Low-cycle fatigue-crack indications by strain gages operating in elastic strain fields

Abstract

This paper describes the response of bonded electrical-resistance strain gages to low-cycle fatigue cracking of four, cyclically pressurized, boxlike structures. The strain-pressure “signatures” of gages installed on flexural parts of the structures were recorded semicontinuously. By observing the changes in signatures of certain gages, it was possible to follow indirectly the initiation and propagation of internal low-cycle fatigue cracks. The described technique appears feasible as a crack-detection system, or as an adjunct to some other type of damage-warning system. It is suggested that the sensitivity and accuracy of this technique make it possible to substitute “crack initiation” for “through cracking” as a criterion for fatigue failure in design.