The growth of small fatigue cracks in A286 steel

Fatigue crack propagation in high-strength A286 steel was studied by comparing crack growth rates determined from: (1) conventional long-crack propagation tests, (2) closure-free long-crack tests at constant K_max, and (3) small-crack propagation tests. Small-crack growth rates were measured by following the growth of surface cracks in samples cycled from near-zero stress to 0.5 or 0.8σ_y. While most of the surface cracks became dormant shortly after nucleation, some grew into long cracks, and some of these propagated at cyclic stress intensities below the long-crack threshold, ΔK_th (or ΔK _th ^eff , the threshold cyclic stress intensity after crack closure effects have been removed). Surface cracks grew more rapidly than long cracks at the same ΔKor ΔK_eff. The small-crack effect disappeared when the crack-tip plastic zone size became greater than the grain size. The results show that the absence of crack closure is only one of several factors that influence short-crack growth in A286 steel. Both peak stress and microstructural effects are important. Microstructural effects are apparently responsible for subthreshold crack growth; the cracks that grow at ΔK < ΔK _th ^eff form and grow in statistically weak regions of the microstructure.     

Microstructural effects and crack closure during near

Near threshold fatigue crack growth behavior of a high strength steel under different tempered conditions was investigated. The important aspect of the study is to compare the crack growth behavior in terms of the closure-free component of the threshold stress intensity range, ΔK _th,eff While a systematic variation in the absolute threshold stress intensity range with yield strength was observed, the trend in the intrinsic ΔK _th or ΔK _th,eff exhibited a contrasting behavior. This has been explained as due to the difference in fracture modes during near threshold crack growth at different temper levels. It is shown that in a high strength and high strain hardening microstructure, yielding along crystallographic slip planes is difficult and hence it exhibited a flat transgranular fracture. In a steel with low strain hardening characteristics and relatively low strength, a tendency to crystallographic planar slip is observed consequently resulting in high ΔK _th. Occurrence of a predominantly intergranular fracture is shown to reduce intrinsic ΔK _th drastically and increase crack growth rates. Also shown is that crack closure can occur in high strength steels under certain fracture morphologies. A ‘transgranular planar slip’ during the inception of a ‘microstructure sensitive’ crack growth is essential to promote intergranular and faceted fracture. The occurrence of a maximum in the fraction of intergranular fracture during threshold crack growth corresponds to the ΔK value at which the cyclic plastic zone size becomes equal to the prior austenitic grain size.     

The influence of interparticle spacing on cyclic deformation and fatigue crack propagation in an aluminum-4 Pct copper alloy

A high purity Al-4 pct Cu alloy has been overaged for two different times at 400°C giving interparticle spacings (λ) of about 0.53 and 1.37 μm. Cyclic plasticity of the alloy with the smaller interparticle spacing can be explained in terms of plastic deformation behavior controlled by the structure whereas that for the alloy with the larger interparticle spacing is controlled by the matrix. The fatigue lives of the weaker alloy (λ = 1.37 μm) may be accurately predicted using the models of Coffin-Manson and Tomkins, however, these models are not applicable to the stronger alloy (λ = 0.53 μm). It was found that the crack tip opening displacement at the threshold stress intensity range (ΔK_th) was equivalent to the interparticle spacing. ΔK_th is related to the cyclic yield stress, σ_cy and the interparticle spacing in the following manner: ΔK_th ≈ (2 Eλσ_cy)^1/2, whereE is the modulus of elasticity. In the present case, the term λσ_cy is constant, giving the impression that ΔK_th is independent of the mechanical properties and microstructure. At very low growth rates, however, the fatigue crack growth is independent of these parameters and also the method of cyclic deformation. A transition to higher crack growth rates occurs when the plastic zone size reaches approximately one-seventh of the specimen thickness, allowing a nonplanar crack front to be developed. The value of the stress intensity range (ΔK_T) at this transition was found to be dependent upon the interparticle spacing according to the relation: ΔK_Tλ = 9.6 Pa-m^3/2. Formerly Lecturer and Research Associate, Department of Mechanical Engineering, University of Waterloo     

