PSI - Issue 33

Julio A. Ruiz Vilchez et al. / Procedia Structural Integrity 33 (2021) 658–664 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 7. Fracture surfaces for two testing specimens: 389 MPa (a), and 585 MPa (b). From Figure 7, it can be observed that the circular zone decreases, as the applied load increases; this behaviour may be attributed to the fact that at lower load levels the slow crack propagation is amplified and covers larger area of the total crack propagation area. Comparative ultrasonic fatigue results obtained by Ishii et al. (2002), under similar conditions, reveals that results obtained in this study present higher fatigue endurance, particularly for the high applied stress (585 MPa). These results may be related to two factors: a) the difference in chemical composition between the two studied materials, and, b) the intermittent ultrasonic fatigue testing modality used in Ishii reference. In order to estimate the number of cycles in the “Stable Zone of Crack Growth”, under the described loading conditions, the Paris Gomez-Anderson equation was used: �� � � ��� � � � � � � � ������ � � � �√��� � � ��� � � � ��� � � (1) Where: N II is the number of cycles in this stable zone. Taking Y( a ) the geometry factor which is assumed constant during crack propagation: Y( a ) ≈ Y( a 0 ) = 1.12. An approximation of  K is:  K ≈  K 0 ( a / a 0 ) 1/2 ; taking the expression:  K 0 = Y( a 0 ) Δσ (π a 0 ) 1/2 , and a 0 = 7  m, a end = 1200  m (Figure 7a, for low applied load), equation 1 becomes: �� � �� � ��������� � � � � �� � � � � � ��� � � � �� � (2)  K 0 ≈ K TH ≈ 2.2 MPa (m) 0.5 (K TH =  K TH /2 for R = -1), Spriestersbach et al. (2014), n and C are the constants for the Paris Law: n = 5 and C = 4.5 x 10 -15 . The number of cycles calculated in the “Stable Zone” is close to : 20 million of cycles, which represent only ≈ 2.5 % of total fatigue life under low applied load (389 MPa and 700 – 900 million of cycles, Figure 5). Conclusions The following conclusions can be draw from the present study:  Ultrasonic fatigue endurance has been obtained in maraging 300 steel, at room temperature, without control of environmental humidity and with zero mean stress (R = -1),