An experimental methodology for quantitative characterization of multi‐site fatigue crack nucleation in high‐strength Al alloys

Abstract

A B S T R A C T An experimental method was developed to quantify the multi-site fatigue crack initiation behaviour of engineering alloys in four-point bend fatigue testing under stress control. In this method, fatigue crack initiation sites [fatigue weak-links (FWLs)] were measured on the sample surface at different cyclic stress levels. The FWL density in an alloy could be best described using a three-parameter Weibull function of stress, although other types of sigmoidal functions might also be used to quantify the relation between weak-link density and stress. The strength distribution of the FWLs was derived from the Weibull function determined by fitting the weak-link versus stress curve experimentally obtained. As material properties, FWL density and strength distribution could be used to characterize the fatigue crack nucleation behaviour of engineering alloys quantitatively and evaluate the alloy quality in terms of weak-link density and strength distribution. In this work, the effects of environment, types of microstructural heterogeneities and loading direction on FWLs were all studied in detail in AA7075 T651, AA2026 T3511, A713 alloys, etc. It was also found that FWL should be quantified as a Weibull-type function of strain instead of stress, when the applied maximum cyclic stress exceeded the yield strength of the tested alloy.Keywords A713 cast Al alloy; four-point bend fatigue test; fatigue weak-link (FWL) density; high strength Al alloy; multi-site fatigue crack nucleation; strength distribution. N O M E N C L A T U R EC = a scaling constant FIB = focused ion beam FEA = finite element analysis FWL = fatigue weak-link HCF = high-cycle fatigue k = constant L = rolling m = Weibull modulus n = the characteristic strength distribution of FWLs N = the number of fatigue weak-links at a specific stress level N 0 = the fatigue weak-link density OM = optical microscopy RH = relative humidity S = short transverse SEM = scanning electron microscopy T = long transverse ε = the strain associated with the applied stress σ ε 0 = the strain measured at stress of the fatigue limit σ 0 σ = stress level σ s = the ultimate tensile strength σ 0 = fatigue limit Correspondence: T. Zhai.

Failure statistics for commercial lithium ion batteries: A study of 24 pouch cells

“…However, these models, similar to models that predict the life expectancy of a newborn baby, are at best rough guides to the actual durability of any given cell. This is because failure is generally a statistical process [16][17][18] , where life is determined by "hidden" variables over which we typically exercise little control [19][20][21] . In batteries these variables could be microscale, such as heterogeneities within particles [22][23][24] ; or mesoscale, such as variation in local porosity 17 or state of charge 25 ; or macroscale, such as the location in a pouch 26 or cell-to-cell variation in the time-temperature history in a pack 9,27 .…”

An overview of fatigue in aircraft structures

“…63 Fracture toughness decreases with increasing strength. 68 The substitution of older by newer, improved materials, is of interest in refurbishing exercises, as a substitution of wings described in 69 where the selection of 7249-T6511 is discussed in the context of the wings of the US Navy aircraft, Lockheed P-3C. [64] or more generally by ref.…”

“…113 An exclusively European research effort, around that time, is GARTEUR report TP104. [68] for Al alloys 7075, 2026. This finding reinforced the movement towards the definition of a bound to the operational life of the aircraft, actually undefined up to the 1988 version of section 25.571 (amendment 96).…”

“…7075 is a common alloy for aero structures, where multi-site fatigue crack nucleation is of interest. 68 The substitution of older by newer, improved materials, is of interest in refurbishing exercises, as a substitution of wings described in 69 where the selection of 7249-T6511 is discussed in the context of the wings of the US Navy aircraft, Lockheed P-3C. This aircraft is involved in an extensive life-extension program being conducted by the US Navy, including the replacement of the 7075 T651 wings with the 7249-T76511 alloy due to its improved corrosion resistance over the 7075 alloy.…”

“…in ref. [68] for Al alloys 7075, 2026. As a consequence of the Aloha accident, many studies have been performed in riveted plates, e.g.…”