Mode III fatigue crack propagation in low alloy steel

To provide a basis for estimating fatigue life in large rotating generator shafts subjected to transient oscillations, a study is made of fatigue crack propagation in Mode III (anti-plane shear) in torsionally-loaded spheroidized AISI4340 steel, and results compared to analogous behavior in Mode I. Torsional S/N curves, determined on smooth bars containing surface defects, showed results surprisingly close to expected unnotched Mode I data, with lifetime increasing from 10⁴ cycles at nominal yield to 10⁶ cycles at half yield. Fatigue crack growth rates in Mode III, measured on circumferentially-notched samples, were found to be slower than in Mode I, although still power-law related to the alternating stress intensity(△K _III) for small-scale yielding. Mode III growth rates were only a small fraction (0.002 to 0.0005) of cyclic crack tip displacements(△CTD _III) per cycle, in contrast to Mode I where the fraction was much larger (0.1 to 0.01). A micromechanical model for Mode III growth is proposed, where crack advance is considered to take place by a Mode II coalescence of cracks, initiated at inclusions ahead of the main crack front. This mechanism is consistent with the crack increment being a small fraction of △CTD_III per cycle. Formerly with Massachusetts Institute of Technology, Cambridge, MA Formerly with M.I. T.     

Fatigue crack propagation in 4140 steel

The fatigue crack propagation rates, da/dN, of 4140 steel were measured in dry argonvs tempering temperature. In specimens 3.2 mm thick at a given ΔK between 15 and 30 MN/ m^3/2, da/dN decreases with increasing tempering temperature, reaches a shallow minimum for tempering at 400°C. The rate for as-quenched specimens increases withR ratio; this is not the case for the 400, 550 and 650°C tempers. Reducing the specimen thickness to 1.3 mm has little effect on the 650°C temper but causes a large decrease in da/dN for the asquenched condition and 200°C temper. Edge notch specimens tempered at 550 and 650°C are subject to crack arrest from cycling prior to crack initiation. The results are discussed in terms of the metallurgical structures and various fatigue crack propagation equations which have been proposed. The results cannot be explained on the basis of da/dN being determined only by Young’s modulus andK _c.     

Influence of heat treatment on the fatigue crack growth rates of a secondary hardening steel

The relationships between microstructure and fatigue crack propagation behavior were studied in a 5Mo-0.3C steel. Microstructural differences were achieved by varying the tempering treatment. The amounts, distribution, and types of carbides present were influenced by the tempering temperature. Optical metallography and transmission electron microscopy were used to characterize the microstructures. Fatigue fracture surfaces were studied by scanning electron microscopy. For each heat treatment the fatigue crack growth properties were measured under plane strain conditions using a compact tension fracture toughness specimen. The properties were reported using the empirical relation of Paris [da/dN = C_oΔK^m]. It was found that secondary hardening did influence the fatigue crack growth rates. In particular, intergranular modes of fracture during fatigue led to exaggerated fatigue crack growth rates for the tempering treatment producing peak hardness. Limited testing in a dry argon atmosphere showed that the sensitivity of fatigue crack growth rates to environment changed with heat treatment.     

The effects of temperature on fatigue crack propagation in linepipe steel

Fatigue crack propagation rates in an A.P.I. 5L Grade steel were investigated by means of constant deflection amplitude bending fatigue tests at 640 c.p.m. on single edge notched specimens at — 50, — 10, 20 and 70°C in argon. The data were evaluated in terms of the crack propagation rate (da/dN) as a function of the stress intensity range (Δ/K), according toda/dN = ΔK ^m . It was found that dynamic strain aging has a major influence on fatigue crack propagation, resulting in a maximum of the crack propagation rate at room temperature. Similarly, the cyclic plastic zone size is a maximum at room temperature. D. H. Andreasen, formerly with the Department of Mining and Metallurgy, University of Alberta, Edmonton, Alberta, Canada.     

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.     

Crack deflection: Implications for the growth of long and short fatigue cracks

The influences of crack deflection on the growth rates ofnominally Mode I fatigue cracks are examined. Previous theoretical analyses of stress intensity solutions for kinked elastic cracks are reviewed. Simple elastic deflection models are developed to estimate the growth rates of nonlinear fatigue cracks subjected to various degrees of deflection, by incorporating changes in the effective driving force and in the apparent propagation rates. Experimental data are presented for intermediate-quenched and step-quenched conditions of Fe/2Si/0.1C ferrite-martensite dual phase steel, where variations in crack morphology alone influence considerably the fatigue crack propagation rates and threshold stress intensity range values. Such results are found to be in good quantitative agreement with the deflection model predictions of propagation rates for nonlinear cracks. Experimental information on crack deflection, induced by variable amplitude loading, is also provided for 2020-T651 aluminum alloy. It is demonstrated with the aid of elastic analyses and experiments that crack deflection models offer a physically-appealing rationale for the apparently slower growth rates of long fatigue cracks subjected to constant and variable amplitude loading and for the apparent deceleration and/or arrest of short cracks. The changes in the propagation rates of deflected fatigue cracks are discussed in terms of thelocal mode of crack advance, microstructure, effective driving force, growth mechanisms, mean stress, slip characteristics, and crack closure.     

