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The paper contains the results of experimental tests of the fatigue crackbehaviour examined for 10HNAP steel under tension and bending in the conditions ofdifferent stress ratios R. For the tests under tension the plane specimens with central,symmetric stress concentrator in the from of the slots with side notches were used, while forbending plane specimens with external unilateral V-notch as stress concentrator weretested. While testing a constant stress ratio R was kept, and accounted for -0.5, -0.3, 0, 0.5.The results of the experimental tests have been described by a nonlinear formula based onÄJ-integral range. The change of the stress ratio R from the value of - 0.5 and - 0.3, 0 to0.5 caused the fatigue crack growth rate in ÄJ-integral range for tension: 4.5.10-3 MPa.m .ÄJ . 1.78.10-1 MPa.m, and for bending: 9.10-3 MPa.m . ÄJ . 1.78.10-1 MPa.m. Theproposed formula for the description of fatigue crack growth rate, including ÄJ-integralrange, satisfactorily describes the results obtained experimentally for 10HNAP steel.