Resonant fatigue testing of riveted joints

A special test facility was constructed for evaluating the fatigue strength of turbine-blade components under steady pullout and vibratory bending loadings. The use of mechanical resonance for these applications was particularly attractive in that failure in inaccessible areas could be detected by the change in natural frequency of the specimen. The sensitivity of the method allowed determination of a fatigue crack as early in life as possible. The application of this facility to the determination of the fatigue strength of a type of riveted connection employed on certain types of blades is described in this paper. A steady load representing the centrifugal force was applied hydraulically through a long flexible pullrod which in combination with the test specimen was excited laterally at its resonant frequency by an electromagnetic shaker. An accelerometer provided closed-loop control on both vibration amplitude and resonant frequency through special circuitry. Fatigue-strength reduction factors at 10⁷ cycles were determined for three types of riveted test specimens in air at room temperature. These values exceeded the elastic stress-concentration factors estimated from earlier photoelastic tests for all three cases.