On the development of crack closure and the threshold condition for short and long fatigue cracks in 7150 aluminum alloy

In an attempt to analyze the behavior of physically “short” cracks, a study has been made of the development, location, and effect of crack closure on the behavior of fatigue cracks arrested at the “long” crack threshold stress intensity range, ΔK _TH , in underaged, peak aged, and overaged microstructures in a 7150 aluminum alloy. By monitoring the change in closure stress intensity,K _cl, during thein situ removal of material left in the wake of arrested threshold cracks, approximately 50 pct of the closure was found to be confined to a region within ∼500 μm of the crack tip. Following wake removal, previously arrested threshold cracks recommenced to propagate at low load ratios even though nominal stress intensity ranges didnot exceed ΔK _TH , representing the behavior of physically short cracks emanating from notches. No such crack extension at ΔK _TH was seen at high load ratios. With subsequent crack extension, crack closure was observed to redevelop leading to a deceleration in growth rates. The development of such closure was found to occur over crack extensions comparable with microstructural dimensions, rather than those associated with local crack tip plasticity. Such results provide further confirmation that the existence of a fatigue threshold and the growth of physically short cracks are controlled primarily by crack closure, and the data are discussed in terms of the micro-mechanisms of closure in precipitation hardened alloy systems. Formerly Graduate Student with the Department of Mechanical Engineering, University of California, Berkeley     

The influence of aqueous environments on low δK and high ΔK fatigue crack propagation behavior in low carbon structural steels

The influence of aqueous environments on fatigue crack propagation behavior was investigated for two types of structural steel (SB42 and HT80) in pure and 3 pct NaCl water under freely corroding conditions. In the intermediate to high ΔK region, fatigue crack propagation rates were higher in both aqueous environments and in 1 atm hydrogen than in air for both types of steel, and the acceleration effect increased power functionally with decreasing frequency from 5 to 0.0005 Hz. Such a crack growth acceleration property was explained by the mechanism of cyclically induced hydrogen embrittlement, as shown by the brittle striations formed on the fracture surfaces. On the other hand, in the lower ΔK region, both aqueous environments inversely suppressed crack growth and enhanced the threshold stress intensity factor range ΔK _th just above the ΔK _th in air, while only in aerated 3 pct NaCl water was the crack observed to grow even under the condition below the ΔK _th in air, not showing the threshold. Probable mechanisms for such fairly complex environmental effects were also suggested.     

Effects of temper level on the dependence of fatigue crack growth threshold and crack closure on the prior austenitic grain size

An attempt has been made to systematically investigate the effects of microstructural parameters, such as the prior austenite grain size (PAGS), in influencing the resistance to fatigue crack growth (FCG) in the near-threshold region under three different temper levels in a quenched and tempered high-strength steel. By austenitizing at various temperatures, the PAGS was varied from about 0.7 to 96 μm. The microstructures with these grain sizes were tempered at 200 °C, 400 °C, and 530 °C and tested for fatigue thresholds and crack closure. It has been found that, in general, three different trends in the dependence of both the total threshold stress intensity range, ΔK _th , and the intrinsic threshold stress intensity range, ΔK _{eff, th} , on the PAGS are observable. By considering in detail the factors such as cyclic stress-strain behavior, environmental effects on FCG, and embrittlement during tempering, the present observations could be rationalized. The strong dependence of ΔK _th and ΔK _{eff, th} on PAGS in microstructures tempered at 530 °C has been primarily attributed to cyclic softening and thereby the strong interaction of the crack tip deformation field with the grain boundary. On the other hand, a less strong dependence of ΔK _th and ΔK _{eff, th} on PAGS is suggested to be caused by the cyclic hardening behavior of lightly tempered microstructures occurring in 200 °C temper. In both microstructures, crack closure influenced near-threshold FCG (NTFCG) to a significant extent, and its magnitude was large at large grain sizes. Microstructures tempered at the intermediate temperatures failed to show a systematic variation of ΔK_th and ΔK_{eff, th} with PAGS. The mechanisms of intergranular fracture vary between grain sizes in this temper. A transition from “microstructure-sensitive” to “microstructure-insensitive” crack growth has been found to occur when the zone of cyclic deformation at the crack tip becomes more or less equal to PAGS. Detailed observations on fracture morphology and crack paths corroborate the grain size effects on fatigue thresholds and crack closure. K.S. RAVICHANDRAN, formerly Research Scholar, Department of Metallurgy, Indian Institute of Science     

