Abstract
A new method for monitoring fatigue life under service load is proposed and examined experimentally. Through FEM strain analysis around a crack under stress, the best positions to attach strain-gages are chosen, where one gage is put parallel-wise close to a crack and the other is put away from the crack. By observing the product of two gage outputs, the crack propagation prosess is monitored, since the signal changes from positive to negative corresponding to the crack length, independent of stress amplitude. An electrical circuit is designed, and examined under fatigue tests. The circuit proved to be successful to detect a critical fatigue crack in the range from 72 to 99% of the fatigue life depending on the distance between the crack and the gage. The circuit costs about one tenth of a strain meter.
