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.”

The recent revolution in high temperature superconducting materials has generated a wave of intense excitement and activity that has swept through the scientific community, attracting the attention of the news media and general public as well. The reason for this is twofold: the unexpected occurrence of superconductivity at such high temperatures is of immense scientific interest, and the new high temperature oxide superconductors may have important technological applications.

Based on a large amount of experimental information and (presumed!) theoretical understanding, the prevailing view prior to 1986, when high temperature superconductivity in oxides was discovered, was that the maximum value of the superconducting transition temperature Tc of any material would not increase much above ˜23 K, the high Tc record held since 1973 by the A15 compound Nb3Ge. In fact, between 1911 (the year H. Kammerlingh Onnes discovered superconductivity) and 1986, Tc only increased at an average rate of ˜0.25 K per year. However, within the last two years the maximum Tc value of the new copper oxide super-conductors has risen at an average rate of ˜50 K per year to its present value of ˜125 K! Thus, superconductivity near or above room temperature no longer seems out of the question, as it did a few short years ago! Moreover, the oxides were generally regarded as the least likely candidates for high Tc superconductivity due to their low concentrations of charge carriers. An understanding of the origin and nature of high Tc superconductivity in the new oxide compounds constitutes one of the most important and challenging scientific problems that has emerged in recent years.