Variability of large-crack fatigue-crack-growth thresholds in structural alloys

The fatigue threshold of large cracks is known to show substantial variations due to microstructural variability in structural alloys. The ΔK _th variations are dependent on the stress ratio (R); they are extremely large at low R ratios, e.g., R<0.5, but are drastically reduced at high R ratios (R>0.8). The origins of these large variations due to intrinsic and extrinsic mechanisms are examined by theoretical analyses. First, an intrinsic fatigue crack growth (FCG) threshold model is developed for structural alloys by considering the cyclic slip process at the crack tip. Second, the effects of extrinsic mechanisms such as residual plastic stretch, crack deflection, fracture-surface roughness, and oxide wedging are considered both individually and concurrently in order to delineate their relative contributions to threshold variability. The theoretical results indicate that the intrinsic threshold depends on the elastic properties, magnitude of the Burgers vector, yield stress, and Taylor factor (i.e., texture), but is independent of the R ratio or the maximum applied stress intensity factor, K _max. The large variability of ΔK _th at low R ratios and their corresponding dependence on K _max appear to arise from various crack closure mechanisms. Applications of the threshold models to structural alloys show good agreement between theory and experimental data from the literature for steels, Ti, Al, Ni, Cu, Nb, and Mo alloys.     

Fatigue crack propagation in carburized high alloy bearing steels

Fatigue cracks were propagated through carburized cases in M-50NiL (0.1 C,4 Mo, 4 Cr, 1.3 V, 3.5 Ni) and CBS-1000M (0.1 C, 4.5 Mo, 1 Cr, 0.5 V, 3 Ni) steels at constant stress intensity ranges, ΔK, and at a constant cyclic peak load. Residual compressive stresses of the order of 140 MPa (20 Ksi) were developed in the M-50NiL cases, and in tests carried out at constant ΔK values it was observed that the fatigue crack propagation rates,da/dN, slowed significantly. In some tests, at constant peak loads, cracks were stopped in regions with high compressive stresses. The residual stresses in the cases in CBS-1000M steel were predominantly tensile, probably because of the presence of high retained austenite contents, andda/dN was accelerated in these cases. The effects of residual stress on the fatigue crack propagation rates are interpreted in terms of a pinched clothespin model in which the residual stresses introduce an internal stress intensity, K_i where K_i, = σ_id _i ^1/2 (σ_i = internal stress, d_i = characteristic distance associated with the internal stress distribution). The effective stress intensity becomes K_e = K_a + K_i where K_a is the applied stress intensity. Values of K_i were calculated as a function of distance from the surface using experimental measurements of σ_i and a value of d_i = 11 mm (0.43 inch). The resultant values of K_e were taken to be equivalent to effective ΔK values, andda/dN was determined at each point from experimental measurements of fatigue crack propagation obtained separately for the case and core materials. A reasonably good fit was obtained with data for crack growth at a constant ΔK and at a constant cyclic peak load. The carburized case depths were approximately 4 mm, and the possible effects associated with the propagation of short cracks were considered. The major effects were observed at crack lengths of about 2 mm, but the contributions of short crack phenomena were considered to be small in these experiments, since the two steels were at high strength levels, and short cracks would be expected to be of the order of 10 μm. Also, the two other steels behaved differently and in a way which followed the residual stress patterns. Both M-50NiL and CBS-1000M have a high fracture toughness, with K_lc = 50 MPa · m^1/2 (45 Ksi · in^1/2), and the carburized cases exhibit excellent resistance to rolling contact fatigue. Thus, M-50NiL, carburized, may be useful for bearings where high tensile hoop stresses are developed, since fatigue cracks are slowed in the case by the residual compressive stresses, and fracture is resisted by the relatively tough core.     

