Jan D. Achenbach#

Laudatio by Emmanuel E. Gdoutos#

Over a period of almost forty-five years, Achenbach has been a worldwide leader in the general area of waves in elastic solids. Among his early works was his book Wave Propagation in Elastic Solids, published in 1973 (Elsevier Science, Ltd., Amsterdam) [1]. It was the first book on elastic waves to be published since the much earlier work by Kolsky. lt covered the then state of the art. The book was extremely well received. lt is still in print, in paperback form. Since 1973, numerous other books on elastic waves have been published, but Achenbach’s book is still the most frequently referenced book in the general area of waves in elastic solids, with a total of 1443 citations. His total output of publications has been cited 13,094 times, with a h-index of 43.

Achenbach was the founder and first director (until 2006) of the Center for Quality Engineering and Failure and Prevention at Northwestern University. For almost twenty-five years, the Center has been at the forefront of research and development of ultrasonic techniques for applications to quantitative non-destructive evaluation and structural health monitoring of safety-critical structures such as aircraft, bridges and nuclear reactors. His most recent work has been concerned with the probability of detection of defects, and probabilistic methods in the evolution of fatigue damage.

Achenbach studied Aeronautical Engineering at Delft University of Technology, the Netherlands, and earned his Ph.D. at Stanford University in Aeronautics and Astronautics (1962). He has received numerous highly prestigious Awards and other recognitions. His pioneering work on the theory and applications of quantitative non-destructive evaluation was recognized by the two highest presidential awards in science and technology in the United States, the US National Medal of Technology (2002) and the US National Medal of Science (2005). He is the only person in the field of mechanics currently alive who has received both these medals.

Achenbach’s international impact is evidenced in many ways. His scholarly papers and books are well known to the international community. Almost every year he has presented lectures at international meetings and universities, all over the world. He was elected a Correspondent Member of the Royal Dutch Academy of Arts and Sciences in 1999. His former students and Post-Docs have positions in several countries in Europe and South-East Asia, He has been an active participant in the work of the international Union of Theoretical and Applied Mechanics as a member of paper committees, the Congress Committee and as Chair of the Solids Symposia Selection Committee. He was elected a Member at Large of the General Assembly of IUTAM in 2008. He is the Founder and Editor-in-chief of Wave Motion, an International journal reporting research on wave phenomena. This journal has a Board of Editors of 23 members, consisting of 5 members from the USA and 18 from other countries. Just recently he was the chair of a Committee to review the research programs of the departments of mechanical engineering at the three Technological Universities in the Netherlands. He has been a visiting professor at universities in China, the Netherlands and South Korea.

Achenbach’s contributions to various aspects of his field of research may be summarized as follows

1. Acoustic Microscopy. The uniqueness of his work on this topic rests on the combination of measurements by line-focus acoustic microscopy with a novel theoretical measurement model for very accurate determination of thin-film elastic constants, A number of papers were published, and eventually an invited review paper was written (with J. Kim and Y.C. Lee), "Measuring Thin-Film Elastic Constants by Line-Focus Acoustic Microscopy," (in Advances in Acoustic Microscopy, ed. By Andrew Briggs, Plenum Press, pp, 153-208, 1995 [8]). The principal applications of this work were for the characterization of the thin film coatings that are widely used in surface engineering to extend the life or enhance the performance of components. For example, transition metal carbides and nitrides have extremely high melting points and extreme hardness, excellent high-temperature strength, and good corrosion resistance. They are used for thin film coatings to produce wear-resistant surfaces, for thermal barriers, to provide corrosion protection against harsh environments, and for other applications.

2. Practical Applications of Quantitative Ultrasortics. Achenbach and his co-workers have come up with new techniques for practical applications of ultrasonics to detection and sizing of cracks and corrosion in metal structures. An example is the detection of corrosion and stress-corrosion cracks in the wing box of the DC-9, without entry or disassembly. The team was awarded the 1997 Model of Excellence Award by the McDonnell-Douglas Co., now part of the Boeing Co. A second example is a successful project carried out for the U.S. Air Force with Warren Robins Air Logistics Center, in which an ultrasonic technique was developed to detect fatigue cracks in weep holes in the internal wing structure of C141 aircraft. That work received the 2002 Best Paper Award from the American Society for Nondestructive Testing.

3. Laser-Based Ultrasonics. Ultrasound generation by laser irradiation in the thermoelastic regime is of great interest for applications in quantitative non-destructive evaluation, since it provides a number of advantages over conventional generation by piezoelectric transducers. A Scanning Laser Source technique (SLS), which greatly improved detectability of surface-breaking cracks, was developed and applied.

4. Diffraction Coefficients. In 1896, Arnold Sommerfeld published a seminal paper that solved the problem of edge diffraction of electromagnetic and acoustic waves by a semi-infinite flat screen. The result proved to be extremely important in radar technology, optics, and acoustics. The corresponding three-dimensional problem of diffraction by a crack in an isotropic elastic solid, which is much more complicated than the Sommerfeld problem, was solved in 1976 by Achenbach and Gautesen. The 1977 paper [6] was included in the list of major contributions to physical acoustics compiled by the Acoustical Society of America.

