Mechanisms of corrosion fatigue crack propagation in Al-Zn-Mg alloys

Corrosion fatigue crack propagation tests were performed on commercial 7075 alloys. Testing was done in a 3.5 pct sodium chloride solution under constant impressed potential and under reversed anodic-cathodic current conditions. Results indicated that a cathodic potential of -1.400 V vs SCE was sufficient to reduce corrosion fatigue crack growth rates to the level observed in dry argon. By alternately impressing anodic and cathodic currents, it was shown that anodic potentials enhance the crystallographic dependence of the fracture mode, resulting in brittle striations, while cathodic potentials result in ductile striations formed by shear. Modification of the alloy chemistry and lower impurity content resulted in a two-fold reduction in crack growth rates. Thermomechanical treatment of these alloys to refine the grain size proved detrimental. Adding an inhibitor to the sodium chloride solution was found to be the most effective means for reducing corrosion fatigue crack growth rates. A model for the environment-surface interaction is suggested.     

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 effect of microstructure on fatigue crack propagation in iron-carbon alloys

The dependence of fatigue crack growth rate on the cyclic stress intensity factor was determined for six iron-carbon alloys ranging in carbon content from 0.23 to 1.08 wt pct carbon. Both ferrite/pearlite and ferrite/free iron carbide microstructures were studied. Scanning electron microscope fractography studies correlated the fatigue mechanism with microstructure. It was found that when the predominant mode of crack growth was ductile, the crack growth rateda/dN could be related to the cyclic stress intensity factor ΔK by an equation of the formda/dN = (ΔK)_m where andm are constants. The constantm was approximately equal to four when the crack growth mechanism presumably was the blunting and resharpening of the crack tip by slip processes. The constantm was greater than four when the crack growth mechanism was void coalescence in the interlamella ferrite of pearlite colonies. The preferred fatigue crack path through the pearlitic alloys was through the free ferrite phase. formerly Research Assistant at Materials Science and Engineering Department and Materials Research Center, Northwestern University.     

Plastic work of fatigue crack propagation in steels and aluminum alloys

The plastic work per unit area of fatigue crack propagation,U, is one of the parameters controlling the rate of fatigue crack propagation,dc/dN. The equation,dc/dN = A ΔK ⁴/(σf_y ²μ U), was previously shown to fit the data for 7 iron and aluminum base alloys for the range of thedc/dN vs ΔK curve where the Paris relation is valid. Values ofU are now available for 6 additional alloys covering a much wider range of σ_y 42 to 868 MN/m². For the total populationA = (2.8 ± 0.9) X 10^-3 where 2.8 is the mean and 0.9 is the standard deviation. In this equation, σ_y is the 0.2 pct offset cyclic yield stress and μ is the shear modulus. The parameterU is related to microstructure and should be of interest to the metallurgist. Generally,U varies oppositely to σ_y due to decrease in the plastic zone size; however, the plastic strain amplitude and degree of localization of the plastic strain in the plastic zone are also important.     

Fatigue crack propagation near fatigue threshold in various structural steels

The investigations have been conducted by measuring fatigue crack propagation near fatigue threshold in various structural steels differing in chemical composition and strength level. The fatigue crack propagation measurements were carried out using the constant-load-amplitude test in Paris-region, R-constant and K_max-constant method in near fatigue threshold region. Scanning electron microscopy at fatigue crack front on fracture surface was applied to interpret the influence of crack closure effects on the measured fatigue threshold. Marked fretting oxide deposits distributed on the fracture surface at threshold level were observed in a low load ratio resulting from the combined action of plasticity- and oxide-induced crack closure under laboratory atmosphere. Fatigue threshold dependent on the load ratio appeared to be related to the extent of the crack closure effect. By considering the relationship of reversed plastic zone size and grain size the fatigue threshold in region of crack closure was calculated theoretically. The result has shown a good agreement with the experimentally measured values.     

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.     

