A Study on the Change of Fatigue Fracture Mode in Two Titanium Alloys

Abstract

The appearance of the fatigue fracture surface and crack growth curve have been examined for a Ti–2.5Cu alloy with different microstructures (two equiaxed and two lamellar microstructures), and for TIMETAL 1100 with a lamellar microstructure. With increasing ΔK, a slope change in the crack growth curve correlates with a transition in the fracture surface appearance (induced by a fracture mode transition); this being found in each microstructure. The microstructure size that controls the fatigue fracture is found to be the grain size for equiaxed microstructures and the lamella width for lamellar microstructures. The transitional behaviour can be interpreted in terms of a monotonic plastic zone size model in microstructures having a coarse microstructure size and in terms of a cyclic plastic zone size model for microstructures having a fine microstructure size.

Understanding the “blue spot”

“…For ST1, LCF133 and other specimens [11], elevated 16 O -, 35 Cl -, and 23 Na + ions were identified in the profiles close to the fracture surface within the side-face patches (figure 18), although not found for specimen ST2. The same procedure on the control specimen LCF127 did not identify any significantly elevated 35 Cl -or 23 Na + compared to the datum trenches.…”

“…Initial investigation deemed that fractographic characteristics within the blue spot resembled microstructure ("structure-sensitive"; figure 2) [15,16]. A stress-corrosion mechanism was considered, but at the time no corrosive species were identified using the available techniques on either ST1 or ST2.…”

The effect of crack length and maximum stress on the fatigue crack growth rates of engineering alloys

“…a change in exponent, in the region II fatigue crack growth rate curve when plotted on a double logarithmic scale [10]. The change in slope in region II was observed for multiple steel-, nickel-, titanium-and aluminium alloys [11][12][13][14][15]. Yoder et al showed that the FCGR curves for 7XXX-series aluminium alloys (AA) exhibit a multilinear form when plotted double logarithmic over a sufficiently broad spectrum of ΔK (see Fig.…”

Fatigue Crack Growth Behaviour and Role of Roughness-Induced Crack Closure in CP Ti: Stress Amplitude Dependence

“…A fatigue crack path may be a powerful resource determining all the aforementioned features. For instance, roughness-induced crack closure (RICC) is attributed to crack path deflection, especially near the threshold range, at which a serrated or zigzag crack path is induced by microstructure-sensitive crack growth [2][3][4]. The tilt angle of the crack path in 2124 Al alloy is reported to be a key controlling factor for RICC [5].…”

“…The tilt angle of the crack path in 2124 Al alloy is reported to be a key controlling factor for RICC [5]. Wang and Müller [2,6] reported that RICC occurs due to a serrated crack path, which significantly affects crack growth rates in Ti-2.5 Cu (wt%) and TIMETAL 1100 alloys. Fatigue crack growth rates are reported to be decreased by crack path deflection [3,4,7], as the crack path causes a direct reduction of the local driving force for crack propagation and an increase in the total crack path length, which results in lower crack growth rates and induces RICC.…”