Fatigue crack growth rate in miniature specimens using resonance

Abstract The method of crack growth rate measurement in miniature single edge notched (SEN) cantilever specimen at stress ratio R ∼ −1 is presented. The resonance circuit is formed by the cantilever specimen with inertial yoke at its free end excited by alternating electromagnetic field. The parameters of the harmonic excitation are controlled by the phase shift technique. The crack length is estimated from specimen compliance added by the crack opening. The stress intensity factor is computed from the applied bending moment. Linear elastic fracture mechanics with standard shape functions corresponding to the particular specimen geometry and applied loading are used. The analysis of signal from single piezoelectric accelerometer provides data for the estimation of crack length and applied stress intensity factor, making the method very simple and inexpensive, yet reasonably accurate. The crack length measurement was calibrated using fractographic marking technique. The presented method was validated by performing crack growth rate tests in miniature 4 × 3 × 32 mm beam specimens with 150° chevron notch. 7075 aluminium alloy and 4043 high strength steel were tested at load frequency of 110 and 160 Hz respectively. The crack growth curves were measured in both load increase and load reduction mode and the resulting crack compare well with the baseline data available for standard central crack (CCT) specimens.