A. Neidel, M. Giller, S. Riesenbeck
April 28, 2017
The damaged bolt failed due to material fatigue. Due to frequently repeated tightening/loosening during long-term use, the component was cyclically plastically deformed. This led to strain und necking (tapering) near the later fracture surface (similar as in a tensile test) where a reduction in the bolt's cross section occurred. Hence, the extremely low-cycle, but high cycling stress during tightening and loosening increased. This resulted in incipient cracks caused by fatigue in the (most highly stressed) threads adjacent to the fracture surface. After this circumferential low-cycle fracture (LCF) penetrated up to 2 mm in the bolt shaft, the residual forced fracture was caused by almost uniaxial overload. The deformations and damages in other parts of the thread confirm this assessment.