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DISCUSSION : EXPERIMENTAL TECHNIQUES AND MATHEMATICAL MODELLING
A. Vincent
To cite this version:
A. Vincent. DISCUSSION : EXPERIMENTAL TECHNIQUES AND MATHEMATI- CAL MODELLING. Journal de Physique Colloques, 1987, 48 (C8), pp.C8-383-C8-386.
�10.1051/jphyscol:1987857�. �jpa-00227161�
DISCUSSION :
EXPERIMENTAL TECHNIQUES AND MATHEMATICAL MODELLING
A. VINCENT
GEMPPM, INSA
de
Lyon, CNRS, F-69621 Vflleurbanne Cedex, France Sixteen papers were contributed to the session "Experimental Techniques and Mathematical Modelling".The discussion concerning this session can be divided into two types of contributions:
-
comments or questions related to the techniques themselves;-
discussion about physical questions that arise when using new techniques or new methods.TECHNIQUES
Bias stress techniques
About techniques themselves, at first, I would like to emphasize various contributions pointing out multiple developments of the so-called "bias stress technique" or "coupling technique". A compression/tension apparatus coupled with ultrasonic measurements working down to 15 K with a load range z 500 N was presented by BECKER (Aachen group). Also, a push-pull system working with elastic kinematics and coupled with acoustic measurements, with a load range : 10 N allowing cyclic 4 plasticity studies was described by CHICOIS (Lyon group). Finally, BENOIT and the POITIERS group presented a recent application of the coupling principle to a low frequency WOIRGARD's4type pendulum, consisting of applying a very low frequency torsion stress (10- Hz) and to measure simultaneously the damping using a superposed "high1' frequency (20 Hz) torsion stress.
Discussing this technique, on the one hand, LENZ asked whether the term
"signature" is suitable or not to designate the cyclic response from the high frequency measurement. In response to this terminological question, BENOIT pointed out that one cannot distinguish between different peaks, such as a BORDONI peak, a HASIGUTI peak or a
P
peak, only on the basis of their shape. On the contrary, the shape of the cyclic Surve corresponding to the high frequency response allows the experimenter to identify the mechanism responsible for the peak, for instance kink pair formation, depinning mechanism, dragging mechanism. In that sense this cyclic curve can be called a "signature". On the other hand, referring to the symmetrical shape of the ultrasonic "signatures" obtained in LYON, FOUGERES asked BENOIT if he can explain the asymmetrical shape of the cyclic response curve observed in his results: BENOIT answered that internal stresses could play a role in this phenomenon. Then, a discussion was led by BENOIT and FOUGERES, about the comparison of the ultrasonic "signatures" obtained in LYON and those obtained in the low frequency pendulum. From this discussion it appeared that the comparison is rather difficult especially because both the amplitudes of the high frequency measuring solicitations and the temperatures are quite different in the POITIERS and the LYON experiments; moreover, from the results presented orally by FOUGERES during the session DISLOCATIONS, it seems that the likeness between the shapes of these"signatures", which is mentioned by BENOIT in his paper, is not so good when the comparison is made for the same amplitude of low frequency deformations as when this comparison is made considering an eight shaped ultrasonic "signature8' obtained at a higher amplitude than in the POITIERS pendulum.
Ultrasonic attenuation measurements
Concerning attenuation measurements in the MHz range, we have to mention the fully automatized measuring system using the continuous wave method described orally by KULIK (LAUSANNE group).
In another contribution BECKER emphasized the irreversible behaviour of the
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987857
C8-384 JOURNAL DE PHYSIQUE
attenuation in pure copper when classical pulse-echo measurements are carried out as a function of temperature. This problem arises from the difference in thermal expansion between transducer and sample leading to considerable mismatch stresses when the bond is cooled below its solidification temperature. In order to avoid this problem, a quite new solution was proposed by BECKER: it consists in using PVDF foil transducers which induce much lower mismatch stresses than quartz, usually used in this field, because of their thinness and low elastic modulus.
Free decay oscillations
Finally, concerning the internal friction (1.F.)
-
--
measurements carried out in free decay of natural vibrations of specimens in vacuum, the comments and the results of CAPPECHI have to be reported. Replying to a question of GREMAUD, he emphasized the "terrible" interaction between the sample and the suspension device using simple nodes characterized by zero displacement and non-zero rotation. This effect can be reduced working with bi-null oscillators such as disks or cross shaped specimens. Damping obtained with these specimens is lower in comparison with bars oscillating in flexure. Moreover, these results are no more in agreement with the predictions of ZENER's theory on transverse thermal diffusion. In order to corroborate this point CAPPECHI presented a sandwich disk made of two sheets glued together, thus having something similar to a barrier to the thermoelastic transverse current. The I.F. damping obtained in this disk is exactly the same as that obtained in a monolithic disk. In this context, CAPPECHI's work raises some doubts on the previous results and the ZENER's theory.PHYSICAL QUESTIONS
Let us consider, now, the physical questions which were approached during the discussion because of experimental procedures.
Origin of the time and heating rate dependent Internal Friction
A heated discussion was devoted to the interpretation of a time and heating rate dependent component in the internal friction which is observed during various transformations such as martensitic transformation, precipitation or dissolution, and recrystallization. As far as the martensitic transformation is concerned, it is well-established that a transient I.F. is superposed to the quasi-static I.F. BENOIT put forward and defended the idea that the transient effect is a general phenomenon occurring in every transformation. The main argument is that the transient term has been observed during various transformations and it is approximately proportional to
?/o as in the case of martensitic transformation.
