This study deals with the SEM and optical microscopic characterization of fatigue
plastic deformation process during fatigue crack initiation to understand where, why and how
cracks initiate under conditions of low cycle fatigue. Samples were prepared from the
13Х11Н2В2МФ high-chromium stainless steel used for fusion power applications. The low-cycle
tests were conducted at room temperature with the standard V-notched samples prepared from
conventional stainless steel. The following characteristics were studied during fatigue tests: 1
macrocrack propagation, 2. interaction between macrocrack and isolated microcracks, 3. interaction
between macrocrack and slip bands, 4. interaction between macrocrack and microstructure elements
of the steel.
The above experiments show that during macrocrack propagation a plastic zone is formed around it,
where isolated microcracks and slip bands of 2-3 different directions are observed. Measurement of
plastic zone dimensions after different number of cycles of deformation show that plastic zone size
increases during the first stage of cyclic deformation (until definite number of cycles are
completed), and then remains unchanged. The observations show that main crack is composed of
individual micro-components, the lengths of which are in a good correlation with the dimensions of
microstructure elements of the steel (former austenite grains, martensite crystals). It was revealed
that during growth, as a rule, macrocrack rarely propagates along isolated microcracks and slip
bands. Direction of macrocrack propagation changes while passing from one microstructure
element to another, so that main direction is the same.
No preferable transcrystalline or intercrystalline propagation of macrocrack has been observed in
the investigated steel. It is shown that after subsequent fatigue tests, dimensions of the previously
created slip bands increase, and additional new slip band are also formed. The sites and frequency
of slip bands’ formation in plastic zone are also studied. It was observed that the boundaries and
mainly the sites of intersection of martensite crystals are the sites of isolated (rough) microcracks’
formation. The dimensions of slip bands are comparable with those of martensite crystals. The
angles between the main crack propagation direction and slip bands varied from 30o to 60o,
however, most of the slip bands were oriented at 45o to the main crack.
Based on the obtained results a conclusion is made that plastic deformation in samples go
inhomogeneously. In plastic zones, along with the heavily deformed areas, almost non-deformed
areas are also observed. The speed of fatigue fracture increases with the increase in frequency and
amplitude of deformations. Generally, the annealed samples are destructed prematurely in
comparison with non-annealed ones of the investigated steel.
Effect of Microstructure on Low-cycle Fatigue Property of Structural Steels under Large Plastic Deformation
