44 Matching Results

Search Results

Advanced search parameters have been applied.

Influences of gaseous environment on low growth-rate fatigue crack propagation in steels. Annual report No. 1, January 1980. Report No. FPL/R/80/1030

Description: The influence of gaseous environment is examined on fatigue crack propagation behavior in steels. Specifically, a fully martensitic 300-M ultrahigh strength steel and a fully bainitic 2-1/4Cr-1Mo lower strength steel are investigated in environments of ambient temperature moist air and low pressure dehumidified hydrogen and argon gases over a wide range of growth rates from 10/sup -8/ to 10/sup -2/ mm/cycle, with particular emphasis given to behavior near the crack propagation threshold ..delta..K/sub 0/. It is found that two distinct growth rate regimes exist where hydrogen can markedly accelerate crack propagation rates compared to air, (1) at near-threshold levels below (5 x 10/sup -6/ mm/cycle) and (2) at higher growth rates, typically around 10/sup -5/ mm/cycle above a critical maximum stress intensity K/sub max//sup T/. Hydrogen-assisted crack propagation at higher growth rates is attributed to a hydrogen embrittlement mechanism, with K/sub max//sup T/ nominally equal to K/sub Iscc/ (the sustained load stress corrosion threshold) in high strength steels, and far below K/sub Iscc/ in the strain-rate sensitive lower strength steels. Hydrogen-assisted crack propagation at near-threshold levels is attributed to a new mechanism involving fretting-oxide-induced crack closure generated in moist (or oxygenated) environments. The absence of hydrogen embrittlement mechanisms at near-threshold levels is supported by tests showing that ..delta..K/sub 0/ values in dry gaseous argon are similar to ..delta..K/sub 0/ values in hydrogen. The potential ramifications of these results are examined in detail.
Date: January 1, 1980
Creator: Ritchie, R.O.; Suresh, S. & Toplosky, J.
Partner: UNT Libraries Government Documents Department

Mechanistic dissimilarities between environmentally-influenced fatigue-crack propagation at near-threshold and higher growth rates in lower-strength steels

Description: The role of hydrogen gas in influencing fatigue crack propagation is examined for several classes of lower strength pressure vessel and piping steels. Based on measurements over a wide range of growth rates from 10/sup -8/ to 10/sup -2/ mm/cycle, crack propagation rates are found to be significantly higher in dehumidified gaseous hydrogen compared to moist air in two distinct regimes of crack growth, namely (i) at the intermediate range of growth typically above approx. 10/sup -5/ mm/cycle, and (ii) at the near-threshold region below approx. 10/sup -6/ mm/cycle approaching lattice dimensions per cycle. Both effects are seen at maximum stress intensities (K/sub max/) far below the sustained-load threshold stress intensity for hydrogen-assisted cracking (K/sub Iscc/). Characteristics of environmentally influenced fatigue crack growth in each regime are shown to be markedly different with regard to fractography and the effect of such variables as load ratio and frequency. It is concluded that the primary mechanisms responsible for the influence of the environment in each regime are distinctly different. Whereas corrosion fatigue behavior at intermediate growth rates can be attributed to hydrogen embrittlement processes, the primary role of moist environments at near-threshold levels is shown to involve a contribution from enhanced crack closure due to the formation of crack surface corrosion deposits at low load ratios.
Date: November 1, 1981
Creator: Suresh, S. & Ritchie, R. O.
Partner: UNT Libraries Government Documents Department


Description: It is generally accepted that the fatigue or endurance strength of planar slip materials, such as steel and brass, is increased by refining the grain size, whereas in wavy slip materials, such as pure copper and pure aluminum, the fatigue strength is unaffected. However, there is little similar evidence of an effect of grain size on fatigue crack propagation. In both wavy and planar slip metals, growth rates appear independent of grain size. For example, variations in grain size from 10 to 200{micro}m in 70/30 brass (6), and from 45 to 480{micro}m in austenitic stainless steel produce no measurable change in fatigue crack propagation rates over a range of growth rates from 10{sup -5} to 10{sup -2} mm/cycle. Recently, however, there have been indications in the literature that grain size may indeed influence crack propagation behavior at growth rates less than 10{sup -5} to 10{sup -6} mm/cycle approaching the threshold for crack propagation, {Delta}K{sub 0}. Robinson and Beevers report an order of magnitude decrease in near-threshold growth rates in {alpha}-titanium after coarsening the grain size from 20 to 200{micro}m. Similar effects have been seen in Ti-6Al-4V. Furthermore, Masounave and BaIlon have observed a marked increase in threshold {Delta}K{sub 0} values in a range of low strength steels by increasing ferrite grain size. In all the above studies however, no attempt was made to control strength; and the effect of coarsening the grain size may well have been caused by a concurrent decrease in material strength, particularly since it is known that, in steels at least, near-threshold fatigue crack growth is markedly decreased by reducing the yield strength. A comparison at constant yield strength between coarse and fine-grained materials has been made in ultra-high strength steel (300-M) where it was found that, on enlarging the (prior austenite) grain size from 20 ...
Date: January 1, 1977
Creator: Carlson, M.F. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

