13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- samples exhibited a slightly lower fatigue life than that of the aging samples at lower strain amplitudes due to the coarser α phase. Based on Basquin equation and Coffin-Manson relation, the total strain amplitude could be expressed as elastic strain amplitude and plastic strain amplitude [15], i.e., c f f b f f p e t N E N 2 2 2 2 2 , (1) where E is the Young’s modulus, f N is the fatigue life or number of cycles to failure (the entirety of 2 f N is the number of reversals to failure), f is the fatigue strength coefficient, b is the fatigue strength exponent, f is the fatigue ductility coefficient, and c is the fatigue ductility exponent. In addition, cyclic deformation behavior is normally considered to be related to the portion of the plastic strain amplitude and is independent of the elastic strain amplitude, which could be expressed by the following equation [15], n p K 2 2 , (2) where 2 is the mid-life stress amplitude, 2 p is the mid-life plastic strain amplitude, n' is the cyclic strain-hardening exponent and K' is the cyclic strength coefficient. The obtained fatigue life parameters evaluated on the basis of Equs (1) and (2) were summarized in Table 3. It is also observed that the cyclic strain hardening exponent (n′), the cyclic strength coefficient (K′), fatigue strength coefficient ( f ), and cyclic yield strength ( y ) of the post-welded joint after aging were nearly the same as that of STA. In general, a smaller absolute value of fatigue strength exponent (b) and fatigue ductility exponent (c) and a larger value of fatigue strength coefficient ( f ) and fatigue ductility coefficient ( f ) represent a longer fatigue life. This implies that a longer fatigue life of a material in the strain-controlled tests requires a good combination of both high strength and superior ductility. In spite of such a seemingly conflicting effect of the exponent pair (b and c) and the coefficient pair ( f and f ) on the fatigue life, the exponent pair would be expected to play a greater role in the sense of exponential functions. In comparison with STA, the absolute value of c in the aging condition was smaller (while b was nearly the same), and the value of f was also smaller (while f remained nearly the same). The combined role of these fatigue life parameters gave no substantial difference between the aging and STA conditions, as also seen in Fig.7. It should be noted that in evaluating the above fatigue life parameters the strain amplitude was limited to a range in-between 0.4% and 1.2%, excluding the strain level with run-out data at which some fatigue samples did not fail at or above 107 cycles. 3.6 Fractography The fracture location of both types of post-welded joints mostly lay in the Ti-6Al-4V BM. Only one sample in the STA condition failed in the HAZ at Ti-6Al-4V side, which might be due to the formation of the coarse α in the HAZ (Fig.3). Fracture surfaces of the fatigued specimens were examined using SEM. Fig.8(a) and Fig.9(a) show an overall view of fracture surfaces of the post-welded joints in the aging and STA conditions at a total strain amplitude of 0.4%, containing regions of fatigue crack initiation, propagation, and final fast fracture. It is seen from these low magnification images that fatigue crack initiated from the specimen surface or near-surface defect, and the river line patterns appeared in both aging and STA conditions which were irregular and
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