BY JIANMIN QU AND ZDENĚK P. BAŽANT

JAN DREWES ACHENBACH, Walter P. Murphy Professor and Distinguished McCormick School Professor Emeritus-in-Service at the McCormick School of Engineering and Applied Science, Northwestern University, passed away peacefully in Evanston, Illinois, on August 22, 2020, just 2 days after his 85th birthday.

A renowned educator, celebrated researcher, and respected leader of the scientific community, Jan had an illustrious career. He dedicated his entire professional life to science and engineering education. In this memorial of our dear colleague and friend, we celebrate his life and his contributions to the scientific literature and to engineering education and practice.

Jan was born August 20, 1935, in Leeuwarden, a provincial town in the northern part of the Netherlands, which was occupied by Germany during World War II, from May 1940 until April 15, 1945, when the Canadian Army liberated this part of the country. The town was in the path of British and American bombers flying from England to Germany. In addition, a German airbase for fighter planes was established on the  outskirts of the town and, toward the end of the war, Jan and his young friends would get as close as the barbed wire and mine-fields would allow to see the German fighter planes take off to engage the Allied planes overhead. The excitement of watching the planes in action generated his lifelong interest in aviation.

After schooling in Leeuwarden Jan studied aeronautical engineering at Delft University of Technology (TU Delft). The launch of Sputnik in 1957 and the ensuing space race that stimulated rapid growth in both fundamental and applied research at US universities inspired him. He applied to Stanford University for his graduate studies, was awarded a scholarship, and received his PhD in aeronautics and astronautics in 1962.

After a year as a postdoctoral fellow at Columbia University, in 1963 Jan was appointed assistant professor in the Department of Civil and Environmental Engineering at Northwestern University, where he remained except for sabbatical leaves to the University of California at San Diego and TU Delft.

As an early member of the solid mechanics group at Northwestern, Jan was instrumental in building a team of scholars that led to the university’s national and international leadership in solid mechanics. Established in the early 1960s by George Herrmann (NAE 1981), Northwestern’s Theoretical and Applied Mechanics (TAM) Program became a hub of outstanding research. In addition to Jan, the members of the group in the Civil Engineering Department in the 1970s included John Dundurs, Leon Keer (NAE 1997), Toshio Mura (NAE 1986), Zdeněk Bažant (NAE 1996), Sia Nemat-Nasser (NAE 2001), and Ted Belytschko (NAE 1992), and, in the Materials Science Department, Johannes (Hans) Weertman (NAE 1976). The group’s largest, multimillion-dollar effort, joint with Los Alamos National Laboratory, was funded by NSF’s RANN (Research Applied to National Needs) program and led by Weertmann (1974–79). The goal was to analyze a proposed hot-dry-rock geothermal energy scheme and drill in the Jemez caldera in northern New Mexico. The findings had a major influence on policy and research in the United States and Japan.

Beginning in the mid-1960s and continuing for the next two decades, the solid mechanics group in the Civil Engineering Department, with Jan at its helm, was very collaborative—and social. Every Friday at 4 p.m., there was a mechanics seminar, usually led by a guest speaker. Long discussions often followed, in a group standing at a three-leaf blackboard filled with equations and sketches in chalk (this mode of presentation, from which one could actually understand the speaker’s argument, unfortunately disappeared with the arrival of transparencies and PowerPoint). Dundurs made sure that after each seminar there was a party with the speaker at someone’s home. One memorable party in 1972, at Toshio’s home, lasted until 4 a.m. as the speaker, Ronald Rivlin (NAE 1985), entertained all with anecdotes about other mechanicians.

In 1981 Jan was designated the Walter P. Murphy Professor in the Departments of Civil and Environmental Engineering and Mechanical Engineering. In 1985, with support from the Federal Aviation Administration, he founded at Northwestern the Center for Quality Engineering and Failure Prevention (QEFP). It initially focused on developing nondestructive evaluation (NDE) technologies for the aerospace industry, then gradually expanded its scope to many areas of engineering applications, including structural health monitoring of civil infrastructures and nuclear power facilities. The center quickly attracted many promising students, postdoctoral fellows, and visiting professors from all over the world. In the NDE community it was well known that there were two centers of excellence: at Northwestern the QEFP focused on fundamental research, and at Iowa State on applied research.

