Abstract：The fatigue crack propagation threshold test of 25Cr2Ni2MoV steel is carried out. The sample size is the compact tensile specimen. The frequency of the test is 20 Hz， the waveform is the sine wave， the test environment medium is air， and the test temperature is 23 ℃. The process is tested using a continuous decrease K program. The test results show that the effective threshold values ?Keff are 5.30，5.10，3.68，3.34，2.72 and 2.43 MPa·[m] when the stress ratio R are 0.05， 0.1， 0.3， 0.5， 0.7 and 0.8， respectively. As the stress ratio increases， the effective threshold value decreases gradually， and the da/dN curve moves toward the smaller ?K direction. As the stress ratio increases， the crack closure effect is gradually reduced. When the stress ratio R≥0.7， the crack closure effect disappears， and the actual measured threshold value is the effective threshold. Using the modified McEvily-Groeger formula， the empirical equation of the effective threshold value of 25Cr2Ni2MoV steel can be obtained.
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