The mechanisms of fatigue-crack propagation are examined with particular emphasis on the similarities and differences between cyclic crack growth in ductile materials, such as metals, and corresponding behavior in brittle materials, such as intermetallics and ceramics. This is achieved by considering the process of fatigue-crack growth as a mutual competition between intrinsic mechanisms of crack advance ahead of the crack tip (e.g., alternating crack-tip blunting and resharpening), which promote crack growth, and extrinsic mechanisms of crack-tip shielding behind the tip (e.g., crack closure and bridging), which impede it. The widely differing nature of these mechanisms in ductile and brittle materials and their specific dependence upon the alternating and maximum driving forces (e.g., ΔK andK
 max) provide a useful distinction of the process of fatigue-crack propagation in different classes of materials; moreover, it provides a rationalization for the effect of such factors as load ratio and crack size. Finally, the differing susceptibility of ductile and brittle materials to cyclic degradation has broad implications for their potential structural application; this is briefly discussed with reference to lifetime prediction.

Mechanisms of fatigue-crack propagation in ductile and brittle solids

Microstructural effects on the mechanical behavior of B-modified Ti–6Al–4V alloys

— The problem associated with short crack growth, defined as situations in which the intensity of the crack tip field is underestimated by linear elastic fracture mechanics analyses, is briefly reviewed.



Two cases are identified, cracks growing in plastically strained materials, such as occurs in high strain fatigue studies and at notch roots, and small cracks growing in single grains as occurs close to the fatigue limit in plain specimens.



Important mechanical and metallurgical features of short cracks are discussed with particular reference to the upper and lower bound definition of a short crack.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1460-2695.1982.tb01250.x

The short crack problem

The high-cycle fatigue (HCF) of titanium alloy turbine engine components remains a principal cause of failures in military aircraft engines. A recent initiative sponsored by the United States Air Force has focused on the major drivers for such failures in Ti-6Al-4V, a commonly used turbine blade alloy, specifically for fan and compressor blades. However, as most of this research has been directed toward a single processing/heat-treated condition, the bimodal (solution-treated and overaged (STOA)) microstructure, there have been few studies to examine the role of microstructure. Accordingly, the present work examines how the overall resistance to high-cycle fatigue in Ti-6Al-4V compares between the bimodal microstructure and a coarser lamellar (β-annealed) microstructure. Several aspects of the HCF problem are examined. These include the question of fatigue thresholds for through-thickness large and short cracks; microstructurally small, semi-elliptical surface cracks; and cracks subjected to pure tensile (mode I) and mixed-mode (mode I+II) loading over a range of load ratios (ratio of minimum to maximum load) from 0.1 to 0.98, together with the role of prior damage due to sub-ballistic impacts (foreign-object damage (FOD)). Although differences are not large, it appears that the coarse lamellar microstructure has improved smooth-bar stress-life (S-N) properties in the HCF regime and superior resistance to fatigue-crack propagation (in pure mode I loading) in the presence of cracks that are large compared to the scale of the microstructure; however, this increased resistance to crack growth compared to the bimodal structure is eliminated at extremely high load ratios. Similarly, under mixed-mode loading, the lamellar microstructure is generally superior. In contrast, in the presence of microstructurally small cracks, there is little difference in the HCF properties of the two microstructures. Similarly, resistance to HCF failure following FOD is comparable in the two microstructures, although a higher proportion of FOD-induced microcracks are formed in the lamellar structure following high-velocity impact damage.

Influence of microstructure on high-cycle fatigue of Ti-6Al-4V: Bimodal vs. lamellar structures

The two main effects of stress on phase transformation, kinetics modification and transformation plasticity, are reviewed for both diffusional and non-diffusional transformations. Results for these interactions during the pearlitic and martensitic transformation of steels under uniaxial tensile stress are analysed from a metallurgical point of view. These results are used to produce a model for a triaxial stress state, and in a finite element program for calculating internal stresses during quenching. Transformation plasticity is introduced in the calculation of internal stresses as an additional strain related to the stress state and to the progress of transformation, and the kinetics of martensitic transformation are also related to the stress state. The calculated results show that these phenomena have important consequences on the stress and plastic strain histories during quenching.MST/9

Stress–phase-transformation interactions – basic principles, modelling, and calculation of internal stresses

Quantitative analysis of microstructural effects on fatigue crack growth in widmanstätten Ti-6A1-4V and Ti-8Al-1Mo-1V

On microstructural control of near-threshold fatigue crack growth in 7000-series aluminum alloys

: Though a number of investigators have examined the influence of microstructural variables on near-threshold fatigue-crack growth rates in steels, a comprehensive understanding of the dependence of near-threshold growth rates on grain size, yield strength and microstructural morphology in steels has yet to emerge -- as noted in an excellent review by Ritchie. Recently, however, from our own extensive studies with alpha/beta titanium alloys, the basis for microstructural dependence of widely different fatigue crack growth rates was established for titanium alloys. Inasmuch as the micromechanistic model from that work does not depend uniquely on alloy family, it is of great interest to explore its applicability to steels -- especially since it predicts quantitatively the influence of yield strength and grain size in the near-threshold region for steels. Thus, the purpose of this paper is to critically analyze the near-threshold fatigue crack growth behavior, as reported in the literature, for steels of widely different strength level, grain size and microstructural morphology -- in the search for a systematic ordering of near-threshold fatigue crack growth rates that pertains to the whole gamut of steels. (Author)

A Critical Analysis of Grain-Size and Yield-Strength Dependence of Near-Threshold Fatigue-Crack Growth in Steels.

Prediction of slip-band facet angle in the fatigue crack growth of an AlLi alloy

A eutectoid steel exhibited abnormally large tensile extensibility in three quite dissimilar circumstances: i) micrograin plasticity, defined by a strain-rate sensitivity exponent m = 0.42, was found at 716°C for straining the ferrite-cementite aggregate at the rate e = 4.4 x 10-3 min-1, giving 133 pct elongation; ii) at a much greater strain rate, e = 25 min-1, superplasticity appeared in austenite strained at 927°C, giving 142 pct elongation; iii) a new type of transformation plasticity was predicted and experimentally verified: 490 pct elongation resulted from thermal cycling 21 times across Ae1. Plastic stability analysis distinguishes it from micrograin plasticity by showing that it owes to strain-hard ening during plastically stable flow; hence, there are no restrictions e or m. Furthermore, it is not necessary to perform the straining during the transformation, since the strain-hardening capacity can be regenerated by thermal cycling through the phase transformation if the transformation serves to recover the flow stress. Additional work showed that straining during austenitizing fails to increase m above pretransformation levels.

G.R. Yoder

Papers

Quantitative analysis of microstructural effects on fatigue crack growth in widmanstätten Ti-6A1-4V and Ti-8Al-1Mo-1V

On microstructural control of near-threshold fatigue crack growth in 7000-series aluminum alloys

A Critical Analysis of Grain-Size and Yield-Strength Dependence of Near-Threshold Fatigue-Crack Growth in Steels.

Prediction of slip-band facet angle in the fatigue crack growth of an AlLi alloy

Superplasticity in eutectoid steel