FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

peaks which are easily measurable.
3. APPLICATIONS
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3.1 Lattice strain measurements in overrolled cold-worked Zr 2.5 wt% Nb
pressure tubes
Several measurements of lattice strains in model engineering components by neutron
diffraction have recently been reported. [1,2]. Our experiments were
initiated in response to a pressing engineering problem: the understanding of
the strains that developed in overrolled présure tubes in CANDU reactors in
Ontario. In a CANDU reactor, the seal between the zirconium alloy tube and its
steel end-fitting is a rolled joint, made by inserting a rolling tool and
expanding the tube into the end-fitting. The mouth of the fitting was slightly
tapered for easy location and insertion of the tube in the fitting. In many
cases the rolling tool was inserted too far, so that the tube was expanded into
the taper, giving rise to a large circumferential residual tensile stress [3].
Under operating conditions, hydrogen diffused to the regions of high tensile
stress. Platelets of zirconium hydride were formed perpendicular to the
residual tension, and cracking eventually occurred. We set out to map the
lattice strain in the overrolled region with the neutron diffraction technique.
Figure 2 shows measurements of lattice strain in the hoop direction in a complete
overrolled tube as a function of distance from a point in the tube within
the end fitting (0 mm) through the point of maximum Insertion of the rolling
tool (17.5 mm) to a region characteristic of the production tube. The lattice
strain is determined with respect to the interplanar spacing at the free
"unstrained" end and corresponds to the incremental elastic strain induced by
rolling. Well within the end-fitting, where the tool presses the tube against
the fitting the lattice is compressed, as one would expect; however, in the
unsupported region there is a sharp increase In the (0002) interplanar spacing
corresponding to the hoop direction. This tensile strain extends beyond the
insertion point of the tool. A coupon was subsequently cut from the complete
overrolled tube for X-ray line broadening measurements and Fig. 2 also shows
the lattice strain In the hoop direction of this coupon. Although the macroscopic
hoop stresses no longer exist, as much as 50% of the original lattice
strain Is still present In the coupon. We believe that this phenomenon arises
from grain-to-grain interactions. One conventional method of measuring
residual stress involves observing the change in strain gauge readings when
such a coupon Is cut from a tube, and assuming that no stresses remain in the
coupon. This approach would clearly fail to reveal the effects we have
observed with neutron diffraction. Also shown In Fig. 2 is the lattice strain
profile for a complete tube that was given a stress-relief heat treatment after
the rolled joint was made.
Measurements of the anlsotropy of the residual strain were made with this
coupon and some of these are shown in Fig. 3 for the plane in the coupon