PSI - Issue 51

Abdelhak Nehila et al. / Procedia Structural Integrity 51 (2023) 152–159 A.Nehila and W. Li / Structural Integrity Procedia 00 (2022) 000–000

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2.2. Fatigue testing method The axial loading test of notched-specimens was carried out by using high electromagnetic resonant fatigue testing machine with a loading frequency is about 100Hz. The stress ratio is R = ‒1 and the testing temperature is the room temperature, 25°C. The applied stress σ a ranges from 200 MPa to 500 MPa and the resulted fatigue life ranges between 10 4 -10 8 cycles. After experiment, the fracture surfaces were carefully observed by using the optical microscope (OM) and the scanning electron microscopy (SEM). Then, the crack origins and the failure modes were investigated. After that, figures of fracture surfaces were processed in order to measure the different defect sizes. 3. Results 3.1. S-N diagram The S-N characteristics of the notch-fatigue tests were given in Fig. 2. The run-out specimen with the stress amplitude of 200 MPa was stopped when the fatigue life reached 10 9 cycles. When the stress amplitude, σ a , ranges from 500 MPa to 300 MPa, the fatigue life decreases rapidly, then tends to decrease slower at the range of 300-200 MPa to finally reach an inflection point at around 200 MPa and stabilizes at this level. According to the conventional fatigue theory, for a value of the elastic stress concentration factor, K t , equals to 1.89, the notched specimen of the carburized 17CrNi steel admits a fatigue limit at the stress amplitude σ a = 200 MPa and it’s considered as the fatigue limit ( σ w ) for this steel. Therefore, the fitted curve was established using the equation given as: 9 1 2.34 3.58 10 ( ) 200 a N σ × = + （ 1 ）

600

Surface failure Run out

500

200 Stress amplitude σ a , MPa 300 400

9 / N ) (1/2.34) ×

σ a = 200+ (3.58 10

100

10 2

10 3

10 4

10 5

10 6

10 7

10 8

10 9

Number of cycles to failure N f , cycles

Fig. 2. S-N curve of 17CrNi notch specimen under axial loading R = ‒1.

3.2. Observation of fracture surfaces the primary observation of fracture surface reveals that the final fracture of all the notched specimens was induced from surface unlike the smooth specimens (Nehila et al. (2018)). This is due to the higher stress concentration near the root of the notch. A propagating crack was found on each surface, as shown in Figs. 3(a,b). The fatigue failure in the case of notch specimens is very sensitive to the surface state around the notch tip and the crack generally initiates from surface defects such as cavities and scratches. The crack initiate in the vicinity of the defect on the surface and propagates in a stable manner. When the size of the crack reaches a certain critical value, the propagation starts to be more instable to finally leading to fracture. Furthermore, it can be seen that both the stable