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.     

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 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.     

Effect of thermomechanical processing on fatigue crack propagation

The effect of thermomechanical processing on fatigue crack propagation (FCP) is examined for 70/30 brass and 305 stainless steel. It is found that grain size and cold work induced changes in yield strength, ductility, and preferred orientation have a minor effect on FCP. Rather, cyclically stabilized properties of material in the crack tip plastic zone are believed to control the FCP process. Although mechanical processing fails to significantly alter the rate of FCP, it is apparently responsible for the unique fracture path observed in specimens oriented at an angle(A) to the rolling direction. Deviation of the crack path out of the plane of maximum net section stress is believed to be associated with mechanical fibering andJor crystallographic texturing effects. The complex fracture mode transition observed in cold worked 70/30 brass also is associated with the deformation texture of the starting material. For the cold-worked 305 stainless steel, striation spacings are correlated with the stress intensity range for specimens tested in the longitudinal, transverse, and “angle” orientations. Comparison of these data with corresponding macroscopic data indicate that an approximately one-to-one correspondence exists between macroscopic and microscopic fatigue crack growth rates over the range investigated.     

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

Metallurgical and Materials Transactions A -     

Effect of hydrogen on fatigue crack propagation in vanadium

The influence of hydrogen on fatigue crack propagation in unalloyed vanadium and several hydrogen-charged vanadium alloys has been investigated. The Paris-Erdogan equation,da/dN =C(Δ.K)^m, was approximately obeyed for all alloys. Crack growth rates were lowest in vanadium and dilute vanadium-hydrogen alloys, and were not very sensitive to volume fraction of hydrides in more concentrated alloys. The crack growth exponent,m, is inversely proportional to the cyclic strain hardening rate,n′, and the rate constantC is inversely proportional to the square of the ultimate tensile stress, σ_UTS: Metallographic examination showed hydride reorientation and growth in the originally hydrided alloys. No stress-induced hydrides were observed in V-H solid-solution alloys. Fractures in hydrided materials exhibited cleavage-like features, while striations were noted in unalloyed vanadium and dilute solid-solution alloys.     

Effects of microstructure on fatigue crack initiation and propagation of 16Mn steel

In the present study, the effect of microstructure of 16Mn steel on fatigue crack initiation (FCI) life and fatigue crack propagation (FCP) rates was experimentally investigated under two different conditions,i.e., as-received condition and high-temperature normalized (H.T.N.) condition. The microstructure of 16Mn steel under the as-received condition is ferrite and pearlite, which corresponds to that of the base metal of welded elements, and the microstructure under the H.T.N. condition is mainly coarse Widmanstätten structure, which can be thought of as the simulated microstructure at the weld toe. The fatigue test results show that the high-temperature normalization results in the increase of FCP rates in near-threshold region and the decrease of both FCI and FCP thresholds, and FCI life of 16Mn steel. Little effect of the microstructure is observed on the FCP mechanism in the intermediate range (da/dN=10^?8 to 10^?6 m/cycle). Based on the test results and analysis, the general expressions are given for both FCI life and FCP rates under the two conditions. It is pointed out that which of the test results should be applied to prediction of FCI life and FCP life depends upon the FCI location and FCP path in the welded elements.     

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.     

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.     

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. 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.     

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.     

Closure-affected fatigue crack propagation behaviors of powder metallurgy-processed Al-Li alloys in various environments

The environment-affected fatigue crack propagation (FCP) behavior of rapid solidification-processed (RSP) Al-2.6Li-1.0Cu-0.5Mg-0.5Zr (RSP 644-B) and mechanically alloyed (MA) Al-4.0Mg-1.5Li-1.1C-0.8O₂ (MA 905-XL) were examined in air, in vacuum, and in an aqueous 3.5 pct NaCl solution at R=0.1 and a sinusoidal frequency of 20 Hz. The emphasis was placed on the effect of environment-sensitive crack closure on the FCP behavior of fine-grain-sized powder metallurgy (P/M)-processed Al-Li alloys. The present study suggests that closure is extremely sensitive to environmental factors and significantly alters the environment-affected da/dN-ΔK relationships for both alloys. In the submicron grain-sized MA 905-XL, for example, increased corrosion product-induced closure in aqueous NaCl appeared to overwhelm the detrimental environmental effects in low- and intermediate-ΔK regimes. The environment-sensitive closure contribution alone, however, cannot completely explain the FCP behavior of P/M-processed Al-Li alloys. The intrinsic environmental effects also need to be considered for further understanding of this behavior.     

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