ABSTRACT
Preface Quasibrittle materials include many well-known traditional materials as well as high-tech advanced materials: concretes and mortars, rocks, toughened ceramics, certain fiber composites, ice, wood and wood particle board, paper, and even some advanced tough metallic alloys. These are materials that fail by fracture but the fracture does not have a sharp tip. Rather, due to heterogeneity of the microstructure of the material, the fracture is preceded by a large zone of distributed damage, mainly micro-cracking. The advance of fracture consists of localization of distributed cracking into a major continuous crack. This behavior causes the mathematical description of the failure process of quasibrittle structures to be considerably more complicated and difficult than it is for either ductile structures, which are described by the theory of plasticity, or for brittle structures, which are described by linear elastic fracture mechanics. In recent years it has become clear that understanding of failure of quasibrittle materials is of paramount importance in many engineering fields. It is required for a rational design against failure of concrete structures as well as geotechnical structures. It is particularly important for modern toughened ceramics, and especially for the design of better ceramic materials. The same could be said of many fiber composites, as well as ice, especially sea ice. Simply, progress in the understanding and mathematical modeling of quasibrittle materials plays a major role in many technological advances. The field of fracture and damage mechanics of quasibrittle materials has recently been undergoing an explosive development. The impetus for this development has come from various directions: (1) The recognition, among the structural analysis specialists, that without some form of nonlinear generalization of failure mechanics or reformulation of the classical concept of damage, the existing finite element codes cannot correctly capture damage localization phenomena and cannot yield objective, that is, mesh-independent predictions of quasibrittle structural failures. (2) The realization that these computer codes cannot correctly predict the effect of structure size on the maximum load, structure ductility and energy absorption capability in quasibrittle type of failures. (3) The recognition that understanding of quasibrittle failures is particularly important for various modern high-performance materials, such as high strength concretes, toughened ceramics or fiber reinforced composites, and that this quasibrittle behavior cannot be ignored if the capacity of these new materials is to be exploited effectively and safely. (4) The realization that the way toward developing better high-performance materials cannot bypass understanding of the micromechanics of fracture, particularly the progressive spread of microcracks, interface and bond breakages, frictional slip, pull-out of fibers, etc. Although there is a great similarity in the failure process among all quasibrittle materials, the progress has been to a large extent happening separately for various types of materials. The fracture behavior of concrete has been studied more deeply and for a longer time than that of ceramics, and in some respects advanced farther, for example in the development of nonlocal finite element codes for quasibrittle fracture in structures of arbitrary shapes, or in the theory of size effect, which is especially important because of the large sizes of many concrete structures. Some analogies with these results might prove profitable in the field of ceramics. In other respects, though, for example in micromechanical analysis and analytical descriptions of various mechanisms of crack tip shielding, the role of inclusions and their interactions with cracks, recent researches in ceramics have become more sophisticated and more advanced. So, vice versa, emulations of some approaches might be profitable for the study of concrete, although the greater degree of complexity and disorder in the microstructure of concrete poses considerable difficulties. The problems of fracture and damage in quasibrittle materials impinge on many scientific disciplines: microstructure observations and characterization, micromechanical analysis of the processes in the microstructure, continuum mechanics, theory of composites, bifurcation theory, statistical modeling, and computational algorithms. It is thus clear that interdisciplinary exchanges of ideas may be expected to accelerate progress. It has been with this objective that the present workshop has been organized. The U.S.-Europe Workshop on Fracture and Damage in Quasibrittle Structures: Experiment, Modeling and Computater Analysis, which will be held at the Czech Technical University in Prague (CVUT), represents a sequel to the France-U.S. Workshop on Cracking and Damage. This preceding workshop was held in 1988 at the Laboratory of Mechanics and Technology at E.N.S. de Cachan, France, under the joint chairmanship of J.Mazars and Z.P.Bazant, and under the co-sponsorship of C.N.R.S. (Conseil National de Recherche Scientifique, France) and N.S.F. (U.S. National Science Foundation). This highly successful workshop, locally organized by J.Mazars, led to the publication of a proceedings volume which contained many outstanding contributions1 and was summarized in a journal article2. Financial support for the present workshop has been obtained in the United States from the National Science Foundation, and in Europe from the European Community, through the ALERT-Geomaterials Laboratories Network of the European Union. Grateful appreciation is due to these sponsors for making the Workshop possible. The Workshop is organized under the auspices of the International Association for Fracture Mechanics of Concrete and Concrete Structures (IA-FraMCoS, incorporated in Illinois), as part of the series of FraMCoS Workshops. The Workshop will continue the discussion of many problems which were the focus of the First International FraMCoS Conference held in Breckenridge, Colorado, in 19923, although now the scope will be broadened to all quasibrittle materials. The choice of Prague as the site of the Workshop reflects the bright new political situation in Europe, which is particularly gratifying to both of us, natives of Prague. Prague is a beautiful, ancient European city with an old cultural tradition, which we both love and would like our colleagues in research to taste and appreciate. The choice of the site should make it clear that a free Europe-wide research community has come into existence. It reflects the desire of Czech researchers as well as all the researchers in the newly emerged democracies of Central and Eastern Europe to be integrated into the world-wide international scientific community. Achieving this goal is of course equally important to the researchers in Western Europe and America. Although the provenance of financial sponsorship of the Workshop implies the majority of participants to be from Europe and the United States, as
reflected in the title of the workshop, it has been possible to attract some prominent guests from other continents and countries. Their contributions are very welcome, even though their small number still leaves their research establishments under-represented, which is of course inevitable due to the choice of sponsors. We would like to express our thanks to the International Scientific Committee of the workshop, which was chaired by
J.Mazars (Cachan), and had the following members: T.B.Belytschko (Evanston), R.de Borst (Delft), G.J.Dvorak (Troy), G.Maier (Milano), M.Ortiz (Providence), I.G.Vardoulakis (Athens) K.Willam (Boulder). The members of this Committee reviewed the abstracts of the papers submitted for the present volume and rated them. The ratings were then averaged. This made possible an impartial selection of papers for presentation at the workshop and the choice of principal lecturers. Regrettably, due to the limitation on the number of papers, dictated by the format of a workshop, and the decision to avoid parallel sessions, not all worthy abstracts of papers proposed for presentation could be accepted. But at least some of those are featured as poster session papers. It is a particular pleasure to both of us to hold this workshop at the Czech Technical University in Prague (CVUT), one of the oldest engineering schools in Europe (founded by an imperial decree in 1707). This University, from which we both graduated and to which we feel deep emotional attachment, was at one time one of the major centers of the free intellectual discourse in Europe. After decades of forced isolation, this former position is now being regained. We would like to express our particular thanks to Milan Jirásek, Assistant Professor of Structural Mechanics at the CVUT, for his essential help in the preparation of the present volume as well as local organization of the workshop. Furthermore, thanks are due to Jirí Šejnoha, Professor of Structural Mechanics at the Czech Technical University, for his valuable advice and help in organization. Our grateful appreciation is also due to Robin Ford, Workshop Secretary at Northwestern University, for her expert, meticulous and dependable secretarial assistance. Finally, we wish to express our thanks to all those participants of the Workshop who contributed excellent articles to the present volume. To achieve the best possible appearance, the organizers decided to require that all the contributions be done in one and the same word processor, namely LaTeX. This is a superior word processor for mathematical type-setting, which is rather demanding for the authors. We deeply appreciate that most authors complied, either preparing the text in this word processor, or providing diskettes for conversion in Prague. Thus our particular thanks are due to all those authors who were cooperative in this respect.
