Materials Week '97: Tuesday 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 Tuesday morning, September 16.

HIGH CYCLE OF FATIGUE OF STRUCTURAL MATERIALS: Session I: Overviews and Perspectives

Session Chairs: Prof. Richard Hertzberg, Lehigh University; Materials Department, Whitaker Lab #5, Bethlehem, PA 18015; Prof. Wol Soboyejo, The Ohio State University, Dept. of Materials Science and Engineering, Columbus, OH 43210

OPENING REMARKS: Prof. Wol Soboyejo, The Ohio State University, Dept. of Materials Science and Engineering, Columbus, OH 43210

8:00 am INVITED

THE PARIS LAW FOR FATIGUE CRACK GROWTH IN TERMS OF THE CRACK TIP OPENING DISPLACEMENT: F.A. McClintock, Prof. Emeritus of Mechanical Engr., Massachusetts Institute of Technology, Cambridge, MA 02139

A convenient normalization of the Paris law is to divide the growth rate by the Burgers vector b and the range of stress intensity factor by not only E (Barsom, Speidel), but also by ÷(2b) where 2 is an empirical factor to make the threshold knee occur at DK_th/E÷(2b)=1, da/(bdn)=1. For non-normalized data, the reference point E÷(2b), b is compared with actual threshold knees for different alloys and conditions. Microstructurally short fatigue cracks, with ranges of stress intensity below the long crack threshold, grow faster than expected form the Paris law. This faster growth per cycle is compared to the crack tip displacement from several different idealized crack tip deformation mechanisms: pure slip on one plane, alternating slip on two symmetrical planes, and continuum sliding off from six fans at a non-hardening plastic crack tip. The fortunate deficit in fatigue crack growth rate from the CTOD is due to a combination of roughening, closure, and obstacles requiring polyslip. Apparently the smaller deficit in harder alloys, due to relatively less resistance contributed by polyslip, exceeds the benefit of smaller CTOD for a given DK.

8:25 am INVITED

COMMENTARY ON THE PARIS RELATION: M.E. Fine, Dept. of Materials Science and Engineering, Northwestern Univ., Evanston, IL 60628

The empirical establishment of the Paris Relation relating fatigue crack propagation rate to stress intensity amplitude is the basis for much experimental, theoretical, and modeling of fatigue failure in metals that continues to this day. A number of issues concerning the relation will be discussed. In the history of a fatigue failure when does the Paris regime begin? Can the Paris relation be extended to small cracks? What is the relation between flaws and cracks? How do flaws become cracks? When can the Paris Relation be integrated to give a useful estimation of fatigue life? What are the fundamental parameters that determine the crack propagation rate in the Paris regime? What determines the hysteretic plastic work during a loading cycle?

8:50 am INVITED

THE PARIS EXPONENT AND DISLOCATION CRACK TIP SHIELDING: J. Weertman, Dept. of Materials Science and Engineering and Dept. of Geological Sciences, Northwestern Univ., Evanston, IL 60208

A crack blunting dislocation emission mechanism likely leads to a Paris fatigue crack growth rate law of exponent 2 if dislocation shielding is not important. This talk discusses how dislocation crack tip shielding may affect an increase in the Paris exponent with a simultaneous reduction in the fatigue crack growth rate.

9:15 am INVITED

PARIS LAW EXTENSIONS FOR LOW (K) AND HIGH (K) REGIMES: A.B.O. Soboyejo, Dept of Aerospace Engr., W.O. Soboyejo, Dept. of Materials Science and Engr., The Ohio State University, Columbus, OH 43210

Fatigue crack growth and fracture in structural metallic materials is a stochastic process. The applicability of Paris law will be demonstrated to cover the entire zones of low stress intensity factor range (K) and high intensity factor range (DK) in structural metallic materials, provided appropriate modifications are made to account for the "death rate" in the stochastic process model for the characterization of the low (K) regime, and the "birth rate" in the stochastic process model for the characterization of the high (K) regime. Stochastic process models which incorporate the paris law, including the essential effects of multi-parameter variables and their possible statistical variabilities, which can contribute to the effective driving force which can cause fatigue crack growth and fracture, are presented in this technical paper. From the stochastic process model proposed, appropriate reliability functions are developed, in order to quantify the probability of survival, or structural metallic materials, under fatigue conditions. Possible applications of the principles developed here, in the area of engineering design, in order to minimize and control crack initiation, propagation and failure, in metallic materials and structures, will be discussed.