Fatigue crack propagation in 99.99+ and 1100 aluminum at 298 and 77 k

The fatigue crack propagation rate,dc/dN, in cold-rolled and annealed 99.99+ Al is about 80 times slower at 77 K than at 298 K. In annealed 1100 Al which contains constituent particles,dc/dN decreases by a factor of 20 on cooling from 298 to 77 K. At 298 and 77 K, annealed 99.99⁺ Al and 1100 Al cyclically harden but the amount is greater at 77 K. Cold-rolled 99.99⁺ Al cyclically hardens at 77 K but cyclically softens at 298 K. The much slower fatigue crack propagation rate at 77 K in aluminum is attributed in part to the increase in cyclic yield stress, σ_y′, on cooling. At 77 K the high rate of work hardening at large strains is also thought to result in high plastic work per unit area of fatigue crack thereby reducing the fatigue crack propagation rate. Rice’s theory for a Mode I plane stress crack predicts the measured plastic zone size if the local stress corresponding to zero plastic strain in the cyclic stress-strain curve is employed in the formula.     

Effects of pre-existing grain boundary microvoid distributions on fracture toughness and fatigue crack growth in low alloy steel

The role of dispersions of pre-existing grain boundary microvoids is investigated in fracture toughness and fatigue crack propagation behavior in a low alloy steel. Microvoid damage is achieved by prior exposure of the steel to gaseous hydrogen atmospheres at high temperatures and pressures, where carbon within the steel reacts with ingressed hydrogen to nucleate methane bubbles along prior austenite grain boundaries (hydrogen attack). It is shown that, whereas the crack initiation and crack growth toughness (i.e. K_Ic and the tearing modulus) are severely degraded, even for comparatively mild degrees of microvoid damage, rates of sub-critical crack growth by fatigue remain relatively unaffected. Such results are interpreted in terms of a mutual competition between microstructural damage generated by the grain boundary microvoids, which promotes crack growth by lowering the intrinsic resistance of the microstructure, and the resulting tortuous crack paths, which extrinsically retard crack growth at low stress intensities by lowering the local crack tip “driving force” (crack tip shielding). As shielding effects are minimized at high stress intensities, the degradation in intrinsic toughness is related to changes in ductility by means of a stress-modified critical strain model for ductile fracture, where the presence of small microvoid clusters is shown to promote coalescence through the easier onset of plastic strain localization. Fatigue behavior, conversely, is dominated by extrinsic shielding mechanisms and is modeled in terms of two-dimensional models of crack deflection and roughness-induced crack closure.     

The effect of tempering temperature on near- threshold fatigue crack behavior in quenched and tempered 4140 steel

Fatigue crack growth in compact tension samples of high purity 4140 steel quenched and tempered to various strength levels was investigated. Tempering temperatures of 200, 400, 550, and 700 °C produced yield strengths from 1600 to 875 MPa, respectively. Crack propagation and crack closure were monitored inK-decreasing tests performed underR = 0.05 loading conditions in laboratory air. Results indicated that as the yield strength increased the crack growth rate increased at a given ΔK and ΔK_th decreased. Threshold values varied from 2.8 MPa m^1/2 (200 °C temper) to 9.5 MPa m_1/2 (700 °C temper). Cracks in the 200 °C tempered samples grew by an intergranular mechanism following prior austenite grain boundaries probably caused by hydrogen embrittlement or tempered martensite embrittlement. Tempering above 200 °C produced transgranular fatigue crack growth. The level of crack closure increased with tempering temperature and with crack propagation in a given tempered condition. Crack closure was caused by a combination of plasticity-induced and oxide-induced mechanisms. The use of an effective stress intensity range based on crack closure consolidated the fatigue crack growth curves and the threshold values for all tempering temperatures except 200 °C. Formerly Graduate Research Assistant, Department of Materials Science and Engineering, Stanford University, Stanford, CA. Formerly Professor, Department of Materials Science and Engineering, Stanford University, Stanford, CA.     