An in situ transmission electron microscopy study of the thermalstability of near-surface microstructures induced by deep rolling andlaser-shock peening

Description: Mechanical surface treatments are known to be effective at improving the fatigue resistance of metallic alloys at elevated temperatures ({approx}550-600 C), even though the near-surface compressive residual stress fields have been annealed out. We have investigated the thermal stability of near-surface microstructures induced by deep rolling and laser-shock peening in an austentic stainless steel (AISI 304) and a titanium alloy (Ti-6Al-4V) using in situ hot-stage transmission electron microscopy. It is found that the improvements in fatigue resistance at elevated temperature are related to the high-temperature stability of the work-hardened near-surface microstructure in each case.
Date: February 24, 2003
Creator: Altenberger, I.; Stach, E.A.; Liu, G.Y.; Nalla, R.K. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

Effect of orientation on the in vitro fracture toughness ofdentin: The role of toughening mechanisms

Description: A micro-mechanistic understanding of bone fracture thatencompasses how cracks interact with the underlying microstructure anddefines their local failure mode is lacking, despite extensive research nthe response of bone to a variety of factors like aging, loading, and/ordisease.
Date: January 28, 2003
Creator: Nalla, R.K.; Kinney, J.H. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

Determining the toughness of ceramics from Vickers indentationsusing the crack-opening displacements: An experimental study

Description: Recently, a method for evaluating the fracture toughness of ceramics has been proposed based on the computed crack-opening displacements of cracks emanating from Vickers hardness indentations. In order to verify this method, experiments were carried out to determine the toughness of a commercial silicon carbide ceramic, Hexaloy SA, by measuring the crack-opening profiles of such Vickers indentation cracks. While the obtained toughness value of Ko = 2.3 MPavm was within 10% of that measured using conventional fracture toughness testing, the computed crack-opening profiles corresponding to this toughness displayed poor agreement with those measured experimentally, raising concerns about the suitability of this method for determining the toughness of ceramics. The effects of subsurface cracking and cracking during loading are considered as possible causes of such discrepancies, with the former based on evidence observed for secondary radial cracking which affected the crack opening profile and deduced toughness values.
Date: October 30, 2002
Creator: Kruzic, J.J. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

Stress-corrosion fatigue-crack growth in a Zr-based bulk amorphousmetal

Description: Electrochemical and mechanical experiments were conducted to analyze the environmentally-influenced cracking behavior of a bulk amorphous metal, Zr41.2Ti13.8Cu12.5Ni10Be22.5. This study was motivated by a scientific interest in mechanisms of fatigue-crack propagation in an amorphous metal, and by a practical interest in the use of this amorphous metal in applications that take advantage of its unique properties, including high specific strength, large elastic strains and low damping. The objective of the work was to determine the rate and mechanisms of subcritical crack growth in this metallic glass in an aggressive environment. Specifically, fatigue-crack propagation behavior was investigated at a range of stress intensities in air and aqueous salt solutions by examining the effects of loading cycle, stress-intensity range, solution concentration, anion identity, solution de-aeration, and bulk electrochemical potential. Results indicate that crack growth in aqueous solution in this alloy is driven by a stress-assisted anodic reaction at the crack tip. Rate-determining steps for such behavior are reasoned to be electrochemical, stress-dependent reaction at near-threshold levels, and mass transport at higher (steady-state) growth rates.
Date: September 21, 2005
Creator: Schroeder, V. & Ritchie, R. O.
Partner: UNT Libraries Government Documents Department

Effects of the amorphous oxide intergranular layer structure and bonding on the fracture toughness of a high purity silicon nitride

Description: The microstructural evolution and structural characteristics and transitions in the thin grain-boundary oxide films in a silicon nitride ceramic, specifically between two adjacent grains and not the triple junctions, are investigated to find their effect on the macroscopic fracture properties. It is found that by heat treating a model Si3N4-2wt percent Y2O3 ceramic for {approx}200 hr at 1400 degrees C in air, the fracture toughness can be increased by {approx}100 percent, coincident with a change in fracture mechanism from transgranular to intergranular.
Date: November 18, 2002
Creator: Ziegler, A.; Kisielowski, C.; Hoffmann, M.J. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

