Local yielding attending fatigue crack growth

Fatigue crack growth rate measurements were performed at 100°C on an Fe-3Si steel in three thickness conditions and at different ΔK-levels. The test pieces were subsequently sectioned and etched to reveal the plastic deformation attending crack growth both on the surface and in the interior. Unlike preceding studies, the Fe-3Si steel displayed classical cyclic crack growth: well-defined fatigue striations with a spacing close to the per-cycle growth rate, and essentially the same growth rates that have been reported for low and medium strength steels. A highly strained region, approximately one-fifth the size of the monotonie plastic zone, is identified as the cyclic plastic zone. On this basis three regions with distinct cyclic strain histories that precede the crack are identified: a microstrain region wherein the material receives ∼10³ to 10⁴ strain cycles in the range 0 < Δε _P ≲ 10^-3; a cyclic plastic zone corresponding to ∼200 cycles in the range 10^-3 < Δ∈ _P ≲ 10^-1, and a COD-affected zone that receives ∼10 strain cycles in the range 10^-1 ≲ Δ∈ _P ≲ 1. It is suggested that the damage associated with the instabilities in the fatigue substructure to overstrain contribute to the growth mechanism.     

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.     

A theory of fatigue crack initiation at inclusions

The dislocation dipole accumulation model for fatigue crack initiation previously proposed by the authors is extended to an analysis of the fatigue strength reduction due to inclusions in high strength alloys. The initiation of a fatigue crack is determined by an energy criterion under the assumption that the crack initiation takes place when the self strain energy of dislocation dipoles accumulated at the damaged part in the material reaches a critical value. Explicit formulae for the crack initiation criterion in several cases are derived as functions of the applied stress, the inclusion size, the slip band shape, and the shear moduli of the inclusion and matrix. The following three types of fatigue crack initiation at inclusions are considered: the slip-band crack emanating from a debonded inclusion, the inclusion cracking due to impinging of slip bands, and the slip-band crack emanating from an uncracked inclusion. The first mechanism was reported to be operative in high strength steels, while the last two mechanisms were reported in high strength aluminum alloys. The present theoretical results are in good agreement with the experimental data published for each case of fatigue crack initiation at inclusions.     

Photoelectron microscopy of plastic zone around a fatigue crack

The photoelectron emission from a metal is enhanced by fatigue cycling. This increase, which is often referred to as exoelectron emission, has been observed directly in a photoelectron microscope. The increase of emission results primarily from the rupturing of the thin surface oxide, thereby revealing a fresh metal surface of lower work function. During the propagation of a fatigue crack there are two sources of exoelectrons. 1) The crack itself, which emits very strongly and is easily identified. 2) The deformation generated in the nearby material,i.e. the plastic zone. In addition, the active portion of the plastic zone at the crack tip may be distinguished from the relatively inactive wake of deformed material.     

Photoelectron microscopy of plastic zone around a fatigue crack

The photoelectron emission from a metal is enhanced by fatigue cycling. This increase, which is often referred to as exoelectron emission, has been observed directly in a photoelectron microscope. The increase of emission results primarily from the rupturing of the thin surface oxide, thereby revealing a fresh metal surface of lower work function. During the propagation of a fatigue crack there are two sources of exoelectrons. 1)The crack itself, which emits very strongly and is easily identified. 2) The deformation generated in the nearby material,i.e. the plastic zone. In addition, the active portion of the plastic zone at the crack tip may be distinguished from the relatively inactive wake of deformed material.     

