PSI - Issue 54

Mihaela Iordachescu et al. / Procedia Structural Integrity 54 (2024) 52–58 Mihaela Iordachescu et. al / Structural Integrity Procedia 00 (2023) 000–000

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The subsequent observation of the fracture surfaces in the SEM allowed the fatigue and stress corrosion cracks to be measured. In three of the SSRT-FIP tests, the crack growth was estimated from the high-resolution digital images used by the video-extensometer to measure the lateral COD.

Fig. 2. a) Load – COD plot and fracture surface of a fatigue pre-cracked SENT specimen subjected to SSRT-FIP, unloading and final fracture in air; b) Crack size and stress intensity factor vs. COD in the SSRT-FIP stage of the same test. 3. Experimental results Fig. 2a shows the load-crack opening displacement (COD) curve obtained in the interrupted SSRT-FIP test of a notched, fatigue pre-cracked specimen, including the unloading and the subsequent reloading until breaking in air. According to Eq [1] (Tada et al., 2000), the slopes of the straight lines corresponding to the initial loading and the interruption unloading provide the crack depth a of the specimen throughout these test stages:

1 . 46 + 3 . 42 1 − cos π ! a 2 ( ) cos π ! a 2 ( ) 2

(1)

F ′ E B

COD = 4 ! a

where ã = a/W is the relative crack depth and E´ is the generalized elastic modulus for plane stress or strain. The crack depth values obtained from the slopes fully coincide with the measurements made on the fracture surface (Fig. 2a), once averaged along the cracking front. According to ASTM E399 (2022), for the SENT specimens used in the present research, the crack tip plastic zone allows the stress concentration at the crack front to be dominated by the singularity of the linear-elastic stress field as the stress intensity factor K does not exceed the limit value K Issy :

K ≤ K Issy = R p 0 . 2 0 . 4 B

(2)

whose value for the yield strength of the steel and the thickness of the specimens is 31 MPa·m ½ . Since in SSY regime, the COD, the load F and the crack depth a verify Eq (1), the measured values of COD and F also verify it and determine the crack depth of the specimen along the test provided that he stress intensity factor K is under the limit value of 31 MPa·m ½ . For the specimen in Fig. 1b, the stress intensity factor K is (Tada et al., 2000):

0 . 752 + 2 . 02 ! a + 0 ,. 371 − sen π ! a 2 ( ) 3 cos π ! a 2

tg π ! a 2

2 W

K = F B

(3)