J. Gittus – författare

The scientific work of Jean Mandel has been exceptionally rich in the area of the mechanics of solids; the subjects which he has treated have been extremely diverse, from the theory of plasticity, buckling, soil mechanics, visco-elasticity, the theory of reduced models, and thermo­ dynamics, to percolation in porous media. But throughout this com­ prehensive work Jean Mandel has always maintained his interest in forming connections between the properties of materials (strength, deformability, viscosity) and the properties of their basic constituents. What is sometimes referred to in materials science as the transition from the microscopic to the macroscopic has for him been a very constant direction of research, which he never ceased to encourage in the Laboratoire de Mecanique des Solides of which he was the director. It is known that in the plasticity of metals permanent deformations must be sought in intercrystalline slip and more generally in disloca­ tions and the various microstructural defects. Before deformation of polycrystals is tackled, it is necessary to understand the mechanisms which take place within the crystal: the different systems of slip which may be activated and also the elementary mechanisms of twinning. Jean Mandel has shown how to make the transition from the behaviour of the single crystal to that of the polycrystal and has given the relation­ ships between the overall permanent deformation of the polycrystal and the plastic deformation of the single crystal.

A central problem in engineering is the deformation of structures. These may be structures made of metal, from concrete or other buildingmaterials,orfrom soilforexample. Generallyspeaking, the engineerrequiresthedeformationofastructuretoberelativelysmall, predictable, tolerable and non-damaging. Professor Jean Mandel devotedalargepartofhisprofessionalcareertostudiesofdeforma- tionandhewassuccessfulinidentifyingprinciplesandproceduresof wideapplicability.Accordingly,itisveryappropriatetobringtogether as we dointhis volume papers by world authorities concerned with deformationinmemoryofProfessorMandel. The papers in this volume were all invited contributions to an international CNRS colloquium which was held at the Ecole Poly- techniqueinParis, 30September-2October1985. Thevolumeconsidersthedeformationofmetals,rocks,composites, soils, sand and wood. The microscopic processes and theory of deformationaretreated, asarethegenerallawsrelatingdeformation with parameters such as stress system and temperature.A central problemwhichhasbeensystematicallyattackedinthecaseofmetalsis the relationship between the behaviour of crystal defects such as dislocations and the deformationofa large specimenorengineering component.Itshould be possible to produce accurate predictionsof macroscopic deformation from a microscopic model and substantial progresstowardsthisendhasbeenmadeinrecentyears.Thefirsttwo sectionsofthe bookare largelyconcerned with progress in this very importantarea. A parallel theme which was established in earlier days is the developmentofcontinuummodelsfordeformation.Suchmodelswere proposedatatimewhenmicroscopyhadnotdevelopedtoitspresent levelofsophisticationsothat,forexample,itwasnotestablishedthat v VI PREFACE crystalsactuallycontaineddislocations.Thecontinuumtheorieswhich datebackmorethanacenturysoughttoexplainmicroscopicdeforma- tion in terms of abstract models involving mechanical elements of whichthespringand the dashpot wereprominentexamples.Froma strictly practical standpoint these continuum models still have great utilitytoday, particularlyinareaswhere the materialsaresocompli- cated that the preferred route, linking microscopic behaviour with macroscopicbehaviour, is notyet available. Section3ofthe book is concernedthereforewiththecontinuumpointofviewformetals.