Influence of cyclic overloads on the strength characteristics of structural steel

Abstract

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 αc = 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 Nfrac) 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.