Materials Week '97: Monday AM Session

Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following session will be held Monday morning, September 15.

GEORGE R. IRWIN SYMPOSIUM ON CLEAVAGE FRACTURE: Session I: Fracture Mechanics

Sponsored by: SMD Mechanical Metallurgy Committee, MSCTS Flow & Fracture and Computer Simulation Committees

Program Organizer: Kwai S. Chan, Southwest Research Institute, San Antonio, TX 78238

Room: 211

Session Chairs: Kwai S. Chan, Southwest Research Institute, San Antonio, TX 78238; John D. Landes, Department of Mechanical and Aerospace Engineering, University of Tennessee, Knoxville, TN

OPENING REMARKS: Kwai S. Chan, Southwest Research Institute, San Antonio, TX 78238

8:30 am INVITED

BRITTLE CLEAVAGE FRACTURE: AN OVERVIEW OF SOME HISTORICAL ASPECTS: Paul C. Paris, Department of Mechanical Engineering, Washington University, St. Louis, MO 63130

Many aspects of cleavage fracture were not well understood by the mid-1950s when George R. Irwin coined the term "Fracture Mechanics." This discussion will attempt to relate some of the common concepts and misconceptions of that period. The leadership of Dr. Irwin in developing new concepts and methods for clarifying the understanding of "brittle fracture" will be emphasized here. An essential part of the progress made was to bring together the Engineering Mechanics, Metallurgy, and Physics of the subject into a reasoned balance in trying to further understand the nature of the phenomena. This is illustrated by recalling some of the "personalities" of that time period and their concepts and claims and how they interacted to make progress toward today's state of knowledge. Indeed today's closer ties between fields such as mechanics and metallurgy were in part caused by these requirements of analysis of "BRITTLE CLEAVAGE FRACTURE." We should all be thankful that Irwin was around to bring us together!

9:00 am

INTRODUCTION OF PROFESSOR GEORGE R. IRWIN: Professor Frank McClintock, Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307

9:10 am INVITED

MICROSTRUCTURAL MECHANICS DESCRIPTION OF CLEAVAGE FRACTURING: R.W. Armstrong, G.R. Irwin, X.J. Zhang, University of Maryland, College Park, MD 20742

Grain size and particle size/distribution influences are considered in relation to the ductile-brittle transition and cleavage fracture toughness properties of steel materials. Lower ductile-brittle transition temperatures and greater fracture toughnesses are obtained at smaller grain sizes because of the greater microstructural stress intensities required for cleavage fracturing as compared with plastic yielding. Stereosection fractographic observations have revealed the spread of cleavage from single particle fractures and, more interestingly, from local hole-joining failures at particle clumps where rapid load transfer is reasoned to produce the necessary stress elevations for transition to cleavage.

9:40 am

INTERPRETATION OF RIVER LINE STEPS ASSOCIATED WITH THE GROWTH OF CRACKS: Derek Hull, Dept. of Materials Science & Engr., University of Liverpool, Liverpool L69 3BX, United Kingdom

The development of river line steps on fracture surfaces is common to cleavage in crystalline solids and to the fracture of amorphous and semi-crystalline solids. A brief review is given of the development of ideas about river lines, with particular reference to work on crystalline cleavage and the interpretation of river line patterns in terms of the intersection of cracks with arrays of screw dislocations. Topographical studies on fracture surfaces using optical microscopy, SEM, AFM and profilometry, are described which provide a more general understanding of the origin and development of river line steps. It is shown, following Sommer], that river line patterns are a consequence of local mixed mode I/III conditions and the geometrical constraints associated with the 'no twist' condition. These ideas about river line steps are extended to the interpretation of crack propagation under dynamic conditions and, in particular, to the influence of dynamic stress intensity and crack velocity on fracture surface topography.

10:00 am

SEM STEREO-SECTION FRACTOGRAPHY (SSF) OBSERVATIONS: X.J. Zhang, R.L. Tregoning, Naval Surface Warfare Center, Carderock, MD 20034; R.W. Armstrong and G.R. Irwin, University of Maryland, College Park, MD 20742

Cleavage initiation is a local microstructural event occurring within engineering materials that often are not macroscopically nor microscopically uniform. This increases the difficulty of quantifying the relationship between cleavage fracture initiation toughness measurements and microstructure. The SEM stereo-section fractography (SSF) technique allows simultaneous observations of both the fracture surface and the underlying microstructure. Longitudinal sectioning within one micrometer of the local cleavage initiation site is possible, which allows a direct correspondence to be established between fracture event and microstructure. The information obtained from SSF provides critical input for the micromechanical modeling of microstructural influences on fracturing behavior. A compendium is presented of such SSF results and their consequences for fracturing in A533B steel, HY-80 multi-pass weldment, and Ti₆Al₄V.

