Microstructural Characterization and Cause Analysis of Fracture Surface Burrs in Lockbolts

DOI: https://doi.org/10.62517/jes.202602320

Author(s)

Shengyuan Yang1, Qin Zhang1, Yafeng Fan1, Xu He1, Xueliang Rong2, Yongzhan Zhu3

Affiliation(s)

1CRRC Meishan Fastening System Co., Ltd., Meishan, Sichuan, China 2Department of Bridge Engineering, Shijiazhuang Tiedao University, Shijiazhuang, Hebei, China 3China Railway Fifth Survey and Design Institute Group Co., Ltd., Beijing, China

Abstract

Lockbolts are a category of rivets widely applied in bridge steel structures, mining machinery, rail transit and other fields. After installation, its fracture surface must be regular and free of sharp burrs. Previous inspections identified sharp burrs on the fracture surfaces of some production batches after pintail break-off, indicating noncompliance with the product specifications. Samples exhibiting abnormal fracture surfaces were subjected to systematic failure analysis, including low-magnification examination of acid-etched longitudinal sections, longitudinal metallography, macroscopic and microscopic fractography, and energy-dispersive X-ray spectroscopy of longitudinal sections. The abnormal fracture surfaces were ultimately attributed to banded segregation of Mn or C in the lockbolt core. Differences in solute content disrupted the homogeneity of the steel and produced alternating bands with different ductility and toughness. Under tensile loading, sliding along the segregation bands generated axial cracks. These cracks interacted with transverse cracks originating at the root of the breakneck groove, ultimately producing sharp fracture surface burrs inclined at approximately 45°.

Keywords

Lockbolt; Banded Segregation; Fractography; Failure Analysis of Metallic Materials

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