Structural variations ahead of crack tip during monotonic and cyclic fracture tests of AISI 304LN stainless steel

This investigation presents examination of structural variations ahead of crack tip during monotonic fracture toughness and cyclic fracture toughness tests of AISI 304LN stainless steel. The monotonic fracture toughness tests are carried out using standard J-integral technique; whereas the cyclic fracture toughness tests are evaluated using periodic unloading to different extents fixed by pre-determined stress-ratio, R. The associated structural variations adjacent to fracture surfaces are characterized in terms of nature and amount of deformation induced martensite using micro structural analysis, X-ray diffraction, hardness measurement, ferrofluid technique and transmission electron microscope. The obtained results reveal that monotonic fracture toughness specimens exhibit higher amount of deformation induced martensite compared to cyclic fracture toughness specimens, and the latter exhibit variation in martensite content with change in R ratio. The dislocation substructure also changes rapidly with decreasing stress ratio for cyclically deformed samples. The associated significant structural variations in cyclic fracture toughness specimens has been explained by variation in energy expended during crack propagation and different residual stress fields ahead of cracks in cyclic fracture toughness compared to monotonic fracture toughness specimens.