10:20 am BREAK

10:30 am INVITED

A THREE-DIMENSIONAL MODEL FOR POLYCRYSTALLINE CLEAVAGE AND PROBLEMS IN CLEAVAGE AFTER EXTENDED PLASTIC FLOW OR CRACKING: Frank A. McClintock, Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307

Fractures in the Northridge and Kobe earthquakes, as well as in new laboratory configurations, show the need for predicting cleavage after plastic flow. Fitness-for-service rather than traditional design and maintenance necessitate quantitative predictions. Those from micromechanisms revealed by micro-fractography provide insight, are limited, and require so much microstructural data as to be impractical. (An addition to this armamentarium is a model presented here for the reduction, as the grain size decreases, in the lower shelf work due to twisting between adjacent cleavage facets.) Thus fracture mechanics is needed, which predicts cleavage in large structures from tests on small specimens. K(T) and J(Q) mechanics are valuable for elastic-plastic initiation. The limit of non-hardening fracture mechanics for fully plastic flow or crack growth before cleavage problems include perturbations due to cleavage and the effects of strain reversals, aging, and statistics. Stability and arrest require elastic-plastic analysis.

11:00 am

THE ROLE OF PLASTIC DEFORMATION IN CLEAVAGE CRACK PROPAGATION AND ARREST IN FERRITIC STEELS: E. Smith, Manchester University-UMIST Materials Science Centre, Grosvenor Street, Manchester M1 7HS, United Kingdom

The paper reviews and presents new perspectives, related to the role of plastic deformation in the propagation and arrest of cleavage cracks in ferritic steels. Thus, the paper addresses, amongst others, the following issues: (a) The interpretation of K_Ia, the crack arrest toughness, (b) Temperature versus mechanics related arrest criteria, (c) The role of plasticity in the context of cumulative versus non-cumulative modes of crack extension, (d) The role of shear lips in cleavage crack propagation, (e) The relation between the upper shelf for initiation toughness and the maximum temperature at which a crack is able to propagate by the cleavage mechanism, in the context of the ASME K_IC and K_Ia curves.

11:20 am

AN INSTABILITY AND ENERGY RATE MODEL FOR CLEAVAGE FRACTURE: C.E. Turner, Mech. Eng. Dept., Imperial College, London SW7 2BX, United Kingdom

The problem addressed is cleavage fracture in steel, with appreciable plasticity. A deterministic model is offered to relate different configurations. The view point is that although a criterion such as a critical stress over a small region or process zone ahead of the crack tip must be satisfied, a critical energy rate must also be available to move forward the crack-tip plastic zone to embrace the next small process zone, before a crack can run in an unstable manner. The driving force for this process in real-elastic-plastic material can only be the energy rate available for linear elastic unloading. In LEFM (such as on the lower shelf) this rate is G. In a partly plastic state (once off the lower shelf) it is I=G+G_p where G is the LEFM term and G_p is a transferral rate of elastic to plastic energy associated with crack growth but not a 'release rate' available for crack separation, per se. G_p is a function of both configuration and system compliance, as well of course, of the fracture load. Some estimates of I will be given from simple theory supported by two-dimensional elastic-plastic finite element studies. This energy rate, I, is then used to relate cleavage data at a given temperature, taken from the literature, for deep notch bending, compact tension, shallow notch bending and centre-cracked tension configurations.

11:40 am INVITED

THE INFLUENCE OF A GREAT AMERICAN SCIENTIST AND ENGINEER IN GERMANY: Erwin Sommer, Fraunhofer-Institut für Werkstoffmechanik, Freiberg, Germany

In March 1961 the German Society for Material Testing (DVM) organized the conference DVM-Tag 1961 in Würzburg. In one of the invited papers Dr. George R. Irwin co-authored by John E. Srawley discussed the basic principles of fracture mechanics. At this time, only a few scientists in Germany mainly physicists had studied his article on the fundamentals of fracture in the "Handbook of Physics." This first scientific visit to Germany in 1961 left deep traces because of a personal contact to another speaker of the conference: Frank Kerkhof, who had proven by his investigations that due to their extreme brittleness glasses were ideal materials for applying the methods of fracture mechanics. This contact lead to a second visit of Dr. Irwin to Germany in 1965 at Freiburg, followed up by a regular exchange of ideas and have stimulated many projects in the field of fracture mechanics some examples will be discussed.