Fatigue crack growth behavior in niobium-hydrogen alloys

Near-threshold fatigue crack growth behavior has been investigated in niobium-hydrogen alloys. Compact tension specimens (CTS) with three hydrogen conditions are used: hydrogen-free, hydrogen in solid solution, and hydride alloy. The specimens are fatigued at a temperature of 296 K and load ratios of 0.05, 0.4, and 0.75. The results at load ratios of 0.05 and 0.4 show that the threshold stress intensity range (ΔK _th ) decreases as hydrogen is added to niobium. It reaches a minimum at the critical hydrogen concentration (C _cr ), where maximum embrittlement occurs. The critical hydrogen concentration is approximately equal to the solubility limit of hydrogen in niobium. As the hydrogen concentration exceeds C _cr , ΔK _th increases slowly as more hydrogen is added to the specimen. At load ratio 0.75, ΔK _th decreases continuously as the hydrogen concentration is increased. The results provide evidence that two mechanisms are responsible for fatigue crack growth behavior in niobium-hydrogen alloys. First, embrittlement is retarded by hydride transformation-induced and plasticity-induced crack closures. Second, embrittlement is enhanced by the presence of hydrogen and hydride.     

Fatigue crack propagation in carburized X-2M steel

The growth rates of fatigue cracks propagating through the case and into the core have been studied for carburized X-2M steel (0.14 C, 4.91 Cr, 1.31 Mo, 1.34 W, 0.42 V). Fatigue cracks were propagated at constant stress intensities, ΔK, and also at a constant cyclic peak load, and the crack growth rates were observed to pass through a minimum value as the crack traversed the carburized case. The reduction in the crack propagation rates is ascribed to the compressive stresses which were developed in the case, and a pinched clothespin model is used to make an approximate calculation of the effects of internal stress on the crack propagation rates. We define an effective stress intensity, K_e = K_a + K_i, where K_a is the applied stress intensity, K_i = σ_id _i ^1/2 , σ_i is the internal stress, and d_i is a characteristic distance associated with the depth of the internal stress field. In our work, a value of d_i = 11 mm (0.43 inch) fits the data quite well. A good combination of resistance to fatigue crack propagation in the case and fracture toughness in the core can be achieved in carburized X-2M steel, suggesting that this material will be useful in heavy duty gears and in aircraft gas turbine mainshaft bearings operating under high hoop stresses.     

Significance of the small crack growth law and its practical application

The effects of microstructure and specimen size on the fatigue crack growth rate of an annealed 0.42 C steel were investigated under uniaxial fatigue loading in air. Although a dramatic fluctuation of crack growth rate was found in the propagation process of microstructurally small cracks, the mean value of crack growth rate can be evaluated by a simple mechanical parameter, σ _a ⁿ l (l, crack length; n, constant), under high stress levels where small-scale yielding conditions are exceeded. This parameter is also effective for cracks larger than 1 to 2 mm under high stress levels, as long as the finite boundary effect of a specimen on the driving force of crack propagation is considered. The crack growth rate of the alloy was described as a function of stress amplitude and crack length in terms of two mechanical parameters, σ _a ⁿ l and ΔK. The applicable conditions of the two parameters were discussed and manifested.     

The initiation and growth of fatigue cracks in a titanium aluminide alloy

The micromechanics of small, naturally initiated fatigue cracks and large through-thickness fatigue cracks have been studied in the titanium aluminide alloy Super Alpha 2. The microstructure investigated had equal volume fractions ofα ₂ and Β phases. Crack growth rates were higher than through α-Β titanium alloys. Initiation of small cracks was found always to occur in theα ₂ phase, and small cracks grew belowΔK _th, the minimum cyclic stress intensity required for growth of large fatigue cracks. A method previously proposed for reconciling the growth rates of large and small cracks is applied to these results.     

