Crack Formation and Propagation

Extract

After Griffith's explanation of the decrease in the strength of a loaded material containing a disbonded area (a crack), it took about another quarter century before “Fracture Mechanics” as an engineering discipline became established. At that time it was the catastrophic failure of the Liberty ships and the basic contributions by Irwin2 that created a high interest in the quantification of the process by which a crack will grow. A series of accidents involving bridges, pressure vessels, generator rotors, aircraft, etc., contributed greatly — not only to further research but also to the opinion that cracks are basically “bad” and should be avoided under all circumstances.

I started to become acquainted with cracks in the early 1970s. The U.S. Air. Force had recently lost a few F-lll aircraft in southeast Asia due to wing-box cracking. In reaction, the Air Force decided that the new bomber, the B1, should be designed according to fatigue and fracture criteria. Since the materials were specified mostly by metallurgical engineers, they had to become concerned with the effects, for example, of the materials' microstructures on fatigue and crack propagation. At the same time, the interest in crack formation began to evolve.

Furthermore, the capabilities for quantitative nondestructive evaluation of a structure in service increased sharply since it was realized that we have to live with cracks and that not all cracks are necessarily “bad.”