EFFECT OF HYDROSTATIC PRESSURE ON THE CREEP...crystals, application of a hydrostatic pressure caused...
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ABSTRACTS OF PAPERS PRE SENTE D 7 01 (5) A quantitative fit of th e ex perimental test r es ult s to the dislo ca tion model gives values of 3- 4 for the ratios of total di slo ca tion density to th e r ecovera bl e com ponent and values of 7- 8 dynjcm2 for the restoring for ce consta nt for th e recoverable di slo cat ions. Th e r es toring force con sta nt s and recove rabl e di slo cat ion densiti es we re, within ex perimenta l e rror , found to be indep en dent of temperature, m eas ur eme nt st r ess and st rain - rat e, a nd predefo rmation l eve l. Valu es found for the param eters for cree p and mec hani c al after-effect tests were equal w ithin exp er ime ntal error to th e values found from consta nt s train-rat e and str ess -r e laxation tests ca rried out on the same sam ple. If the r ecov era ble dislo cat ion co mponent is id entified as bowed-out di slocation segme nt s whose end s are fixed, the restoring force co uld be accounted f or by the elastic line tension of di slo cat ion seg ment s of length s of about 8 X 10 - 3 cm. No f eat ur e of th e experime ntal res ult s was in co nsiste nt with this di slo ca tion mod el, and the results of the s tudy are all in agreement with the theory proposed by Wee rtman in which the dislocation drag force is very la rge b ecause of th e st r ess -induced ordering of water molecules 111 the stress fi eld of the mov ing dislo cat ion. This pap er is to be submitt ed for publi cat ion in full in another jo urnal. DIS CU SSION N. K. SINHA: During the tests I found it diffic ult to maintain the imp osed s train-r ate to a co ns tant leve l. It was al so observed that it was imp ossibl e to maintain the imp osed st rain to a constant level in a str ess -re laxati o n test. Th ese are because of the inh ere nt e la sticity of or dinar y test ing ma chin es. Wer·e the expe rim e nt s on str ess relaxation and strain-rat e t es ts performed with a closed-loop testing ma chin e? If n ot, the expe rim e ntal r es ults may re pr ese nt only the r es pons e of the ice- ma c hin e system. J. HOLD E R: Th e tests were ca rri ed out in an MTS testing ma chin e, in a closed loop co nfigura- tion. All s tr a in s were monitored by capa citor plat es attached dir ect to the sp ec imen (as o pp osed to the cross-head). J . PER EZ : You h ave use d the Orowan relation ; did you take into co nsideration , at least at high er temperatures, dynami c r ecove ry phenom e na leading to values of A de pending on s tr ess and on temperature in the stea dy stat e? HOLDER : No syste mati c st ud y of di slo cat ion density as a function of predeformation stress was ca rri ed out; our co ncern was to maintain a co nsta nt density during the meas ur eme nt s after the predeformation . Our contl'ol in this rega rd was to co mp are s train-rat es during the actual tests to the s train-rates meas ur ed during the pr ede formati on. The densiti es dete rmin ed by the eq u at ion were th e same, within o ur un cer tainty. EFFECT OF HYDROSTATIC PRESSURE ON THE CREEP OF ICE By STEPHEN J. JO NE (Glaciology Divi sion, Inland VVat ers Directo rat e, D e partme nt of Fisheries and En viro nm e nt , Ottawa , Ontari o KIA oE7, Ca na da ) ABSTRACT. Uniaxial co mpr ess iv e creep tests on single crysta ls a nd polyc rys tal s of ice at abo ut - 10 °C und er differe nt hy drostat ic pr ess ur es are d esc rib ed . After cree ping und er a co n stant load at atmospheric pr ess ur e for so me ho ur s, a hydr ostatic pr ess ur e of the or d er of 35 MN m- 2 wa s applied and th e c hange in s train-rat e was n ote d. Some hours lat er the
Transcript of EFFECT OF HYDROSTATIC PRESSURE ON THE CREEP...crystals, application of a hydrostatic pressure caused...
ABSTRACTS OF PAPERS PRE SENTE D 701
(5) A quantitative fit of the exp erimental test r esults to the dislocation model gives values of 3- 4 for the ratios of total dislocation density to the recoverable com ponent and values of 7- 8 dynjcm 2 for the restoring force constant for the recoverable dislocations. The res toring force constants and recoverable dislocation densities were, within experimental error, found to be independ en t of temperature, measurement stress and strain-rate, and pred eformation level. Values found for the parameters for creep and mechanical after-effect tests were equal within experim ental error to the values found from constant strain-rate and stress-relaxation tests carri ed out on the same sample. If the recoverable dislocation component is identified as bowed-out dislocation segments whose ends are fixed, the restoring force could be accounted for by the elastic line tension of dislocation segments of lengths of about 8 X 10- 3 cm.
