VHCF properties of nitrided 18Ni maraging steel thin sheets with different Co and Ti content

Abstract

High cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of two 18Ni maraging steels with different cobalt and titanium content and similar static strength are investigated. Ultrasonic fatigue tests are performed with thin sheets with nitrided surfaces at load ratio R = 0.1. The specimens are mounted on a carrier and are forced to joint vibrations at approximately 20 kHz. The increase of Co content and the elimination of Ti improved the HCF and VHCF strength of 18Ni maraging steel. TiN inclusions if Ti is present and Al2O3 inclusions in the Ti free material with sizes (areaINC)1/2 smaller than 10 µm were preferential crack initiation locations. Considering inclusions as initial cracks, the minimum stress intensity range for VHCF failure is 1.2 MPam1/2 for TiN inclusions and 1.8 MPam1/2 for Al2O3 inclusions. Data scatter may be slightly reduced if lifetimes are presented versus stress amplitudes multiplied by (areaINC)1/12 rather than in an S–N diagram.

Estimation of P‐S‐N curves in very‐high‐cycle fatigue: Statistical procedure based on a general crack growth rate model

“…Figure depicts the estimated curves, which confirm the appropriateness of the proposed model: Except for one case ( = 46.2 μm), experimental data are within the plotted curves that, according to the literature, move downward (to smaller VHCF strength values) and leftward (to smaller VHCF life) if the initial defect size increases. Moreover, the trend followed by the curves in the finite VHCF region (from 10 8 to 10 10 cycles) is in good agreement with the Basquin's model (linear trend in a bilogarithmic S‐N plot), which is commonly assumed for fitting VHCF data …”

VHCF response of Gaussian specimens made of high‐strength steels: comparison between unrefined and refined AISI H13

“…Following a typical procedure adopted in the literature, 45 the experimental data are gathered together at a unique stress level (σ ref ) by considering the power relationship between the mean fatigue life and the stress amplitude. Following a typical procedure adopted in the literature, 45 the experimental data are gathered together at a unique stress level (σ ref ) by considering the power relationship between the mean fatigue life and the stress amplitude.…”

The formation mechanism of characteristic region at crack initiation for very-high-cycle fatigue of high-strength steels

“…Fatigue failures of metallic materials may happen beyond 10 7 loading cycles under a relatively low cyclic stress below conventional fatigue limit [1][2][3][4]. This is the new research topic of very-high-cycle fatigue (VHCF) that receives increasing research attentions due to not only engineering requirements but also scientific interests [5][6][7][8][9][10].…”