Effects of load ratio and temperature on the near-threshold fatigue crack propagation behavior in a CrMoV steel

The influence of temperature in the range of 24 to 260 °C and load ratio on the near-threshold fatigue crack growth rate behavior of a CrMoV steel was characterized. At all temperatures investigated, the threshold stress intensity range, ΔK _th, for fatigue crack growth decreased with increasing load ratio. The near-threshold crack growth rates increased significantly at 149 °C when compared with the rates at room temperature. However, the crack growth rates at 260 °C were comparable to those at 149 °C. These observations are rationalized in terms of the concepts of roughness and oxide-induced crack closure. Extensive fracture surface characterization using SEM, oxide thickness measurements by Auger spectroscopy, and roughness measurements by light-section-microscopy were conducted to substantiate the explanations.     

The influence of tempering temperature on small fatigue crack behavior monitored with surface acoustic waves in quenched and tempered 4140 steel

Small surface fatigue crack growth in specially designed cantilevered bending samples of high-purity 4140 steel quenched and tempered to various strength levels was investigated. Tempering temperatures of 200 °C, 400 °C, 550 °C, and 700 °C were used to produce a range of yield strengths and microstructures. Crack propagation and crack closure were monitored with a surface acoustic wave ultrasonic technique. The small crack results were compared to those of long cracks in compact tension samples. Small cracks in the 200 °C and 400 °C tempers grew at ΔK levels above their long crack thresholds. Small cracks in the 550 °C tempers grew at ΔK levels slightly below the long crack threshold. The surface cracks in the 700 °C temper grew well below the long crack threshold showing the “small crack effect.” Small crack growth occurred in a very narrowda/dN- ΔK scatter band showing much less variation with microstructure than the long crack data. The differences between the long and small crack data were due to the high, relatively similar closure values for the small cracks and the variation of long crack growth with microstructure. Formerly Graduate Research Assistant, Department of Materials Science and Engineering, Stanford University, Stanford, CA.     

Fatigue crack propagation in aluminum-lithium alloy 2090: Part II. small crack behavior

A study has been made of the mechanics and mechanisms of fatigue crack propagation in a commercial plate of aluminum-lithium alloy 2090-T8E41. In Part II, the crack growth behavior of naturallyoccurring, microstructurally-small (2 to 1000μm) surface cracks is examined as a function of plate orientation, and results compared with those determined in Part I on conventional long (≳5 mm) crack samples. It is found that the near-threshold growth rates of small cracks are between 1 to 3 orders of magnitude faster than those for long cracks, subjected to the same nominal stress intensity ranges (at a load ratio of 0.1). Moreover, the small cracks show no evidence of an intrinsic threshold and propagate at ΔK levels as low as 0.7 MPa{ie563-01}, far below the long crack threshold ΔK_TH. Their behavior is also relatively independent of orientation. Such accelerated small crack behavior is attributed primarily to restrictions in the development of crack tip shielding (principally from roughness-induced crack closure) with cracks of limited wake. This notion is supported by the close correspondence of small crack results with long crack growth rates plotted in terms of ΔK_eff (i.e., after allowing for closure above the effective long crack threshold). Additional factors, including the different statistical sampling effect of large and small cracks with microstructural features, are briefly discussed.     

Near- threshold fatigue crack growth in copper and alpha- brass: Grain- size and environmental effects

The fatigue propagation rates and fatigue threshold ( ΔK _th) values were studied (R = 0.1 and frequency = 20 Hz) on copper and 70-30 α-brass of two different grain sizes in laboratory air and dry argon. With decreasing grain size, the threshold increased in copper, while it decreased in α-brass. These results suggest that in copper, crack tip plasticity considerations were more important in determining the threshold values than crack closure effects. Dry argon increased ΔK _th slightly in copper and more significantly in α-brass. A transition from completely transgranular to partially intergranular and back to completely transgranular cracking was observed with decreasing crack growth rates in both materials and environments. The growth rates for which intergranular cracking was obtained were found to be consistent with a hydrogen embrittlement mechanism, associated with adsorption of water molecules and dislocation transport of hydrogen.     

