been chosen conservatively. However, in any particular application, the designer is given
the option of using a lower value of r if it can be so justified.
The justification for a different r consists of estimating the peak values of strain and stress
conservatively using elasto-plastic analysis (B 3024.2), using Eq. 1 to evaluate R, and taking
r as the maximum R in the structure. The designer should use judgment in selecting a
suitable sub-model and loading of the structure. See BÊ3025 for a discussion of calculation
zones and sub-models. In addition, the designer should use an appropriate stress-strain law
that corresponds to the lowest work hardening (which, for steel, occurs at the maximum
fluence) under consideration. The analysis must be carried out either to the load level of
interest or to demonstrate that the r-value has peaked prior to reaching the load level of
interest.
SDC-IC, Appendix B. Guidelines for Analysis page 28