In 1992 Jan was named Distinguished McCormick School Professor. During his career he supervised over 40 PhD students and numerous postdocs. He was a strict mentor with very high standards—and his high expectations inspired many to achieve beyond their potential. Some of his former students, such as Ben Freund (NAE 1994, NAS 1997) and C.T. Sun, went on to become distinguished researchers themselves.

Jan was a preeminent researcher in solid mechanics for over half a century, doing both analytical and experimental work. His doctoral dissertation at Stanford in 1962 dealt with waves and vibrations in viscoelastic solids, a problem of considerable interest for the dynamic response of solid-rocket propellants. He solved the three-dimensional problem by means of a visco-elastic correspondence principle for transient waves and presented simplified solutions based on a novel viscoelastic constitutive model.¹

After making important advances in dynamic fracture mechanics early in his career, he went on to major contributions in the field of propagation of mechanical disturbances in solids; achieved important results in quantitative NDE of materials, damage mechanisms in composites, and vibrations of complex structures; and developed methods for flaw detection and characterization by ultrasonic scattering methods. He also achieved valuable results on earthquake mechanisms, the mechanical behavior of composite materials under dynamic loading conditions, and vibrations of solid propellant rockets. In addition to numerous journal and conference papers, he published several books that consolidated and synthesized many of his advances in these areas.²

During 1964–75 his work focused on dynamic behavior of composite materials. In the mid-1960s these inhomogeneous materials were represented by a homogeneous anisotropic material via the “effective modulus” theory. This representation, however, was often unacceptable at higher frequencies. Jan developed a better model for laminated media and fiber-reinforced composites based on a generalized continuum theory and formulated a method to calculate the material constants from the elastic constants of the constituents and geometrical parameters. His new theory properly represented dispersion of wave motion at high frequencies. He published numerous papers on this subject and eventually summarized them in an influential monograph of lasting value.³

During the period 1968–80 Jan was one of the first investigators to advance understanding of the dynamic effects on fracture caused by either high crack propagation speeds or dynamic external excitation. He derived expressions for elastodynamic stress intensity factors and combined them with energy conditions for the propagation of a crack. His first paper on this subject set a new direction,⁴ and many other researchers (primarily Freund) developed his approach further.

He also obtained important results on diffraction coefficients. Working with his postdoctoral assistant Arthur K. Gautesen and student Harry McMaken, he generalized the geometrical theory of diffraction to elastodynamics by solving two canonical problems, both concerned with the diffraction by a semi-infinite crack of a plane wave incident under an arbitrary angle with the edge of the crack. Along with applications to scattering by cracks of finite dimensions, he consolidated this work in their coauthored 1982 book.

Jan’s 1973 book on elastic waves, written during a sabbatical at TU Delft, covered the then state of the art and was extremely well received. It is still in print and remains the most frequently referenced book on waves in elastic solids.

Jan became widely known for his groundbreaking contributions to acoustic microscopy. Around 1985 he started a laboratory in quantitative ultrasonics. Among the advanced instrumentation that he used and further developed was a line-focus acoustic microscope. He supplemented the experimental work on measurement of the V(z) curve by a theoretical analysis based on measurement models. His theoretical and experimental research, particularly on the determination of the elastic constants of thin films, resulted in a number of significant papers (coauthored with graduate students). A novel feature was Jan’s use of an accurate measurement model with all the systemic features of the actual measurement process, but based on rigorous analysis. His model made it possible to obtain the material parameters with great accuracy by systematically minimizing the difference between the measured and calculated V(z) curves. He eventually summarized his results in an influential review paper.⁵

Beginning in 1985 the emphasis of his work was on the theory and applications of ultrasonic methods to quantitative NDE, particularly on the measurement of elastic properties of thin films by acoustic microscopy, and the detection of cracks and corrosion in safety-critical structures. He also began to work on the development of probabilistic methods for structural health monitoring of fatigue damage in structural components for the purposes of diagnostics and prognostics. His pioneering ideas often defined new directions of research.

