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 the
Griffith theory by incorporating a plastic work of fracture in addition to the classical surface energy of
crack formation. As part of this work, Irwin defined the fundamental concept of a
Stress Intensity Factor and the critical plane-strain stress intensity factor (K_{IC}) which is a
material property. He was involved in the development of several standards and led several committees for the
American Society for Testing and Materials (ASTM). In 1967, Irwin was recruited to
Lehigh University as the Boeing University Professor by long-time collaborator
Paul C. Paris, the father of modern methods for predicting crack growth and its control in aircraft structures. Irwin served for five years before reaching mandatory retirement age. In his tenure, he continued his collaboration with Paris, and collaborated with, influenced, or assisted many notable individuals in the fracture mechanics community, including: • F. Erdogan on cracks in thin-walled shell structures; • A. A. Wells of the British Welding Institute on characterizing fracture in normally ductile steel structures; • F. A. McClintock,
Massachusetts Institute of Technology and
John W. Hutchinson,
Harvard University, on the development of fracture mechanics procedures in the presence of substantial ductility; •
James R. Rice, Harvard University, on developing the
J integral approach for characterizing the onset of crack growth in ductile materials; • L. B. Freund,
Brown University, and M. F. Kanninen,
Southwest Research Institute, on the dynamics of inertial limited crack propagation and arrest. In 1972, Irwin joined the faculty of the
University of Maryland, College Park where he worked in the field of
dynamic fracture, specifically concerned with crack arrest and the implications in a
loss-of-coolant accident in a
nuclear power plant. He contributed to The Stress Analysis of Cracks Handbook. ==Memberships and honours==