PSI - Issue 22

C.L. Niu et al. / Procedia Structural Integrity 22 (2019) 361–368 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Table 4. Fitting results of equivalent initial cracks for undercut defects Weld number Defect size (mm)

a/t

1 2 3 4 5 6

0.27 0.34 0.39 0.42 0.47 0.50

0.026 0.037 0.050 0.064 0.087

0.094 In the table: a is equivalent initial crack length, t is specimen plate thickness.

Table 4 shows that the fatigue life of undercut specimens can be fitted to the corresponding equivalent initial crack value. Similarly, as long as the equivalent initial crack value is obtained for other welding defects through experiments, the structural fatigue life assessment and stress grade assessment with welding defects can be carried out. Based on the fatigue damage prediction results of welded frame under 13 fatigue load cases, the prediction results are corrected by considering welding defects with different undercut depths. The results show that the undercut defect significantly reduces the fatigue resistance of the product, and the fatigue cumulative damage value increases with the increase of the undercut depth. Table 5. Contrast of Stress state grade of Welding Seam of Frame and Its Change with undercut depth

Stress State Grade of Different Undercut Depth h (mm)

Weld number

Defectless Stress State Grade

0.27 low low low

0.34 low low low

0.39 low low low

0.42 low low low

0.47

0.50

1 2 3 4 5 6

low low low low

medium

medium

low low

low low

medium medium medium

medium medium medium

medium medium

medium

medium

medium

medium medium

high high

high high

high

high high From the results in Table 5, it can be seen that the stress state grade increases with the depth of undercut defect, which increases the potential safety hazards in service. 4. Conclusion 1) Based on ASME-BPVC-VIII-2:2015 and IIW-2008 standards and according to the definition of stress state grade of welded joints in EN15085-3, a quantifiable method for evaluating stress state grade is proposed, and two methods for evaluating stress state grade are programmed by using C and APDL languages. 2) The programmed evaluation method has been applied in the design of truck frame welding. Finally, it is concluded that the evaluation method based on IIW-2008 standard is more stringent. The stress factor of weld seam located in cross beam and traction rod seat is the largest, with a value of 0.881. The force state grade is medium. 3) The fatigue life assessment method of welded joints with defects is given. The fatigue life assessment and stress state grade assessment of welded structures with defects are carried out by using the equivalent initial crack substitution principle. 4) In the design stage, the method of evaluating the stress state grade of welded joints of complex structures based on ASME-BPVC-VIII-2:2015 and IIW-2008 standards proposed in this paper is of great engineering significance in the fatigue resistance design of welded joints of railway vehicles and other complex products. References [1] Zhen Lixiong ． Fatigue strength assessment method study of railway vehicle frame welded joint [D]. Southwest Jiaotong University, 2016 ． [2] Chengji Mi,Zhengqi Gu,Qingquan Yang,Duzhong Nie ． Frame fatigue life assessment of a mining dump truck based on finite element