Analytic And Experimental Methods In Elastodynamic Crack Problems

It is weH known that the traditional failure criteria cannot adequately explain failures which occur at a nominal stress level considerably lower than the ultimate strength of the material. The current procedure for predicting the safe loads or safe useful life of a structural member has been evolved around the discipline oflinear fracture mechanics. This approach introduces the concept of a crack extension force which can be used to rank materials in some order of fracture resistance. The idea is to determine the largest crack that a material will tolerate without failure. Laboratory methods for characterizing the fracture toughness of many engineering materials are now available. While these test data are useful for providing some rough guidance in the choice of materials, it is not clear how they could be used in the design of a structure. The understanding of the relationship between laboratory tests and fracture design of structures is, to say the least, deficient. Fracture mechanics is presently at astandstill until the basic problems of scaling from laboratory models to fuH size structures and mixed mode crack propagation are resolved. The answers to these questions require some basic understanding ofthe theory and will not be found by testing more specimens. The current theory of fracture is inadequate for many reasons. First of aH it can only treat idealized problems where the applied load must be directed normal to the crack plane.

The rapid pace of current developments in the theoretical, analytical, numerical and experimental fields of dynamic failure of materials called for an international seminar of workshop style aimed at improving the finding and understanding of solutions to the basic physical processes involved in dynamic failure. The Vienna Seminar DFM-l was held at the Technical University Vienna in the historic administration building in the city center under the auspices of the university. More than 30 international experts from all over the world followed the invitation to participate at this seminar. High in the list of priorities was the common desire for ample time for discussions after each technical presenta tion, a fact and a chance frequently made overextensive use of during the seminar. Thus, opportunity was given to the seminar participants to present and expose ideas and results of their original research work, either terminated, ongoing, proposed or conceived and intended, to an international forum of experts for critical discussions, evaluation and appraisal. The technical program included dynamic failure of polymers and steel, numerical modelling of fracture processes, experimental techniques and analytical/numerical investigation of crack/wave interaction problems. The scope of the contributions stretched from implementation of advanced mathematical techniques in the theoretical developments to most direct applications in various fields of engineering practice. Papers published in this volume represent revised, updated and expanded versions of the seminar contributions.

A higher order asymptotic analysis of the transient deformation field surrounding the tip of a crack running dynamically along a bimaterial interface is presented. An asymptotic methodology is used to reduce the problem to one of the Riemann-Hilbert type. Its solution furnishes displacement potentials which are used to explicitly evaluate the near-tip transient stress field. Crack-tip fields corresponding to crack speeds up to the lower of the two shear wave speeds are investigated. An experimental study of dynamic crack growth in PMMA/ steel interfaces using the optical method of CGS and high speed photography, is also described. Transonic terminal speeds (up to 1.4c sub s(PMMA) and initial accelerations (approx. 10(8)m/secz92)) are reported and discussed. Transient effects are found to be severe and more important than in homogeneous dynamic fracture. For subsonic crack growth, these experiments are used to demonstrate the necessity of employing a fully transient expression in the analysis of optical data to accurately predict the complex dynamic stress intensity factor history.

New developments in the applications of fracture mechanics to engineering problems have taken place in the last years. Composite materials have extensively been used in engineering problems. Quasi-brittle materials including concrete, cement pastes, rock, soil, etc. all benefit from these developments. Layered materials and especially thin film/substrate systems are becoming important in small volume systems used in micro and nanoelectromechancial systems (MEMS and NEMS). Nanostructured materials are being introduced in our every day life. In all these problems fracture mechanics plays a major role for the prediction of failure and safe design of materials and structures. These new challenges motivated the author to proceed with the second edition of the book. The second edition of the book contains four new chapters in addition to the ten chapters of the first edition. The fourteen chapters of the book cover the basic principles and traditional applications, as well as the latest developments of fracture mechanics as applied to problems of composite materials, thin films, nanoindentation and cementitious materials. Thus the book provides an introductory coverage of the traditional and contemporary applications of fracture mechanics in problems of utmost technological importance. With the addition of the four new chapters the book presents a comprehensive treatment of fracture mechanics. It includes the basic principles and traditional applications as well as the new frontiers of research of fracture mechanics during the last three decades in topics of contemporary importance, like composites, thin films, nanoindentation and cementitious materials. The book contains fifty example problems and more than two hundred unsolved problems. A "Solutions Manual" is available upon request for course instructors from the author.

Fracture and Mechanics is concerned with the experimental method of static and dynamic caustics and the Distribution of Determinant (Det.)-criterion of fracture. The Det.-criterion determines the conditions causing a crack and gives information on the expected angle of crack propagation. The object of this publication is to present the latest results to the re.earch and development community and to assist the teaching of experimental fracture mechanics and stress analysis in undergraduate and postgraduate courses. After discussing the basic theoretical considerations on the subject, experimental techniques and applications are introduced. Most of the results presented are based on the author's own investigations in the field of experimental mechanics since 1973 at the National Technical University of Athens.