The work on diffraction coefficients and applications to scattering by cracks of finite dimensions was consolidated in a book by Achenbach, Gautesen and McMaken: Ray Methods for Waves in Elastic Solids - With Applications to Scattering by Cracks, Pitman Advanced Publishing Program, Boston/London/Melbourne, 1982.

The solutions for the diffraction coefficients allowed the authors to generalize the geometrical theory of diffraction to scattering by cracks of arbitrary shape. Other workers in the USA, the UK and Sweden subsequently used the results in computer programs for the detection of cracks in nuclear reactors, engine components, and other safety-critical structures.

5. Dynamic Behavior of Composite Materials (1964-1975). In the mid-sixties these inhomogeneous materials were represented by a homogeneous but anisotropic material, via the "effective modulus" theory. This was often not acceptable at higher frequencies. In the period l964-l966, Achenbach and co-workers developed a more general model based on a generalized continuum theory [4]. A number of papers was published and eventually the theory was summarized in a monograph entitled A Theory of Elasticity with Microstructure for Directionally Reinforced Composites, CISM Monograph l67, Springer-Verlag, Vienna/New York, 1975 [2].

6. Dynamic Effects on Fracture (1968-1980). With Eshelby (UK) and Btoberg (Sweden), Achenbach was one of the first to study dynamic effects on fracture, both due to high crack propagation speeds or due to dynamic external excitation. Expressions were derived for elastodynamic stress intensity factors, and these were combined with energy conditions for the propagation of a crack [5]. Work in the field eventually expanded to include shear rupture on fault planes for earthquake mechanisms, non-linear and plasticity effects, dynamic branching and fragmentations and large-scale numerical simulations.

7. Elastodynomics. In recent work Achenbach has returned to the classical theory of waves in elastic solids. A new formulation was derived to express guided waves in terms of a carrier wave propagating along the waveguide. The carrier wave, which is the solution of a simple reduced wave equation, carries the thickness motion. Examples of applications are Lamb waves in a layer and Rayleigh surface waves. This work was carried out parallel to a novel application of elastodynamic reciprocity whereby a problem is solved by combining its solution with a virtual wave solution in the reciprocity theorem. The combination of the two new formulations was extremely useful in deriving expressions for wave motion generated by internal and surface excitation of a solid body [9]. The results have also appeared in his recent book entitled Reciprocity in Elastodynamics, Cambridge University Press, 2003 [3].

8. Probabilistic Considerations of QNDE and SHM. Probabilistic considerations play a dominant role in the four stages of the diagnostics and prognostics of fatigue damage in metals. In recent work Achenbach has given considerable attention to the evolution and detection of pre-crack fatigue damage and the probabilistic aspects of subsequent macrocrack formation (Stage l). For Stage 2 (macrocrack growth and detection), he has shown that Paris law for crack growth under cyclic loading conditions can be useful, particularly if it is placed in a probabilistic context, and if the constants in the law are represented by probability distributions. By introducing the probability of detection concept, various probabilities related to the existence, alter N cycles, of cracks larger than a critical size, were determined in Stage 3 for purposes of prognostication [10]. Another recent paper dealt with the probabilistic aspects of the optimization of inspection scheduling for Stage 4.

[1] Wave Propagation in Elastic Solids, North-Holland Publishing Company/American Elsevier, Amsterdam/New York, 1973 (referenced 1443 times).

[2] A Theory of Elasticity with Microstructure for Directionally Reinforced Composites, CISM Monograph 167, Springer Verlag, Vienna/New York, 197 5.

[3] Reciprocity in Elastodynamics, Cambridge University Press, Cambridge, UK, 2003.

[4] "Continuum Theory for a Laminated Medium," J of Applied Mech., 35, pp. 467- 475, 1968 (with C. T. Sun and G, Herrmann), (referenced 205 times in ISI Web of Knowledge).

[5] "Dynamic Effects in Brittle Fracture," in Mechanics Today, 1, ed. by S. Nemat- Nasser, Pergamon Press, Oxford, UK, pp. 1-57, 1974,

[6] "Geometrical Theory of Diffraction for Three-D Elastodynamies," J Acoust. Soc. Am., 61, pp. 413-421, 1977 (with A. K. Gautesen), (referenced 44 times in ISI Web of Knowledge).

[7] "Effect of Interfacial Zone on Mechanical Behavior and Failure of Fiber-Reinforced Composites" J Mech. Phys. Solids, 37, pp. 381-393, 1989 (with H. Zhu), (referenced 175 times in ISI Web of Knowledge).

[8] "Measuring Thin-Film Elastic Constants by Line-Focus Acoustic Microscopy," Advances in Acoustic Microscopy, ed, by Andrew Briggs, 1, pp. 153-208, 1995 (with J. Kim and Y.-C. Lee).

[9] "Combination of a Virtual Wave and the Reciprocity Theorem to Analyze Surface Wave Generation on a Transversely isotropic So1id," Philosophical Magazine, 85, pp. 4143-4157, 2005.

[10] "Structural Health Monitoring and Damage Prognosis in Fatigue," Structural Health Monitoring, 7, pp. 37-49, 2008 (with S.S. Kulkarni).

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