Modeling of corrosion fatigue crack initiation under passive electrochemical conditions

A combined mechanical/electrochemical model has been developed which successfully predicts the corrosion fatigue initiation behavior of an iron-base superalloy using readily measurable electrochemical and mechanical properties. In particular, the model uses the current decay curve, the initial or bare metal corrosion rate, and the critical slip step height, a parameter associated with the transition from an intense slip band to an incipient crack. The exponential parameter,α, used to fit the early (short time) portion of the current decay curve has been found to scale with the fatigue crack initiation time, suggesting thatα could be used as a valuable screening aid to assess the corrosion fatigue susceptibility of any alloy under passive electrochemical conditions. The model permits accurate prediction of both the shape and magnitude of fatigue life (S-N) curves. The limitations and theoretical implications of the approach of this model are also discussed. Formerly Postdoctoral Associate, Carnegie Mellon University Formerly Associate Professor, Carnegie Mellon University Formerly Professor, Carnegie Mellon University     

Metallography of fatigue crack initiation in an overaged high-strength aluminum alloy

Crack initiation was observed by optical microscopy using Nomarski interference contrast during fatigue cycling of an overaged 2024 aluminum alloy. The number of cracks more than five microns long at any given fraction of the fatigue life, and the distribution of cracks among various possible initiation sites, both depend on the applied stress amplitudeσa). The crack density at failure falls from approximately 300/mm² when σ^a is 90 pct of the yield strength, to less than I/mm² when σ^a is less than 60 pct of the yield strength. Cracks may begin in the matrix, in grain boundaries, or at constituent particles. At all stress amplitudes, however, the most common initiation sites areβ (Al₇Cu₂Fe) constituent particles. At low stress amplitudes in particular, fatigue cracks develop from the interface between closely-spaced fragments of β particles broken during prior processing (cluster sites). The stress-raising effect of voids which often occur at cluster sites may be responsible for their effectiveness in initiating fatigue cracks. Formerly Graduate Students in the Department of Metallurgical Engineering at Wayne State University During 1982–83 he is on leave as Associate Director of the Metallurgy Program, Division of Materials Research, National Science Foundation, Washington, DC 20550.     

Influence of microstructure on fatigue crack initiation in fully pearlitic steels

The effect of microstructure on the fatigue crack initiation of fully pearlitic steels was studied through independent variation of the prior austenite grain size, pearlite colony size, and the pearlite interlamellar spacing. Increasing yield strength (controlled by decreasing the pearlite interlamellar spacing) was seen to increase the smooth and notched-bar crack initiation endurance limit. Grain and colony size variations, at constant yield strength, were seen to exhibit no effect on crack initiation. Scanning Electron Microscopy revealed smooth-bar cracks to have initiated at surface inclusions. The influence of the pearlite interlamellar spacing, reflecting a change in the effective slip length, and the differences between notched and smooth-bar fatigue specimens for studying the effects of microstructure on crack initiation are discussed. Formerly with Carnegie-Mellon University     

A study of subsurface crack initiation produced by rolling contact fatigue

Results of subsurface crack initiation studies produced by pure rolling contact fatigue in 7075-T6 aluminum alloy are presented in this article. Microstructural changes and subsequent crack initiation below the contacting surface in cylindrical test specimens subjected to repeated rolling contact are illustrated. The rolling conditions are simulated in a three-dimensional elastic-plastic finite element model in order to estimate the plastic strains and residual stresses in the test material. The numerically estimated distribution of plastic strains in the model correlate well with the extent of microstructural changes observed in the test specimen. Results also indicate that a combination of plastic strains and low values of residual stresses is conducive to subsurface crack initiation and growth.     

