13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- KImax = 0.65σ0 area π (7) Thus, considering the mechanical equivalence between small crack and defect as discussed in the previous section, KTH for surface defects associated with KImax is expressed by KTH = 0.65σf area π (8) Figure 15 shows the KTH values both for the artificial surface defects and nonmetallic inclusions as a function of the defect size area [12]. The data are shown in a double logarithmic diagram, as has widely been used for the arrangement of ΔKth data of the small fatigue cracks [7, 8, 10, 16]. It is well known that in fatigue crack the threshold stress intensity factor range ΔKth has the following relation with the defect size in the small crack regime, e.g. area < 1 mm. ΔKth ( area ) 1/3 (9) Also in the present results for KTH, we can see a definite size dependence in the range between about 10 and 500 μm in Fig. 15, i.e. KTH was reduced with a decrease in area . However, the exact dependence of the threshold value on the defect size is not clear at present since the threshold value can be varied also with the hydrogen concentration at the location of fracture origin, as will be discussed in the next section. As a reference data, in Fig. 15, the shaded area shows the Moody et al.’s KTH data for a large crack in the standard CT and WOL specimens of AISI4340 steel with the yield strength σYS of 1500 ~ 1900 MPa, which was determined from tests in hydrogen gas, NaCl solutions and H-precharged specimens [4]. The values of KTH for both the nonmetallic inclusions and artificial defects were definitely smaller than those for the large cracks, though the effect of some influencing factors, e.g. material type, strength level, hydrogen content, strain rate etc., should further be clarified in the future. Figure 14. Comparison of stress field ahead of tip of large and small cracks under a stress intensity factor KI = 10 MPa m 2a = 0.02 mm 2a = 20 mm yy (MPa) Distance from crack tip, r (m) Large crack, 2a = 20 mm, 0 = 56 MPa Small crack, 2a = 0.02 mm, 0 = 1784 MPa x y Remote stress 0 a a 10 20 30 40 50 1000 2000 3000 4000 5000 Long crack (Moody et al. [4]) (m) Fracture from artificial defects SAE52100 HV=712 1 10 100 10 100 1000 10000 Symbol Defect type (m) CH, R (ppm) Type A(Rectangularslit) (Fracture origin: Nonmetallic inclusion) 35.0 (Inclusion*: 12.4) 3.03 Type B (Semi-circular slit + FIBslit) 93.0 3.12 186 3.20 463 3.20 Type C (Drill Hole + FIB slit) 116 3.39 196 3.95 Type D (Electro-discharged hole + FIB slit) 513 3.77 Fracture from non-metallic inclusions Figure 15. Relationship between KTH and area for both short and long cracks in SAE52100. : The specimen had the artificial defect, but was fractured from a non-metallic inclusion [12]
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