13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- 2.1. Test specimens da/dN tests were conducted for compact specimen [C(T)] with different HPS 485W thickness under load ratio of R=0.1, 0.5 and 0.8. The prefabricated crack for all the specimen is oriented perpendicular to the rolling direction of the steel plate (L-T). The dimension and specimen are shown in Figure 1 and Table 1. 1.2W B 1.25W 1.2W an (a) Dimension of C(T) specimen (b) Fabricated C(T) specimens Figure 1. Dimension of C(T) specimen Table 1. Specimens dimensions (mm) B 1.25W 1.2W an 7.5 50 48 9 12.5 75 72 15 19.5 125 120 23 The loading frequency during test is 15 Hz, considering low frequency of the alternating cycle load in practical traffic transportation. Fatigue crack precracking is conducted before test to provide a sharpened fatigue crack. During the test, the electro-hydraulic servo-controlled fatigue testing machine was used and the crack growth length is read by the microscope with min scale of 0.01mm. The load cycle number and corresponding crack length are recorded at certain intervals from near-threshold to fatigue crack propagate unstably. 2.2. Test result After the test, the stress-intensity factor range ΔKi under each crack length is calculated, according to Eq.1. In Eq. 1, α=a/W错误!未找到引用源。, and a/W≥0.2. 3/2 2 3 4 (2 ) / (1 ) (0.886 4.64 13.32 14.72 5.6 ) P K B W α α α α α α Δ + Δ = ⋅ − + − + − (1) Using Paris equation to fit tested ΔKi and (da/dN)i data. The parameters are obtained including material constant m and C, the standard deviation S, and correlation coefficient. The da/dN test result for HPS 485W is shown in Table 2 and Figure 2. The fracture surface of da/dN specimen presents fibrous, and grey or dark grey. Figure 3 shows fracture surface for one of test specimens. The fatigue fracture is composed with crack propagation phase and ultimately quickly failure phase, corresponding to smoothing region and intense coarse region. The fracture surfaces of specimens, shown in Figure 3, are characterized with secondary
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