Small fatigue cracks: mechanics, mechanisms and engineering applications

Description: Damage-tolerant design and life-prediction methodologies have been practiced for metallic structures for decades, although their application to brittle materials, such as ceramics, and intermetallic alloys, still poses particular problems, primarily because of their extreme flaw-sensitivity.
Date: May 1, 2001
Creator: Ritchie, R.O. & Peters, J.O.
Partner: UNT Libraries Government Documents Department

In situ measurement of fatigue-crack growth rates in a silicon carbide ceramic at elevated temperatures using a D.C. potential system

Description: The understanding of the mechanisms of fatigue-crack propagation in advanced ceramics at elevated temperatures (>800 degrees C) has in part been hampered by the experimental difficulty in directly measuring crack lengths, and hence crack-growth rates, at such high temperatures.
Date: October 12, 1999
Creator: Chen, D.; Gilbert, C.J. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

Atomic resolution transmission electron microscopy of the intergranular structure of a Y{sub 2}O{sub 3}-silicon nitride ceramic

Description: High-resolution transmission electron microscopy (HRTEM) employing focus-variation phase-reconstruction methods is used to image the atomic structure of grain boundaries in a silicon nitride ceramic at a resolution of 0.8 Angstrom
Date: May 1, 2002
Creator: Ziegler, A.; Kisielowski, C.; Hoffmann, M.J. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

Mo-Si-B Alloy Development

Description: Mo-Si-B silicides consisting of the phases {alpha}-Mo (Mo solid solution), Mo{sub 3}Si, and Mo{sub 5}SiB{sub 2} have melting points on the order of 2000 C and have potential as ultra-high temperature structural materials. Mo-Si-B alloys can be processed such that the {alpha}-Mo is present in the form of isolated particles in a silicide matrix, or as a continuous matrix ''cementing'' individual silicide particles together. The latter microstructure is similar to that of WC-Co hard metals. This paper focuses on the relationship between the topology as well as scale of the microstructure of Mo-Mo{sub 3}Si-Mo{sub 5}SiB{sub 2} alloys, and their creep strength and fracture toughness. For example, the creep strength of Mo-Si-B alloys is improved by reducing the {alpha}-Mo volume fraction and by making the {alpha}-Mo phase discontinuous. The fracture toughness is improved by increasing the {alpha}-Mo volume fraction and by making the {alpha}-Mo phase continuous. Room temperature stress intensity factors as high as 21 MPa m{sup 1/2} were obtained. The room temperature fracture toughness of Mo-Si-B alloys can also be improved by microalloying with Zr. The room temperature ductility of Mo itself can be improved by adding MgAl{sub 2}O{sub 4} spinel particles suggesting yet another way to improve the ductile phase toughening of Mo-Si-B alloys.
Date: April 24, 2003
Creator: Schneibel, J. H.; Kruzie, J. J. & Ritchie, R. O.
Partner: UNT Libraries Government Documents Department

Mechanism of fatigue in micron-scale films of polycrystalline silicon for microelectromechanical applications

Description: Reported nearly a decade ago, cyclic fatigue failure in silicon thin films has remained a mystery. Silicon does not display the room temperature plasticity or extrinsic toughening mechanisms necessary to cause fatigue in either ductile (e.g., metals) or brittle (e.g., ceramics and ordered mintermetallic) materials.
Date: August 2, 2001
Creator: Muhlstein, C.L.; Stach, E.A. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

Fracture and fatigue properties of Mo-Mo{sub 3}Si-Mo{sub 5}SiB{sub 2} refractory intermetallic alloys at ambient to elevated temperatures (25-1300 degrees Centigrade)

Description: The need for structural materials with high-temperature strength and oxidation resistance coupled with adequate lower-temperature toughness for potential use at temperatures above {approx} 1000 degrees C has remained a persistent challenge in materials science. In this work, one promising class of intermetallic alloys is examined, namely boron-containing molybdenum silicides, with compositions in the range Mo (bal), 12-17 at. percentSi, 8.5 at. percentB, processed using both ingot (I/M) and powder (P/M) metallurgy methods. Specifically, the oxidation (''pesting''), fracture toughness and fatigue-crack propagation resistance of four such alloys, which consisted of {approx}21 to 38 vol. percent a-Mo phase in an intermetallic matrix of Mo3Si and Mo5SiB2 (T2), were characterized at temperatures between 25 degrees and 1300 degrees C. The boron additions were found to confer superior ''pest'' resistance (at 400 degrees to 900 degrees C) as compared to unmodified molybdenum silicides, such as Mo5Si3. Moreover , although the fracture and fatigue properties of the finer-scale P/M alloys were only marginally better than those of MoSi2, for the I/M processed microstructures with coarse distributions of the a-Mo phase, fracture toughness properties were far superior, rising from values above 7 MPa sqrt m at ambient temperatures to almost 12 MPa sqrt m at 1300 degrees C.
Date: August 1, 2002
Creator: Choe, Heeman; Schneibel, J.H. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

