Strain mode analysis for crack identification in the upper frame of a pantograph

The pantograph is a critical subsystem of high-speed trains. During train operation, the pantograph undergoes dynamic interactions with the overhead contact wire, including the effects of alternating loads and crosswinds. These interactions can lead to various faults, such as contact strip wear and upper frame cracking. Conventional detection of upper frame cracks primarily relies on visual inspections during routine maintenance, which may result in missed detections and lack timeliness. To address this issue, this paper proposes a strain mode-based method for identifying upper frame cracks in pantographs. Compared to displacement mode, strain mode exhibits higher sensitivity to cracks, even at the early stage of crack initiation, and thus enables more effective detection by comparing with healthy components. Further discussion reveals that low-order strain modes demonstrate good identification performance for both single and dual cracks. Subsequently, two additional methods are proposed: one based on the rate of change in strain modes and the other combining the second-order strain mode with symmetry analysis. Both methods show strong capabilities for crack identification. Moreover, the paper analyzes crack-induced damage severity based on strain mode characterization, revealing that the damage severity increases with crack propagation. Finally, an online monitoring scheme based on this method is briefly discussed, offering a pre-detection approach to support crack detection during maintenance.