13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- The C(T) test results for all R-ratios are presented in Figure 3: (a) da/dN-KApp (before ACR correction); and (b) da/dN-KACR (after ACR correction). The comparison shows that the ACR method reduced the data scatter especially the “fanning” effect at the near-threshold region, and gave lower Kth values by removing the closure and residual stress effects. The scatter reduction is in the low stress ratios (R0.05-0.45) than in the high stress ratios (R0.6-0.8). It is also noted that some ACR corrections are very small possibly due to the positive residual stress effects. 2.2. Small crack tests using SENT coupons A SENT coupon, as shown in Figure 4-(a), was used for fatigue tests with naturally nucleating small surface cracks. A test matrix, shown in Figure 4-(b), was designed using a simple center composite circumscribe (CCC) design of experiment (DOE) method aiming to provide a complete set of data under six stress ratios (R) and five stress levels. The test loading spectra were constant amplitude loading with marker band (MB) loading sequences. The silicon-based replication technique (RepliSetTM) was used to measure the surface crack length, and marker bands (MB) on the fracture surfaces were reconstructed to determine the crack depth. Group No. Replicate Applied Max. Stress (ksi) Applied Min. Stress (ksi) Stress Ratio (R) 1 6 23.00 1.15 0.05 2 3 20.00 -6.00 -0.30 3 3 26.00 -7.80 -0.30 4 3 20.00 10.00 0.50 5 3 26.00 13.00 0.50 6 3 18.76 0.94 0.05 7 3 27.24 1.36 0.05 8 3 23.00 -23.00 -1.00 9 3 23.00 18.40 0.80 (a) SENT coupon (dimensions in inches, 1in=25.4mm) (b) SENT test matrix (1ksi=6.89MPa) Figure 4. SENT coupon and test matrix ‐100 900 1900 2900 3900 4900 5900 6900 ‐100 900 1900 2900 3900 4900 5900 6900 µm µm Outline 10 MB 4 MB 6 MB ORIGIN ‐100 900 1900 2900 3900 4900 5900 6900 ‐100 900 1900 2900 3900 4900 5900 6900 µm µm J407‐1D‐5 Outline 10 MB 4 MB 6 MB Origins c1 c2 (a) single crack origin (pores) (b) multiple crack origins (pores/particles) Figure 5. Typical SENT coupon marker bands and nucleation sites The smallest replica crack detection was about 12 m, and the smallest MB measured crack was about 20 m. The MB crack detection resolution varied with stress levels and stress ratios, but
RkJQdWJsaXNoZXIy MjM0NDE=