Objective The structural safety of pantograph has a significant impact on the operational reliability of trains. Fatigue cracks are prone to occur at the elbow joints of pantograph on high-speed trains. Therefore, it is necessary to utilize their vibration characteristics to accurately identify the hardly detectable small cracks in their early-stage initiation and to analyze their characteristics.

Method The damage identification theory based on strain mode is introduced. Taking the upper frame of a certain type of pantograph as the research object, a finite element model is established to perform modal simulation analysis. The accuracy of the simulated modal frequency results is verified through a physical modal test. The displacement mode and strain mode shapes of the pantograph upper frame are calculated and analyzed. The first two strain mode orders are selected, and the mean value of the strain mode change rate is adopted as the damage index to identify cracks. The influence of different crack sizes correspond to strain modes is calculated.

Result & Conclusion  The occurrence of cracks has no significant effect on mode shape or frequency. At the crack location, the strain mode exhibits a pronounced mutation, whereas the displacement mode does not change. The strain mode is more sensitive to damage than the displacement mode. The strain mode of upper frame shows a distinct mutation at the crack damage location, with the effect being most evident in the lower-order modes, while the changes caused by higher-order mode damage may be concealed. As the crack size increases, the mutation of the strain mode at the crack location becomes more significant. The damage index value increases with the increase in crack depth; when the crack length is less than 10.00 mm, the difference in damage index value is small, and when the crack length exceeds 10.00 mm, the damage index value will increase with the crack length.