Fatigue Test of Thin Plate for Welded Bellows.

Abstract

It is commonly difficult to carry out a fatigue test of a very thin plate of approximately 0.1 mm to 0.3 mm thickness by means of a conventional test apparatus. In order to test the thin plate for welded bellows, the authors developed a new fatigue test system utilizing the resonance vibrations of the test plate. The system is composed of a speaker, an amplifier, a function generator, a laser displacement meter and a personal computer. One end of the test plate is fixed like a cantilever on the cone of the speaker and vibrates at a designated frequency. First, the characteristics of this test apparatus were examined. As materials of the test plate, Ni base alloy A and Ni base alloy B for welded bellows were used. The calculated bending stress based on the deflection curve of the cantilever with notch was in close agreement with the measured stress by the strain gage. Further-more, it was confirmed that the displacement amplitude of the test plate decreases with the crack propagation and the occurrence of fatigue crack is detected accurately during the test. The results were arranged in the S-N curves, where S is the stress amplitude calculated using the measured displacement amplitude and N is the number of cycles to surface crack judged by decrease in the displacement amplitude. Fairly different S-N curves were obtained between Ni base alloy plates A and B, but these curves had the same tendency as the results of the repeated displacement test of the practical welded bellows.