No feature of the experimental results was inconsistent with this dislocation model, and the results of the study are all in agreement with the theory proposed by W eertman in which the dislocation drag force is very la rge because of the stress-induced ordering of water molecules 111 the stress fi eld of the moving di slocation.
This paper is to be submitted for publication in full in another j ournal.
DIS CU SSION
N. K . SINHA: During the tests I found it difficult to maintain the imposed strain-rate to a constant level. It was also observed that it was impossible to maintain the imposed strain to a constant level in a stress-relaxation test. These are because o f the inherent elastici ty of ordinary testing machines. Wer·e the experiments on stress relaxation and strain-rate tes ts performed with a closed-loop testing machine? If not, the experim ental results may represent only the response of the ice- machine system.
J . HOLDER: The tests were carried out in an MTS testing machine, in a closed loop configuration. All stra ins were monitored by capacitor plates attached direct to the specimen (as o pposed to the cross-head) .
J . PEREZ : You have used the Orowan relation ; did you take into consideration , at least at higher temperatures, dynamic recovery phenomena leading to values of A depending on stress and on temperature in the steady state?
H OLDER : No systematic study of dislocation density as a fun ction of predeformation stress was carried out; our concern was to maintain a consta nt density during the measurem ents after the predeformation . Our contl'o l in this regard was to compare strain-rates during the actual tes ts to the strain-rates measured during the pred eformation. The d ensiti es determined by the eq uation were the same, within o ur uncertainty.
EFFECT OF HYDROSTATIC PRESSURE ON THE CREEP OF ICE
By STEPHEN J. JONE
(Glaciology Division, Inland VVaters Directorate, D epartment of Fisheries and Environment, Ottawa, Ontario KIA oE7, Canada)
ABSTRACT. Uniaxial compressive creep tests on single crystals a nd polycrystals of ice at about - 10°C under different h ydrostatic pressures are described . After creeping under a constant load at atmospheric pressure for some hours, a hydrostatic pressure of the order of 35 MN m- 2 was applied and the change in strain-rate was noted. Some hours later the
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hydrostatic pressure was removed and the test continued at atmosph eric pressure. From these changes in strain-rate E, an activation volume V was determined from the equation
(0 In E)
V = - RT 3P ' a, T
where R is the gas constant, T is the absolute temperature, P is the hydrostatic pressure and T
is the superimposed uniaxial creep stress . V is negative if E increases with P. For single crystals , which show an accelerating creep curve, that is the strain-rate increases
continuously with time, no sensible a c tivation volume was determined b ecause the strain-rate inc reased both on the application of the hydrostatic pressure, and on its removal. For polycrystals , application of a hydrostatic pressure caused an increase in strain-rate which gave an a ctivation volume of about - 10 cm 3 mol- J • However, on removal of the pressure the strainrate did not return to its original atmospheric-pressure value implying that steady-state creep had not been reached.
This value of activation volume is in general agreement with two previously published values for creep d eformation , but is of opposite sign to the activation volume for dielectric relaxation. Further tes ts are in progress.
DISCUSSION
D. TABOR: In calculating the activation volume does the author take into account the fact that hydrostatic pressure raises the m elting temperature, i. e. does he use the homologous temperature rather than the absolute temperature? Secondly, ice crystals have markedly anisotropic deformation properties. Consequently polycrystalline specimens subjected to a hydrostatic pressure w ill experience shear stresses. Could these play a part in the creep behaviour of the polycrystalline material ?
S. ]. ]ONES : I hope, of course, to obtain activation volumes both close to the melting p oin t, as shown here, and some considerable distance from it. This wi ll tell u s what is the effect of approaching the pressure-melting point. Experimentally, one has to be careful not to exceed the pressure-melting point. In the dielectric experiments one has a similar problem.
] . ' !\T. GLEN : In your table you include results for high-pressure phases of ice. Since these, where they have a phase boundary with the liquid, do not contract on m elting, the argument about vacancies having a negative effect on volume is not applicable to them.
You did not attempt to deduce an activation volume from your single-crystal data for obvious reasons-but if you had you would have got one positive and one negative value for activation volume. I was very impressed by the consistent results on dislocation movem en t of pre-stressed ice single crystals presented in this symposium by ] oncich and others (19 78) . It might be a good idea to use such pre-stressed crystals under relatively low stress to determine an activation volume.
VV. F. B UDD: R egarding the decrease of strain-rate with time which you observe at I ba r, we find from long-term creep tests, d esigned to reach tertiary creep , that the time required to reach a minimum strain-rate greatly increases with d ecreasing stress. At I bar compression at - rooC we expect that at least several thousand hours would be required to r each a mInImum.
]ONES : W e have started experiments at high stresses to avoid this problem.
REFERE C E
J oncich, D . M., and others. 1978. Plas ti c behavior of predeformed ice crystals, by D. M . J oncich, J. H older and A. V. Granato. Journal oJ Glaciology, Vo!. 21, No. 85, p. 700- 0 I.
