FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

Results and Discussion
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Ultrasonic attenuation results are given in Fig. 4 as a function of temperature
for frequencies ranging from 10 to 50 MHz. The attenuation exhibits a
strong dependence on frequency but the variations with temperature are small
(within experimental error) and somewhat random. The frequency dependence of
the attenuation is illustrated in Fig. 5 using values averaged over the
temperature range studied and corrected for diffraction losses by the method
described in (15). The boundary that distinguishes Rayleigh scattering from
stochastic scattering, X = 2it6, corresponds for the case of a grain size 6 =
25 ym to a value of f = 38.7 MHz. We see, on Fig. 5, the initial f 1 * dependence
of a typical of Rayleigh scattering, gradually transforming into an f
dependence in the X = 2IT6 region where stochastic scattering is expected.
Thus, the frequency dependence of a is consistent with grain boundary
scattering theory.
In order to examine the temperature dependence of a in the perspective of the
temperature related changes in plastic properties and fracture toughness, the
values of 0£ were computed, as described in ref. 2, and compared with the
empirical relation given in the introduction, equation (2). Fig. 6 is a plot
of V£0£ values against Kic /cv. The data points lie more or less
along a horizontal line indicating no net effect on ultrasonic attenuation of
the almost 3 fold change in fracture toughness.
It is well-known O0) that ultrasonic attenuation in steel at typical
N.D.E. frequencies (1-50 MHz) is mainly due to scattering by grain boundaries
and the present results appear to corroborate this fact. Although scattering
in a multiphase material such as type 403 stainless steel is not easily
calculated theoretically because of the complexity of the structure and
insufficient knowledge of elastic constants, the results of Fig. 5 are
consistent with measurements by Papadakis and others ( 10 ), which indicate
that the main scattering features are the original austenitic grains, and that
the effect of the intragranular structure is to modify the effective anisotropy
of the grains. In general, the presence of multiple phases is to reduce
the anisotropy compared to single phase materials. This explains the
relatively low attenuation of the present sample. Based on the anisotropy, A,
of pure iron (**•) where A = Cilit-(Cn-C12)/2 and the Cy 's are the usual
elastic constants for a cubic system, any increase of =6% in attenuation with
temperature is expected between -60°C and +40°C. This is within experimental
error of the results of Fig. 4.
The temperature dependence of yield stress (and therefore indirectly that of
Kic) may be expected to correlate better with absorption, which can be
related to dislocation density and mobility ( 17 \ rather than scattering
which is associated with a constant grain size and varying anisotropy. An
effort was therefore made to separate any absorption contribution to a by
analyzing the data of Fig. 5 in the form of plots of ct/f versus f 2 as in
reference (18). Such an analysis, however, did not give a clear separation of
scattering and absorption, largely because of the lack of data lying clearly
^ ideated that the absorption contribution is