Effect of in situ material properties on fatigue damage modes in titanium matrix composites

Titanium matrix composites (TMC) and their behavior under mechanical fatigue loads was the subject of this research. The primary objective was to explain fatigue damage modes in center-notched TMC specimens. Two modes of damage have been observed in continuously reinforced, zero-degree unidirectional, SCS-6/Ti-15V-3Cr-3Al-3Sn (SCS-6/Ti-15-3) laminates. The fatigue specimens were destructively analyzed using optical microscopy to determine where cracks originated and how they grew throughout the specimen. A micromechanical model was developed to explain the fatigue crack patterns observed in the interface region surrounding the fibers of the woven and acrylic-binder TMC material systems. A two-dimensional (2-D) model of a longitudinal lamina with a center hole was used to obtain a set of displacement boundary conditions for an element near the notch, yet within the net section where the spiral crack patterns were observed. These boundary conditions were then used on a three-dimensional (3-D) unit cell model of the fiber, matrix, and interface. This article is based on a presentation made in the symposium “Fatigue and Creep of Composite Materials” presented at the TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee.     

Slow crystallographic fatigue crack growth in a nickel-base alloy

Fatigue crack propagation rates at very low cyclic stress intensity levels (1 to 3 MNm^-372) have been measured in cube-oriented, planar slip nickel-base superalloy monocrystals using a high frequency (20 kHz) resonant fatigue testing technique. It is found that crack propagation is entirely along the crystallographic slip planes and the crack growth rate does not drop off into a threshold behavior but follows a power law with a power law exponent close to 4, which is similar to the functional dependency observed at higher cyclic stress intensity levels in similar superalloys. The observed behaviors are discussed with respect to a new theory on threshold and the effects of strong crystallographic constraints on crack propagation behavior.     

Formation of crystallographic texture in titanium nickelide single crystals during rolling

The texture formation in the single crystals of the Ti-48 at % Ni-2 at % Fe alloy rolled at a temperature of 350°C up to a strain of 80% in eleven different initial orientations is examined. There are three stable initial orientations that remain unchanged in rolling: {011}〈011〉, {111}〈011〉, and {111}〈112〉. The TiNi single crystals are shown to be deformed by means of a combined action of slip in systems {011}〈001〉 and twinning in systems {114}〈221〉 and {118}〈441〉. The types of formed rolling texture in the single crystals depend on their initial orientation and strain.     

Electrochemical detection of fatigue damage

Electrochemical reoxidation currents are shown to provide a new method of detecting fatigue damage in metals. The basic principle is demonstrated by measurements on anodized samples of aluminum. The initial anodic oxide film is ruptured at the site of plastic deformation and the flow of charge is measured during subsequent reanodization, or healing of the oxide. In a preliminary series of experiments on tensile specimens, the total flow of charge is shown to be the sum of two components: i) that associated with charging the capacitor created by the metal specimen, the oxide film, and the electrolyte ii) that associated with reoxidation of the microcracks in the oxide film. The reoxidation component is in agreement with a simple model calculation of oxide rupture, and provides a quantitative measure of the total area of the microcracks in the oxide film. In the fatigue experiments these two components of charge flow are separated by a simple immersion scanning procedure, which also reveals the localized distribution of the damage. The flow of charge to these localized regions correlates with observations of the severity of the damage by scanning electron microscopy. Fatigue cracks ∼50 μm long or longer can be detected. In our specimens, these occurred after 10 percent of the fatigue life. The magnitude of the associated reanodization current transient is an accurate measure of the crack length and can predict the fatigue life within ±50 percent.     

Thermal activation of fatigue damage

The effect of temperature on the fatigue of aluminum alloys results from a combination of thermally induced changes in the microstructure and the intrinsic temperature dependence of the fatigue process. These two effects are separated for the first time, and it is shown that the intrinsic fatigue process is thermally activated. Two distinct regimes are identified. For fatigue lives <3 × 10⁶ cycles, the activation energy is 86 kJ/mole in 339 aluminum/15 pct Kaowool composites and 120 kJ/mole in unreinforced 5086 aluminum, i.e., in the range reported for diffusion in aluminum. For fatigue lives >3 × 10⁶ cycles, the activation energy is 240 kJ/mole. The magnitude of all three activation barriers decreases in direct proportion to the applied cyclic stress. These results are consistent with a dislocation model of jog formation at low cyclic stresses and the diffusion-assisted motion of jogs at high cyclic stresses. The activation volumes correspond to dislocation loop lengths of 10 to 30 nm.     

Analysis of crystallographic high temperature fatigue crack growth in a nickel base alloy

Crack growth behavior of a nickel-base alloy, Udimet 700, was studied at room temperature and 850 °C in air and vacuum. Crack growth rates were higher in air than in vacuum but this increase in growth rates was nearly the same at both temperatures. In contrast to the effect of environment, an increase of temperature from 25 to 850 °C has a much larger effect on growth rates although the mode of crack growth did not change with temperature or with environment. A detailed analysis of the fracture surfaces indicated that the growth rates under all of the above experimental conditions occurs by a crystallographic faceted mode with the plane of the facet identified to be the {100} cleavage plane rather than a slip plane. Also the increase in growth rates with temperature appears not to be directly related to an environmental effect, creep effect or variation of elastic modulus with temperature.     

