Threshold for fatigue macrocrack propagation in some aluminum alloys

Measurement of the threshold for fatigue macrocrack propagation, ΔK_o, in a number of aluminum alloys has shown an increase with grain size and decrease with increase in strength as with steels. The results are not primarily due to environmental enhancement of fatigue crack growth because an even larger variation in ΔK_o with microstructural change is noted at 77 K than at 300 K. In particular, ΔK_o of high purity 2124-T4 increases much more on cooling from 300 to 77 K than does ΔK_o of 2024-T4. It is suggested that ΔK_o is determined by the stress necessary to operate a dislocation source near the crack tip. A Frank-Read type source is proposed for 2024-T4 with constituent particles acting as pinning points while double cross-slip, a thermally activated process, is proposed for the source in high purity 2124-T4.     

An Evaluation of the summation of cyclic plastic strain damage in two high strength aluminum alloys

A damage equation based upon the integration of low cycle fatigue plastic strain ranges was verified experimentally for two high strength aluminum alloys 2024-T4 and 7075-T651. The damage equation which has been used extensively for many fatigue crack propagation theories assumes cyclic damage under increasing plastic strain ranges. In order to verify the damage equation, low cycle fatigue specimens were subjected to a fully reversed strain cycle in which the total strain-range was increased linearly by a constant amount Δ[Δε_d] per cycle. An excellent agreement was obtained between the predicted and observed fatigue lifetimes. The stress-strain response of these alloys was also measured. The experimental results showed that these two alloys cyclically harden substantially and that the single strain increment stress-strain curve is a fair lower bound approximation of the cyclic stress-strain curve.     

Fractography of fatigue crack propagation in 2024-T3 and 7075-16 aluminum alloys in air and vacuum

Electron fractography and transmission electron microscopy were used to study fatigue crack propagation processes in 2024-T3 and 7075-T6 aluminum alloys in vacuum and air. There was evidence that crack growth occurs cycle-by-cycle in vacuum, but only for fa tigue in air was it possible to relate dislocation substructure band spacings, immediately below the fracture surface, to cyclic crack growth. A discussion of crack propagation mechanisms suggested that the Tomkins and Biggs model of striation formation comes closest to explaining the fractographic features.     

Fractography of fatigue crack propagation in 2024-T3 and 7075-16 aluminum alloys in air and vacuum

Metallurgical and Materials Transactions A - Electron fractography and transmission electron microscopy were used to study fatigue crack propagation processes in 2024-T3 and 7075-T6 aluminum alloys...     

Intrinsic fatigue crack growth

The influences of microstructure and deformation mode on inert environment intrinsic fatigue crack propagation were investigated for Al-Li-Cu-Mg alloys AA2090, AA8090, and X2095 compared to AA2024. The amount of coherent shearable δ (Al₃Li) precipitates and extent of localized planar slip deformation were reduced by composition (increased Cu/Li in X2095) and heat treatment (double aging of AA8090). Intrinsic growth rates, obtained at high constantK _max to minimize crack closure and in vacuum to eliminate any environmental effect, were alloy dependent;da/dN varied up to tenfold based on applied ΔK or ΔK/E. When compared based on a crack tip cyclic strain or opening displacement parameter (ΔK/(σ_ys E)^1/2), growth rates were equivalent for all alloys except X2095-T8 which exhibited unique fatigue crack growth resistance. Tortuous fatigue crack profiles and large fracture surface facets were observed for each Al-Li alloy independent of the precipitates present, particularly δ, and the localized slip deformation structure. Reduced fatigue crack propagation rates for X2095 in vacuum are not explained by either residual crack closure or slip reversibility arguments; the origin of apparent slip band facets in a homogeneous slip alloy is unclear. Better understanding of crack tip damage accumulation and fracture surface facet crystallography is required for Al-Li alloys with varying slip localization.     

