Fatigue fracture at a screw - 1.5511
A fatigue fracture (endurance failure) occurs because of a fatigue stress. In the area of the fatigue incipient crack the fracture surface mostly has a smooth structure and the residual fracture is usually coarse structured. The arrest lines are characteristic for a fatigue fracture. The macroscopic phenomenon, however, isn't always distinctive. Triggers for these fractures are imperfections at the surface or also in the workpiece. Imperfections are for example decarburization at the surface, slag inclusions, cracks or microstructure anomalies. In the case of a decarburization the ferrite works as trigger due to a worse endurance strength. In the course of the fracture origin the material cross sectional area reduces itself increasingly (arrest lines). At the end it comes, by overloading, to the fracture (residual fracture) of the component. In the following case a screw is broken after some years of use.
Screw M8 - quenched and tempered