PSI - Issue 64

Yining Zhang et al. / Procedia Structural Integrity 64 (2024) 1334–1338 Y. Zhang and L. Hu/ Structural Integrity Procedia 00 (2019) 000 – 000

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4. Experiment setup The experiment scheme is listed in Table 4. Two groups of tests were designed for each UHSS, representing maximum stress σ max =200 MPa and stress level σ max / f y =50%. The following rule governs the specimens: in the example of “U/D960 - 200”, “U” indicates the un - strengthened specimen, “D” indicates the double -sided strengthened specimen, “960” indicates the UHSS Q960, and “200” indicates σ max =200 MPa.

Table 4. Experiment scheme and result. Group Specimen Steel grade

f y (MPa)

σ max (MPa)

σ max / f y (%)

N (cycles)

U960-200 D960-200 U960-449 D960-449 U890-200 D890-200 U890-427 D890-427

358202 930142 29756 39878 309849 585513 32345

1

200

22.3

Q960

898

2

449

50.0

3

200

23.4

Q890

854

4

427

50.0

34480 The experiment was conducted at a hydraulic test machine under uniaxial tensile loading, and the loading frequency was set as 10-15Hz. To record the crack propagation traces, the beach marking method was adopted. The stress ratio of reference load and mark load was 0.1 and 0.5, respectively. Here, σ max denotes the maximum stress, N r denotes cycles of the reference load, and N m denotes cycles of the mark load, which equals 0.5 N r . N r =10000 cycles for specimens with σ max =0.5 f y and N r =30000-40000 cycles for specimens with σ max =200 MPa. 5. Experiment results The experimental fatigue life is listed in Table 4. For the unstrengthened specimens, U890-200 has a smaller fatigue life than U960-200 due to lower yield strength, and U890-427 has a bigger fatigue life than U960-449 due to subjecting to lower stress amplitude. After being strengthened with CFRP, the fatigue life of Q960 and Q890 was improved by 1.34 times and 1.07 times at σ max =0.5 f y , and by 2.60 times and 1.89 times at σ max =200 MPa. The poorer strengthening effect of D890-427 may be due to the early debonding of CFRP. Comparing the two groups, it can be concluded that CFRP works more effectively under a lower stress amplitude. The common failure modes found in the strengthened specimens were CFRP delamination and rupture. For the specimen with σ max =200 MPa, the main failure mode was CFRP delamination; while for the specimen with σ max =0.5 f y , the main failure mode was a combination of delamination and fracture. 6. Conclusion In this paper, the fatigue behavior of CFRP-strengthening butt-welded UHSS Q890 and Q960 was studied. When subjected to a maximum stress of 0.5 f y , the fatigue life decreases as the steel grade increases, and the improved fatigue life of Q890 and Q960 after CFRP strengthening is 1.07 and 1.34 times, respectively; while under a maximum stress of 200 MPa, the fatigue life decreases as the steel grade decreases, and the fatigue life was improved by 1.89 and 2.60 times for Q890 and Q960, respectively. For specimens subjected to a maximum stress of 0.5 f y , the fracture mode was a mix of CFRP delamination and fracture, and for specimens subjected to a maximum stress of 200 MPa, the fracture mode was mainly CFRP delamination.