Erastus H. Lee, professor emeritus and a prominent researcher, with
fundamental contributions to plasticity, viscoelasticity and wave
propagation, died at the age of 90 on May 17, 2006 in Lee, New
Hampshire.

Ras Lee was born on February 2, 1916, in Southport, England. He
graduated from Cambridge University in 1937 with a bachelor degree
in mechanical sciences and mathematics. After a further year of
postgraduate study at Cambridge with Professor C. E. Inglis, Ras was
awarded a fellowship from the Commonwealth Fund of New York to study
with Professor Stephen Timoshenko at Stanford University. He
completed his Ph.D. degree in 1940 and immediately thereafter became
involved in the British war efforts during World War II. He worked
first as a progress officer in the British Purchasing Commission in
New York and later in the British Air Commission in Washington.
Officer Lee was concerned with planning aircraft deliveries from
U.S. companies and keeping records of modifications required to meet
British needs. He and his wife, Shirley, returned to England during
the war, where Ras first worked at the Ordnance Board and then at
the Armaments Research Department. He was elected a Fellow of his
College, Gonville and Caius at Cambridge, in 1944, and became
Assistant Director in charge of the Technical Engineering Section of
the Production Department of the newly established British
Department of Atomic Energy in 1946.

After an offer from Professor William Prager, Lee and his family
returned to the United States in 1948, where he was a Professor of
Applied Mathematics at Brown University for 14 years (1948-1962). He
served as Chairman of the Applied Mathematics Department for five
years. During these years, faculty members in the Divisions of
Applied Mathematics and Engineering, which included W. Prager, D.C.
Drucker, H. Kolsky, E.H. Lee, A. Pipkin, P. Symonds, R.S. Rivlin,
R.T. Shield, and E. Sternberg, made Brown the worldwide center for
research in solid mechanics. In 1962, Ras was appointed as a
Professor in the Division of Applied Mechanics and the Department of
Aeronautics and Astronautics at Stanford University, joining J.N.
Goodier, W. Flugge, N.J. Hoff, and M. Hetenyi in making the
Stanford applied mechanics group a wildly acclaimed group in
mechanics research. Almost every graduate student in solid mechanics
during that time took his sequence of three courses (each two
quarters long) in nonlinear continuum mechanics, viscoelasticity,
and plasticity. He remained at Stanford for 20 years (1962-1982),
retiring at the mandatory retirement age of 65. For the last 10
years of his professional career, Lee was the Rosalind and John J.
Redfern Jr. Chair Professor of Engineering at Rensselaer Polytechnic
Institute.

In his early work, Lee made fundamental contributions to the
development of the solutions for elastic-plastic problems and the
slip-line field solutions for metal forming processes by plastic
deformation. This includes a series of papers, written in England
with R. Hill and S. Tupper, on the theory of autofrettage process,
wedge indentation in ductile metals, and compression of a block
between rough plates. This was followed by research with his
students at Brown on the stress discontinuities in plane plastic
flow, the analysis of plastic flow in deeply notched bars, and
discontinuous machining and chip formation. At Brown, Lee also made
significant contributions to the analysis of boundary value problems
in the theory of plastic wave propagation, including the
determination of the moving plastic-elastic boundaries, known as
loading and unloading waves, with particular application to normal
impact between a cylinder and rigid target at rest. Extending his
research interests to polymers, he greatly contributed to the
development of solution methods for viscoelastic stress analysis, by
reducing them to more tractable elastic problems, which is now known
as the correspondence principle. He studied the effects of residual
stresses and temperature variations on viscoelastic response (the
well-known time-temperature shift), viscoelastic contact problems,
and viscoelastic wave propagation problems. His research papers in
this field are regularly referenced in contemporary publications,
monographs, and books devoted to viscoelasticity.

Lee continued his research on inelastic wave propagation at
Stanford, by developing a finite-strain elastic-plastic theory with
application to plane-wave analysis of dynamic plate impact problems,
which culminated in his 1969 paper "Elastic-plastic deformation at
finite strain," published in the Journal of Applied Mechanics.
Through this research, he developed a framework for the constitutive
analysis of large elastic-plastic deformations based on the
multiplicative decomposition of the deformation gradient
(F=F_eF_p), now commonly referred to as Lee's decomposition. This
decomposition had a great impact on subsequent developments of
elastoplastic constitutive theories for polycrystalline materials
and single crystals. With his students at Stanford and RPI, Lee
applied the decomposition to develop the rate-type theories of
elastoplastic deformation at finite strains for both isotropic and
anisotropic materials. His other contributions to mechanics include
the studies of shock waves in elastic-plastic solids, wave
propagation in composite materials with periodic structure,
elastic-plastic stress and deformation analysis of metal-forming
processes, with the first calculations of the residual stresses, and
the modeling of anisotropic strain hardening.

Lee was elected to the National Academy of Engineering in 1975 and
was awarded the Timoshenko Medal in 1976, in recognition of his
distinguished contributions to the field of applied mechanics. He
was a Fellow of the American Academy of Mechanics, and a Life Fellow
of the American Society of Mechanical Engineers, with frequent
publications in the Journal of Applied Mechanics throughout his
career. With the contributions from his colleagues and former
students, an anniversary volume entitled "Topics in Plasticity,"
was published in 1991 by AM Press on the occasion of his 75th
birthday. He delivered invited lectures throughout the world, was a
Guggenheim Fellow in 1975, and an Alexander von Humboldt Fellow in
1986.

Ras Lee is survived by his four children and four grandchildren. He
was predeceased by his wife, Shirley, whom he met and married at
Stanford in 1940. Memorial service for Ras was held on May 21, 2006
in Lee, under the direction of Purdy Memorial Chapel. Professor Lee
was liked by his many colleagues, and admired by his students. He
shall be missed, but his mechanics legacy will remain alive.

V. A. Lubarda