Corrosion fatigue crack growth of titanium alloys in aqueous environments

The rate of fatigue crack propagation for Ti-6Al-6V-2Sn and Ti-6 A1-4V in aqueous environments has been measured as a function of solution chemistry, frequency, and stress wave form. Depending on the specific encironment, three types of fatigue crack growth rate behavior have been observed as a function of frequency. Crack growth rates increase with decreasing frequency in distilled water, while addition of Na₂SO₄ produces frequency-independent behavior. In solutions containing chloride or bromide ions, a reversal in frequency-dependence takes place at ΔK_scc. Below this transition ΔK level, crack growth rates decrease with decreasing frequency due to passive film formation at the crack tip. Above ΔK_scc corrosion fatigue crack growth is due to SCC under cyclic loading. The ΔK transition in fatigue is lower than the static stress corrosion threshold because of repeated rupture of the passive film at the crack tip, approaching K_Isco only for very slow cycling frequencies. This paper is based upon a thesis submitted by D. B. Dawson in partial fulfillment of the requirements of the degree of Doctor of Science at Massachusetts Institute of Technology.     

A probabilistic approach to evaluate creep and fatigue damage in critical components

The subject of reliability assessment of plant components is of considerable importance to maintenance and operations engineers. There exist different degradation mechanisms such as creep and fatigue in the plant components operating under conditions of high pressure and temperature. Hence, the safety issue of these components must be addressed by a realistic reliability model so that the frequency of in-service inspection activities can be optimized. In this work, a probabilistic module based on first order reliability method and Markov model is used in order to calculate different failure state probabilities of an in-service thermal power plant pipe-bend. This procedure helps in online risk assessment of critical plant components. The method of determination partial safety factors of the above component for optimum design based on a target reliability level is also discussed.     

A population-based incidence study of chronic fatigue

BACKGROUND: Although isoflurane may cause subendocardial hypoperfusion in the presence of coronary stenosis because of its coronary arteriolar dilatory effects, it is not known how the subendocardial microcirculation is affected. The authors examined the effects of isoflurane on poststenotic subendocardial microvessels with coronary stenosis. METHODS: The authors observed subendocardial microvessels in in situ beating swine hearts with or without critical stenosis of the left anterior descending coronary artery (LAD) with a needle-type videomicroscope during isoflurane- (ISO-H), adenosine- (ADE-H), and nitroglycerin- (NTG-H) induced hypotension (mean arterial pressure, 55 mmHg). Regional myocardial function, oxygen balance, and lactate metabolism in the region perfused by the LAD also were determined. RESULTS: In swine with stenosis, there were no differences in heart rate, cardiac output, and LAD blood flow among the three types of hypotension. Regional lactate production and anterior interventricular venous pO2 were similar during ISO-H and NTG-H but higher during ADE-H. With videomicroscopy, about half as many subendocardial microvessels could be visualized during ADE-H as with ISO-H and NTG-H. The average decrease in the systolic diameter of subendocardial microvessels of greater than 100 microm was 9 +/- 6% during ISO-H and 12 +/- 5% during NTG-H, but no consistent phasic diameter changes were observed during ADE-H. In swine without stenosis, a systolic diameter decrease was observed during all three types of hypotension. CONCLUSIONS: These findings suggest that hypotension induced by isoflurane or nitroglycerin preserves phasic diameter changes in subendocardial microvessels in the presence of critical coronary stenosis, whereas that induced by adenosine does not.     

Microstructure and crystallographic texture of titanium subjected to combined severe plastic deformation processing

Structural studies of the nanocrystalline titanium powders produced by cryogenic milling followed by severe plastic deformation consolidation are performed. The results obtained are compared with the results obtained for monolithic titanium subjected to severe plastic deformation by torsion.     

Wedge type creep damage in low cycle fatigue

A model is proposed to quantify the accumulation of wedge type creep damage in low cycle fatigue. It is proposed that such damage is produced primarily during the ramp periods of the cycle. Equations are developed for estimating incremental accumulation of damage per cycle in fully reversed, multiaxial loading. The rate of accumulation of damage depends on the strain-rate, the temperature, and the microstructure. The analysis is kept simple by making physically reasonable assumptions. Cycles to failure are predicted by invoking a fracture criterion. The model is applied to two sets of data; one set is a well characterized life test data on an aluminum alloy, and the other is phenomenological data on austenitic stainless steels. In both cases the predictions are good enough to prompt further experimental evaluation of the model. This paper deals with only one mechanism of creep-fatigue interaction. Other mechanisms of failure,e.g., ‘r’ type cavitation, or fatigue crack initiation and propagation, are also viable. The model described here may be expected to apply only under those conditions when wedge damage is the dominant failure mechanism.     

