PSI - Issue 22

C.L. Niu et al. / Procedia Structural Integrity 22 (2019) 361–368

366

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Author name / Structural Integrity Procedia 00 (2019) 000 – 000

3.3 Assessment results of stress state grade Before evaluating the stress state grade, the fatigue life of the frame is evaluated by using the program developed by IIW and ASME standards. The fatigue life of six welds under the two standards meets the design requirements. On the premise of satisfying the fatigue life, the stress state grade is evaluated. 1 ） Assessment results of stress state grade based on IIW-2008 standard Using IIW-2008 standard assessment procedure, the results of the six welds are shown in Table 2. The weld stress factor between the traction rod seat and the beam is the largest. Table 2 .Analysis results of stress state grade assessment based on IIW-2008 standard

  Calculation

  Permissible

Weld number

FAT

Stress factor

Stress state grade

1 2 3 4 5 6

34.5 32.9 19.5 48.2 35.9 46.1

80 80 80 80 63 71

58.9 58.9 58.9 58.9 46.4 52.3

0.586 0.559 0.331 0.818 0.775 0.881

low low low

medium medium medium

2 ） Assessment results of stress state grade based on ASME standard According to the evaluation procedure developed before, the stress factors of the welds between the cross beam and brake hanger, and between the cross beam and traction rod base are relatively large. The stress states of the two welds are determined to be "medium", while the stress states of the other welds are "low". As shown in Table 3. Table 3. Based on ASME Standard Weld Stress State grade Weld number Stress factor Stress state grade

1 2 3 4 5 6

0.57 0.43 0.29 0.69 0.76 0.81

low low low low

medium

medium The stress state grade assessment results of IIW standard and ASME standard were compared, the analysis shows that the stress factor value evaluated by IIW-2008 standard is higher than that of the another standards. For defective welded structures, ASME presents a fatigue life assessment method based on equivalent initial crack. This method can modify the fatigue test results of specimens with welding defects and then map them to the actual structure. The equivalent initial crack of a defect is defined as the quantitative effect of the defect on the fatigue life, so the fatigue life of a structure with welding defects can be evaluated. In this paper, the fatigue life assessment and stress state assessment of welded joints with defects are carried out by taking the undercut defects which need to be strictly controlled in welding as an example. Firstly, the specimen with undercut defect is made. The material of the specimen is Q345E, 10 mm thick and 80 mm wide. There are no defects in the inner part of the weld, and the outer part is unilateral undercut defect with different depths that can be identified by naked eyes. Pulse tension fatigue tests were carried out for each group of specimens with reference to GB/T 13816.The minimum tensile force of the tensile fatigue testing machine is 5T and the testing frequency is 10Hz. The finite element model of the specimens with 8-node hexahedron element as the main element and 6-node pentahedron element as the auxiliary element is established. The number of meshes of the finite element model is 368,000 and the number of nodes is 409,308. According to the test results, based on the structural stress method in ASME standard, the master S-N curve of -2  (steel) is selected and the value of a/t is debugged. Finally, the fitting fatigue life similar to the test fatigue life is obtained. The results are shown in Table 4.