FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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containing the tube radius (X^) and the hoop direction (X3). At the place in
the coupon where the (0002) plane spacing in the hoop direction shows residual
tensile strain, the *(2ffO) plane spacing in the radial direction shows a
compressive strain. The third major direction X2 (axial, corresponding to
(1010)) shows a small residual tensile strain- Analysis of 19 interplanar
spacings in three orthogonal planes X1X2, X2X3 and X3X1 gives the complete
strain tensor including sizeable shear terms.
To make this approximate one-to-one correspondence between plane normals and
direction in the sample, we have made use of the marked texture of a production
pressure tube. The vast majority of grains have their [01Î02 axes along the
tube axis, the [0002] axes along a hoop direction and the [2110] axes along a
radial direction. This is the first time such a detailed mapping of the strain
has been carried out and it should lead to a realistic model of the distortion
caused by overrolling.
3.2 Residual stresses in Incoloy-800 steam generator tubes
The steam generator tubes in CANDU nuclear generating stations are formed into
hairpins. Residual stresses in a given tube after it has been bent to its
design shape are undesirable because they make the tube prone to stresscorrosion
cracking when a concentration of salts in areas of localized dryout
coincides with high tensile stress.
The spacings of the {ill} and {200} planes in the longitudinal direction of the
tube were measured as a function of position around the circumference. The
"relaxed" value of plane spacing was measured on a piece of straight tubing.
The strains derived from {220}, {200} and {ill} planes in the radial and hoop
directions were also measured as a function of circumferential position.
Figure 4 shows a schematic view of the experimental arrangement in which the
neutron beam beam passes first through the near wall of the tube but the
incident and scattered beams only intersect in the far wall which in this case
is oriented so that its axial direction lies along the scattering vector. The
neutron counter is shielded so that it cannot see the scattering from the near
wall. This kind of arrangement is very useful for studying tubes nondestructively.
The results are shown in Fig. 5. The longitudinal (or axial)
lattice strain shows a maximum just above the neutral plane of the bent tube
and a minimum just below it. The maximum longitudinal strain derived from
{200} planes is almost twice that derived from {ill} planes. The magnitude of
the strains within the wall came as a surprise, since conventional X-ray
methods [4] had indicated lower strains. The fact that the strain in the
longitudinal direction could be different in differently aligned grains of a
cubic material was also a surprise. Model calculations for a thin walled tube
with elastic/plastic bending [5], following by elastic unloading, confirm the
shape of the residual strain curve. They suggest, moreover, that the anisotropy
of Young's modulus in Incoloy accounts for the difference between the {ill}
and {200} strains. The Incoloy tubing is textured such that there are 5 times
more {ill} planes oriented along the tube axis than {002} planes. The {ill}
behaviour, showing a balanced bending moment, is more representative of the
tube. However, the texture is not so strong that we can determine the complete
strain tensor. The variation of lattice strain in both the hoop and radial