13th International Conference on Fracture June 16–21, 2013, Beijing, China probability of the Weibull-distribution function; − the exponent of the Paris-Erdogan law, n, is that value belonging the 5 % probability of Weibull-distribution function; − the Paris-Erdogan constant, C, is calculated on the basis of the correlation between C and n; − the critical value of the stress intensity factor range or fatigue fracture toughness, ΔKfc, is that value which belongs to the 5 % probability of the Weibull-distribution function. Sixth step: calculation of the parameters of the fatigue crack propagation limit curves. Simplified method was used for the calculation of the design curves, based on simple crack growth law, using the above mentioned five steps. The details of the curves can be found in the Table 4. and on Fig. 5. Table 4. Details of determined fatigue crack propagation limit curves (simple law) Grade/Mark ΔKth n C ΔKfc MPam1/2 MPam1/2 and mm/cycle MPam1/2 37C base material 10,4 2,98 8,22E-09 53 37C welded joint – (1), (2) 3,16 2,42E-09 70 DP-25156 base material – 2,02 1,68E-07 95 E420C base material 8,0 2,26 9,78E-08 92 E420C welded joint – (1), (3) 2,74 1,16E-08 101 TRIP-28670 base material – 1,84 3,06E-07 250 X80TM base material – 1,78 3,74E-07 129 X80TM welded joint – (1) 1,86 3,13E-07 – QStE690TM base material – 1,82 3,27E-07 – QStE690TM base material(4), (5) – 2,15 1,09E-07 – S960QL base material – 1,80 3,50E-07 94 XABO 1100 base material – 1,76 4,00E-07 104 (1) It can be derived from data concerning to the base metal after the evaluation of characteristic and assessment of magnitude of residual stresses. (2) Average value of 16 tests under compressive residual stress: ΔKth = 16,9 MPam1/2. (3) Average value of 4 tests under compressive residual stress: ΔKth = 16,3 MPam1/2. (4) Under mixed mode I + II (tension and shear) loading condition. (5) ΔK should be replaced by ΔKeff. 4. Discussion For the investigated steels and their welded joints both the tendency of threshold stress intensity factor range ( ΔKth) and the tendency of the Paris-Erdogan exponent (n) decrease with the increase of the strength of material; while the tendency of fatigue fracture toughness ( ΔKfc) has not unambiguous dependence on the strength of material. For the investigated steels both the Paris-Erdogan exponent (n) and the fatigue fracture toughness ( ΔKfc) for welded joints are higher than those of base materials. The proposed method is suitable for determination of fatigue crack propagation design curves under mixed mode I + II (tension and shear) loading condition, too. For this case stress intensity factor range ( ΔK) should be replaced by effective stress intensity factor range ( ΔKeff). The design curves of welded joints in the near threshold region are open. On the one hand, if the threshold stress intensity factor range value ( ΔKth) is not known, values can be found in the -6-
RkJQdWJsaXNoZXIy MjM0NDE=