Influence of inclusion content on fatigue crack

Fatigue crack growth rates were measured at room temperature in dry air for three 7075-T6 aluminum alloys with different inclusion content. Volume fractions of inclusions were determined for each alloy by the point count method with two different automated systems. Plots of the fatigue crack growth rate (da/dN) vs the stress-intensity-factor range (ΔK) show a well defined change of slope at the transition between plane strain and plane stress fracture. This transition is associated with a marked increase in the amount of fracture by void growth around inclusions. The volume fraction and mean spacing of voids within the cyclic plastic zone have been determined as a function of ΔK by quantitative fractography. Fracture by voids is important when the mean spacing of such voids is approximately equal to the width of the cyclic plastic zone in the plane of the crack. It is concluded that the inclusion content increases the fatigue-crack growth rates only within the plane stress range, that is for values of the stress-intensity-factor range ΔK \s> 20 kpsi√in.     

A notch analysis of fracture approach to fatigue crack propagation

A model of fatigue crack growth is proposed that utilizes the recent developments in notch analysis of fracture and a concept of size effect that results from the changes in the critically stressed volume ahead of a crack tip. Accordingly, the fatigue crack growth mechanism involves local stresses reaching the theoretical cohesive strength and causing brittle fracture of atomic bonds at nominal stresses near the threshold, whereas slip-plane decohesion and plastic blunting and resharpening of the crack tip process may occur at stresses above the threshold range. The model contains three material parameters σFF nF, and ρF, that conveniently extend continuum analysis to situations where inhomogeneity of the material structure can influence the behavior appreciably. The analytical expression from the model was found to correlate fatigue crack growth data reasonably well in the low and intermediate stress ranges in Al 2024-T3, Al 7075-T6 and 250 grade maraging steel. The fracture modes observed are in agreement with the predictions from the model. The same fatigue crack growth model can be extended to estimating the threshold stress intensity factor range, ΔK_o and fatigue notch sensitivity of different materials.     

Fatigue threshold and low-rate crack propagation properties for structural steels in 3 Pct sodium chloride aqueous solution

The fatigue threshold and low-rate crack propagation properties for a carbon steel, two high-strength steels, and two stainless steels were investigated in a 3 pct sodium chloride aqueous solution at frequencies between 0.03 and 30 Hz. Tests were conducted in a manner designed to avoid crack closure. Under freely corroding conditions, the effective values of the threshold stress intensity factor range, ΔK_th,eff, were lower than in air for all of the steels. In particular, the ΔK_th,eff values for the carbon and high-strength steels were almost equal to the theoretical ΔKth value of about 1 MPa m^1/2 calculated on the basis of the dislocation emission from the crack tip. At a given ΔK level higher than the threshold, the fatigue crack propagation rates accelerated with decreasing frequency for all of the steels. Under cathodic protection, the threshold and fatigue crack propagation properties were coincident with those in air regardless of material and frequency. The observed fatigue crack propagation behavior in a 3 pct NaCl solution was closely related to the corrosion reaction of the bare surface formed at the crack tip during each loading cycle.     

The Effect of Grain Size on Fatigue Growth of Short Cracks

The influence of alloy grain size on growth rates of surface cracks 20 to 500 μm in length was studied in Al 7075-T6 specimens prepared in 12 and 130 μn grain sizes. Grain boundaries temporarily interrupt the propagation of cracks shorter than several grain diameters in length. Linear elastic fracture mechanics is inadequate to describe resulting average growth rates which must instead be characterized as a function of cyclic stress amplitude, σ_a, and alloy grain size as well as stress intensity range, σK. These observations are rationalized using two models, one that relates crack closure stress to alloy grain size, and a second that relates the development of microplasticity in a new grain in the crack path to grain size. In addition, growth rates were found to be faster in fully reversed loading than in tension-tension loading, especially in the large grained material. Evidence is presented to demonstrate that this is a consequence of the fatigue induced development of a compressive residual surface stress during tension-tension loading. These complex effects, and the role of grain size in determining short crack growth, are discussed.     