Fatigue-crack propagation in advanced aerospace materials: Aluminum-lithium alloys

Description: Characteristics of fatigue-crack propagation behavior are reviewed for recently developed commercial aluminum-lithium alloys, with emphasis on the underlying micromechanisms associated with crack advance and their implications to damage-tolerant design. Specifically, crack-growth kinetics in Alcoa 2090-T8E41, Alcan 8090 and 8091, and Pechiney 2091 alloys, and in certain powder-metallurgy alloys, are examined as a function of microstructure, plate orientation, temperature, crack size, load ratio and loading sequence. In general, it is found that growth rates for long (> 10 mm) cracks are nearly 2--3 orders of magnitude slower than in traditional 2000 and 7000 series alloys at comparable stress-intensity levels. In additions, Al-Li alloys shown enhanced crack-growth retardations following the application of tensile overloads and retain superior fatigue properties even after prolonged exposure at overaging temperatures; however, they are less impressive in the presence of compression overloads and further show accelerated crack-growth behavior for microstructurally-small (2--1000 {mu}m) cracks (some three orders of magnitude faster than long cracks). These contrasting observations are attributed to a very prominent role of crack-tip shielding during fatigue-crack growth in Al-Li alloys, promoted largely by the tortuous and zig-zag nature of the crack-path morphologies. Such crack paths result in locally reduced crack-tip stress intensities, due to crack deflection and consequent crack wedging from fracture-surface asperities (roughness-induced crack closure); however, such mechanisms are far less potent in the presence of compressive loads, which act to crush the asperities, and for small cracks, where the limited crack wake severely restricts the shielding effect. 50 refs., 21 figs.
Date: October 1, 1988
Creator: Venkateswara Rao, K.T. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

Influence of extrinsic crack deflection and delamination mechanisms on the cryogenic toughness of aluminum-lithium alloy 2090: Behavior in plate (T81) vs sheet (T83) material

Description: Cryogenic strength-toughness relationships are examined in 1.6-mm- thick sheet of commercial 2090-T8 aluminum-lithium alloy, and results compared with behavior in 12.7-mm-thick rolled plate. Unlike the significant increase in L-T fracture toughness exhibited by thick place sections at cryogenic temperatures, the thin sheet (of normally similar composition and microstructure) shows a marked decrease in toughness between 298 and 77 K. Such contrasting observations are attributed primarily to the low short-transverse toughness of the 2090-plate material, which results in enhanced through-thickness intergranular splitting during low-temperature fracture and hence to a prominent role of crack-divider delamination toughening. 23 refs., 6 figs., 1 tab.
Date: February 1, 1989
Creator: Venkateswara Rao, K.T. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

Mechanics and mechanisms of cyclic fatigue-crack propagation in transformation-toughened zirconia ceramics

Description: Damage and cyclic fatigue failure under alternating loading in transformation-toughened zirconia ceramics is reviewed and compared to corresponding behavior under quasi-static loading (static fatigue). Current understanding of the role of transformation toughening in influencing cyclic fatigue-crack propagation behavior is examined based on studies which altered the extent of the tetragonal-to-monoclinic phase transformation in MG-PSZ through subeutectoid aging. These studies suggest that near-tip computations of the crack-driving force (in terms of the local stress intensity) can be used to predict crack-growth behavior under constant amplitude and variable-amplitude (spectrum) loading, using spatially resolved Raman spectroscopy to measure the extent of the transformation zones. In addition, results are reviewed which rationalize distinctions between the crack-growth behavior of preexisting, ``long`` (> 2 mm), through-thickness cracks and naturally-occurring, ``small`` (1 to 100 {mu}m), surface cracks in terms of variations in crack-tip shielding with crack size. In the present study, the effect of grain size variations on crack-growth behavior under both monotonic (R-curve) and cyclic fatigue loading are examined. Such observations are used to speculate on the mechanisms associated with cyclic crack advance, involving such processes as alternating shear via transformation-band formation, cyclic modification of the degree of transformation toughening, and uncracked-ligament (or grain) bridging.
Date: May 1, 1992
Creator: Hoffman, M. J.; Dauskardt, R. H.; Ritchie, R. O. & Mai, Y. W.
Partner: UNT Libraries Government Documents Department

