2000 Timoshenko Medal Lecture by Rodney J. Clifton
Applied Mechanics Research and Researchers
2000 Timoshenko Medal Lecture by Rodney J. Clifton
November 9, 2000
To begin, I would like to express my appreciation to the members of the Applied Mechanics Division who somehow came to the conclusion that I should be awarded the Timoshenko Medal. I would also like to thank all those who must have written letters or, through other means, provided supportive input that contributed to my being named the recipient of such a prestigious award. I hope that everyone understands that experimental research involves a team effort so that this award should be viewed as being shared by the many excellent graduate students that I have had the privilege of advising. They, along with very supportive technical staff members, are the ones who have done the experiments for the research that is being recognized by this award.
I have been in a state of shock ever since I opened the letter congratulating me on my selection. I was totally surprised. Having assumed the role of Dean of Engineering at Brown two and a half years ago, much of my thinking and energy has been focused on the needs of the Division of Engineering. The possibility of receiving such a high honor from the applied mechanics community was not even on my radar screen. Even before I became pre-occupied with administrative responsibilities, I never thought of myself as a likely choice for the Timoshenko Medal. One look at the list of prior recipients is enough to humble nearly all of us and certainly me. I am deeply grateful --- and want to move on before the Committee decides to reconsider its choice!
The talk given by the Timoshenko Medalist at the Applied Mechanics dinner is one of the few opportunities that we have to come together as a community and reflect on the challenges and opportunities of our discipline. This tradition bears some resemblance to one that I first encountered when I was an undergraduate at the University of Nebraska. I was a member of a social fraternity and at the end of our meetings we had a time for what we called "Remarks for the Good of the Fraternity". Some of the talks were really quite good, a few were very funny --- all were well intentioned. I remember one by a particularly talented English major who gave an impassioned plea for buying pink toilet paper as a way of urging us to put more emphasis on the finer qualities of life and less on such mundane issues as keeping operating costs at a minimum. While most of the talks were less memorable, I still remember the good spirit and the good intentions with which they were delivered.
My talk tonight is offered in this same spirit and can be thought of as 'remarks for the good of the applied mechanics community'. I believe that many members of our community feel that applied mechanics has come to a crossroad. As they look back they see a solid record of achievement. As they look forward they see declining student interest in our discipline, particularly among graduate students and especially among American students. As those of us who are nearing retirement age look back we see a time in the '60s when we had multiple opportunities for good jobs in universities, corporate research centers, and national labs. As some of our younger members look forward they are apprehensive about finding good jobs and those going into academic positions are concerned about obtaining financial support for a research program.
So, what do we make of this? First, we should feel good about the contributions of applied mechanics to the technological society in which we live. During this first year of the new millennium many of the professional societies have identified the outstanding contributions over the past century. On almost any of these lists one can see the supporting hand of applied mechanics. For example, the National Academy of Engineering named 20 of the greatest engineering achievements of the 20th century. Of these, approximately one-third have direct connection to applied mechanics (e.g. the automobile, the airplane, agricultural mechanization, spacecraft, household appliances, and high-performance materials) and even more have an indirect connection through the role of applied mechanics in manufacturing and in ensuring the reliability of the products being made.
Just mentioning these sectors of our economy whose development owes much to applied mechanics does not do justice to the dramatic advances that have occurred in our field over, say, the past forty years --- to choose the time since I first became a graduate student. During this time our field has been transformed by the almost unfathomable increase in computing power and its accessibility to all of us. While we have others to thank for the development of the computers, we have done our part by developing the software that makes it possible for designers to use computers to provide rational, safe, economical designs for airplanes, automobiles, spacecraft, and a wide range of other structural and mechanical systems. On the experimental side we have benefited from advances in lasers and digital electronics, for example, but here too we have developed the techniques that have turned these tools into powerful aids for measuring and understanding the flow of fluids and the deformation and failure of solids ---- from which new and better designs have emerged.
If we are so confident of our contributions in the past, why is there so much hand wringing over our future? Have we gone about as far as we can go? Have the laws of mechanics been repealed? Have they lost their importance? Have other fields become much more important? If so, do those fields not need mechanics? No, we have neither learned all that needs to be known nor have the laws of mechanics become obsolete. Our problem is not with our discipline but with the limitations that we put on it when we decide to stay with what is familiar and comfortable instead of tackling what is unfamiliar and risky. We stand on the threshold of what could be the most exciting time in mechanics since classical mechanics lost some of its luster nearly a century ago with the development of quantum mechanics.
Today we are entering a new age in biology, and a wholly new technology --- called nanotechnology --- appears to be emerging. Both areas call for new understanding from the mechanics community. In biology the greatest excitement is at the level of cells, molecules and genes. Mechanics at this level in biological systems is clearly in its infancy. Members of our community are beginning to look at the mechanics of cell membranes and even the mechanics of individual molecules. Such studies may provide the foundations for understanding how to combat viruses and how to inject drugs and genes where they are needed. While determining the structure of the DNA molecule may have been a problem in electron microscopy, understanding the operation of the DNA molecule is a problem in mechanics --- as is the great unsolved problem of the folding of proteins. Such are the problems I speak of as being unfamiliar and risky --- but possibly holding the key to great payoffs. Furthermore, from my experience in working on the restructuring of our program in biomedical engineering at Brown I believe that the biology community is receptive, even eager, for the participation of those who can make measurements and do computer simulations that will help them understand the processes that occur at the cellular and molecular level.
