1992 Timoshenko Medal Lecture by Jan D. Achenbach
The Wages of Wave Analysis
by Jan D. Achenbach, Northwestern University
The text of the Timoshenko Medal Acceptance Speech delivered at the Applied Mechanics Dinner of the 1992 Winter Annual Meeting of ASME.
Ladies and Gentlemen, Friends and Fellow Members of the Applied Mechanics Division, I am grateful to the Applied Mechanics Division for honoring me with the Timoshenko Medal. When I think of the past recipients of this award, I must, however, stand here with a great deal of humility.
It appears that I am the first member of a next generation, the redoubtable sputnik generation, squeezed in between the elder statesmen and the baby boomers, to receive the medal. Undoubtedly many of my contempo¬raries, who have kept the field of applied mechanics on the move, will follow soon.
Two of my favorite colleagues, Ben Freund and Dan Drucker, participated in tonight's proceedings. Ben, who was one of my first Ph.D. students, has become famous for his work on dynamic fracture. I feel some kind of vicarious pride in his achievements. Ben is a very generous person, as was obvious from his introduction. I am happy Dan Drucker was here to present the medal. I have admired Dan since my graduate student days, not only for his achievements in applied mechanics, for which he received the Timoshenko medal many years ago, but also because he has been a consistent and forceful spokesman for basic and applied research in the high councils, in which he more than anyone else in applied mechanics has taken the time and effort to participate.
Let me tell you of the experience of a colleague who passed away some years ago. He had lived a virtuous life, and he was admitted to Heaven. When he entered the Gate he was briefed by an Angel who explained to him that the pace was relaxed in Heaven. There was plenty of funding for research. Researchers were taking their time, and they studied mechanics problems carefully and in depth. So the Angel said why don't you go to work, and give a seminar a year from now. No problem, our colleague replied, I did some work on the way over here and I can give a seminar tomorrow. The Angel thought for a minute and then said: Fine, but keep one thing in mind, Timoshenko, von Karman and G. I. Taylor will be in the audience. Our colleague decided to take some more time.
In a sense Timoshenko, von Karman and G. I. Taylor, as well as all those other recipients of the medal are in the audience tonight. But then again these luminaries of applied mechanics are in that same sense always in the audience in talks that you and I may give in the AMD sessions that we are attending this week. They have set the standards. Judging from the Sessions I attended we are, however, admirably living up to these standards.
I believe I am the first recipient of the Timoshenko medal who has never seen Timoshenko in person. When I arrived at Stanford as a Graduate Student in 1959, Timoshenko, who was on the faculty, had long since retired. I never saw Timoshenko, but I certainly saw his books. They were used in courses on advanced strength of materials, elasticity, shells, stability, vibrations and dynamics. My exposure to the Timoshenko approach was sandwiched in between a more classical European viewpoint and a more modern American one. Before I came to Stanford I studied in Delft, and took courses from Biezeno and Koiter, both got the Timoshenko medal years ago. The core course in solid mechanics was then and usually still is linear elasticity. Biezeno taught elasticity broadly based on Biezeno and Grammell: "Technische Dynamik", written in German. It was an excellent course but it still was a refreshing experience to be exposed by J.N. Goodier to the Timoshenko approach. After Stanford, I went to Columbia as a Post-Doc, and I decided to listen in on elasticity as taught by Ray Mindlin and I learned a good deal more, particularly since Mindlin included the inertia term. Biezeno, Goodier and Mindlin, all three were incomparable teachers with their own style and particular interests.
If I wanted to stretch the point of connections to Timoshenko, I could tell you that my advisor (C.C. Chao) did his work with Bruno Boley, who worked for Nick Hoff, who was a Ph.D. student of Timoshenko. The genealogy is right. Time marches on.
I have always been happy that, when asked what I do for a living, I can answer I work on waves, particularly when asked by someone outside the field. Waves conjure up thoughts of possibly destructive rapidly moving energies, and visions of sweeping motions propagating towards far horizons, or deep into unknown territories.