9:40 am INVITED

PROGRESS IN UNDERSTANDING CORROSION FATIGUE CRACK GROWTH: R.P. Wei, Dept. of Mechanical Engineering and Mechanics, in conjunction with AFOSR, Lehigh University, Bethlehem, PA 18015

The introduction and promotion of the use of the linear fracture mechanics parameter DK by Paul C. Paris and his associates in the early 1960s to characterize the driving force for fatigue crack growth profoundly affected fatigue research and design. The impact of this contribution is highlighted through a perspective overview of the progress in understanding corrosion fatigue crack growth in metallic alloys and its application to design over the past 30 years. Directions for future research are discussed.

10:05 am BREAK

10:20 am INVITED

PERSONAL REFLECTIONS OF PAUL PARIS' FIRST GRADUATE STUDENT: R.W. Hertzberg, New Jersey Zinc, Dept. of Materials Science and Engineering, Materials Department, Whitaker Lab #5, Bethlehem, PA 18015

The introduction and promotion of the use of the linear fracture mechanics parameter DK by Paul C. Paris and his associates in the early 1960s to characterize the driving force for fatigue crack growth profoundly affected fatigue research and design. The impact of this contribution is highlighted through a perspective overview of the progress in understanding corrosion fatigue crack growth in metallic alloys and its application to design over the past 30 years. Directions for future research are discussed. The early post-graduate years were highlighted by collaborative failure analyses of assorted structural components, including the famous 1969 failure analysis of the F-111 wing box. Simultaneous participation in fracture mechanics short courses broadened my technical training and prepared me for my teaching and research career at Lehigh University. The focus of my textbooks and my research interests will be examined in the context of my training with Paul Paris. Finally, a secret involving Paul Paris and one of my earlier failure analyses will be revealed.

10:45 am INVITED

HISTORY OF CONSTANT LIFE DIAGRAMS: G.P. Sendeckyj, Wright Laboratory, Materials Directorate, WL/MLLN, Wright-Patterson AFB, Dayton, OH 45433

A historical review of the development of constant life diagrams (variously referred to as Goodman, Smith, Haigh, etc. diagrams) is presented. It shows that neither Gerber nor Goodman published the first constant life diagram. Goodman never drew what is now called the Goodman diagram and the so-called Goodman law was in general engineering use before Goodman's book first appeared. Similar comments are shown to hold for the various other recent constant life diagrams.

11:10 am INVITED

CRACK GROWTH UNDER VARIABLE-AMPLITUDE AND SPECTRUM LOADING IN 2024-T3 ALUMINUM ALLOYS: J.C. Newman Jr., Mechanics of Materials Branch, NASA Langley Research Center, Hampton, VA

The damage-tolerance approach used today began when Professor Paul Paris proposed that fatigue-crack growth could be correlated with the "stress-intensity factor range." This concept has revolutionized the treatment of crack growth in aircraft structures. When Elber discovered "crack closure", the effective stress intensity factor range began to explain many crack growth load-interaction effects. This paper is dedicated to these achievements. The present paper is concerned with the application of a "plasticity-induced" crack closure model to study fatigue crack growth under various load histories. The model was based on the Dugdale model but modified to leave plastically deformed material in the wake of the advancing crack. The model was used to correlate crack growth rates under constant-amplitude loading and then used to predict crack growth under variable-amplitude and spectrum loading on thin sheet 2024-T3 aluminum alloys. Predicted crack-opening stresses agreed well with test data from the literature. The crack growth lives agreed within a factor of two for single and repeated spike overloads/underloads and within 20 percent for spectrum loading. Differences were attributed to fretting-product-debris-induced closure and three-dimensional affects not included in the model.

11:40 am

INTRODUCING THE K_MAX SENSITIVITY CONCEPT FOR CORRELATING FATIGUE CRACK GROWTH DATA: J.K. Donald, Fracture Technology and Associates, 2001 Stonesthrow Road, Bethlehem, PA 18015; Gary H. Bray and Ralph W. Bush, Alcoa Technical Center, 100 Technical Drive, Alcoa Center, PA 15069