Mechanical relaxation of localized residual stresses associatedwith foreign object damage

Description: Foreign-object damage associated with the ingestion ofdebris into aircraft turbine engines can lead to a marked degradation inthe high-cycle fatigue life of turbine components. This degradation isgenerally considered to be associated with the premature initiation offatigue cracks at or near the damage sites; this is suspected to be dueto, at least in part, the impact-induced residual stress state, which canbe strongly tensile in these locations.
Date: May 1, 2002
Creator: Boyce, B.L.; Chen, X.; Peters, J.O.; Hutchinson, J.H. & Ritchie,R.O.
Partner: UNT Libraries Government Documents Department

Fatigue failure in thin-film polysilicon is due to subcriticalcracking within the oxide layer

Description: It has been established that microelectromechanical systems (MEMS) created from polycrystalline silicon thin-films are subject to cyclic fatigue. Prior work by the authors has suggested that although bulk silicon is not susceptible to fatigue failure in ambient air, fatigue in micron-scale silicon is a result of a ''reaction-layer'' process, whereby high stresses induce a thickening of the post-release oxide at stress concentrations such as notches, which subsequently undergoes moisture-assisted cracking. However, there exists some controversy regarding the post-release oxide thickness of the samples used in the prior study. In this Letter, we present data from devices from a more recent fabrication run that confirm our prior observations. Additionally, new data from tests in high vacuum show that these devices do not fatigue when oxidation and moisture are suppressed. Each of these observations lends credence to the '''reaction-layer'' mechanism. Recent advances in the design of microelectromechanical systems (MEMS) have increased the demand for more reliable microscale structures. Although silicon is an effective and widely used structural material at the microscale, it is very brittle. Consequently, reliability is a limiting factor for commercial and defense applications. Since the surface to volume ratio of these structural films is very large, classical models for failure modes in bulk materials cannot always be applied. For example, whereas bulk silicon is immune to cyclic fatigue failure thin micron-scale structural films of silicon appear to be highly susceptible. It is clear that at these size scales, surface effects may become dominant in controlling mechanical properties. The main reliability issues for MEMS are stiction, fatigue and wear. Fatigue is important in cases where devices are subjected to a large number of loading cycles with amplitudes below their (single-cycle) fracture stress, which may arise due to vibrations intentionally induced in the structure (i.e. a resonator) or those which arise ...
Date: January 11, 2005
Creator: Alsem, D.H.; Muhlstein, C.L.; Stach, E.A. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

How Tough is Human Cortical Bone? In-Situ Measurements on Realistically Short Cracks

Description: Bone is more difficult to break than to split. Although this is well known, and many studies exist on the behavior of long cracks in bone, there is a need for data on the orientation-dependent crack-growth resistance behavior of human cortical bone which accurately assesses its toughness at appropriate size-scales. Here we use in-situ mechanical testing in the scanning electron microscope and x-ray computed tomography to examine how physiologically-pertinent short (<600 mu m) cracks propagate in both the transverse and longitudinal orientations in cortical bone, using both crack-deflection/twist mechanics and nonlinear-elastic fracture mechanics to determine crack-resistance curves. We find that after only 500 mu m of cracking, the driving force for crack propagation was more than five times higher in the transverse (breaking) direction than in the longitudinal (splitting) direction due to major crack deflections/twists principally at cement sheathes. Indeed, our results show that the true transverse toughness of cortical bone is far higher than previously reported. However, the toughness in the longitudinal orientation, where cracks tend to follow the cement lines, is quite low at these small crack sizes; it is only when cracks become several millimeters in length that bridging mechanisms can develop leading to the (larger-crack) toughnesses generally quoted for bone.
Date: May 10, 2008
Creator: Ritchie, Robert O.; Koester, K. J.; Ager III, J. W. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

On the development of life prediction methodologies for the failure of human teeth

Description: Human dentin is known to be susceptible to failure under cyclic loading. Surprisingly, there are few reports that quantify the effect of such loading, considering the fact that a typical tooth experiences a million or so loading cycles annually. In the present study, a systematic investigation is described of the effects of prolonged cyclic loading on human dentin in a simulated physiological environment. In vitro stress-life (S/N) data are discussed in the context of possible mechanisms of fatigue damage and failure.
Date: September 18, 2002
Creator: Nalla, R.K.; Imbeni, V.; Kinney, J.H.; Marshall, S.J. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department