On August 6, 1985 Prof. Dr. Hans Ziegler from Zurich, Switzerland, died at the age of 75 in Denver, Colorado.

He was born on the 5th of September 1910 in Winterthur, Switzerland. From 1930 to 1936 he studied mechanical engineering and physics at the famous “Swiss Federal Institute of Technology” (ETH). In 1938 he received the degree of Doctor of Mathematical Sciences while working in the field of applied mechanics under Prof. Dr. E. Meissner of Switzerland and Prof. Dr. R. Grammel of Germany. In 1942 he was appointed Full Professor of Mechanics at the ETH and remained in this position until his retirement in 1977.

Hans Ziegler was a perfect teacher and an extremely talented scientist of world renown. From 1948 to 1956 he published a number of papers in the area of structural and dynamical stability, which have now become classics in this field together with his book “Principles of Structural Stability”, Blaisdell 1968 and Birkhäuser 1977.

These contributions had a decisive influence on the development of basic concepts and applications in stability theory during the last thirty years.

From 1956 to 1957 Prof. Dr. Ziegler was a Visiting Professor at Brown University, Providence. There he became acquainted more closely with Professor William Prager and the field of plasticity. This influence had a very productive effect on his numerous subsequent scientific contributions. For instance, many experts in plasticity use today one of his results under the name of “Prager-Ziegler's hardening rule”. Besides plasticity, Hans Ziegler also devoted himself during the sixties and seventies to irreversible thermodynamics and especially to the nonlinear generalization of Onsager's reciprocity relations, thereby establishing an orthogonality principle as a criterion to derive nonlinear constitutive relations for solids and fluids. From 1963 to 1964 he was Jerome Clarke Hunsaker Professor at MIT, Cambridge, USA and from 1970 to 1971 Distinguished Visiting Professor at Ohio State University, Columbus, USA. After his retirement in 1977 Hans Ziegler spent his summers in his second home in Glen Haven, Colorado and continued to do research in his various fields of interest.

Hans Ziegler was a great scientist, a great man. The high ethical principles and ideals to which he devoted all his life as a scientist, as a teacher, and as a human being have become a leading model for all his students, colleagues, and friends.

Mahir Sayir