Theoretical model for FCGR near the threshold

A theoretical model for fatigue crack growth rate at low and near threshold stress intensity factor is developed. The crack tip is assumed to be a semicircular notch of radius ρ and incremental crack growth occurs along a distance 4ρ ahead of the crack tip. After analysis of the stress and strain distribution ahead of the crack tip, a relationship between the strain range and the stress intensity range is proposed. It is then assumed that Manson-Coffin cumulative rule can be applied to a region of length 4ρ from the crack tip, where strain reversal occurs. Finally, a theoretical equation giving the fatigue crack growth rate is obtained and applied to several materials (316L stainless steel, 300M alloy steel, 70-30 α brass, 2618A and 7025 aluminum alloys). It is found that the model can be used to correlate fatigue crack growth rates with the mechanical properties of the materials, and to determine the threshold stress intensity factor, once the crack tip radius α is obtained from the previous data.     

Fatigue crack propagation in trip steels

The fatigue crack propagation behavior of a class of metastable austenitic steels called TRIP steels has been investigated. The alloy composition was chosen to have theMs well below room temperature and theMD above room temperature after thermomechanical processing. A simple theoretical model of fatigue crack propagation (FCP) based on fracture mechanics was developed. Fatigue crack propagation tests on SEN specimens at various stress intensity ranges (ΔK) were carried out, and two stage plastic-carbon replica were used to observe the fracture surface of the FCP specimens. To a first approximation, both the experimental and theoretical results followed the usual relationship between ΔK and FCP rates;i.e. da/dn ∝ (ΔK).⁴ The fatigue fracture surface contained fatigue striations, quasicleavage and elongated dimples; a reflection of the complex structure of TRIP steels. A beneficial effect of strain induced martensite transformation with regards to fatigue crack propagation was found. TRIP steels showed better FCP properties than a number of alloy steels of similar strength levels and compared favorably with mar aging steels in the low ΔK range.     

Fatigue of annealed and cold worked stable and unstable stainless steels

Fatigue crack growth rates (FCGR) in AISI 301 and 302 austenitic stainless steel alloys have been measured in controlled load cycles withR = 0.05. Both annealed and cold rolled conditions were examined. The austenite phase of the AISI 301 alloy was unstable under stress and transformed martensitically to α′ to a much greater extent than the AISI 302 alloy. At low values of mean stress the unstable alloy had a lower FCGR than the more stable 302 alloy. The FCGR increased with mean stress until values of mean stress ⪞70 MPa, where the FCGR was independent of mean stress and was the same for both alloys. Various metallographic and macroscopic measurements were made to try to understand this behavior. It was concluded that residual compressive stress due to transformation at the crack tip was responsible for the lower crack growth rates of the unstable 301 alloy. Cold worked specimens had significantly lower crack growth rates than the annealed specimens, and both alloys behaved identically. Formerly with the Department of Metallurgy, University of Illinois at Urbana-Champaign     

The role of dislocation substructures in fatigue crack propagation in copper and alpha brass

The effect of dislocation substructures on fatigue crack propagation (FCP) behavior in copper and alpha brass was studied. Various dislocation substructures were obtained by prestraining in tension. Dislocation cells were formed by this prestraining in copper and 90/10 brass and when they formed the resistance to FCP at intermediate propagation rates (5×10⁻⁹ to ∼10⁻⁷ m/cycle) increased with increasing prestrain. Planar dislocation arrays were observed in 70/30 brass instead of cells, and the effect of prestraining on the FCP resistance was insignificant. From the FCP data for each material it was observed that, regardless of the difference in the dislocation substructures and grain sizes, the two constantsC andm in the Paris equation,da/dN=C(ΔK) ^m, were interrelated. Possible relations between the cyclic strain hardening exponent andm are discussed. The influence of both prestrain and grain size on threshold behavior was also studied.     

The effect of heat treatments on the corrosion fatigue properties of 13 Pct Chromium Stainless Steel in 3 Pct NaCI Aqueous Solution

The effect of austenitizing or tempering temperature on the corrosion fatigue properties of 13 pct chromium stainless steel was studied. Three pct NaCI aqueous solution was used as the corrosive environment, and the results were compared with the atmospheric fatigue properties. Strong influence of the tempering temperature on the S-N and FCP behavior of this blading material was found. The damage ratios (corrosion fatigue limit divided by endurance limit) of these various heat treated specimens became very low by this environment. Especially, extremely low corrosion fatigue strength of the specimen tempered at 600 °C was noticed. This microstructure was strategically used to clarify the reduction of pH inside the corrosion pits which were generally formed at the fatigue crack initiation sites. FCP data in the corrosive environment showed higher resistance than the atmospheric ones at the stress intensities below 18 MPa · m^1/2, and which is opposite to the generally known influence of the corrosive environments. As for the fractographic feature, an appearance of the intergranular facets was especially noticed in NaCI aqueous solution environment. The fraction of this intergranular cracking was obtained as a function of the stress intensity factor.     

