The problems arising as a result of aging aircraft, rail and civil infrastructure have focused attention on tools for predicting the growth of cracks from small naturally occurring material discontinuities. To this end, the present paper discusses on the difference between the analysis tools needed for ab initio design and sustainment, modelling of cracks that grow from small naturally occurring material discontinuities and ways to determine the short crack da/dN versus ΔK data from long crack American Society for Testing and Materials (ASTM) tests. It also discusses how existing equations can be used to predict short crack growth and how to account for the variations seen in crack growth histories. Attention is also focused on the recent Federal Aviation Administration limit of validity ruling and the effect of the environment on widespread fatigue damage in civil transport aircraft.

Fatigue crack growth and damage tolerance

Aluminium-lithium alloys are a class of low density, high strength, high stiffness monolithic metallic materials that have been identified as prime candidates for replacing 2000 and 7000 series aluminium alloys currently used in commercial and military aircraft. In this review, the cyclic fatigue strength and fatigue crack propagation characteristics of aluminium-lithium alloys are reviewed in detail with emphasis on the underlying micromechanisms associated with crack advance and their implications to damage tolerant design and lifetime computations. Compared with traditional aerospace aluminium alloys, results on the fatigue of binary Al-Li, experimental Al-Li-Cu, and near commercial Al-Li-Cu-Zr and Al-Li-Cu-Mg-Zr systems indicate that alloying with Li degrades the lowcycle fatigue resistance, though high-cycle fatigue behaviour remains comparable. The alloys, however, display superior (long crack) fatigue crack growth properties, resulting from a prominent role of crack tip shielding, principa...

Fatigue of aluminium—lithium alloys

Fatigue crack propagation in ARALL® LAMINATES: Measurement of the effect of crack-tip shielding from crack bridging

Ambient-temperature fracture toughness and fatigue crack propagation behavior are investigated in a wide range of (γ+α 2) TiAl microstructures, including single-phase γ, duplex, coarse lamellar (1 to 2 mm colony size (D) and 2.0 µm lamellar spacing (λ)), fine lamellar (D ∼ 150 µm, λ=1.3 to 2.0 µm), and a powder metallurgy (P/M) lamellar microstructure (D=65 µm, λ=0.2 µm). The influences of colony size, lamellar spacing, and volume fraction of equiaxed γ grains are analyzed in terms of their effects on resistance to the growth of large (>5 mm) cracks. Specifically, coarse lamellar microstructures are found to exhibit the best cyclic and monotonic crack-growth properties, while duplex and single-phase γ microstructures exhibit the worst, trends which are rationalized in terms of the salient micromechanisms affecting growth. These mechanisms primarily involve cracktip shielding processes and include crack closure and uncracked ligament bridging. However, since the potency of these mechanisms is severely restricted for cracks with limited wake, in the presence of small (<300 µm) cracks, the distinction in the fatigue crack growth resistance of the lamellar and duplex microstructures becomes far less significant.

/pdf/the-effect-of-microstructure-on-fracture-toughness-and-47bteotd6f.pdf

The effect of microstructure on fracture toughness and fatigue crack growth behavior in γ-titanium aluminide based intermetallics

https://www2.lbl.gov/ritchie/Library/PDF/IJFat_Nalla_MixMod.pdf

Mixed-mode, high-cycle fatigue-crack growth thresholds in Ti-6Al-4V: Role of small cracks

https://escholarship.org/content/qt25c8796t/qt25c8796t.pdf?t=p0hhod

K.T.Venkateswara Rao

Papers

On the behavior of small fatigue cracks in commercial aluminum-lithium alloys