In the late 1990s Jan returned to classical elastodynamics. He derived a new formulation to express Lamb waves in terms of a carrier wave propagating in the midplane of the layer. The carrier wave, which is the solution of a simple reduced-wave equation, carries the thickness motion of the layer.⁶ This information, in conjunction with a novel application of elastodynamic reciprocity, was extremely useful in deriving expressions for wave motion in an elastic layer (generated by a time-harmonic point load of arbitrary direction) in terms of superposition of wave nodes.

Jan was also very successful in practical applications of his results on quantitative ultrasonic NDE. With his colleagues, he made major contributions to practical applications of ultrasonics to detection and sizing of cracks and corrosion damage in metal structures. An example is the detection of corrosion and stress-corrosion cracks in the wing box of the DC-9. In the mid-1990s Northwest Airlines had in operation more than 125  DC-9 aircraft older than 20 years. These aircraft needed periodic inspections for corrosion in the inner layers of the wing box, which is a fuel compartment. The old way was to enter the wing box for visual inspections or to disassemble the wing from the fuselage, a procedure that took about 800 hours.

Jan led a team that developed an ultrasonic technique for nondestructive testing of the wing box from the outside of the wing, without entry or disassembly. This reduced the inspection time to 50 hours and saved Northwest Airlines millions of dollars. The technique is now also used by other airlines and by the US Air Force. Vital details of this technology were published⁷ and his team received the 1995 McDonnell-Douglas Aerospace Model of Excellence Award.

In June 1999, the night before a planned flight to Korea, Jan suffered serious cardiac arrhythmia with a loss of consciousness. He was kept for 2 weeks in an induced hypothermic coma. Upon waking, his first phone call was to his secretary to check on his project funding. It took him a year, but his recovery was remarkable. It is a testimony to Jan’s will, determination, and perseverance that he successfully restarted his research programs.

In his later years Jan focused his research on applying the elastodynamic reciprocity to obtain closed-form solutions for the scattering of elastic waves by surface cracks in plates and pipes.⁸ In 2009 he switched to emeritus status to enable his departments to hire younger faculty, but continued his research, supervised PhD students, and published numerous papers.

Throughout his career, a distinctive feature of Jan’s research was the elegant application of rigorous mathematical methods in engineering applications. For example, traditional ultrasonic nondestructive methods are based on empirical measurements and heuristic analysis based on signal processing. It was Jan who introduced quantitative analysis of scattering of ultrasonic waves by defects to nondestructive evaluation. Late in his career, when asked what work of his he was most proud of, he answered “I added the letter Q to NDE.” Indeed, the research field is now called quantitative nondestructive evaluation (QNDE). In 2008 Jan delivered the plenary lecture at the 27th Annual Review of Progress in Quantitative Nondestructive Evaluation, the prime annual gathering of the international QNDE community. His lecture, titled “NDE with a Q,” was a brilliant blend of science with a retrospective on progress in engineering.

Jan was also active in the professional community, as a member of the ASME-AMD Executive Committee and US National Committee on Theoretical and Applied Mechanics (1987–94), founder and editor of the Journal of Wave Motion, and member of the editorial board of the Proceedings of the National Academy of Sciences (2003–04). He served on several NAE and NRC committees, including the Committee on Membership (1997–2001), Mechanical Engineering Peer Committee (1995–96; vice chair, 1996–97; chair, 1997–98), National Materials and Manufacturing Board (1994–97), Committee on Application of Expert Systems to Materials Selection During Structural Design (1991–95), and Committee on Army Basic Scientific Research (1982–88).

Jan was extensively honored for his myriad contributions. In addition to his NAE election in 1982, he was elected to the National Academy of Sciences in 1992 and a fellow of the American Academy of Arts and Sciences in 1994. He became a corresponding member of the Royal Dutch Academy of Arts and Sciences in 1999, and an honorary foreign member of the National Academy of Sciences of the Republic of Korea in 2010.

