Macroscopic stress measurements by neutron diffraction and the part played by the "stress-free" reference

Holden, T. M.*; Suzuki, Hiroshi  ; Carr, D. G.*

In the last decade a more sophisticated understanding of how to calculate the macroscopic stress from strains measured in neutron diffraction experiments has developed. Strains measured with the aid of a particular reflection may have a component corresponding to the stress field as well as a component corresponding to a type-2 or intergranular stress. The latter originates in inhomogeneous plastic deformation among different grain orientations. One approach is to cut small reference coupons from the component, small in size compared with the variation of the stress field. Cutting destroys the stress field but leaves any chemistry and intergranular effects intact since they are on the scale of the grain size. The difference between the intact sample and the coupon then gives an approximation to the strain associated with the macroscopic stress. Examples of how the intergranular and macroscopic strains and stresses are superposed, are presented for the cases of slightly bent and severely bent steam generator tubing, and for a deformed pressure tube. Welds often represent a situation where plastic deformation has occurred on cooling. In addition, microstructural and compositional changes are often associated with melting, so it is important to have reference coupons as a function of position through the weld zone. Examples of austenitic stainless steel and Zr-alloy welds are presented.