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Franssen Raymond 113 1993.Pdf GEOLOGICA ULTRAIECTINA Mededelingen van de Faculteit Aardwetenschappen Universiteit Utrecht No. 113 RHEOLOGY OF SYNTHETIC ROCKSALT with emphasis on the influence ofdeformation history and geometry on the flow behaviour Raymond Franssen 2 b - 0 0 5. - .. RHEOLOGY OF SYNTHETIC ROCKSALT with emphasis on the influence of deformation history and geometry on the flow behaviour Reologie van synthetisch steenzout met de nadruk op de invloed van defonnatie geschiedenis en geometrie op het vloeigedrag {met een samenvatting in het NederlandsJ PROEFSCHRIFT TER VERKRUGING VAN DE GRAAD VAN DOCTOR AAN DE UNIVERSITEIT UTRECHT, OP GEZAG VAN RECTOR MAGNIFICUS, PROF. DR. I.A. VAN GINKEL, INGEVOLGE HET BESLUIT VAN HET COLLEGE VAN DEKANEN IN HET OPENBAAR TE VERDEDIGEN OP DINSDAG 23 NOVEMBER 1993, DES NAMIDDAGS TE 12.45 UUR door RAIMUNDUS CHRISTIANUS MARIA WILHELMUS FRANSSEN Geboren op 20 februari 1959 te Heerlen IV PROMOTOR: PROF. DR. S.H. WHITE COPROMOTOR: DR. C.J. SPIERS v Hurrah for positive science! long live exact demonstration! Fetch stonecrop mixt with cedar and branches of lilac, This is the lexicographer, this the chemist, this made grammar of old cartouches, These mariners put ship through dangerous unknown seas This is the geologist, this works with the scalpel, and this is a mathematician. Gentlemen, to you the first honours always! Your facts are useful, and yet they are not my dwelling, I but enter by them into an area ofmy dwelling. Walt Whitman, Song ofMyself (1881). VI Vormgeving: Bert Janssen, Utrecht Druk: Drukkerij Elinkwijk, Utrecht CIP-GEGEVENS KONINKLIJKE BIBLIOTHEEK, DEN HAAG Franssen, Raimundus Christianus Maria WiIhelmus Rheology of synthetic rocksalt with emphasis on the influence of deformation history and geometry on the flow behaviour / Raimundus Christianus Maria Wilhelmus Franssen. - Utrecht: Faculteit Aardwetenschappen Universiteit Utrecht (Geologica Ultraiectina, ISSN 0072-1026; no. 113) Proefschrift Universiteit Utrecht. - Met Lit. opg. - Met samenvatting in het Nederlands. ISBN 90-71577-67-8 Trefw.: synthetisch steenzout; reologie. Contents VII CONTENTS SUMMARY 1 Chapter 1 INTRODUCTION: DEFINITION OF PROBLEMS AND AIMS 1.1 General background and scope 7 1.2 The role of salt in tectonic processes . 9 1.3 Geotechnically related questions 11 1.4 Generalization of laboratory creep equations 12 1.5 Anisotropy development and shear localization during dislocation creep 13 1.6 Aims of the present study 15 Chapter 2 UNIAXIAL DEFORMATION EXPERIMENTS ON SYNTHETIC POLYCRYSTALLINE SALT Abstract 17 2.1 Introduction 19 2.2 Experimental methods. 20 2.2.1 Samples 20 2.2.2 Deformation apparatus 22 2.2.3 Testing procedure 22 2.2.4 Data processing 24 2.2.5 Other techniques 24 2.3 Mechanical data 25 2.3.1 Stress - strain behaviour 25 2.3.2 Dependence of flow stress on strain rate and temperature 31 2.4 Microstructural observations . 35 2.4.1 Microstructural development during constant strain-rate experiments 35 VI/I Contents 2.4.2 Microstructural development during strain-rate stepping experiments 43 2.4.3 Subgrain size vs. stress data 50 2.4.4 Crystallographic preferred orientation data 51 2.5 The observed flow behaviour: data analysis and discussion 53 2.5.1 Analysis of flow data . 53 2.5.2 Deformation mechanisms and rate-controlling processes 55 2.5.3 Two-mechanism flow law 59 2.5.4 Comparison with previous studies 60 2.6 Microstructural and texture development: discussion of operating processes 63 2.6.1 Mechanisms of dynamic recrystallization 63 2.6.2 The relationship between migration recrystallization and creep behaviour . 66 2.6.3 Subgrain-size evolution and strain-rate history effects . 71 2.6.4 Crystallographic preferred orientation development 74 2.7 Conclusions 77 Chapter 3 SIMPLE SHEAR DEFORMATION EXPERIMENTS ON SYNTHETIC POLYCRYSTALLINE HALITE Abstract 79 3.1 Introduction 81 3.2 Experimental methods 83 3.3 Experimental results . 87 3.3.1 Mechanical behaviour 87 3.3.2 Microstructural observations 88 3.3.3 Textures 99 3.4 Analysis of flow behaviour 104 3.5 Discussion 107 3.5.1 Deformation mechanisms and rate-controlling processes 107 3.5.2 Microstructural development . 112 3.5.3 Crystallographic preferred orientations 117 3.5.4 Comparison with other shear experiments on rocksalt 119 3.6 Conclusions 120 Chapter 4 DEFORMATION OF POLYCRYSTALLINE SALT IN COMPRESSION AND IN SHEAR AT 250-350 DC Abstract 123 4.1 Introduction 125 4.2 Experimental methods 126 4.3 Experimental results . 129 4.3.1 Mechanical data 129 4.3.2 Optical microstructures 130 4.3.3 Textures 135 4.4 Discussion 137 4.4.1 Deformation mechanisms and microstructure. 137 4.4.2 Textures 137 4.4.3 Mechanical behaviour. 138 4.4.4 Comparison 139 4.5 Conclusions 143 Chapter 5 INFLUENCE OF DEFORMATION GEOMETRY ON THE FLOW BEHAVIOUR OF POLYCRYSTALLINE HALITE AT TEMPERATURES IN THE RANGE 250-600 DC Abstract. 