Issue 41

M. Sakane et alii, Frattura ed Integrità Strutturale, 41 (2017) 16-23; DOI: 10.3221/IGF-ESIS41.03 18 Figure 2 : Cracking modes in tension-torsion LCF of SUS304 stainless steel at 923K. Table 1 : Principal strain ratio at which crack changes propagation plane. Note that the above discussion does not hold for all materials listed in the table. Cracks of Inconel 738LC conventional cast superalloy remain on the principal strain plane in all principal strain ratios. Tab. 1 mainly summarizes the crack transition in elevated temperature LCF so that oxidation may bring some influence on cracking direction [2,3] but this paper is not discuss the oxidation effect in more detail because there are conflicting reports on the oxidation effect on cracking direction [2–4]. Material anisotropy also has some influence on the cracking direction as seen in the literature [10]. Cracking direction is a physical background for developing a multiaxial LCF damage model and therefore studying the relationship between cracking direction and fatigue life is meaningful. The relationship of the three materials is shown in Fig.3 [7]. The ordinate of the figure is the life ratio obtained by dividing the multiaxial LCF life by the uniaxial LCF life in a test with the same von Mises strain amplitude. The dashed lines indicate a factor of two band from unity value of the ordinate. As stated earlier, Inconel 738LC showed principal cracking in all principal strain ratios and most of all fatigue lives fall into the factor of two band. Fatigue lives of SUS 304 and 1Cr-1Mo-1/4V steels with principal cracking also fall Materials Temperature, K  =  3 /  1 Ref. 1Cr-1Mo-1/4V 838  0.79 [3] 1Cr-1Mo-1/4V* 293  0.74 [3] 1Cr-1Mo-1/4V** 823  0.74   0.70 [4] SUS 304 923  0.74 [2] SUS 304 823  0.64 [5] AISI 316 673  0.74 [3] Mild steel R.T.  0.74 [6] Inconel 738LC 1123 N. T. [7] Rene’80H DS 1173  0.70 [8] Mar-M247LC DS 1173  0.70 [9] *    1.2 %, **    1.2 %, N.T. No Transition (Shear crack), DS Directionally solidified superalloy