疲劳裂纹萌生尺寸确定方法研究

以缺口件为研究对象 ,根据J积分理论 ,对Dowling法进行了改进 ,并根据疲劳非扩展裂纹的性质 ,给出裂纹萌生尺寸的定义 ,介绍疲劳裂纹萌生尺寸的确定方法     

Strength distribution of fatigue crack initiation sites in an Al-Li alloy

The stress-number of cycles to failure (S-N) curves were measured along the short-transverse (S) and rolling (L) directions of a hot-cross-rolled AA 8090 Al-Li alloy plate (45-mm thick). The alloy was solution heat treated, quenched in water, strained by 6 pct, and peak aged. Fatigue tests were carried out in four-point bend at room temperature, 20 Hz, R=0.1, in air. It was found that the fatigue limits in the S and L directions were 147 and 197 MPa, respectively. The crack population on the surface of a sample at failure increased with the applied stress level and was found to be a Weibull function of the applied maximum stress in this alloy. The strength distribution of fatigue weakest links, where cracks were initiated, was derived from the Weibull function determined by the experimental data. The fatigue weakest-link density was defined as the crack population per unit area at a stress level close to the ultimate tensile stress and can be regarded as a materials property. The density and strength distribution of fatigue weakest links were found to be markedly different between the L and S directions, accounting for the difference in fatigue limit between the directions in this alloy. They were also found to be different between S-L and S-T samples, and between L-T and L-S samples of this alloy, which could not be revealed by the corresponding S-N curves measured. These differences were due to the anisotropy of the microstructures in different directions in this alloy.     

Fatigue crack initiation and strain-controlled fatigue of some high strength low alloy steels

Initiation and growth of fatigue microcracks were investigated in several Nb and V alloyed high strength low alloy steels, including conventional and dual phase microstructures. Fatigue microcracks initiated along prominent slip bands. Macrocracks formed by linking up of small microcracks. At low applied stress or strain, the number of cycles to crack initiation increased with the cyclic yield stress. Comparing the cyclic stress-strain curves to the monotonie stress-strain curves, cyclic hardening or softening occurred, depending upon strain amplitude. Plateau regions were observed in plots of cyclic stress amplitudevs cyclic plastic strain amplitude obtained by increasing the total strain amplitude in steps after 30 cycles at each step. In polycrystalline 0.03 pct Nb steel, the plateau region was identified with prominent slip band formation, as others have observed in single crystals of copper, C-doped iron, and other metals.     

Role of microstructure and residual stresses on fatigue crack initiation of carbonitrided steels

In this paper the role of microstructure and residual stresses on fatigue crack initiation of gaseous carbonitrided steels is examined. Optimum for austenite content in the case is investigated for microstructures containing bainite and for those without bainite. Fractographic study suggests subsurface initiation sites for notched specimens when austenite content is high and initiation sites at the surface when austenite is low.     

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.     

The plastic zone and residual stress near a notch and a fatigue crack in HSLA steel

The plastic zone and residual stress around a notch under load and with the load removed, and around a fatigue crack (at the same stress intensity factor as for the notch) have been examined, with automated X-ray techniques and a microbeam. There is good agreement between the measured plastic zone size and Hutchinson's theory for a work hardening material. Residual stresses exist well behind the tip, and vary with depth, so that measurements of crack closure on a surface may not be directly related to closure stress (which samples the bulk). Instabilities in the dislocation arrangement can be detected by comparing X-ray line broadening of bulk specimens under load, and with the load removed. formerly Research Assistant, Department of Materials Science and Engineering, The Technological Institute, Northwestern University, Evanston, IL     

氢致钢内部疲劳裂纹萌生和扩展的有限元分析

对氢致钢内部疲劳裂纹的萌生和扩展进行了数值模拟.首先用有限元法分析了氢在疲劳载荷作用下向钢中缺陷处扩散富集的过程,然后计算得到氢含量分布结果.根据夹杂理论将氢富集区视为在缺陷附近分布的弹性夹杂,用有限元法计算得到的氢含量场求出夹杂处的应力强度因子,进而建立疲劳裂纹萌生和扩展的判据.比较了在不同加载条件下氢致疲劳裂纹萌生和扩展的规律.用梯形法修正了Sofronis和McMeeking的瞬态扩散有限元公式,发现用梯形法可以缓解加载初期较高的浓度梯度和应力梯度引起的计算结果震荡的情况,这对于计算开裂判据是十分重要的.最后讨论了提高模拟精度和改进模型的方法.