Low cycle fatigue, fatigue crack propagation and substructures in a series of polycrystalline Cu-Al alloys

Low Cycle Fatigue (LCF) on smooth hour glass specimens and Fatigue Crack Propagation (FCP) studies on Single Edge Notch (SEN) specimens were carried out at room temperature on four Cu-Al polycrystalline alloys to investigate the effects of Stacking Fault Energy (SFE) and mechanical property variations on fatigue characteristics. Significant improvements in fatigue properties were observed for alloys of low SFE. A microhardness technique was used to delineate the fatigue plastic zone ahead of stopped cracks at several stress intensity ranges for all the alloys. Planar slip was associated with a less than a second power dependence of plastic zone size on the stress intensity range. Transmission Electron Microscopy (TEM) was used to observe the substructures that developed both in LCF at different strain ranges and also ahead of fatigue cracks at different stress intensity ranges. Fractography was carried out to study the micromechanisms of crack propagation using a two stage replication technique. The experimental results were in good agreement with a theoretical model for FCP developed previously by the authors which incorporates mechanical and microstructural variables. AXENA, formerly Graduate Student, Dept. of Materials Science and Metallurgical Engineering, University of Cincinnati is This paper is based on a thesis submitted by Ashok Saxena in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Cincinnati.     

Environmental fatigue crack propagation of aluminum alloys at low stress intensity levels

Environmental fatigue crack propagation in 2024-T3, 7075-T6, and 7178-T6 has been studied at low levels of cyclic amplitude of stress intensity, ΔK. Both wedge force loading and remote loading techniques were employed to achieve the desired ΔK levels, and preliminary experiments were designed to test their compatibility. Testing was carried out in humid air, distilled water, and 3.5 pct sodium chloride solution, and the observed crack growth rates compared with those in desiccated air. Later studies were also conducted in an inert reference environment with a total water content of less than 2 ppm. When the data are plotted as log ΔK vs log d2a /dN, alloy 2024-T3 exhibits a marked slope transition, alloy 7075-T6 a slight slope transition, and alloy 7178-T6 a rectilinear behavior throughout the whole range of ΔK studied. The basic shape of these curves is discussed in terms of state-of-stress conditions at the crack tip, frequency effects, environmental effects, strain rate sensitivity, and metallurgical structure. An attempt is also made to correlate the rate of fatigue crack propagation in a particular environment and at a particular ΔK level with the fracture topography.     

Low cycle fatigue,fatigue crack propagation and substructures in a series of polycrystalline Cu-Al alloys

Low Cycle Fatigue (LCF) on smooth hour glass specimens and Fatigue Crack Propagation (FCP) studies on Single Edge Notch (SEN) specimens were carried out at room temperature on four Cu-Al polycrystalline alloys to investigate the effects of Stacking Fault Energy (SFE) and mechanical property variations on fatigue characteristics. Significant improvements in fatigue properties were observed for alloys of low SFE. A microhardness technique was used to delineate the fatigue plastic zone ahead of stopped cracks at several stress intensity ranges for all the alloys. Planar slip was associated with a less than a second power dependence of plastic zone size on the stress intensity range. Transmission Electron Microscopy (TEM) was used to observe the substructures that developed both in LCF at different strain ranges and also ahead of fatigue cracks at different stress intensity ranges. Fractography was carried out to study the micromechanisms of crack propagation using a two stage replication technique. The experimental results were in good agreement with a theoretical model for FCP developed previously by the authors which incorporates mechanical and microstructural variables.     

On the influence of fracture mechanisms on fatigue crack propagation in aluminum alloys

Metallurgical and Materials Transactions A -     

Influence of gaseous environments on rates of near-threshold fatigue crack propagation in nicrmov steel

The influence of hydrogen environment (448 kPa) on near-threshold fatigue crack propagation rates was examined in a 779 MPa yield strength NiCrMoV steel at 93 °C. An automatically decreasing and increasing stress intensity technique was employed to generate crack growth rates at three load ratios(R = 0.1, 0.5, and 0.8). Results show that the crack propagation rates in hydrogen are slower than those in air for levels of stress intensity range, ΔK, below about 12 MPa√m. The crack closure concept does not explain the slower crack growth rates in hydrogen than in air. Near-threshold growth rates appear to be controlled by the levels of residual moisture in the environments. In argon and air, the fracture morphology is transgranular, while in H₂ the amount of intergranularity varies with ΔK and achieves a maximum when the cyclic plastic zone is approximately equal to the prior austenite grain size.     

