Objective As the operating mileage of metro trains in China is increasingly approaching their designed lifespan, in order to evaluate the remaining service life mileage of metro vehicle main load-bearing components and provide reference for subsequent vehicle maintenance, it is necessary to carry out fatigue strength assessments of welded structures in combination with the vehicle specific operating lines.

Method The FKM (Forschungskuratorium Maschinenbau) criterion, compiled by the German Research Board for Mechanical Engineering, is a strength assessment criterion applicable to materials such as steel, stainless steel, cast iron, aluminum, and cast aluminum. Compared with other welded structure assessment standards, the FKM criterion covers a more comprehensive range of materials and provides a more detailed evaluation procedure. Taking the bogie frame of a metro project in China as an example, the processing of test data from the operating line, the calculation methods for equivalent stress and fatigue limits of welded joints are described in detail. For both the original and improved frame welded structures subjected to alternating stress amplitudes, the utilization ratios at key locations of the frames are calculated using the FKM criterion, and fatigue assessment is carried out. The purpose and selection principle of each parameter involved in the assessment process are explained in detail, and the key factors for improving the accuracy of this assessment method are identified.

Result & Conclusion  Reasonable and effective reduction of the stress level at the assessed locations can significantly increase theservice life mileage. The improvement scheme reduces the stress level from 13.572 MPa to 7.680 MPa, resulting in an increase in service life mileage by nearly 6 times.