A method for the determination of the cyclic stress‐strain curve of presistent slip bands in fatigued single crystals

Abstract

Single crystals of pure nickel oriented for single slip were fatigued a t constant total strain amplitude (eat) at room temperature. The dependence of the saturation resolved shear stress amplitudes (tas) on the saturation plastic resolved shear strain amplitudes (yaps) (cyclic stress-strain curve -CSSC) was determined. In the plateau range of the CSSC the volume fraction f of persistent slip bands (PSBs) was found to be linearly related to yaps. The reduction of the total strain amplitude after saturation in the plateau range of t>he CSSC leads to the so-called secondary CSSC with a slope n > 0 in the log-log plot.From the secondary CSSC the "true" cyclic stress-strain curve of the PSB-volume (PSB-CSSC) can be determined. The method is based on the two assumptions that firstly the PSB-groups (or "macrobands") penetrate the whole cross-section of the specimen and secondly the volume fraction of PSBs in the secondary range of the CSSC remains constant.Nickeleinkristalle (99,99yo Ni) des Orientierungsfaktors 0,5 wurden bei Raumtemperatur mit konstanter Gesamtdehnungsamplitude (Fat) zyklisch verformt. Die Abhangigkeit der Sattigungs-Schubspannungsamplituden (78s) von den Abgleitungsamplituden im mechanischen Sattigungsstadium (yaps) (zyklische Spannungs-Dehnungs-Kurve -ZSD) wurde ermittelt. I m Plateau-Bereich der ZSD hangt der Volumenanteil f der persistenten Gleitbiinder (PSBs) linear von der Abgleitungsamplitude ab. Die Verringerung der Gesamtdehnungsamplitude nach dem Erreichen des Sattigungsstadiums im Plateau-Bereich der ZSD fuhrt zur sog. sekundaren ZSD mit dem Anstieg n > 0 in doppelt-logarithmischer Darstellung. Aus der sekundiiren ZSD kann die ,,wahre" zyklische Spannungs-DehnungsKurve des PSB-Volumens (PSB-ZSD) bestimmt werden. Diese Methode basiert auf den zwei Annahmen, daJ3 erstens die PSB-Gruppen (oder ,,Makrobiinder") den gesamten Probenquerschnitt durchqueren und daJ3 zweitens der Volumenanteil der PSBs im Bereich der sekundaren ZSD konstant bleibt.

3D discrete dislocation dynamic investigations of persistent slip band formation in FCC metals under cyclical deformation

“…Winters model has been cited in many publications but the fatigue test with a decreasing loading is rarely tried in practice. Blochwitz [10] has tested the evolution of PSBs with a decreasing loading for Ni and he has observed the changes induced on the PSBs. Our DDD simulations could reproduce these experimental results, especially, for the four thicker PSBs which stay at the same positions, but reduces in thickness after reduction of the plastic strain amplitude.…”

“…One remarkable observation during experiments of the early stages of fatigue damage is the formation of slip bands, which was firstly evidenced by Ewing [7] at the surface of a Swedish iron. During the last fifty years, the fatigue behavior of FCC (face-centered cubic) metals has been deeply investigated [8,9,10] because of the relatively uniformed fatigue behaviors and wide applications in industry. The development of persistent slip bands (PSBs) [8,11,12,13] is considered as a characteristic feature for FCC metals at early stages of fatigue damage.…”

Influence of Plastic Strain Control on Martensite Evolution and Fatigue Life of Metastable Austenitic Stainless Steel

“…However, the performance of stress-controlled or total strain-controlled fatigue tests disregards changes in the cyclic plastic strain caused by cyclic hardening or softening of materials. In particular, for ductile metallic materials in single (copper, e.g., [5,6], nickel [7]) and polycrystalline (AISI 316L [8], nickel [9], copper [6], aluminum [10]) conditions, a broad variety of investigations were performed, demonstrating the influence of cyclic plastic strain on the dislocation arrangement in the microstructure and, therefore, also on the crack initiation and propagation. However, in the case of materials undergoing a strong cyclic hardening, such as metastable austenitic stainless steels, the majority of fatigue life investigations are performed under stress control or under strain control at constant total strain amplitude [11].…”

Dislocation Structure and Strain Localisation in Nickel Single Crystals Cyclically Deformed at 77 K

“…for example MUGHRABI 1981, BLOCHWITZ and KAHLE 1980, BLOCHWITZ and VEIT 1982, BASINSKI and BASINSKI 1992, SCHWAB et al 1998). for example MUGHRABI 1981, BLOCHWITZ and KAHLE 1980, BLOCHWITZ and VEIT 1982, BASINSKI and BASINSKI 1992, SCHWAB et al 1998).…”