Mechanisms of Slow Fatigue Crack Growth in High Strength Aluminum Alloys: Role of Microstructure and Environment

The role of microstructure and environment in influencing ultra-low fatigue crack propagation rates has been investigated in 7075 aluminum alloy heat-treated to underaged, peak-aged, and overaged conditions and tested over a range of load ratios. Threshold stress intensity range, ΔK₀, values were found to decrease monotonically with increasing load ratio for all three heat treatments fatigue tested in 95 pct relative humidity air, with ΔK ₀ decreasing at all load ratios with increased extent of aging. Comparison of the near-threshold fatigue behavior obtained in humid air with the data forvacuo, however, showed that the presence of moisture leads to a larger reduction in ΔK₀ for the underaged microstructure than the overaged condition, at all load ratios. An examination of the nature of crack morphology and scanning Auger/SIMS analyses of near-threshold fracture surfaces revealed that although the crack path in the underaged structure was highly serrated and nonlinear, crack face oxidation products were much thicker in the overaged condition. The apparent differences in slow fatigue crack growth resistance of the three aging conditions are ascribed to a complex interaction among three mechanisms: the embrittling effect of moisture resulting in conventional corrosion fatigue processes, the role of microstructure and slip mode in inducing crack deflection, and crack closure arising from a combination of environmental and microstructural contributions.     

Finite element prediction of high cycle fatigue life of aluminum alloys

A new method is described for calculating the long fatigue life (>10⁵ cycles) portion of the stress-life (S-N) fatigue curve for precipitation-hardened aluminum alloys. It is based upon a finite element model of the deformation of a persistent slipband (PSB), and the only material parameter required is the ultimate tensile strength (UTS) of the alloy. The stress dependence of the plastic strain at the tip of a PSB is shown to be very pronounced and to closely match that of anS-N fatigue curve. Very good agreement is obtained for 6061-T6, 2014-T6, 2024-T4, and 7075-T6 aluminum, and the fatigue strength (at 10⁸ cycles) is calculated to be 26 pct of the tensile strength of each alloy, in agreement with experimental data. By contrast, the plastic strain at a crack tip has a much weaker stress dependence. Thus, these calculations also confirm that the elongation of a PSB, and not crack growth, is the rate-controlling process in high cycle fatigue.     

Finite element prediction of high cycle fatigue life of aluminum alloys

A new method is described for calculating the long fatigue life (>10⁵ cycles) portion of the stress-life (S-N) fatigue curve for precipitation-hardened aluminum alloys. It is based upon a finite element model of the deformation of a persistent slipband (PSB), and the only material parameter required is the ultimate tensile strength (UTS) of the alloy. The stress dependence of the plastic strain at the tip of a PSB is shown to be very pronounced and to closely match that of anS-N fatigue curve. Very good agreement is obtained for 6061-T6, 2014-T6, 2024-T4, and 7075-T6 aluminum, and the fatigue strength (at 10⁸ cycles) is calculated to be 26 pct of the tensile strength of each alloy, in agreement with experimental data. By contrast, the plastic strain at a crack tip has a much weaker stress dependence. Thus, these calculations also confirm that the elongation of a PSB, and not crack growth, is the rate-controlling process in high cycle fatigue.     

Effect of cyclic stress wave form on corrosion fatigue crack propagation in al-zn-mg alloys

Fatigue crack growth rates of a 7075 type aluminum alloy were measured as a function of environment, frequency, stress wave form, alloy chemistry, and thermomechanical treatment. At low ΔK values (belowK _ISCC ), the crack growth rates in a 3.5 pct sodium chloride solution were ten times greater than those in a reference argon environment. Comparison of the effects of a square wave, a negative-sawtooth wave, and a positivesawtooth wave at different frequencies indicates that the synergistic interaction with the environment occurs during the loading part of each cycle. Overaging the alloy and limiting the alloy impurity content results in a reduced corrosion fatigue crack growth rate, but a thermomechanical treatment leading to a grain size refinement increases it.     

