Issue34

S. Kikuchi et alii, Frattura ed Integrità Strutturale, 34 (2015) 261 - 270; DOI: 10.3221/IGF-ESIS.34.28 267 Figure 9 : A xonometric drawing of fracture surfaces of the (a) IP and (b) Harmonic series ( R = 0.1). Figure 10 : Inverse pole figure (IPF) maps of the crack profiles in the Harmonic series ((a) R = 0.1 and (b) R = 0.5). Estimation of the threshold stress intensity range of the material with harmonic structure Based on these results obtained in the present study, one might expect that a fine-grained structure in the harmonic structure dominates the fatigue crack propagation of the Harmonic series. To quantify the effects of fine-grained structure on the fatigue crack propagation of Ti-6Al-4V alloy, the threshold values of  K obtained from the bulk homogeneous materials are re-plotted against the square root of the grain size, d in Fig. 11. For each R value, the value of  K th increased with square root of the grain size. The relation between  K th and d in the bulk homogeneous material are expressed as Eqs. (3) and (4), respectively.  K th = 2.86 + 0.578 d 1/2 : R = 0.1 (3)  K th = 2.61 + 0.259 d 1/2 : R = 0.5 (4) 100 200 300 400 500 600 200 400 600 [  m] 150 100 50 0 50 100 150 (a) 100 200 300 400 500 600 200 400 600 [  m] 150 100 50 0 50 100 150 (b) 50 μm Crack tip (a) 20 μm Coarse-grained structure Coarse-grained structure (b) {0001} {0110} {1230} RD TD Propagation Analyzed