Prediction of fatigue crack formation in 304 stainless steel

Strain-controlled fatigue tests have been conducted on center-holed 304 stainless steel specimens. The fraction of total fatigue life spent until formation of an “engineering” crack ranged from about 15 to 85 pct, indicating the potential importance of being able to predict the fatigue crack formation life. A “just formed engineering crack,” as defined here, is a through crack long in the thickness direction, which has just emerged from the center hole. An energy based parameter, ΔσrΔε,, has been shown to correlate with the appearance of fatigue cracks in the center-holed 304 stainless steel specimens. This parameter is suggested to be more useful in predicting fatigue crack formation life than Δσ or Δε, alone. A good correlation was found over the limited range of data for two types of 304 stainless steel, a powder metallurgy (PM) stainless steel with higher than normal strength prop-erties and an ingot metallurgy (IM) stainless steel with normal strength properties. A better correlation was found for strain-controlled fatigue tests which did not go into compressive strain than for com-pletely reversed fatigue. Formerly a graduate student with the Materials Science and Engineering Department, Northwestern University, is     

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.     

The effects of residual macrostresses and microstresses on fatigue crack propagation

The effects of residual microstresses and tensile residual macrostresses on fatigue crack propagation (FCP) are examined in a high-carbon steel. Phase-specific diffraction measurements show that uniaxial deformation and radial cold expansion produce predominantly microstress and tensile macrostress fields, respectively. Microstresses are found to have little effect on FCP rates, while tensile macrostresses increase crack growth rates in a manner that depends systematically on ΔK. The increases are partly attributed to crack closure, which was found to be appreciable near the surface of control samples but absent in the presence of tensile residual stresses. Both the ΔK dependence and absence of microstress effects were explored by X-ray microbeam measurements around propagating fatigue cracks and found to stem from fading and/or redistribution of residual macrostresses and microstresses during fatigue crack growth.     

Fatigue crack propagation in austenitic and ferritic stainless steel single crystals

The orientation-dependence of fatigue crack propagation rates in single crystals of austenitic and ferritic stainless steels has been investigated at low stress levels on standard fracture mechanics specimens. The stress intensity factor required for a crack growth rate of 10⁻⁶ mm/cycle may vary by up to 50% according to the crystal orientation. In general, relatively high crack growth rates are associated with macroscopically plane fracture surfaces for both the f.c.c. and b.c.c. crystal structures. The crystallographic conditions for the formation of macroscopically plane fracture surfaces at low stress levels do not always correspond with the predictions of the original alternating slip model. In particular, for the ferritic steel, only those orientations for which two 〈111〉 slip directions are normal to the crack front and symmetrically inclined to the crack plane allow the formation of macroscopically plane fracture surfaces. The macroscopic appearance of the austenite fracture surfaces depends upon the crystal orientation and the crack growth rate: at low stresses, plane fracture surfaces are developed for most of the orientations tested. A model of crack propagation by alternating shears, in which the shears are accommodated by multiple slip processes at the crack tip is proposed to explain these results.     

无镍高氮不锈钢弯曲疲劳裂纹萌生与扩展行为

 采用四点弯曲疲劳试验研究了不同应力水平下无镍高氮不锈钢的疲劳行为,并对材料疲劳裂纹的微观形貌、萌生位置及扩展路径进行了分析。结果表明,试验钢疲劳为多裂纹起裂,随着应力水平的升高,裂纹总长度逐渐增加,当应力水平接近材料屈服极限时,裂纹长度趋于稳定;裂纹大多数在滑移带处萌生,裂纹在扩展过程中产生了扭曲、偏移和分叉现象;裂纹在晶内主要沿单滑移带或多滑移带交替扩展,穿过晶界或孪晶界时大多发生了偏转。     

Existence of the coaxing effect and effects of small artificial holes on fatigue strength of an aluminum alloy and 70-30 brass

In order to study the existence of the coaxing effect and the effects of small defects on the fatigue strength of an aluminum alloy and 70-30 brass, plain specimens and special holed specimens containing one or more very small drilled holes with diameters of 40,50,100,200, and 500 ώm were prepared. Contrary to commonly accepted knowledge, the existence of a distinct coaxing effect was confirmed in the fatigue test on 2017-T4 aluminum alloy. However, the coaxing effect was not confirmed in the fatigue test on 70-30 brass, though specimens with small artificial holes could contain non-propagating cracks at the fatigue limit. It was found that the appearance of the coaxing effect depends on the endurance at a higher stress level of small cracks initiated at a lower stress level. The very small drilled holes with diameters of 40 and 50 ώm had no harmful effect on the fatigue strength of both the aluminum alloy and 70-30 brass; that is, the fatigue limits of specimens containing one or more drilled holes with diameters of 40 and 50 ώm were identical with those of the plain specimens.     