Several participants, as BESHERS, agreed with the fact that the transient observations are often neglected whereas useful information could be obtained from the transient effect.
On the other hand different comments or remarks were opposed to the BENOIT proposal. First, LENZ raised the question about the role of the mechanical vibration in the transformation i.e. does the energy taken from the oscillations contribute to the transformation? In response to this question, referring again to the martensitic transformation. BENOIT said that in this case it is quite clear that the transient term is proportional to the quantity of phase which is transformed during one cycle;
so, it means that the stress could itself participate to the deformation or to the change of phase or to the change of state of the sample when it is heating, but it does not induce the phenomenon. In fact, this point is not so clear if we add to this statement further comments from BENOIT himself and SCHALLER in response to a question of MAGALAS: there is a zero point drift of the pendulum in the case of recrystallization, precipitation and martensitic transformation but generally this parameter was not systematically studied. Nevertheless, such systematic measurements have been done recently in the case of the martensitic transformation of a Ti-Ni alloy: the experiments revealed a large irreversible autotwisting of the specimen during the transformation, in addition to the* 'f effect on the internal friction.
SCHALLER added that the autotwisting was not T dependent, but in my opinion, this result is not surprising because the autotwisting is very likely proportional to the total quantity of phase transformed which is itself independent of
%.
In conclusion, the question concerning the role of the mechanical oscillation
approached neither in the oral discussion nor in the paper defended by BENOIT, could provide useful informations.
Finally, let us mention two additional questions to BENOIT's general idea:
BESHERS said that, in the case of twinning, he doubts that the twinning phenomenon directly contributes to the I.F. damping he observed. This conclusion was deduced from the fact that twinning was something that came very rapidly and occurred occasionally by bursts in BESHERS's experiments. BENOIT replied that he did not make experiments on twinning, but in the case of the phase transition in cobalt, which is very similar to twinning, he observed exactly the same T/w effect.
Also, SEEGER commented on the fact that there are actually two types of mechanisms that could contribute to the I.F.: one has to do with the progression of the transformation which involves the propagation of dislocations (in the case of the cobalt phase transition for example); the second one has to do with the transformed material which should not be perfect and contain defects left over from the transformation. So, SEEGER said that he can understand that both effects arise, but how to separate them? BENOIT replied that the first effect is the transient one whereas the second one, which is a structure effect, is a static effect: so, according to the general idea that the transient effect varies as T/o, if measurements are done in the high frequency range, this transient effect becomes negligible. Then, comparing high and low frequency measurements it is possible to reveal quite clearly the effect of the transformation and the effect of the change of structure. BENOIT added that this was exactly the goal of such measurements carried out in his group on cobalt because, in this case, the situation is much more simple than in Cu-Zn-A1 or Ni-Ti alloys, for example, which present a lot of post-transformation changes due to diffusion.
Moreover, carrying over SEEGER's comments to other transfonnations, especially to recrystallization and precipitation, leads the discussion back to the question about identification of mechanisms which are responsible for the transient I.F. In the case of recrystallization, BENOIT put forward the idea that one has to consider the I.F. due to dislocation motions (transient) and that due to the change of their mobility (static); in a similar way, in the case of precipitation, one has to distinguish between the I.F. due to precipitation itself (transient) and that due to the presence of the precipitates (static). Nevertheless, in both cases, further investigations are necessary in order to establish firmly this idea and to identify the mechanisms responsible for the transient I.F.
On the dislocation enhanced Snoek effect in niobium
Finally, from the oral discussion, the last point which is worth being reported, deals with the study of the dislocation enhanced Snoek effect (D.E.S.E.) in niobium. Let us recall that by using the new sealing techniques described by MOLINAS and coworkers (STUTTGART group), which consists in evaporating in U.H.V. a Ti layer of about 1 um thickness on the Nb sample, it was demonstrated that the enhancement of the oxygen Snoek peak in Nb-0 alloy reported previously ought to be considered with caution. Indeed, the enhancement of the oxygen Snoek peak (after annealing treatment), which arises presumably because of residual gas pick-up in the case of the uncoated specimen, is not observed in the case of the coated specimen.
Also, plastic deformation of about 1
X
does not affect noticeably (no more than a few X), the height of the oxygen Snoek peak. Thus, the existence of the oxygen D.E.S.E. in Nb can be seriously questioned.At first, MAGALAS commented on MOLINAS' results which, in his opinion, have not been obtained in conditions favourable for the oxygen D.E.S.E. (too high concentration of oxygen and too small plastic deformation). Indeed, in the case of iron, the D.E.S.E. effect is the more marked the lower is the concentration of foreign interstitial atoms. In response, WELLER said that experiments were carried out also with concentrations lower than those presented in his paper (80 ppm instead of 190 ppm), but they lead to the same conclusion.
So two questions remain open: i) why is the oxygen D.E.S.E. so small in Nb-O in comparison with those observed in Fe-C, Fe-N alloys? ii) what is the role of precipitates (carbides and nitrides) on the carbon or nitrogen D.E.S.E. in iron, as questioned finally by WELLER.
C8-386 JOURNAL
DE
PHYSIQUEMAGALAS claimed that the carbon D.E.S.E. is observed both in deformed precipitation-free Fe-C alloys and in Fe-C alloys containing a large amount of carbides. This point will be clarified in further work on the D.E.S.E. in the LYON group.
Further discussions on the conditions for observing the D.E.S.E. are reported in the session on dislocations (cfr. the discussion report by Magalas and Niblett).
