Trends in fatigue-crack growth for the 2024 Al-alloy after a single tensile overload

Abstract

A single tensile overload (OL) applied during a constant amplitude fatigue test could have a positive or negative influence on improving the subsequent fatigue-crack growth rate (da/dN). The resulting variation in da/dN was very relevant to both the OL level and the fatigue loading conditions. Comprehensive tests on 2024-T4 Al-alloy specimens were carried out to rationalize the variation trend of da/dN after a single tensile OL. It was found that the trend of the fatigue-crack growth after the OL is not unique, but strongly depends on the OL stress-intensity factor KOL and the applied stress-intensity factor Kapp just before OL. Three potential trends of variation in da/dN, related to both the KOL and the KOL/Kapp ratio, were proposed in this study: initial deceleration and then recovery (Type I); initial slight-acceleration, subsequent retardation and final recovery (Type II); and initial major-acceleration, subsequent retardation and final recovery (Type III). Among the three types, the type II with the medium KOL/Kapp ratio and the low Kapp is the optimal OL type for improving the fatigue life of a 2024-T4 Al-alloy specimen. Keywords: overload, fatigue crack growth trend, fatigue retardation, overload-induced fracture zone Med testom utrujanja s konstantno amplitudo so uporabili posamezen natezni preizkus s preobremenitvijo (OL; angl.: single tensile overload) ter ugotovili, da ima lahko le-ta pozitiven ali negativen vpliv na nadaljnjo hitrost rasti razpoke (da/dN). Nastala razlika v da/dN je bila tesno povezana tako z nivojem OL kot tudi s pogoji utrujanja. Avtorji~lanka so izvedli ob{irne preizkuse na vzorcih Al zlitine 2024-T4, da bi razumeli trend variiranja da/dN po OL. Ugotovili so, da trend hitrosti rasti razpoke po OL ni enoten, temve~je mo~no odvisen od razmerja OL-faktor intenzitete napetosti KOL in uporabljenega nivoja napetostnega faktorja intenzitete Kapp tik pred OL. V tej {tudiji so avtorji predlagali tri mo`ne trende variiranja da/dN glede na KOL in razmerje KOL/Kapp: za~etno zmanj{evanje hitrost (zaviranje) in nato popravo (Type I); za~etno rahlo pospe{evanje, odla{anje in nato popravo (Type II) in za~etno izrazito pospe{evanje, kateremu je sledilo odla{anje in kon~na poprava (Type III). Med vsemi tremi je tip II s srednjim KOL/Kapp razmerjem in nizkim Kapp optimalen OL tip za izbolj{anje dobe trajanja vzorcev iz Al zlitine 2024-T4. Klju~ne besede: preobremenitev, trend rasti utrujenostne razpoke, odla{anje utrujanja, s preobremenitvijo inducirana cona loma

Study of overloading retardation on fatigue crack propagation behavior in 300M steel

“…The results show that an increase in the overload ratio or a decrease in the load‐to‐load ratio aggravates the deceleration effect of crack growth. Zhang et al 19 proposed that the ratio of the stress intensity factor at the crack tip under overload and constant amplitude is the main reason for the change of crack growth rate under stress intensity.…”