R elationships between the J-integral and the crack opening displacement δt are obtained by exploiting the dominance of the Hutchinson-Rice-Rosengren singularity in the crack-tip region. The coefficient dn that relates J to δt, is dependent on the material deformation properties and is independent of crack configuration under small-scale yielding conditions. For low hardening materials, dn appears to be configuration dependent in the fully yielded state. Similarly, the slope of the J-resistance curve is relatable to an operationally defined crack opening angle if J-controlled crack growth conditions are met. These relationships are corroborated by finite element results for the complete regime of elastic-plastic deformation and experimental data for A533B steels, HY-80 steels and several other ductile metals.

Relationships between the J-integral and the crack opening displacement for stationary and extending cracks

The concept of oxide-induced crack closure is utilized to explain the role of gaseous and aqueous environments on corrosion fatigue crack propagationat ultralow, near-threshold growth rates in bainitic and martensitic 2 1/4 Cr-1 Mo pressure vessel steels. It is shown that at low load ratios, near-threshold growth rates are significantly reduced in moist environments (such as air or water), compared to dry environments (such as hydrogen or helium gas), due to the formation of excess corrosion deposits on crack faces which enhances crack closure. Using Auger spectroscopy, it is found that at the threshold stress intensity, ΔKo, below which cracks appear dormant, the maximum thickness of excess oxide debris within the crack is comparable with the pulsating crack tip opening displacement. The implications of this model to near-threshold fatigue crack growth behavior, in terms of the role of load ratio, environment, and microstructure are discussed.

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Oxide-Induced Crack Closure: An Explanation for Near-Threshold Corrosion Fatigue Crack Growth Behavior

Powder metallurgy (PM) of titanium is a potentially cost-effective alternative to conventional wrought titanium. This article examines both traditional and emerging technologies, including the prod...

https://www.tandfonline.com/doi/pdf/10.1080/09506608.2017.1366003

Powder metallurgy of titanium – past, present, and future

Analysis of the deformation field consistent with a Prandtl stress distribution travelling with an advancing plane-strain crack reveals the functional form of the near tip crack profile in an elastic-plastic solid. The crack opening δ is shown to have the form δ ∼ r In (const./r) at a distance r from the tip. This observation coupled with data generated from finite element investigations of growing cracks in small-scale yielding permits the construction of a relation characterizing the deformation at an extending crack tip. A ductile crack-growth criterion consisting of the attainment of a critical opening at a small characteristic material distance from the tip is adopted. Predictions of the stability of a growing crack for both small-scale yielding specimens and those subject to general yielding are discussed.

Continuing crack-tip deformation and fracture for plane-strain crack growth in elastic-plastic solids

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

Fatigue crack growth rates in commercial purity Ti-6A1-4V can be substantially reduced with a beta anneal from levels associated with the mill anneal, owing primarily to crystallographic crack bifurcation in the Widmanstatten packets. This microstructurally sensitive fatigue crack growth occurs when the reversed plastic zone is less than the packet size and results in a fracture surface with a faceted morphology. It appears from replica and scanning electron microscopic examination that the facets are comprised of three superposed features,viz cleavage-like river-line patterns, very fine striations and traces of slip lines (and slip-band cracks). Limited X-ray evidence suggests a facet orientation some 8 to 10 degrees off the basal plane.

Observations on microstructurally sensitive fatigue crack growth in a widmanstätten Ti-6Al-4V alloy

Fatigue crack growth rates have been determined for beta-annealed Ti-6Al-4V alloys with respective oxygen contents of 0.06, 0.11, 0.18 and 0.20 wt pct. For each of these alloys, transitional crack growth behavior has been observed which appears to correlate with a critical value of the reversed plastic zone size,viz the Widmanstatten packet size. Moreover, growth rates below transitional levels order in terms of packet size, with lower growth rates associated with larger packets. The present results suggest that intersti-tial oxygen content and prior beta grain size significantly affect fatigue crack growth rates through control of the Widmanstatten packet size.

Fatigue Crack Propagation Resistance of Beta-Annealed Ti-6Al-4V Alloys of Differing Interstitial Oxygen Contents

On the effect of colony size on fatigue crack growth in Widmanstätten structure α+β titanium alloys

Significant enhancement in fatigue crack propagation resistance and plane strain fracture toughness was obtained in commercial purity Ti-6Al-4V and Ti-6Al-6V-2Sn through microstructural modification. Alloys studied were in the form of 25.4 mm-thick plate with interstitial oxygen contents of 0.20 and 0.17 weight percent, respectively. Heat treatments were chosen to provide widely varied microstructures; these included a mill anneal, recrystallization anneal and a beta anneal. The most beneficial heat treatment for improving these crack tolerance properties was found to be the beta anneal. However, the beta anneal resulted in yield strength reductions of 9 to 14 percent, from levels associated with the original mill anneal. The recrystallization anneal provided significant enhancement of plane strain fracture toughness and marginal improvement in fatigue crack propagation resistance with negligible loss of yield strength.

Enhancement of Fatigue Crack Growth and Fracture Resistance in Ti-6Al-4V and Ti-6Al-6V-2Sn Through Microstructural Modification

Fatigue crack growth rates (da/dN) in ambient laboratory air have been determined for a wide variety of materials from four basic α + β titanium alloy systems. Each material was cyclically loaded with a haversine waveform and a load ratio, R = 0.10. The results indicate that, at a constant value of stress-intensity range (ΔK), the width of the da/dN data band exceeds an order of magnitude. For example, at ΔK = 21 MPa·m1/2 , a 50-fold difference in fatigue crack propagation rates is observed. Analysis of the crack growth rate data at this point indicates a systematic dependence on grain size (l ), viz. that da/dN decreases with increasing l . An interpretation of this effect is offered in terms of reversed (cyclic) plastic zone size considerations.

G. R. Yoder

Papers

Observations on microstructurally sensitive fatigue crack growth in a widmanstätten Ti-6Al-4V alloy

Fatigue Crack Propagation Resistance of Beta-Annealed Ti-6Al-4V Alloys of Differing Interstitial Oxygen Contents

On the effect of colony size on fatigue crack growth in Widmanstätten structure α+β titanium alloys

Enhancement of Fatigue Crack Growth and Fracture Resistance in Ti-6Al-4V and Ti-6Al-6V-2Sn Through Microstructural Modification

50-Fold Difference in Region-II Fatigue Crack Propagation Resistance of Titanium Alloys: A Grain-Size Effect