Internal hydrogen-induced subcritical crack growth in austenitic stainless steels

The effects of small amounts of dissolved hydrogen on crack propagation were determined for two austenitic stainless steel alloys, AISI 301 and 310S. In order to have a uniform distribution of hydrogen in the alloys, they were cathodically charged at high temperature in a molten salt electrolyte. Sustained load tests were performed on fatigue precracked specimens in air at 0 ‡C, 25 ‡C, and 50 ‡C with hydrogen contents up to 41 wt ppm. The electrical potential drop method with optical calibration was used to continuously monitor the crack position. Log crack velocityvs stress intensity curves had definite thresholds for subcritical crack growth (SCG), but stage II was not always clearly delineated. In the unstable austenitic steel, AISI 301, the threshold stress intensity decreased with increasing hydrogen content or increasing temperature, but beyond about 10 wt ppm, it became insensitive to hydrogen concentration. At higher concentrations, stage II became less distinct. In the stable stainless steel, subcritical crack growth was observed only for a specimen containing 41 wt ppm hydrogen. Fractographic features were correlated with stress intensity, hydrogen content, and temperature. The fracture mode changed with temperature and hydrogen content. For unstable austenitic steel, low temperature and high hydrogen content favored intergranular fracture while microvoid coalescence dominated at a low hydrogen content. The interpretation of these phenomena is based on the tendency for stress-induced phase transformation, the different hydrogen diffusivity and solubility in ferrite and austenite, and outgassing from the crack tip. After comparing the embrittlement due to internal hydrogen with that in external hydrogen, it is concluded that the critical hydrogen distribution for the onset of subcritical crack growth is reached at a location that is very near the crack tip. Formerly Research Assistant, Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign.     

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.     

The Effect of ITMT’s and P/M Processing on the Microstructure and Mechanical Properties of the X7091 Alloy

The influence of microstructure and texture on the monotonic and cyclic properties of X7091-T651 was investigated. The various structures were developed from conventional ingot metallurgy (I/M), powder metallurgy (P/M) and intermediate thermal mechanical treatments (ITMT). Powder metallurgy produced a finer grain structure and particle distribution than I/M. Intermediate thermomechanical treatment produced a recrystallized, coarse grain structure with a weak texture, compared to the unrecrystallized grain structure and sharp texture obtained with conventional processing (CP). All materials had comparable monotonic properties. The resistance to fatigue crack initiation (FCI) increased with both a reduction in grain size and a finer particle distribution. Smaller grain sizes and finer particle distributions reduced the degree of cyclic strain localization. The CP-P/M alloy had the poorest ductility and FCI resistance of all the materials, although the slip was fairly homogeneous. This may be due to the presence of oxides at the grain boundaries and a sharp texture. The threshold stress intensity, ΔK_th, and the fatigue crack growth rate (FCGR) roughly follow a grain size dependence with the resistance of fatigue crack propagation (FCP) increasing with increasing grain size. It appears that large grains allow more reversible slip and reduce the amount of accumulated plastic strain within the reverse plastic zone. It is also believed that a greater degree of fatigue crack closure, which may be associated with large grains and a rough FCP surface, results in a lower FCGR in the lowΔK region. The intermediate thermomechanical treatment of P/M X7091 produced the optimum microstructure giving the best combination of mechanical properties. The important features include a small recrystallized grain structure, a fine particle distribution, a weak texture, and a low concentration of oxides at grain boundaries. Formerly Director, Fracture and Fatigue Research Laboratory, Georgia Institute of Technology, Atlanta, GA.     

Hydrogen Induced Slow Crack Growth in Stable Austenitic Stainless Steels

The behavior of hydrogen induced slow crack growth in type 310 and type 16-20-10 stable austenitic stainless steels along with type 321 unstable austenitic stainless steel were investigated. It was found that slow crack growth could occur in all three types of stainless steels, and the threshold values wereK _H/K_c = 0.55, 0.7, and 0.78 for type 321, 310, and 16-20-10 stainless steel respectively, when charged under load. Slow crack growth could also occur if the precharged specimens were tested under constant load in air. No slow crack growth occurred in the precharged and then out-gassed specimens. This indicates that delayed cracking in stable austenitic stainless steels is induced by hydrogen. Since there is no hydrogen induced α’ martensite in type 310 and 16-20-10 stainless steel, the existence of a’ martensite is not necessary for the occurrence of slow crack growth in the austenitic stainless steels, although it can facilitate slow crack growth. The mode of hydrogen induced delayed fracture in either the stable or unstable austenitic stainless steel is correlated with theK, value; the fracture surface is changed from ductile to brittle asK ₁ is decreased.     

