L.A. Cooley

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

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 Widmanstätten 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.

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

Abstract Analysis of fatigue crack growth behavior of five beta-annealed alloys indicates significant effects of microstructure upon the logarithmic plot of fatigue crack growth rate (d a /d N ) vs stress-intensity range ( ΔK ). Each plot exhibits a bilinear form with a transition at ΔK T , the position of which lies between 18 and 31 MPa · m 1 2 for the five alloys. Comparison of mean Widmanstatten packet size to cyclic plastic zone size, as analyzed on the basis of different models, indicates their equality at ΔK T . Greater clustering of the packet size distribution about the mean value effects a decrease in hypertransitional exponent ( m A ) in the power law, d a /d N = C ( ΔK ) m , and a simulataneous increase in the sharpness of transition. Hypertransitional exponents for the five alloys lie between 2.9 ≤ m A ≤ 4.7, whereas hypotransitional exponents lie between 5.9 ≤ m B ≤ 7.6. Hypotransitional growth rates vary inversely with mean packet size, with a 20-fold reduction in d a /d N observed for a 3.5-fold increase in mean packet size.

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

Abstract : Fatigue crack growth rate data for 7000-series aluminum alloys can be approximated with a multilinear form, when plotted in conventional logarithmic coordinates over a sufficiently broad spectrum of delta K. Each transition point in the growth-rate curve appears to be associated with a specific microstructural feature that can serve as a barrier to slip-band transmission, in accord with the cyclic plastic zone model of fatigue crack growth. Evidence strongly suggests that transition to the threshold for fatigue crack growth is controlled by dispersoid particles. (Author)

Fatigue crack propagation resistance of beta-annealed Ti-6AI-4V alloys of differing interstitial oxygen contents

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 Widmanstätten 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 Widmanstätten packet size.

Fatigue-crack growth in 2090 AlLi alloy

Abstract The fatigue-crack growth kinetics of the 12.7 mm thick 2090-T8E41 alloy plate vary significantly with plate thickness location. The apparent fatigue-crack growth rates in the midthickness location are substantially lower than those in the surface region at intermediate and high applied stress intensity ranges. The lower crack growth rates are related to the extraordinarily tortuous fatigue profile which induces high crack closure levels. Such a tortuous crack profile appears to be primarily a consequence of the alloys intense crystallographic texture and its great propensity for planar slip.

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

Abstract : 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)

Procedures for Precision Measurement of Fatigue-Crack-Growth-Rate Using Crack-Opening-Displacement Techniques.

This paper describes experimental and analytical procedures whereby the conventional commercial fracture mechanics clip-gage can be used for precision measurement of fatigue crack growth rate in compact-type specimens. Potential sources of error in measuring crack length via crack-opening displacement (COD) techniques are delineated. Comparisons are made between crack-length data obtained via specimen surface observations and COD techniques. Comparisons are also made between data analyzed by the secant and 7-point incremental polynomial methods. It is emphasized that COD techniques can enhance the accuracy of the secant method of data reduction. Steploading procedures using COD techniques are described. Proposed amendments to ASTM Tentative Test Method for Constant-Load-Amplitude Fatigue Crack Growth Rates Above 10 - 8 m/Cycle (E 647-78 T) regarding incrementing of crack-length measurements via COD techniques are discussed.

Effects of Microstructure and Frequency on Corrosion-Fatigue Crack Growth in Ti-8Al-1Mo-1V and Ti-6Al-4V

Abstract : Fatigue crack growth studies were conducted on Ti-8Al-1Mo-1V and Ti-6Al-4V alloys in 3.5% NaCl aqueous solution. Each alloy was studied in two microstructural conditions and at two cyclic frequencies. The Ti-8Al-1Mo-1V was heat treated to produce a fine-grained duplex anneal microstructure and a coarse-grained Widmanstatten microstructure resulting from a beta anneal. The two microstructural conditions for the Ti-6Al-4V were an as-received mill anneal and a beta anneal. The two cyclic frequencies were 01. and 5.0 Hz. Each of the four alloy/microstructure combinations studied has been the subject of prior investigation regarding fatigue crack growth rate/microstructure interactions in ambient air environments. For both alloys, crack growth rates in air were found to be significantly reduced as a result of microstructural modifications associated with the beta anneal heat treatment. Although the salt water environment significantly accelerated crack growth rates for both alloys, this same ranking of fatigue crack growth resistance persisted in the present study. Both microstructures of the Ti-6Al-4V alloy exhibited a frequency crossover effect; in contrast, no significant frequency effects were observed in the Ti-8Al-1Mo-1V, in either microstructure. Similar frequency effects were seen in separate specimens cycled at a single constant frequency or in single specimens cycled at two alternating frequencies. Out-of-plane cracking was observed in both alloys in the beta annealed condition. The effects of varying degrees of out-of-plane cracking on apparent crack growth rates are noted. (Author)

Unusual fracture mode in the fatigue of an Al-Li alloy

ABSTRACT The aluminum-lithium alloy 2090-T8E41, which exhibits uncommonly good resistance to the growth of fatigue cracks over a broad spectrum of stress-intensity range (ΔK), also exhibits a fatigue fracture surface morphology and crack closure levels that are quite different than observed with conventional aluminum alloys such as 7075-T651. For fatigue crack growth at a stress ratio of R = 0.10 in ambient air, the fracture surface of the Al-Li alloy exhibits an extraordinary tortuosity, with considerable oxide debris attributable to fretting —- giving rise to a macroscopically blackish appearance. Associated with this tortuosity, the fracture surface exhibits asperities of unusual height, as comprised of adjacent pairs of slip-band facets. This height is a consequence of two synergetic factors, viz. an extraordinary textural intensity and an uncommon propensity for a planar slip mode in Al-Li alloys. Thus individual, well defined slip-band facets are formed which can traverse multiple grains at a time to give asperities of unusual height — which give rise to high closure levels at stress-intensity ranges much above near-threshold values. Moreover, it is shown that the characteristic included angle between an adjacent pair of slip-band facets which comprise an individual asperity is a consequence of the texture. Thus, important progress has been made to elucidate the vastly superior resistance of the 2090 alloy and its micromechanistic basis.