The role of alpha and beta phases in fatigue crack propagation of Ti-Mn alloys

Crack propagation studies, with compact tension specimens, have been carried out on a series of equiaxed α-β Ti-Mn alloys, containing 0.4, 2.0, 5.6, 8.0, and 10.0 pct Mn, with volume fraction of β, varying from 0.019 to 0.759 for bothLT andTL directions. Textures of both phases were determined and interior plastic zone sizes were measured from the extent, in β, of deformation bands which emanated from the fracture surface. Relative crack propagation rates were rationalized on the basis of the slip systems which had the highest total resolved shear stress and the orientation of these slip systems with respect to the nominal crack propagation direction. The yield strength of the alloys increased considerably as the volume fraction of β increased, because the yield strength of α is one-third that of β.⁷ When Δa/ΔN was plotted against ΔK/YS, the data separated out according to yield stress, and Δa/ΔN for Stages II and III was highest for the 10.0 Mn alloy. Threshold ΔK values were also found to increase with the volume fraction of β and were considered to be determined primarily by slip in α. In Stage II a plot of log Δa/ΔN varied linearly with log √r_p, whererp is the size of the reversed plastic zone. The deformation bands which definerp were found at considerably lower values of ΔK than were the striations. It was proposed that this was due to the need for a sufficiently large plastic zone which would generate sufficient unloading back stress to cause the necessary back flow to produce striations. This back flow was considered to be influenced by Bauschinger behavior. An equation has been proposed to relate the constantC in the theoretical equation forrp to volume fraction of α, equiaxed a particle size, and the average Bauschinger strain. Formerly Graduate Student in the Department of Physical and Engineering Metallurgy, Polytechnic Institute of New York     

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.     

The effect of overload on the fatigue crack propagation in metastable beta Ti-V alloys

The effects of overload on the fatigue crack propagation behavior of two Ti-V alloys having different deformation mechanisms were studied. The results are explained in terms of residual stress effects associated with the overload and the removal of these stresses during post-overload cycling. An additional effect occurs during multiple cycle overload when the deformation structure representative of the strain amplitude is believed to form in the overload reverse plastic zone. This structure must be rearranged during cycling at ΔK _b before the baseline FCGR is reached and the process is responsible for part of the delay period. Formerly Research Scientist at Georgia Institute of Technology.     

Environmentally assisted fatigue crack propagation in steel

The influence of various gasenous environments on fatigue crack propagation has been determined for three quenched and tempered steels with yield strength levels of 800 to 1400 MN/m². The crack growth rate was increased by an order of magnitude in low pressure (13 KPa) hydrogen, and by a factor of two in most mildly aggressive environments relative to the growth rate in vacuum. The gases oxygen, acetylene, carbon monoxide, and nitrous oxide were dominant in a combined environment with hydrogen while methane and carbon dioxide had only a small effect on crack propagation when added to hydrogen. The crack propagation in acetylene was intermediate between that in hydrogen and the mildly aggressive environments. The increase in fatigue crack propagation rate in the hydrogen environment was dependent on the temperature and the cyclic stress intensity. The fracture mode was transgranular for all conditions except the hydrogen influenced HP-9-4-20 fractures. These results are discussed relative to various stages of the hydrogen embrittlement mechanisms. In pacticular, the results are discussed with respect to the adsorption-dissociation of the environment, transport of the gaseous specie within the plasticly deformed zone by mobile dislocations and interaction with segregated impurities within the metal. H. L. MARCUS, formerly with Science center, Rockwell International, Thousand Oaks, CA