George Rankin Irwin (February 26, 1907 – October 9, 1998) was an American scientist in the field of fracture mechanics and strength of materials. He was internationally known for his study of fracture of materials. Early life and education George R. Irwin was born in El Paso, Texas. His family moved to Springfield, Illinois where he went to school. He attended Knox College in Galesburg, Illinois and earned an A.B. degree in English in 1930. After an additional year studying physics, he transferred to the University of Illinois at Urbana-Champaign where he studied from 1931 to 1935. He received his Ph.D. from the University of Illinois in 1937; his thesis was on the mass ratio of lithium isotopes. Career In 1937 he joined the US Naval Research Laboratory (NRL) in Washington D.C. where he worked until 1967. There he worked on ballistics, specifically on the mechanics of projectiles penetrating targets. Here he developed methods for determining the penetration force that a projectile exerts on its target. This work was completed throughout the Second World War. Part of this work led to the development of several nonmetallic armors (see vehicle armor). This coupled with his observation that thick armor plate made from ductile material (such as steel) failed in a brittle manner during test firings initiated his interest in brittle fracture. In 1946 he was made responsible for the project on brittle fracture at the NRL and in 1948 he was promoted from the head of the Ballistics Branch of the NRL to associate superintendent of the Mechanics Division. In 1950 he was again promoted to superintendent of the Mechanics Division. He served in that capacity until his retirement from government service in 1967. The classical approach to brittle fracture in the late 1940s had been developed in the early 1920s, following the work of A. A. Griffith. Griffith had shown that an instability criterion could be derived for cracks in brittle materials based on the variation of potential energy of the structure as the crack grew. The Griffith approach was global and could not easily be extended to accommodate structures with finite geometries subjected to various types of loadings. The theory was considered to apply only to a limited class of extremely brittle materials, such as glasses or ceramics. Irwin observed that the fracture process in metals involved nonelastic work at the crack tip. This observation permitted him to modify