Effects of zinc coatings on the stress corrosion cracking and hydrogen embrittlement of low-alloy steel

The effects of electroplated and hot-dip zinc coatings on the fracture of low-alloy steel AISI 4140 bars tempered to hardnesses in the range Rc 33 to 49 were studied. Either electroplated or hot-dip zinc coatings decrease resistance to stress corrosion cracking,i.e., they reduceK _sc, the threshold stress intensity for stress corrosion cracking in 3.5 wt pct NaCl solution. AboveK _scelectroplated-zinc coatings do not appear to affect the crack-growth rate, although the incubation period prior to the onset of crack growth is reduced. Hot-dip zinc coatings increase stress corrosion crack growth rates slightly because of the additive effect of internal dissolved hydrogen. Hot-dip zinc coatings reduce the critical stress intensity for fracture in the absence of a corrosive environment because of embrittlement by internal hydrogen which is released from traps during hot-dip coating and confined by the inter metallic coatings which form on the steel surface in the hot dip bath. A simple fracture mechanics analysis indicates that either increasing diameter or the presence of a zinc coating lowers the critical hardness at which the stress corrosion cracking of structural bolts can occur.     

The Significance of Crack Initiation Stage in Very High Cycle Fatigue of Steels

Different stages of the Very High Cycle Fatigue (VHCF) crack evolution in tool steels have been explored using a 20 kHz ultrasonic fatigue testing equipment. Extensive experimental data is presented describing VHCF behaviour, strength and crack initiating defects in an AISI H11 tool steel. Striation measurements are used to estimate fatigue crack growth rate, between 10^?8 and 10^?6 m/cycle, and the number of load cycles required for a crack to grow to critical dimensions. The growth of small fatigue cracks within the “fish‐eye” is shown to be distinctively different from the crack propagation behaviour of larger cracks. More importantly, the crack initiation stage is shown to determine the total fatigue life, which emphasizes the inherent difficulty to detect VHCF cracks prior to failure. Several mechanisms for initiation and early crack growth are possible. Some of them are discussed here: crack development by local accumulation of fatigue damage at the inclusion – matrix interface, hydrogen assisted crack growth and crack initiation by decohesion of carbides from the matrix.     

Calorimetric Measurement of the Plastic Vtork of Fatigue Crack Propagation in 4140 Steel

A calorimetric technique has been developed for measurement of the effective surface energy of fatigue crack propagation,U, and the cyclic plastic work in the plastic zone,Q. The technique has several distinct advantages over existing methods. Measurements on 4140 steel (650 °C temper) show thatU andQ are direct functions of the stress intensity factor,hK, and indirect functions of crack growth rate,daldN. Measurement of the change ofU andQ after the application of a tensile overload supports this conclusion and provides strong evidence supporting crack closure theories. Formerly with the Materials Science and Engineering Department and Materials Research Center, Northwestern University, Evanston, IL.     

Influence of impurity segregation on temper em brittlement and on slow fatigue crack growth and threshold behavior in 300-M high strength steel

Interactions between hydrogen embrittlement and temper embrittlement have been examined in a study of fracture and low growth rate (near-threshold) fatigue crack propagation in 300-M high strength steel, tested in humid air. The steel was investigated in an unembrittled condition (oil quenched after tempering at 650°C) and temper embrittled condition (step-cooled after tempering at 650°C). Step-cooling resulted in a severe loss of toughness (approximately 50 pct reduction), without loss in strength, concurrent with a change in fracture mode from micr ovoid coalescence to inter granular. Using Auger spectroscopy analysis, the embrittlement was attributed to the cosegregation of alloying elements (Ni and Mn) and impurity elements (P and Si) to prior austenite grain boundaries. Prior temper embrittlement gave rise to a substantial reduction in resistance to fatigue crack propagation, particularly at lower stress intensities approaching the threshold for crack growth(x0394;K _o). At intermediate growth rates (10^-5 to 10^-3 mmJcycle), propagation rates in both unembrittled and embrittled material were largely similar, and only weakly dependent on the load ratio, consistent with the striation mechanism of growth observed. At near-threshold growth rates (<10⁻⁵ to 10⁻⁶ mmJcycle), embrittled material exhibited significantly higher growth rates, 30 pct reduction in threshold ΔK_o values and intergranular facets on fatigue fracture surfaces. Near-threshold propagation rates (and ΔK_o values) were also found to be strongly dependent on the load ratio. The results are discussed in terms of the combined influence of segregated impurity atoms (temper embrittlement) and hydrogen atoms, evolved from crack tip surface reactions with water vapor in the moist air environment (hydrogen embrittlement). The significance of crack closure concepts on this model is briefly described. ntmis]formerly with the Lawrence Berkeley Laboratory, University of California in Berkeley. Formerly with the Lawrence Berkeley Laboratery, University of California in Berkeley.     

