Failure Analysis of Tower Crane Turn Table Bolts

A metallurgical and root cause analysis was conducted on failed bolts of a tower crane turn table. A first principle’s analysis of peak stresses on the bolts was conducted in addition to examinations of the bolts’ fracture surfaces and tensile testing. Examinations included low magnification fractographic analysis, stereomicroscopy, scanning electron microscope examination and metallurgical and microstructural analysis of fracture surfaces. Tensile testing included flat and round bar samples. The analyses determined that normal operating conditions would not result in tensile overstresses. Examinations identified that all recovered bolts had distinct fatigue zones along with final fast fracture zones. Tensile testing on non-fatigued portions of the recovered bolts returned ultimate strengths which exceeded bolt specifications. The analysis concluded that the root cause of the failure was metal fatigue which resulted in crack propagation through the bolts’ cross-sections, resulting in tensile stresses greater than the material strength of the unaffected cross-sections.