In the emerging world of nanotechnology we will be working with devices and even machines that are smaller than we can see with our eyes, even with the aid of an optical microscope. Deformations and motions need to be described on the scale of nanometers, i.e. on the scale of 3-4 atomic spacings. What mechanics is required to describe forces and motions at this scale? Who is best equipped to contribute at this scale? Clearly the physicists have much to contribute but so do investigators whose primary background is in applied mechanics. To me one of the exciting results of molecular dynamics and lattice dynamics has been to establish that continuum mechanics descriptions are remarkably good down to surprisingly small scales, say two atomic spacings. Boundary value problems need to be solved and who is better at solving such problems than the mechanics community that developed finite element methods.
What do we need to know to contribute to these high profile areas of opportunity? Certainly we should learn some biology and some quantum mechanics. If we are to understand the literature and interact with the researchers from other disciplines we need to know the language and the central results for the types of problems that we are considering. Progress often occurs at the interfaces between fields and we need to get across those interfaces to gain a perspective from both sides. The last four new faculty members that we have hired in the Solid Mechanics Group at Brown have been educated as physicists. One is educated as a soft matter physicist and has turned his attention to problems in cellular and molecular biology. By now all four appear to be equally comfortable doing ab initio calculations of computational physics or finite strain calculations of computational solid mechanics. To me this is the perspective that we need to see more of in mechanics. Twenty years ago when I was on sabbatical leave at Stanford I sat in on the first year courses in Applied Physics. These were courses in quantum mechanics, electrodynamics, and statistical mechanics --- all courses that I had not had before. All were truly exciting. All now seem indispensable for the challenges that I have been describing as opportunities for an applied mechanics community with a lively, stimulating intellectual curiosity.
If I were to stop here I would be leaving the impression that all of our opportunities lie in biomechanics and nanotechnology. That would be like saying all the sunshine is in Florida. We all have different perspectives on the research needs of our society and of potential contributions that applied mechanics can make. To identify a few from my perspective --- the computational design of alloys, the Holy Grail of materials science, may be achievable as computing power continues to increase and we learn better how to include chemistry and microstructural evolution in our numerical simulations. There are clearly needs and opportunities in the mechanics of thin films and functionally graded materials. Better understanding of self assembly of regular structures is an exciting area of study with great potential for valuable contributions to a number of applications. Greater involvement of mechanics in the development of electrical and optical devices is an attractive direction --- especially when there is strong coupling between the mechanical deformation fields and the electro-optical response as, for example, in the effect of strain on quantum wells. Clearly, the list of new and developing areas of mechanics research is long. Also, I have not attempted to comment on exciting directions for fluid mechanics research. From my work on hydraulic fracturing I know that much more needs to be known about the flows of non-classical fluids --- for example, slurries in which particles that are not neutrally buoyant are carried by fluids that are viscoelastic or may even be foams. From our most recent hire in fluid mechanics, a faculty member who works in microfluidics, I have learned of the challenges of trying to understand flows through micron and sub-micron openings.
I am not trying to give a comprehensive list of future directions for mechanics research. I am also not saying that there are not many attractive research directions within our traditional research areas. Certainly the talks at this meeting, and others like it, continue to provide interesting and valuable new insights. Instead, I am trying to give a few examples to make the point that mechanics has exciting opportunities, but that these opportunities often require us to move into unfamiliar areas and to do our homework so that we can take advantage of the understanding that has been developed in other disciplines.
To say just a few words about my own experiences in seeking opportunities at interfaces with other disciplines I would point to career adjustments that I made in the early ‘70s. In ‘71-’72 I spent a sabbatical leave in the Audiology Department of the Institute for Sound and Vibration Research at Southampton in England. That was my first sabbatical and I was looking for new research directions, but directions that would allow me to continue my interest in waves. At Southampton I worked on a mathematical model for waves in the inner ear. Simultaneously, I had a gas gun built at Brown for studies of the shearing and fracture resistance of materials at very high rates of deformation. The work on the inner ear was satisfying in that the stated wave guide problem was solved and insights were obtained that were interesting from a mechanics perspective. However, the model was ultimately abandoned as it did not give the right scaling laws when I tried to apply it to animals ranging in size from bats to elephants. The gas gun led to new plate impact experiments, designed from the perspective of solid mechanics, but taking advantage of the technology that had been developed by the shock wave physicists for their research on high-pressure equations of state. Essentially overnight, we had reduced the time scale of our experiments by three orders of magnitude --- from microseconds to nanoseconds. We had a grand vision: to extend mechanical testing to loading rates that were two to three orders of magnitude higher than accessible with current methods --- while at the same time simplifying the interpretation of the experiments by using plane wave loading. Those were exciting times, as I believe my former students will attest. We thought of problem after problem to which we could apply our new found capability: plastic flow and fracture of metals; rheology of lubricants; micro-cracking of ceramics; shearing resistance of compacted powders; failure waves in glasses; friction; and martensitic phase transformations. And, we could study these phenomena at the high strain rates that occur in such difficult-to-study applications as ballistic impact, high speed machining, and elastohydrodynamic lubrication.
As it turned out, the initiative on waves in the inner ear did not bear much fruit, but that of new plate impact tests for studying the mechanical behavior of materials has had immeasurable impact on the attention that our research has received. My regret is not that one initiative was not very successful, but that I did not seize more opportunities to broaden the reach of mechanics.
The potential reach of mechanics is surely very broad. Even now, mechanics enables us to understand much of our physical world and to respond to many of the physical needs of the world’s people. Aesthetically, the subject has much appeal in the sense that, when practiced well, it embodies truth, beauty, and usefulness. Mechanics is a discipline that we can be proud to be a part of and eager to share with others. It is a discipline that will remain alive and vital if we do not limit it by allowing our focus to be too narrow.
On this note, I would like to conclude by again thanking the Applied Mechanics Division for this extraordinary honor, and by thanking all of you for the attention that you have given to these remarks.