The effect of phase continuity on the fatigue and crack closure behavior of a dual-phase steel

The effect of phase continuity on the low cycle fatigue and fatigue crack growth behavior of a Fe-C-Mn dual-phase steel has been investigated. Two microstructures, one consisting of continuous ferrite and the other continuous martensite, were examined. Although there was no difference in the low cycle fatigue lives between the two microstructures, the continuous martensite structure exhibited an extremely high fatigue threshold value of 20 MPa m^1/2, compared to 16 MPa m^1/2 for the continuous ferrite microstructure. A major effect of phase continuity has also been found in the crack closure levels during fatigue crack propagation studied over three decades of crack growth rates. The continuous martensite microstructure exhibited much higher closure levels due to the martensite constraining the plastic deformation in the ferrite and bearing a larger portion of the applied cyclic load. This effect is similar to the extrinsic toughening phenomenon cited in the literature. After accounting for the closure levels the intrinsic or effective fatigue crack growth rates are similar for the two microstructures. These intrinsic thresholds are predicted by employing experimentally obtained low cycle fatigue parameters and the ferrite grain size.     

High frequency stage I corrosion fatigue of austenitic stainless steel (316L)

High frequency (123 Hz) fatigue crack propagation studies were conducted under rising ΔK conditions (R-ratio = 0.22) on single edge notch specimens of austenitic stainless steel (type 316L) that contained an annealed precrack. Tests were conducted in near neutral (pH 5.5) solutions of 1 M NaCl and 1 M NaCl + 0.01 M Na₂S₂O₃ under potentiostatically controlled conditions and in desiccated air. Attention was directed primarily to the near threshold behavior and the stage I (crystallographic) region of cracking. Good mixing between the crack solution and bulk solution was obtained and crack retardation and arrest effects, due to surface roughness induced closure, were minimized at high anodic potentials by electrochemical erosion. Thermodynamic considerations showed that hydrogen played no role in fatigue crack propagation. Analysis of the results in terms of the estimated effective cyclic stress intensity, ΔK _eff, showed a systematic effect of potential on the average crack growth increment per cycle,da/dN. Anodic dissolution processes were considered to make an insignificant contribution toda/dN. A model was proposed for stage I fatigue cracking based on the effect of oxide nucleation rate on restricted slip reversal. The essential features of the model were considered to be relevant to cracking in aqueous environments and in desiccated air.     

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.     

Fatigue crack initiation and propagation in a quenched and tempered niobium bearing HSLA steel

The fatigue crack initiation and propagation behavior of a niobium bearing HSLA steel heat treated to give two tempered martensitic microstructures presumably with and without fine niobium carbides has been studied by light microscopy, electron microscopy, and strain gage measurements of plastic zone deformation. The high cycle, stress controlled fatigue life of the steel in both heat treated conditions was quite similar with the steel presumably containing the fine niobium carbides having slightly better resistance at low stress amplitudes. This slightly better high cycle resistance is associated with better resistance to fatigue crack initiation for this heat treatment. The fatigue crack propagation behavior of the steel was the opposite. The steel presumably containing the fine niobium carbides exhibited a much faster fatigue crack growth rate than that without them. The difference in growth rates is explained in terms of the plastic work expended during the propagation of the fatigue crack.     

Fatigue behavior of maraging steel 300

The cyclic stress-strain curves, the low cycle and high cycle fatigue lives and the fatigue crack growth rates of annealed (1 h 820°C) and aged (3 h 480°C) maraging steel 300 were determined. Incremental step testing and stable hysteresis loop tip measurements were used to determine the cyclic σ-ε curves. Both annealed and aged maraging steels were found to cyclically soften at room temperature over a plastic strain range from 0.1 to 20 pct. The S-N curves were determined from 10 to 10⁷ cycles to failure by plastic strain controlled low cycle fatigue tests performed in air and load controlled high cycle fatigue tests performed in dry argon. The test results compared very well with the theoretical lifetime predictions derived from Tomkins’ theory. Fatigue crack growth rates were measured in air and dry argon for the annealed and aged alloys. Crack growth rates of annealed maraging steel were found to be equal to those of aged maraging steel at rates between 10^-7 and 10^-5 in./cycle. A significant difference in crack growth rates in the two environments was found at low stress intensity factor ranges, indicating a high susceptibility to corrosion fatigue in the presence of water vapor. The mechanisms of cyclic softening in the two alloys are discussed in terms of dislocations rearrangement in the annealed alloy and dislocation-precipitate interactions in the aged alloy.     

复杂应力状态225Cr 1Mo钢的高温低周疲劳裂纹扩展规律

 用带预裂纹的缺口试件研究225Cr 1Mo 钢高温低周疲劳裂纹扩展规律，通过疲劳试验观察裂纹扩展寿命，应用ANSYS计算裂纹尖端应力应变分量和当量弹、塑性应变范围，利用当量J积分范围表征225Cr 1Mo 钢在复杂应力状态下的低周疲劳裂纹扩展速率。结果表明：疲劳裂纹扩展速率da/dN与当量J积分范围ΔJf的关系不受试件缺口型式和加载应变范围的影响，用当量J积分来评价225Cr 1Mo 钢的高温低周疲劳裂纹扩展规律是有效的。