My address tonight is entitled "The Wages of Wave Analysis'", as in the "Wages of Sin", i.e., the recompense or return. The wages have indeed been many, certainly for me, but also in an infinitely more important sense for many fields of science and technology. I gave some thought to "Riding the Waves." As you probably know this is a surfer's term which would seem appropriate for a talk in Southern California. Some of you may recall that a num¬ber of years ago there was a movie entitled "The Endless Summer", the story of some surfers who go around the world to search for the perfect wave. My activities in the field of applied mechanics, of almost thirty years, have also been an endless summer looking for the perfect wave.
I received an important exposure to waves in solids in a course taught by J.N. Goodier. The book "Stress Waves in Solids" by Harry Kolsky was the textbook. The book was then already out of print and had not yet been published as a Dover paperback. One copy in a decrepit state was available, xeroxing hardly existed and my fellow students and I copied parts of the book by hand. J believe that was the first time the thought occurred to me that there was a need for another book on waves in solids. My own book Wave Propagation in Elastic Solids was published more than ten years later. I am sure that it has been xeroxed many a time, but pardon the plug, it is still available, in paper-back form.
In this country research on waves in solids was already in bloom when I started, thanks to the work of Harry Kolsky, Julius Miklowitz and Ray Mindlin, all departed from this world, and Werner Goldschmidt and C. C. Chao still very much with us. I learned much from these gentlemen, and also from Joe Keller, as well as from contemporaries such as Y. C. Pao, Subhendu Datta and Ajit Mali. Now there is a good-size group of younger workers in the field. The Wave Propagation Committee of the Applied Mechanics Division is more active than ever before.
Over the years I have tried to advance applied mechanics techniques to analyze wave motion in solids and acoustic media in several areas of science and engineering. There were the obvious applications to impact on structures and rapid crack propagation, but there were also applications that reached further from home base to structural acoustics, seismology and quantitative ultrasonics for nondestructive evaluation. These efforts had their ups and downs. They were least successful in seismology and most successful in nondestructive evaluation. It was hard to contribute to seismology in part because seismologists are very good at wave propagation theory, and they have been initiated in the mysteries of earthquake records. There was, however, a period in the seventies and early eighties when applied mechanicians did significantly contribute to the theory of ground motion and to the understanding of earthquake mechanisms with their recently developed models of rapid crack propagation and the associated radiated wave motion.
When I became interested in non-destructive evaluation in the mid-seventies, the field was dominated by applied physicists and electrical engineers. They had excellent abilities in instrumentation and they were interested in analysis and simulation but did not want to spend a lot of time on it. They welcomed help in the area of wave analysis. The perfect match. By now I have learned something about instru¬mentation and measurement techniques and they have adopted our analytical and numerical approaches. Some of the most knowledgeable men in NDE like Don Thompson, Bruce Thompson and Laszlo Adler are among my best friends, and we happily work together.
In the quest for quality of products, especially large expensive products such as planes, bridges and nuclear reactors, and to insure safety of these products, non-destructive testing will play an important role. It is an essential part of life cycle engineering as are other areas of applied mechanics such as fracture mechanics, or in a more general sense failure mechanics, damage tolerant design philosophy and retirement for cause procedures.
I had the privilege of being introduced by a former student. I have been very fortunate with students. It is a great responsibility to find an interesting, challenging and worthwhile topic for a student to work on, particularly since it generally has to be done within the constraints of available funding for specific projects. The choice has long-range consequences for the student. Ideally advisor and student would follow Wayne Gretsky's example. When Wayne Gretsky, who is often said to best hockey player in the USA, was asked the secret of his success, he replied "I never skate to where the puck is, I skate to where it's going to be." Knowing where to go is a good idea, because as Lewis Carroll wrote: "If you don't know where you are going any road will take you there", I might add including many wrong ones. A well defined objective helps. Let me tell another little story. Two men, say a graduate student and his advisor, were looking for work. They were in a flat country, like Holland, where you can see to the horizon. They arrived at a railroad crossing where a third man happened to be standing. The two men explained that they were looking for work and asked where they could find it. The man who was asked pointed to the horizon and said: "over there where the rails get together, that's where you can find work". The two men started to walk along the track, a long way. Finally one of them, probably the professor, stopped and looked back and said "dammit we passed it".