In 2003 he was awarded the US National Medal of Technology and Innovation for engineering research and education in the use of ultrasonic methods, and in 2005 he received the US National Medal of Science for pioneering the field of quantitative nondestructive evaluation. Both were presented by President George W. Bush at the White House.

Other honors include the ASME Timoshenko Medal (1992), SES William Prager Medal (2001), ASCE Raymond D. Mindl Medal (2009) and Theodore von Karman Medal (2010), and ASME Medal (2012) and honorary membership (2002). He became a fellow of the AAAS, the Japan Society for the Promotion of Science, ASA, and AAM. He received honorary doctorates from Zhejiang University (2011) and Clarkson University (2017), and was recognized as an honorary professor at the Beijing Institute of Technology in 2012.

For his teaching and mentoring, he was elected to the Chicago Tribune All-Professor Team in 1993, and in 2004 he received the Tutorial Citation Award from the American Society for Nondestructive Testing. In 2014 Sigma Xi recognized him with the Monie A. Ferst Award for his “notable contributions to the motivation and encouragement of research through education.”

It was on a blind tennis date at Stanford that Jan met his future wife, Marcia Fee; they married in 1961. Both were born abroad, Jan in the Netherlands and Marcia in the Philippines, where her father held a position with Standard Oil, and both were affected by WWII, which ignited in Jan a passion for aviation and in Marcia a drive to help others. They dedicated their lives to service in the public interest—Jan as a scientist and engineer and Marcia as a social worker and community servant.

The couple was very active in the Northwestern community. As they said, Northwestern “has given us lifelong education, culture, music, travel, and other benefits.” Their combined efforts culminated in the establishment of the Jan and Marcia Achenbach Chairs at Northwestern University. They left their entire estate to the university; the planned gift will establish two endowed professorships in mechanics of materials and solids in the Department of Mechanical Engineering and the Department of Civil and Environmental Engineering.

Jan and Marcia found especially enriching their time with family and Jan’s students, many of whom were from China, Japan, and Korea. They also enjoyed traveling, the symphony, lyric opera, and theater.

Marcia passed away July 25, 2019, after 58 years of happy marriage.

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Adapted with permission from Biographical Memoirs of the National Academy of Sciences, available at www.nasonline.org/memoirs. The authors are grateful to Marcia F. Achenbach’s sisters Wendy Baynard and Judy Winslow for sharing Achenbach family stories.
¹ Achenbach JD, Chao CC. 1962. A three-parameter viscoelastic model particularly suited for dynamic problems. Journal of the Mechanics and Physics of Solids 10:245–52.
² Wave Propagation in Elastic Solids (North Holland Publishing Co./ American Elsevier, 1973); Ray Methods for Waves in Elastic Solids–with Applications to Scattering by Cracks (with Gautesen AK, McMaken H; Pitman Advanced Publishing Program, 1982); Reciprocity in Elastodynamics (Cambridge University Press, 2004).
³ A Theory of Elasticity with Microstructure for Directionally Reinforced Composites (CISM Monograph 167; Springer, 1975).
⁴ Extension of a crack by a shear wave. Zeitschrift für Angewandte Mathematik und Physik 21:887–900 (1970). 
⁵ Achenbach JD, Kim J, Lee YC. 1995. Measuring thin-film elastic constants byline-focus acoustic microscopy. In: Advances in Acoustic Microscopy, vol 1, ed Briggs A. New York: Plenum Press.
⁶ Lamb waves as thickness vibrations superimposed on a membrane carrier wave. Journal of the Acoustical Society of America 103:2283–86 (1998).
⁷ Achenbach JD, Komsky I, Andrew G, Grills B, Register J, Linkert G, Huerto G, Steinberg A, Asbaugh M, Moore D, Weber H. 1995. An ultrasonic technique to detect corrosion in DC-9 wing box: From concept to field application. Materials Evaluation 52(7):848–52.
⁸ Reciprocity and related topics in elastodynamics. Applied Mechanics Reviews 59:13–32 (2006); A new use of the elastodynamic reciprocity theorem. Mathematics and Mechanics of Solids 19:5–18 (2014).