## 6 Conclusions  
This paper firstly aimed to analyze the fatigue failure of welded A-type frame based on experiment works and simulation method. The monotonic tensile properties for the weld metal and base metal were both studied and found to be very similar. The fatigue test of the welded joints was implemented to obtain cyclic properties used for the lifetime estimation. The multi-body dynamic model was built to get load history for the welded A-type frame, and its accuracy was validated with the experimental results. The finite element model of this frame was generated to observe the stress distribution, and its accuracy was also verified by comparing the test results with the simulated stresses. The fatigue life of welded A-type frame was predicted and found to be far less than the design requirement, while its predicted failure position was in good agreement with experimental observations.  
Then, considering the multi-uncertainties in geometry dimension, material properties, and bearing load, a co-optimization method for increasing lifetime and lightweight was presented, based on the Kriging approximation model. The multi-objectives optimization function was built. The 50 sample points were obtained by Latin hypercube sampling method, and their responses were calculated by the elastic/plastic finite element analysis. The constructed Kriging approximation model was verified with other 10 sample points, and its accuracy was found to be satisfactory. The NSGA-II was utilized to optimize this multi-objective optimization problem. The distance of weld metal away from the back plates and the thickness of front plates and four absolute cyclic material parameters increased, while the thickness of back plates and four loads at connection sites decreased. Finally, the lifetime of this welded A-type frame increased to 2.40e5 cycles and its weight decreased by 8.2%.  
Conceptualization, C.M. and F.B.; Methodology, C.M.; Software, W.L.; Validation, W.L., X.X. and Z.G.; Formal analysis, C.M.; Investigation, C.M.; Resources, Z.G.; Data Curation, X.X.; Writing--Original Draft Preparation, C.M.; Writing--Review & Editing, F.B.; Visualization, H.J; Supervision, H.J.; Project Administration, C.M.; Funding Acquisition, C.M. and H.J.  
Figure 27: Fatigue life contour of optimized welded A-type frame (cycles).  
**Funding:** This research was funded by [Natural Science Foundation of Hunan Province] grant number [2017]J3059 and 2018]J4060], and [Postdoctoral Science Foundation] grant number [2017M622569], and [State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body] grant number [31715012].  
**Acknowledgments:** The authors gratefully acknowledge the support of the Hunan Province Natural Science Foundation (Grant No.:2017 JJ059 and 2018]J4060), China Postdoctoral Science Foundation (Grant No.:2017M622569), and Open Foundation of State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body (Grant No.:31715012).  
**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.