Elevated-temperature fatigue crack growth

The fatigue crack growth behavior of MAR-M200 single crystals was examined at 982 °C. Using tubular specimens, fatigue crack growth rates were determined as functions of crystallographic orientation and the stress state by varying the applied shear stress range-to-normal stress range ratio. Neither crystallographic orientation nor stress state was found to have a significant effect on crack growth rate when correlated with an effective ΔK which accounted for mixed-mode loading and elastic anisotropy. For both uniaxial and multiaxial fatigue, crack growth generally occurred normal to the principal stress direction and in a direction along which ΔK _II vanished. Consequently, the effective ΔK was reduced to ΔK_I and the rate of propagation was controlled by ΔK _I only. The through-thickness fatigue cracks were generally noncrystallographic with fracture surfaces exhibiting striations in the [010], [011], and [111] crystals, but striation-covered ridges in the [211] specimen. These fracture modes are contrasted to crystallographic cracking along slip bands observed at ambient temperature. The difference in cracking behavior at 25 and 982 °C is explained on the basis of the propensity for homogeneous, multiple slip at the crack tip at 982 °C. The overall fracture mechanism is discussed in conjunction with Koss and Chan’s coplanar slip model.     

Probabilistic micromechanical modeling of fatigue-life variability in an <Emphasis Type="Italic">α+β</Emphasis> Ti alloy

The fatigue-life variability in an α+β Ti alloy (Ti-6Al-4V) has been examined through a probabilistic micromechanical model that treats the crack-initiation and growth processes at the grain-size level. First, a physics-based crack-initiation model is described. This is followed by a summary of a physics-based fatigue-crack-growth model. The combined model is applied to predict the variability of crack initiation and growth lives due to microstructural variations in Ti-6Al-4V. Finally, possible fatigue mechanisms or scenarios that can lead to the worst-case fatigue life are elucidated via probabilistic modeling of the fatigue-crack-initiation process, the driving force of the grain-sized cracks, as well as the intrinsic (closure-free) threshold and the closure-affected threshold of the small cracks. In the absence of preexisting cracks, the worst-case total fatigue life is obtained when two conditions are met: (1) the crack size at initiation is on the order of 1 to 2 times the grain size, and (2) the driving force (applied ΔK) exceeds the intrinsic threshold of the small cracks. The probabilistic results are also used to elucidate the conditions for the occurrence of dual fatigue limits in high-cycle fatigue (HCF) or giga-cycle fatigue.     

The fatigue crack growth behavior of electron-beam welded a286 superalloy

Fatigue crack growth curves(Δa/ΔN =f(K _max )) were measured with 2.5 mm thick sheets of electron beam welded iron base superalloy A286. Fatigue testing frequency was 21 kHz,R = −1 (mean stress zero) and the environment was noncorrosive silicone oil at 20 °C. Two series of samples with different welding conditions were tested. One series was welded perfectly, whereas the second contained microcracks within the weld and the heat affected zone. It was shown that the crack growth rate in the base metal is slower than in the weld. The threshold stress intensity factorK _th of the base metal is 14 MNm^-3/2 and that of the weld, 10 MNm ^-3/2 . However, at higherK _max values, the crack grows more rapidly in the weld than in the base metal; for example, the crack growth rate is 16 times higher at K_max = 20 MNm ^-3/2 . Microcracks introduced by an imperfect welding process do not influence the fatigue cracking behavior in the threshold regime; atK ^max = 15 MNm^-3/2, however, the crack growth rates differ by an order of magnitude. Fractographic examination shows considerable differences in the fracture appearance of weld, heat affected zone, and base material. Weld and base metal display ductile fracture surfaces and the heat affected zone is characterized by crystallographic fracture facets.     

Fracture and fatigue behavior of sintered steel at elevated temperatures: Part II. Fatigue crack propagation

Fatigue cracking resistance of sintered steel as a function of temperature is characterized by crack growth rate vs the stress intensity range, ΔK. The stress ratio effects on fatigue crack propagation (FCP) are investigated from room temperature to 300 °C. The crack closure effects on FCP are evaluated by both theoretical and experimental approaches. We found that the crack closure cannot be fully responsible for the observed increase of fatigue resistance with low stress ratio. Experimental results support that both K _max and ΔK control near-threshold crack growth. Fatigue crack resistance at high ΔK regime decreases with temperature. The apparent increase of fatigue resistance at the near-threshold regime at elevated temperatures might be attributed to microcrack toughening.