Near-threshold fatigue crack growth behavior of SUS304 steel at high temperatures using interferometric strain/displacement gage (1st report, crack closure behavior)

Interferometric strain-displacement gauge (I.S.D.G.) testing system was dev eloped for the measurement of the crack opening displacement at high temper atures. Helium-Neon gas laser and center-cracked specimens of type SUS304 a ustenitic stainless steel were used. Fatigue crack growth tests were conduc ted at 450, 550, 650 and 700 degrees C at stress ratios of R = -0.1 and 0.5 . Using I.S.D.G. for the measurement of crack closure modified using platin um plate and optical filter, crack opening displacement was found to be mea sured with good accuracy at all temperatures tested. Results showed that cr ack closure was observed at R = -0.1, while no crack closure was observed a t R = 0.5 even at 650 degrees C, From the results of the measurement of fat igue crack closure in the near-threshold regime at all temperatures investi gated, it was found that rapid increase and decrease in crack growth rate d a/dN were observed when crack opening ratio U showed rapid increase and dec rease, respectively.