Influence of cyclic overloads on the strength characteristics of structural steel

Conclusions

• 1. Low-cycle fracture of 09G2S steel with a test base of 2·10⁴ cycles in the presence of an inhomogeneous stress pattern and a severe stress concentrator α_c = 15 at normal temperatures (20°C) has a viscous character, with a pronounced zone of cyclic damage. The plastic characteristics of the material are high and the deformation (relative necking) always lies between 13 and 17%. There is no abrupt change in the fracture-inducing deformation over this cyclic range on moving from the region of quasistatic fracture to that of fatigue fracture.

• 2. Preliminary cyclic loading leads to cumulative microscopic and macroscopic damage (development of deformation processes, fatigue damage, and cracking). There is a material increase in the tendency of the material toward brittle fracture and a decrease in its plasticity and fracture-stress level. The work of fracture is reduced by a factor of 2–10.

• 3. Cyclic damage develops during the early stages of prestressing (at N=0.1 N_frac) over a broad load range with a complex stress pattern.

• 3.a A decrease in the cyclic stress (an increase in the number of cycles) leads to localization of the cyclic-damage processes. The tendency of the steel toward brittle fracture increases most rapidly under these conditions.

• 4. Preliminary cyclic loading of structural steel with small test bases substantially (by a factor of 4–5) reduces low-temperature (−40°C) fracture characteristics before visible disruptions of material continuity occur. The initial stage of cyclic damage can thus lead to abrupt brittle fracture.