Effect of silicon particles on the fatigue crack growth characteristics of Al-12 Wt Pct Si-0.35 Wt Pct Mg-(0 to 0.02) Wt Pct Sr casting alloys

Fatigue crack growth (FCG) characteristics and mechanisms in Al-Si-Mg eutectic casting alloys containing 0.35 wt pct Mg and 0 to 0.02 wt pct Sr were investigated as a function of stress ratio,R, stress-intensity-factor range, ΔK, and silicon (Si) particle size. The fatigue crack propagation behavior was compared with that observed in commercial casting alloy A356. At the same applied ΔK level, the crack growth rate was found to increase with increasing stress ratio and Si particle size. Modified (fine Si morphology) and A356 alloys showed better FCG resistance than the unmodified (coarse Si morphology) ones, for a constant applied ΔK, due to increased closure. The effects of roughness-induced and plasticity-induced crack closures, crack branching, and crack meandering on the fatigue crack propagation observed in these alloys have been discussed. The fatigue crack propagation path is found to be dependent on the Si particle characteristics. The mechanisms of silicon particle decohesion and cracking are also discussed. Formerly Research Associate, Département des Sciences Appliquées, Université du Québec à Chicoutimi     

Environmental fatigue of an Al-Li-Cu alloy: part I. Intrinsic crack propagation kinetics in hydrogenous environments

Deleterious environmental effects on steady-state, intrinsic fatigue crack propagation (FCP) rates(da/dN) in peak-aged Al-Li-Cu alloy 2090 are established by electrical potential monitoring of short cracks with programmed constant ΔK andK _maxI loading. Such rates are equally unaffected by vacuum, purified helium, and oxygen but are accelerated in order of decreasing effectiveness by aqueous 1 pct NaCl with anodic polarization, pure water’ vapor, moist air, and NaCl with cathodic polarization. Whileda/dN depend on ΔK^4.0 for the inert gases, water vapor and chloride induce multiple power laws and a transition growth rate “plateau.” Environmental effects are strongest at low ΔK. Crack tip damage is ascribed to hydrogen embrittlement because of acceleratedda/dN due to parts-per-million (ppm) levels of H₂O without condensation, impeded molecular flow model predictions of the measured water vapor pressure dependence ofda/dN as affected by mean crack opening, the lack of an effect of film-forming O₂, the likelihood for crack tip hydrogen production in NaCl, and the environmental and ΔK-process zone volume dependencies of the microscopic cracking modes. For NaCl, growth rates decrease with decreasing loading frequency, with the addition of passivating Li₂CO₃ and upon cathodic polarization. These variables increase crack surface film stability to reduce hydrogen entry efficiency. Small crack effects are not observed for 2090; such cracks do not grow at abnormally high rates in single grains or in NaCl and are not arrested at grain boundaries. The hydrogen environmental FCP resistance of 2090 is similar to other 2000 series alloys and is better than 7075. ROBERT S. PIASCIK formerly Graduate Student, Department of Materials Science, University of Virginia.     

Effect of testing frequency on the corrosion fatigue of a squeeze-cast aluminum alloy

The corrosion fatigue crack propagation behavior of a squeeze-cast Al-Si-Mg-Cu aluminum alloy (AC8A-T6), which had been precracked in air, was investigated at testing frequencies of 0.1, 1, 5, and 10 Hz under a stress ratio (R) of 0.1. Compact-toughness specimens were precracked about 6 mm in air prior to the corrosion fatigue test in a 3 pct saline solution. At some near-threshold conditions, these cracks propagated faster than would be predicted by the mechanical driving force. This anomalous corrosion fatigue crack growth was affected by the initial stress-intensity-factor range (ΔK _i), the precracking conditions, and the testing frequency. The initial crack propagation rate was as much as one order of magnitude higher than the rate for the same conditions in air. This rapid rate was associated with preferential propagation along the interphase interface in the eutectic structure. It is believed that a chemical reaction at the crack tip and/or hydrogen-assisted cracking produced the phenomenon. Eventual retardation and complete arrest of crack growth after this initial rapid growth occurred within a short period at low ΔK values, when the testing frequency was low (0.1 and 1 Hz). This retardation was accompanied by corrosion product-induced crack closure and could be better explained by the contributory stress-intensity-factor range (ΔK _cont) than by the effective stress-intensity-factor range (ΔK _eff).     

