The Merging of Fatigue and Fracture Mechanics Concepts: A Historical Perspective

Faceted features are frequently observed on the fracture surfaces of titanium alloys that have failed by static loading, continuous cycling, dwell fatigue loading, and stress corrosion cracking (SCC). Although the facets formed under different loading conditions seem qualitatively similar, there are significant differences in the spatial and crystallographic orientations of the facets as well as subtle differences in facet surface topography. The current study compares and contrasts facets for various loading conditions (cyclic, creep, SCC, and dwell) in the Ti-8Al-1Mo-1V alloy with the primary motivation being to understand the mechanisms of crack initiation and faceted growth during dwell fatigue. The spatial and crystallographic orientations of the facets were determined using quantitative tilt fractography and electron backscatter diffraction, whereas facet topography was examined using ultra-high-resolution scanning electron microscopy. Collectively, the experimental observations suggest that hydrogen may play an important role in facet formation and accelerating small crack growth rates during dwell fatigue loading.

https://apps.dtic.mil/dtic/tr/fulltext/u2/a532477.pdf

Observations of Facet Formation in Near-α Titanium and Comments on the Role of Hydrogen

The effect of continuous fretting in air at 20°C on fatigue performance has been studied for Ti-17 and T1-6Al-4V, high strength titanium alloys used for gas-turbine fan and compressor disks and blades, respectively. The effect of fretting was to reduce the fatigue stress limit from 700 MPa for plain fatigue to 200 MPa for fretting-fatigue. A number of models, supported by metallographic and fractographic evidence, are proposed which explain (i) how the cyclic loading of individual asperities results in crack initiation; (ii) the formation of multiple cracks; (iii) the existence of non-propagating cracks; and (iv) how fretting influences crack propagation once fatigue cracks have formed.

Mechanisms of fretting-fatigue of titanium alloys

Low ΔK faceted crack growth in titanium alloys

Characteristics of fatigue crack propagation (FCP) have been studied on materials with three different microstructures of a Ti-6A1-4V alloy, prepared with different heat treatments. The effect of microstructure on the FCP behaviour was attributed to the development of crack tip shielding, primarily resulting from the role of crack path morphology in inducing crack closure and crack deflection. Roughness-induced crack closure played an important role on the near-threshold FCP behaviour at a stress ratio of 0.05, but the FCP data plotted in terms of the effective stress intensity factor range, δKeff (allowing for crack closure), still exhibited the effect of microstructure. Fractographic examinations were performed, using a scanning electron microscope (SEM) with the aid of image processing, which enabled a three-dimensional reconstruction of the fracture surface using a stereo pair of SEM micrographs. Fracture surface roughness was evaluated quantitatively by the ratio of the real area of the reconstructed fracture surface to its projected area. As fracture surface roughness was taken into account in evaluating the FCP data in addition to crack closure, the effect of microstructure disappeared, indicating that the intrinsic FCP resistance was the same in all the materials. Thus, it was concluded that fracture surface roughness was a dominating parameter in controlling the FCP of the Ti-6A1-4V alloy.

C.E.W. Looije

Papers

Fractographic and microstructural analysis of fatigue crack growth in a Ti-6Al-4V fan disc forging