# Experimental Investigations on the Effects of Fatigue Crack in Urban Metro Welded Bogie Frame
## 1 Introduction  
With the rapid urbanization and industrialization of China society, a huge metro rail network has been building and running in recent years. By the end of 2019, the operating mileage of China urban rail transit has reached about 6600 km and the vehicles in service have reached more than 35,000. As the most important load-carrying structure for railway vehicles, the welded bogie frame undergoes complex fatigue loads [1; 2]. Under such alternating loads, the safety critical zone inside the frame necessarily experiences cumulated fatigue damage which seriously threatens the operation safety of rail passenger vehicles [3; 4; 5].  
As frequently observed in metro vehicle frames, fatigue cracks are usually initiated from the welded region of the bogie frame due to complex operation loads. Recently, a fatigue crack in the Shinkansen train bogie frame [6] was found with a total length of 44 mm, which is regarded as a serious incident in the Japan Railway system, which could possibly have led to a derailment. Compared with high-speed railway passenger trains with covering super long distances and time, the urban metro vehicles are used to experiencing more abnormal loading due to frequent start-stops and huge passenger uncertainty. Therefore, lots of failures have come out in recent years, for example fatigue cracks observed near the motor suspension seat, beam and positioning block, etc. [7; 8].  
The initiation mechanism of fatigue cracks has been investigated extensively, with the critical attention to rail corrugation in terms of the causes, measurements and solutions [9; 10; 11]. The rail corrugation with the fatigue crack in primary coil springs was tentatively correlated with structuralvibration in experiments and simulations [12]. A linear model of the corrugation of rails was established to reveal the relationship between the initial wheel and rail roughness and the wear rate spectra in the contact patch [13]. The initiation and evolution of rail short-pitch corrugation and the effect on wheelset vibration were further investigated by using the field test and simulations to eliminate the corrugation [14].  
Another argued aspect to buffer the fatigue crack is the wheel polygonalization traditionally found in high-speed railway wheels [15; 16; 17; 18]. By considering the real wheel out-of-roundness, the considerably fluctuating wheel/rail contact force was observed from a vertical vehicle-track coupled dynamics model [17]. Similarly, by using on-site experiments and simulations, the mechanism of metro wheel polygonal wear was carefully explored at the speed of 65-75 km/h and suggested that the wheelset flexibility could accelerate the polygonal wear and abnormal vibration [19; 20]. Such an irregular wheel profile was reasonably believed to produce a detrimental effect on the railway axle and bogie frame [21; 22; 23].  
With regard to the crack of bogie frame [24; 25], initial works have been made through multi-body dynamic system [26] and finite element simulations [27]. In this paper, on-track tests including the dynamic stress and acceleration were performed under different loading conditions to elucidate the cracking mechanism of metro bogie frames. By combining time, frequency and time-frequency domain analyses, the factors inducing the cracks are confirmed and measures are proposed to prevent such similar failures.