Influence of asymmetric cyclic loading on substructure formation and ratcheting fatigue behaviour of AISI 304LN stainless steel

The aim of this report is to examine the nature of accumulation of ratcheting strain (ɛr) vis-à-vis in situ substructural and microstructural variations in 304LN stainless steel under varied asymmetric cyclic loading (ACL) conditions. For this purpose, a series of ACL experiments have been carried out on cylindrical tensile specimens under different combinations of mean stress (σm) and stress amplitude (σa). In one set of experiments, σm has been varied between 50 and 150 MPa keeping σa constant, while in a second set σm has been kept constant and σa is varied between 300 and 400 MPa. From the results it can be inferred that ɛr increases from cycle to cycle for ACL tests and the phenomenon is considered to be governed by the associated mechanics and mechanisms of deformation. Fatigue life of a specimen can either increase or decrease depending upon the test condition, which induces different variations in dislocation density, sub-grain size or micro-damage formation.

Research highlights▶ Accumulation of ratcheting strain depends on mean stress and/or stress amplitude. ▶ Fatigue life may either increase or decrease depending upon the loading condition. ▶ Dislocation density increases with increasing strain accumulation during ratcheting. ▶ Deformation induced martensite forms during ratcheting. ▶ Saturation of strain accumulation occurs around 100 cycles for the investigated steel.