G.R. Yoder

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)

Superplasticity in eutectoid steel

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 è = 4.4 x 10-3 min-1, giving 133 pct elongation; ii) at a much greater strain rate, ε = 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 ε 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.

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)

Fractographic lines in maraging steel—A link to fracture toughness

Lines of constant spacing are exhibited across the surfaces of fracture toughness specimens in an 18 Ni (200) maraging steel. The wavelength associated with this periodic lineage decreases, however, with decreasing fracture toughness as the steel is maraged for greater time to higher strength levels. The periodicity as well as the shape of the lines, which appear to trace successive positions of an advancing crack front, suggests that the crack advances by a repetitive discontinuous unit process such as postulated in the tensile ligament instability theory of fracture toughness. Calculations support interpretation of the wavelength as the process zone size of that theory. The lineage is found in several different types of fracture toughness specimens as well as in tensile specimens. Furthermore, such lineage has been detected for several alloys in addition to maraging steel, and in certain cases for cracking modes other than fast fracture,e.g., stress-corrosion cracking. However, for all cases where the periodic lineage appears, microvoid coalescence is the mode of ultimate separation. The lines are clearly distinguished from Wallner lines.

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.

On fatigue crack growth in a Ti-4.5Al-5Mo-1.5Cr alloy with metastable β-phase

Abstract Fatigue crack growth behavior has been examined in a Ti-4.5Al-5Mo-1.5Cr alloy, for two different levels of β-phase metastability. The resistance to fatigue crack growth appears to be marginally enhanced with the presence of metastable β-phase in a microstructure also containing some primary α-phase (~30 pct) of high aspect ratio. This enhancement appears slightly greater for β-phase water quenched from 899°C, than as more slowly cooled in helium from this same solution treatment temperature, at approximately an air-cooling rate. In the case of the former, nearly full retention of solute in the β-phase is apparent, while in the latter, significant precipitation of secondary α-phase is evident in thin-foil transmission electron micrographs.

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.