Once a good topic has been selected, the work's progress may be characterized by different sports metaphors. One would be like a golf game where the student accurately hits a single ball from hole to hole. The role of the research advisor would be that of the caddy who carries the golf clubs, occasionally advises on the selection of an iron or a wood, warns that the terrain may be rougher than it looks, points out some slopes, warns that the edge of the sand trap is closer than it may seem and applauds the good shots. A second would be like a tennis game where student and advisor bound all over the court to hit the ball in all directions until a point is scored. The final result may be better than in the golf game. I actually prefer to play tennis. Of course many of us dream of the quarterback/running back situation where the ball is handed off by the advisor on the one yard line for a single ninety-nine yards run and a touchdown.
A few hundred years ago a wise man said "Much have I learned from my teachers, even more from my colleagues, but most of all from my students." On a more prosaic contemporary level I might add, and much do I owe to the Agencies that have made my learning possible, particularly the Mechanics Division of the Office of Naval Research and the Basic Energy Sciences Division of the Department of Energy.
As you know there are some important signs on the horizon for changes in research funding from basic to applied research. Some of these changes are already halfway here. The National Science Foundation is presently considering its future. There will be less emphasis on basic science and more on education, applied science and technology transfer. There will be a switch from DOD funding to research for civilian applications. These changes will, it seems to me, offer excellent opportunities for us in applied mechanics. Applications of mechanics pervade every area of science and technology.
A recent article in Business Week dealing with the Federal Government's move toward a new science and technology policy that puts more emphasis on "practical research" was entitled "Hey, you in the ivory tower. Come on down". I believe that we in applied mechanics have always been ready to meet on the first floor with our colleagues in industry. Interaction with industry can be very stimulating and in the future many if not all of us will become more involved with mechanics problems for industrial applications. Remember that Timoshenko worked for many years for Westinghouse and Mindlin based some of his most interesting research on the needs of Bell Labs to understand the vibration of crystals.
An effective cooperation requires, however, the participation of someone on the company's payroll. A major problem is that many medium sized and small companies have long since fired their research and development engineers, including the one that worked in applied mechanics, as part of cost-cutting efforts to improve the bottom line or the last quarterly balance sheet or to service the debt from the last hostile takeover. We at universities can contribute in an important way to strengthen R&D efforts for product development and international competitiveness, but we must have colleagues at companies to cooperate with. So hey you out there in the boardrooms and penthouses of corporate America hire some R&D engineers.
An occasion like this tends to generate retrospection. I have tried to keep it to a minimum. I know I have been very lucky. Somehow I have always stumbled into the right places, the right people and pretty much the right problems to work on. Bruno Boley crossed my path twice, the first time at Columbia, the second time at Northwestern. Back in the early sixties Bruno had an idea for post-doctoral positions that had absolutely no strings attached. They were called preceptorships and they paid better than assistant professor jobs, a princely $1000./month. The money was provided by ONR through Hal Liebowitz, who was then the director of ONR's Mechanics Division. I was one of the first beneficiaries. I used the time primarily to round off my education. After 9 months at Columbia I went to Northwestern, and years later Bruno via a detour to Cornell arrived at Northwestern to become Dean. He established an environment conducive to our research work in mechanics. Special thanks go to Bruno. I also want to thank my former and present colleagues at Northwestern for keeping me on my toes. Starting with George Herrmann, and then John Dundurs, Toshio Mura, Leon Keer, Sia Nemat-Nasser, Zdenek Bazant, Ted Belytschko, John Rudnicki and Isaac Daniel, as well as our younger colleagues, Tak Igusa, Brian Moran and Sridhar Krishnaswamy. I thank them all for providing a challenging environment.
The Applied Mechanics Division was founded in 1927 by S. P. Timoshenko. It has a great tradition. In the Sadam Hussein sense the Applied Mechanics Division is the Mother of all Divisions of the ASME. In the regular sense the Applied Mechanics Division is the Mother of several other Divisions to which it has actually given birth over the years, but the Division remains strong and fertile. The changes in the research environment which I mentioned earlier offer great opportunities to our members for a bright future. I have been a proud member of the Division for almost thirty years, and I hope to be an active member for many years to come. Thank you for honoring me with the Timoshenko Medal. Thank you very much.