Modern technology requires that certain mechanical components (in air- breathing jet engines or nuclear reactors, for example) be subjected to a limited number of cycles at very high loading. Failures in these components begin at stress concentrations; however, very little is known about the effects of the constraint of the surrounding material on localized low-cycle fatigue. The objective of this research is to use a novel, realtime, interferometric strain measuring technique to begin to fill this gap. Strain will be measured over a very short gauge length at the root of stress concentrations for a variety of materials and loading conditions. The research will: evaluate the effect of constraints on the cyclic Neuber relation which is currently used in design of mechanical components; provide design guidance, on an empirical basis, which will permit a more intelligent estimate of the effects of the local stress/strain state on low-cycle fatigue; generate previously unobtainable data which will be valuable to those evaluating or developing theories for predicting localized cyclic behavior.