Failure Analysis of GFRP Composite Current Collector Brackets

A current collector on a transit vehicle failed resulting in a high voltage and a fire. Fracture of the glass fiber-reinforced polymer (GFRP) brackets on the current collector had triggered the incident. Subsequent investigation revealed many other cracked current collectors in the field. Four cracked GFRP brackets from in-service current collector assemblies were submitted to Applied Technical Services (ATS) to determine the failure mechanism, contributing factors, and if possible, the cause of failure.

Various microscopy, mechanical, and chemical analysis techniques were utilized to aid in the current collector failure analysis. Fractography was carried out by stereomicroscopy and scanning electron microscopy (SEM), which revealed large voids in the bracket material along the fracture surfaces. The fracture surfaces exhibited a smooth appearance with poor fiber-to-matrix adhesion. Cross sections were taken from cracked and remote areas of the brackets for microstructural examination by optical microscopy and void content analysis utilizing automated image analysis by SEM. The brackets were further characterized to evaluate for any processing related issues using various chemical analysis techniques including glass fiber content by acid digestion, compositional testing by Fourier-transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and dynamic mechanical analysis (DMA). Utilizing the results from the various tests, a failure mechanism and cause of failure were discovered relating to both the processing and service conditions of the brackets.