Evaluation of Fatigue Properties under Four-point Bending and Fatigue Crack Propagation in Austenitic Stainless Steel with a Bimodal Harmonic Structure

Abstract

Austenitic stainless steel (JIS-SUS304L) with a bimodal harmonic structure, which is defined as a coarse-grained structure surrounded by a network of fine grains, was fabricated using powder metallurgy to improve both the strength and ductility. Four-point bending fatigue tests and Kdecreasing tests were conducted in air at room temperature under a stress ratio R of 0.1 to investigate fatigue crack propagation in SUS304L. The fatigue limit of this harmonic-structured material is higher than that of the material with a homogeneous coarse-grained structure. This is attributable to the formation of fine grains by mechanical milling and to the suppression of pore formation. In contrast, the threshold stress intensity range, Kth, for the harmonicstructured material is lower than that for the homogeneous coarse-grained material, while the crack growth rates, da/dN, are higher at comparable K. These results can be attributed to a reduction in the effective threshold stress intensity range, K eff,th , due to the presence of fine grains in the harmonic structure.

Effect of TiB Orientation on Near-Threshold Fatigue Crack Propagation in TiB-Reinforced Ti-3Al-2.5V Matrix Composites Treated with Heat Extrusion

“…Based on our prior work [39,40,41,42,43,44,45], a disk-shaped compact (DC(T)) sample type was employed in this work, similar to the specimen type referred to in the E399-17 ASTM standard. The composite was machined to produce specimens of this type having the sizes presented in Figure 1, after which the surfaces were polished to a mirror finish using emery papers (#80–#4000) followed by a suspension of SiO 2 .…”

Harmonic Structure Design: A Strategy for Outstanding Mechanical Properties in Structural Materials

“…Nukui et al [72] found that the fatigue strength is increased because the resistance of the material to the initiation of fatigue cracks was improved by the grain refinement at the shell. The beginning of fatigue cracks was also observed in the coarse grains core in harmonic-structured designed materials, including CP titanium [72], Ti-6Al-4V alloys [73,74], and austenitic stainless steel [75,76]. However, in alloys, it can be attributed to both the network topology and strain-induced phase transformations, for example; α-fcc to ε-hcp in CCM [28].…”

Critical Assesment 37: Harmonic-structure materials - idea, status and perspectives

“…In four-point bending high-cycle fatigue testing of Ti64 [49,67], they found that (i) endurance limit of bHS material increases by approximately 50% compared to CG counterpart; (ii) intrinsic crack length in the bHS material is shorter because of the presence of UFG phase; but (iii) small fatigue crack paths and crack growth rate at comparable stress intensity ranges have almost no effect on the microstructure. Commercially pure Ti [69] and stainless steel SUS304L [72] demonstrate virtually the same fatigue performance while a less pronounced increase of endurance limit in bHS materials.…”

“…After a few early works in this direction [36-40], the concept of bHS materials was clearly and concisely formulated in [24]. Following extensive experimental works further confirmed, refined and expanded the approach [41-78]. A few finite-element (FE) simulations have also been carried to further understand the mechanics of plastic deformation in bHS materials [79-82] but more analytical and computer simulation efforts are necessary to understand the phenomena in-depth.…”