On the crystallographic, stage I-like, character of fine granular area formation in internal fish-eye fatigue cracks

Abstract

International audienceThe crystallographic character of fine granular area (FGA) formation from internal particles in martensitic high strength steel has been revealed by an assessment of the plastic zone size at the FGA border. This plastic zone size corresponded to about 3 times the martensite lath width. Tests at different temperatures (20 °C, 200 °C and 400 °C) revealed a decreasing FGA size with increasing temperature at constant applied stress amplitude. As a consequence, the critical stress intensity factor varied as the FGA decreased with temperature. In contrast, the critical plastic zone size remained constant and equal to the sizes of microstructural features. This represents a strong similarity between crystallographic, stage I-like, crack propagation and FGA formation in a vacuum

Strength and fatigue strength of a similar Ti‐6Al‐2Sn‐4Zr‐2Mo‐0.1Si linear friction welded joint

“…The entering air may have changed the surface roughness during fatigue crack growth, because of the influence of oxidation layers in the crack tip (see Figure E). This phenomena is known as “fish‐eye” and has already been documented for martensitic steel by previous studies . A high magnification image at the centre of the central line is shown in Figure B.…”

“…Seemingly, a crystallographic fatigue crack propagation occurred corresponding to stage I‐like fatigue crack propagation at low levels of stress intensity factor and small plastic zones sizes. This phenomena has been approached elsewhere . The final ductile rupture mechanisms are put in evidence by the dimples of Figure F.…”

“…By considering the internal crack as crack in a infinite plate, an approximative and conservative estimation of the stress intensity factor can be made using . Crack size, associated small plastic zone size, and the vacuum environment (internal crack) seem to favour a stage I‐like propagation . The stress intensity factor continued to increase until reaching stage II.…”

“…This phenomena is known as "fish-eye" and has already been documented for martensitic steel by previous studies. [48][49][50] A high magnification image at the centre of the central line is shown in Figure 15B. Three main features are observed: gray round-cornered features resembling to dendrites (see Figure 16A-C), a population of nanometric black voids and flat surfaces reminding cleavage facets (see Figure 16B).…”

Interior induced fatigue of surface‐strengthened steel under constant and variable loading: Failure mechanism and damage modeling

“…Generally, a fisheye‐shaped crack occurs on the fracture surface, in which a peculiar area, named “fine granular area (FGA)” by coauthor Sakai, sometimes can be observed in the vicinity of crack nucleus, but sometimes it does not occur even in the life regime beyond 10 8 cycles . Some ideas or models such as “Polygonization and debonding,” “Depressive decohesion of spherical carbide,” “Continuous grain refinement,” “Numerous cyclic pressing,” “Hydrogen embrittlement‐assisted cracking,” “Repeating contact of crack surfaces,” and “Stage I‐like crystallographic crack propagation” have been proposed to explain the formation mechanism of the FGA from the various viewpoints. Although controversies still exist, most researchers believe that the formation of the FGA represents the onset of a propagating long crack and governs the fatigue property of material in the VHCF regime …”