PURPOSE:To obtain an accurate curve, by substituting the amount of shift into the first term Aa of a Saxena's expression, which obtains the length of a crack, when the amount of crack growth DELTAa at the starting point of the R curve, which is obtained by a load removing compliance method, is shifted by DELTAx from the zero value, and correcting the value. CONSTITUTION:A predicted fatigue crack length is set in an operation controller 6. A repeating load is applied to a compact test (CT) piece 1. The load is detected by a load cell 3. The compliance is obtained by an X-Y recorder 4 from a load waveform. The compliance is substituted in a Saxena's expression an= Aa+Baf1(Ks.Cn)+..., where (an) is the length of a crack, n=1,2,3..., Cn is compliance, Aa and Ba are coefficients and Ks is a material constant. Thus the length of a predicted fatigue crack 1b is computed. When the amount of crack growth DELTAa at the starting point of the R curve, which is obtained by a load removing compliance method, is shifted from the zero value, the amount of shift DELTAx is added to or subtracted from the first term Aa in the expression, and the expression is corrected. Thus the accurate R curve is obtained in a short time, and the measuring accuracy can be improved.
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