Dislocation distribution and prediction of fatigue damage

The dislocation density and distribution induced by tensile deformation in single crystals of silicon, aluminum and gold and by tension-compression cycling in aluminum single crystals and Al 2024-T3 alloys were studied by X-ray double-crystal diffractometry. The measurements of dislocation density were made at various depths from the surface by removing surface layers incrementally. In this way, a propensity for work hardening in the surface layers compared to the bulk material was demonstrated for both tensiledeformed and fatigue-cycled metals. Analysis of the cycled Al 2024 alloy as a function of the fraction of fatigue life showed that the dislocation density in the surface layer increased rapidly early in the fatigue life and maintained virtually a plateau value from 20 to 90 pct of the life. Beyond 90 pct the dislocation density increased rapidly again to a critical value at failure. Evaluation of the dislocation distribution in depth showed that the excess dislocation density in the bulk material increased more gradually during the life. Using deeply penetrating molybdenumK _α radiation, capable of analyzing grains representative of the bulk region, the accrued damage and the onset of fatigue failure could be predicted nondestructively for 2024 Al, cycled with constant stress as well as with variable stress amplitude. The dislocation structure produced in the bulk by prior cycling was unstable when the work-hardened surface layer was removed. It is proposed that the deformation response of the bulk material is controlled by the accumulation of dislocations and associated stresses in the surface layer. Formerly Research Associate, Rutgers College of Engineering.     

Fretting fatigue behavior of surface modified biomedical titanium alloys

Fretting is a form of adhesive wear normally occurring at the contact points gradually leading to premature failure of load bearing medical implants made of titanium alloys. The aim of this work is to characterize the fretting fatigue damage features of PVD TiN coated, plasma nitrided, ion implanted, laser nitrided and thermally oxidized Ti-6Al-4V and Ti-6Al-7Nb contact pairs. The surface layers were characterized. The damage progression during fretting process is apparently explained with tangential force coefficient curves. Plasma nitrided pairs showed highest fretting fatigue life compared to others. PVD TiN coated pairs have experienced early failures due to third body mode of contact interaction with irregular tangential force coefficient pattern. Ion implanted layers showed similar damage as unmodified alloys. Laser nitrided and thermally oxidized pairs experienced early failures due to brittle and irregular modified layers.     

A study of the fatigue limit of copper

This paper shows that, in fatigue tests on copper, a great number of cycles beyond expectation is necessary for determining the fatigue limit. An ultrahigh frequency fatigue testing machine was used andS-N curves covering 10¹⁰ cycles were obtained for annealed specimens and for stretched ones. In annealed copper, the fatigue limit appeared at about 9.8 × 10⁹ cycles. Observations were made on the development of slip bands and substructures in the course of a test at the strain level (3.8 × 10^-4) of the fatigue limit. The slip bands continued to develop up to about 9 × 10⁹ cycles, but remained unchanged if further cycled. On a searching examination of the behavior of microcracks, one end of which stayed in a grain without propagation, it was proved that fatigue hardening was responsible for the existence of the fatigue limit. Contrary to expectation, cell structures were found after 10¹⁰ cycles in such a low strain fatigue. In stretched copper, however, microcracks continued to grow even in the stage beyond 10¹⁰ cycles when fatigued at the strain level of the fatigue limit inferred from itsS-N curve.     

A study of the fatigue limit of copper

This paper shows that, in fatigue tests on copper, a great number of cycles beyond expectation is necessary for determining the fatigue limit. An ultrahigh frequency fatigue testing machine was used andS-N curves covering 10¹⁰ cycles were obtained for annealed specimens and for stretched ones. In annealed copper, the fatigue limit appeared at about 9.8 × 10⁹ cycles. Observations were made on the development of slip bands and substructures in the course of a test at the strain level (3.8 × 10^-4) of the fatigue limit. The slip bands continued to develop up to about 9 x 10⁹ cycles, but remained unchanged if further cycled. On a searching examination of the behavior of microcracks, one end of which stayed in a grain without propagation, it was proved that fatigue hardening was responsible for the existence of the fatigue limit. Contrary to expectation, cell structures were found after 10¹⁰ cycles in such a low strain fatigue. In stretched copper, however, microcracks continued to grow even in the stage beyond 10¹⁰ cycles when fatigued at the strain level of the fatigue limit inferred from itsS-N curve.     

Exoelectron emission from torsional fatigue damage in hardened steels

Exoelectron emission associated with surface fatigue damage in two types of steel was measured by means of an ultraviolet laser scanning system. The localized emission from damage areas in quenched and tempered 1541 steel increased systematically with continued fatigue cycling. Plots of emissionvs cycles were in good agreement with earlier results for 1018 steel. Much of this emission resulted from the accumulation of plastic deformation during the crack initiation stage. In the case of relatively brittle 1144 steel, the dominant source of exoelectrons was a fatigue crack with little associated plasticity. However, shot peening of the 1144 steels completely changed the character of the surface layers. While the core material still cracked in a rather brittle manner, the shot peened layer accumulated a broad distribution of plastic deformation as revealed by the systematic development of exoelectron emission. These results indicate that the accumulation of surface fatigue deformation during the crack initiation stage is a systematic and perhaps universal process when viewed on the appropriate microscale.