Modern technology places great demands on structural material performance. Until the 1950s, structures were invariably designed according to safe-life principles, for which a crack-free service life was required and assumed. However, the tendency to much longer utilisation of aircraft, and latterly the introduction of high strength materials, have focused attention on damage tolerant design in which allowance must be made for possible cracking and its certain detection before catastrophic failure. Especially for high strength materials is this change in philosophy necessary, because the improved static strengths (and hence potentially increased structural efficiency) rarely result in comparable improvements in fatigue properties, and are usually accompanied by a decline in fracture toughness and in greater susceptibility to environment-induced cracking.