Abstract
Interferometric strain-displacement gauge(I.S.D.G.) testing system was developed for the measurement of the crack opening displacement at high temperatures. Helium-Neon gas laser and center-cracked specimens of type SUS304 austenitic stainless steel were used. Fatigue crack growth tests were conducted at 450, 550, 650 and 700°C at stress ratios of R=-0.1 and 0.5. Using I.S.D.G.for the measurement of crack closure modified using platinum plate and optical filter, crack opening displacement was found to be measured with good accuracy at all temperatures tested. Results showed that crack closure was observed at R=-0.1, while no crack closure was observed at R=0.5 even at 650°C. From the results of the measurement of fatigue crack closure in the near-threshold regime at all temperatures investigated, it was found that rapid increase and decrease in crack growth rate da/dN were observed when crack opening ratio U showed rapid increase and decrease, respectively.
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