Near-threshold corrosion fatigue crack growth behavior of type 422 stainless steel at controlled maximum stress intensities

The K_max-controlled near-threshold fatigue crack growth behavior was investigated on 422 stainless steel in a boiling NaCl solution. During the test, there was a transition from corrosion fatigue to stress corrosion cracking. The transition occurred at very high load ratios (R=-0.91) and at very lowAK levels (≤2.1 MPa√m). The characteristics of stress corrosion cracking (SCC) were manifested by time-based crack growth rather than cycle-based crack growth, by crack extension under static loading, and by change in fracture mode. In corrosive environments, the small ripple loading imposed on structural materials should be recognized for engineering designs and failure analyses.     

Fracture mechanics approach to hydrogen-assisted microdamage in eutectoid steel

A fracture mechanics approach to hydrogen-assisted microdamage in eutectoid steel is presented. Fractographic analysis revealed micromechanical effects of hydrogen in the form of tearing topography surface (TTS). The progress of this microdamage is modeled as a macroscopic crack that extends the original fatigue precrack and involves linear elastic fracture mechanics principles. In this case, the change from hydrogen-assisted microdamage (TTS) to cleavagelike topography takes place when a critical stress intensity factor (K _H) is reached, and this value depends on the amount of hydrogen which penetrated the vicinity of the actual crack tip (the fatigue precrack plus the TTS area). It is shown that the value K _H depends on experimental variables—mainly on the fatigue precracking regime—and its value may be associated with a characteristic level of stress intensity factor in the crack growth kinetics curve.     

Investigating and Understanding the Cyclic Fatigue,Deformation, and Fracture Behavior of a Novel High Strength Alloy Steel: Influence of Orientation

In this paper, the results of a recent study aimed at understanding the influence of orientation on high cycle fatigue properties and final fracture behavior of alloy steel Pyrowear 53 is presented and discussed. This alloy steel has noticeably improved strength, ductility, and toughness properties compared to other competing high strength alloy steels having a near similar chemical composition and processing history. Test specimens of this alloy steel were precision machined and conformed to the specifications detailed in the ASTM standards for tension testing and stress‐controlled cyclic fatigue tests. Test specimens were prepared from both the longitudinal and transverse orientations of the as‐provided alloy steel bar stock. The machined test specimens were deformed in cyclic fatigue over a range of maximum stress and under conditions of fully reversed loading, i.e., at a load ratio of ?1, and the number of cycles‐to‐failure recorded. The specific influence of orientation on cyclic fatigue life of this alloy steel is presented. The fatigue fracture surfaces were examined in a scanning electron microscope to establish the macroscopic fracture mode and to characterize the intrinsic features on the fatigue fracture surfaces. The conjoint influence of microstructure, orientation, nature of loading, and maximum stress on cyclic fatigue life is discussed.     

Effect of Grain Size on Mechanical Properties of Nickel-Free High Nitrogen Austenitic Stainless Steel

The fine grained structures of nickel-free high nitrogen austenitic stainless steels had been obtained by means of cold rolling and subsequent annealing.The relationship between microstructure and mechanical properties and gain size of nickel-free high nitrogen austenitic stainless steels was examined.High strength and good ductility of the steel were found.In the grain size range,the Hall-Petch dependency for yield stress,tensile strength,and hardness was valid for grain size ranges for the nickel-free high nitrogen austenitic stainless steel.In the present study,the ductility of cold rolled nickel-free high nitrogen austenitic stainless steel decreased with annealing time when the grain size was refined.The fracture surfaces of the tensile specimens in the grain size range were covered with dimples as usually seen in a ductile fracture mode.     

氢蚀程度对钢疲劳断裂性能的影响

研究了国产钢经不同温度和时间氢暴露后的力学性能、疲劳性能和断裂韧性,用扫描电镜证实了氢蚀后断裂机制发生了变化.研究表明:随氢蚀程度增加,20G钢抗拉强度和塑性降低明显,CrMo钢抗拉强度略有降低,塑性变化不大.氢蚀使20G钢的门槛值有一个最小值,而断裂韧性随氢蚀程度升高而降低,在氢蚀程度较低时,断裂韧性下降程度大;在氢蚀程度较高时,断裂韧性下降程度变缓.碳钢的疲劳性能变化是由于材料损伤作用和氢蚀造成的裂纹表面引起的闭合效应增加二者共同作用的结果.