145 5.1 Introduction 147 5.2 Experimental data for polycrystalline halite deformed in shear and compression 149 5.3 Comparison of the mechanical behaviour in compression and shear 159 5.3.1 Definition of quantities 159 5.3.2 Comparison of stress - strain behaviour 162 5.3.3 Comparison of flow behaviour in compression and shear 163 5.4 Discussion 167 5.4.1 Development of mechanical anisotropy 168 5.4.2 Flow laws in shear and compression 173 5.5 Implications 174 5.6 Conclusions 176 x Contents Chapter 6 GENERAL CONCLUSIONS SUGGESTIONS FOR FURTHER WORK 6.1 Deformation behaviour of synthetic rocksalt 179 6.1.1 Deformation mechanisms and rate-controlling processes 179 6.1.2 Microstructural processes 180 6.1.3 Crystallographic preferred orientations 180 6.1.4 Strain-rate history effects 181 6.2 Validity of the associated flow rule and generalization of laboratory creep laws 182 6.3 Mechanical anisotropy and shear localization. 184 6.4 Suggestions for further work 185 Appendix A Nonlinear inversion method applied to processing uniaxial creep data 189 Appendix B Results nonlinear inversion method applied to simple shear data 193 REFERENCES 197 NEDERLANDSE SAMENVATIING 211 NAWOORD 217 Curriculum vitae 221 Summary SUMMARY The present study consists of an experimental investigation into the deformation behaviour of synthetic polycrystalline rocksalt (NaCl) in uniaxial compression and in simple shear. While there exists an exten­ sive body of experimental work regarding the deformation behaviour of synthetic and natural rocksalt in compression, a number of impor­ tant questions remains unanswered. In particular, though recognized that deformation history and geometry can strongly influence rheologi­ cal behaviour and microstructural and texture development via so-cal­ led "memory' effects", such effects have not been systematically in­ vestigated. In addition, there still is widespread disagreement in the li­ terature regarding the detailed dislocation creep mechanisms, rate-con­ trolling processes and microstructural processes operating in polycry­ stalline NaC!. This study aims at helping to resolve these questions. However, special attention is given to the influence of deformation geometry/his­ tory on the mechanical behaviour of rocksalt and to testing the validity of the assumptions underlying methods normally used to generalize ex­ perimentally obtained (axi-symmetric) flow laws, where stress and strain rate are treated as scalars, into forms suitable for 3-dimensional numerical modelling, where these quantities are treated as tensors. The results not only provide insight into the influence of deformation geo­ metry and/or history on the deformation behaviour of NaC!, but allow general principles to be distilled that are also relevant to the effects of deformation geometry and history on the deformation behaviour of ot­ her rock-forming minerals. In CHAPTER I, the problems addressed in this thesis are introdu­ ced in detail, and the aims of the present study are defined. This is followed in CHAPTER 2 with an investigation of the me­ chanical behaviour of synthetic rocksalt in uniaxial compression, in re­ lation to microstructural evolution, crystallographic preferred orienta­ 2 Summary tion development and strain-rate history. To these ends, synthetic poly­ crystalline halite was deformed in uniaxial compression in the tempe­ rature range 250-780 °e (0.5-0.98 Tm) at strain rates between 10-3 and 10-7 s-l, using both constant strain-rate and strain-rate stepping me­ thods. At temperatures from 250 to 450°C, the dominant mechanism of deformation was inferred to be climb-controlled dislocation creep, where climb is controlled by pipe diffusion through dislocation cores. The corresponding steady state flow law is given by: E= 5.75 exp(-129/RT) (}'5.7 where Eis the strain-rate, the pre-exponential constant is expressed in MPa-ns- 1, the activation energy for creep is in kllmol, the temperature T is in K, the gas constant R is in kJ/Kmol and (}' is the applied stress in MPa. At temperatures in the range 500 - 780°C, the observed steady state flow behaviour follows an empirical creep equation given by: E= 6.57 x 107 exp(-227/RT)(}'4.4 and was inferred to be dominated by lattice diffusion-controlled dislo­ cation creep. With regard to microstructural development, in the tem­ perature range 250-350 °e, polygonization and rotation recrystalliza­ tion are the dominant microstructural processes. At higher temperatu­ res, however, strain-induced grain-boundary migration becomes increa­ singly important. Interestingly, the transition from pipe to lattice diffu­ sion-controlled creep coincides with the transition to migration recry­ stallization dominated microstructures. This is explained in relation to Derby's (1992) dynamic recrystallization model as due to relative changes of grain boundary mobility, nucleation rate and driving force, which allow a balance to be established between the migration rate of nucleated grains and the nucleation rate of new grains. Turning to the strain-rate history, below 450°C this has a profound effect upon the observed microstructural development and mechanical behaviour, na­ mely that stresses do not fall, and subgrain sizes do not increase, to the expected steady state values upon a lowering in strain-rate.
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