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.     

The influence of SiC particulates on fatigue crack propagation in a rapidly solidified Al-Fe-V-Si alloy

The fatigue crack propagation properties of a rapidly solidified aluminum alloy are compared with those of a metal matrix composite (MMC) made of the same base alloy with the addition of 11.5 vol pct SiC particulate. The high-temperature base material, alloy 8009 produced by Allied-Signal, Inc. (Morristown, NJ), is solidified and processed using powder metallurgy techniques; these techniques yield a fine-grained, nonequilibrium microstructure. A direct comparison between the fatigue crack propagation properties of the reinforced and unreinforced materials is possible, because alloy 8009 requires no postprocessing heat treatment. As a consequence, this comparison reflects the influence of the SiC particulate and not differences in microstructure that could arise during processing and aging. The experimental data demonstrate that the SiC-reinforced material exhibits modestly superior fatigue crack propagation properties: slower crack growth rates for a given ΔK, at near-threshold crack growth rates. Even when the data are corrected for crack closure using an effective stress intensity factor, ΔK_eff, the composite exhibits lower crack propagation rates than the unreinforced matrix alloy. Microscopic evidence shows a rougher fracture surface and a more tortuous crack path in the composite than in the base alloy. It is argued that the lower crack growth rates and higher intrinsic threshold stress intensity factor observed in the composite are associated with crack deflection around SiC particles. Formerly Graduate Research Assistant, University of California-Davis     

Creep and fatigue crack propagation in a directionally-solidified carbide eutectic alloy

Creep and fatigue crack growth rates and threshold stress intensity amplitudes have been measured for a directionally solidified carbide-eutectic alloy, C73. Fatigue testing temperatures have been ambient, 750 and 950°C for cracking perpendicular and parallel to the solidification direction. In the former cracking direction comparative propagation rates may be understood in terms of the properties of the matrix, which shows a phase transformation from hexagonal to cubic above ~900°C. A situation where crack growth rates decrease with increasing apparent stress intensity amplitudes (ΔK) has been found to exist for propagation parallel to the solidification direction at low ΔK values and high temperatures only. This phenomenon can be related to the occurrence of crack branching and multiple cracking of the carbide fibers. Considerations of plastic zone sizes and critical defect sizes for crack propagation are consistent with the conditions necessary for such crack deceleration to occur. Transformation of the M₇C₃ fibers, present in the as-cast condition, to M₂₃C₆ at cell boundaries of the solidification structure occurs at a temperature of 950°C. Although M₂₃C₆ carbides are easily cracked and therefore probably reduce propagation rates by causing secondary cracking, their presence is known to be detrimental to creep properties. High cycle fatigue threshold stress intensity amplitudes for C73 in either loading direction at room temperature, 750 and 950°C are ~20 pct lower than for the cast nickel-base alloy, EST 738LC,i.e. critical defect sizes are ~10 pct smaller in C73. Despite the known sensitivity of cracking rates and threshold values to factors such as minor fluctuations in loading amplitude it is believed that these differences are significant.     

Near-threshold fatigue crack growth in 8090 Al-Li alloy

Near-threshold fatigue crack growth was studied in 8090-T8771 Al-Li alloy tested in moist laboratory air. The testing was conducted using (1) the ASTM E-647 load-shedding procedure, (2) a power-law load-shedding procedure, and (3) a constant-amplitude (CA) loading procedure. Crack closure in the three procedures was analyzed. In reconciling fatigue crack growth rates (FCGRs) with different crack closure levels under identical testing parameters, the conventional ΔK _eff (=K _max —K _op) fails to correlate the test data and the modified ΔK eff (=K _max - _χK_op, where χ is the shielding factor, defined by an energy approach) is proven to be the true crack driving force. A parallel slip-rupture model is proposed to describe the mechanism of near-threshold fatigue crack growth in this alloy. The model explains the mode transition from crystallographic slip band cracking (SBC) to subgrain boundary cracking (SGC)/brittle fracture (BF) in terms of a microstructure-environment synergy. The transition is related to the material’s short-transverse grain size.