Dislocation emission and fatigue crack growth threshold

The stress intensity range ΔK below which no cyclic plastic deformation at the crack tip, and hence, no fatigue crack propagation occurs is investigated. The emission of dislocations from the crack tip is assumed as mechanism for the dislocation generation. For a mode III crack, a computer simulation is carried out to study the influence of the number of dislocations, the friction stress and the critical stress intensity, k_e, to emit a dislocation. If during loading only one dislocation is emitted, the return of this dislocation to the crack tip and the emission of a dislocation with an opposite sign and the recombination with the first dislocation are possible during unloading. The ΔK necessary for both mechanisms is about 2k_e. If during loading more than one dislocation is emitted, during unloading at first a certain number of disclocations return to the crack tip before a dislocation of opposite sign is emitted. The necessary ΔK to move one dislocation back to the crack tip during unloading decreases with increasing number of dislocations and reaches a constant values of about 1.1k_e. This value of ΔK then is roughly independent of the friction stress and k_e.     

Near-threshold fatigue crack growth behavior in copper

Near-threshold fatigue crack growth rate data were developed in annealed, quarter-hard, and full-hard copper at various load ratios, (R = σ_min/σ_max). Increasing theR value decreases the resistance to threshold crack growth. At a fixed value ofR, annealed copper has the slowest near-threshold crack propagation rate while full-hard copper has the fastest crack growth rate. Waveform (sine and triangle) and specimen geometry (WOL, CT, and CCT) do not appear to affect the rates of near-threshold crack propagation. The influences of load ratio and material strength on threshold crack growth behavior can be rationalized by crack closure.     

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.     

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     

Influence of environment on fatigue crack growth in the threshold region

Low fatigue crack growth rates (down to 4 × 10⁻¹⁴m/cycle) were produced using a high frequency 20 kHz ultrasonic fatigue testing machine. The influence of non corrosive (silicone oil) and corrosive (3.5% sodium chloride solution) environments was compared. Down to crack propagation rates of some 10⁻¹⁰ m/cycle which corresponds to a crack growth rate of one lattice space per cycle no difference of crack growth rates was found. However, below this rate there seems to exist for non corrosive environments a threshold cyclic stress intensity, below which crack growth becomes diminishingly small, whereas no threshold was found for the corrosive environment. In the first case crack propagation is controlled by plastic deformation processes, in the second case these processes are markedly restricted. For this region, a transition in fracture mode from ductile transcrystalline to intergranular cracking was found.     

An investigation of environmental effects on fatigue crack growth in Q1N (HY80) steel

Fatigue threshold tests have been conducted on through-thickness and semielliptic cracks in laboratory air, vacuum, and salt water at stress ratios(R = K_min/K_max @#@) of 0.2 and 0.7. The effects of stress ratio are rationalized by crack closure concepts. Environmental effects are explained by considerations of the irreversibility of slip at the crack tip and the role of debris on the fracture surfaces. Differences in the fatigue crack growth rates in the three environments are attributed largely to the extent of the irreversibility of slip due to the chemisorption of water/ water vapor at the crack tip. Debris in saltwater solutions is also shown to significantly affect the near-threshold growth through its influence on crack closure and the transportation of environment to the crack tip. formerly Graduate Student, Department of Materials Science and Technology, Cambridge University formerly with the Department of Materials Science and Metallurgy, Cambridge University     

Microstructural effects on fatigue crack growth in a low carbon steel

A study of the influence of microstructure on fatigue crack growth in an AISI 1018 steel has been carried out. Two distinctly different duplex microstructures were investigated. In one microstructure ferrite encapsulated islands of martensite; in the other martensite encapsulated islands of ferrite. The latter structure resulted in a significant increase in threshold level (18 MPa√mvs 8 MPa√m) together with an increase in yield strength. Fractographic analysis was used to investigate the influence of microstructure on the mode of fatigue crack growth. Formerly at the University of Connecticut     

Kinetics of environmental fatigue crack growth in nickel-copper alloy: Part I. In vacuum and oxygen

Fatigue crack growth rates have been determined in a nickel-copper alloy in the mild test environments of vacuum and oxygen. It is found that the fatigue crack growth mode in both vacuum and oxygen is ductile and the growth rate is sensibly independent of the maximum stress intensity levels and stress intensity ratio. The growth rate is found to be lower in vacuum than in oxygen, and the growth rate dependency on the cyclic stress intensity range is more pronounced in vacuum than in oxygen. These differences in behavior may be consistent with differences in crack closure behavior for the two environments. Formerly Senior Staff Associate with the Science Center, Rockwell International Formerly a Graduate Student at Columbia University     

Kinetics of environmental fatigue crack growth in a nickel-copper alloy: Part II. In hydrogen

A systematic matrix of fatigue crack growth rate data in a hydrogen environment has been generated in a nickel-copper alloy and compared with the base line data in the milder oxygen and vacuum environments described in the preceding paper. It is found that crack growth in the hydrogen environment is characterized by high values of the Paris equation exponent and faster crack propagation rates as compared to those in the milder environments. Fractographic examination shows that brittle inter granular separation occurs superimposed on an otherwise ductile crack mode in the hydrogen tests. Quantitative fractographic analysis of the hydrogen affected fracture surfaces indicates that the percentage of intergranular failure (area fraction) is a uniquely related function of the mean stress intensity rather than the maximum stress intensity level of fatigue loading. This dependence is discussed in terms of dislocation sweep-in transport of hydrogen deep into the plastic zone during fatigue cycling. Formerly a Graduate Student at Columbia University Formerly Senior Staff Associate, Science Center, Rockwell International     

Fatigue crack propagation near fatigue threshold in various structural steels

The investigations have been conducted by measuring fatigue crack propagation near fatigue threshold in various structural steels differing in chemical composition and strength level. The fatigue crack propagation measurements were carried out using the constant-load-amplitude test in Paris-region, R-constant and K_max-constant method in near fatigue threshold region. Scanning electron microscopy at fatigue crack front on fracture surface was applied to interpret the influence of crack closure effects on the measured fatigue threshold. Marked fretting oxide deposits distributed on the fracture surface at threshold level were observed in a low load ratio resulting from the combined action of plasticity- and oxide-induced crack closure under laboratory atmosphere. Fatigue threshold dependent on the load ratio appeared to be related to the extent of the crack closure effect. By considering the relationship of reversed plastic zone size and grain size the fatigue threshold in region of crack closure was calculated theoretically. The result has shown a good agreement with the experimentally measured values.     

Effect of strontium modification on near- threshold fatigue crack growth in an Al-Si-Cu die cast alloy

The effects of strontium modification on microstructure and fatigue properties in a die cast com-mercial aluminum-silicon alloy are demonstrated. Strontium additions of 0.010 and 0.018 wt pct drastically change the morphology of the eutectic silicon. The influence of these microstructural changes on fatigue properties is evaluated through fatigue crack growth testing. Examination of the fracture surfaces and the crack path establish distinct fatigue fracture modes for the modified and unmodified eutectic structures. Changes in fracture mode and crack path are correlated to the mi-crostructure changes. A higher energy fracture mode and increased crack path tortuosity explain the observed improvement in fatigue properties for the modified alloys. Strontium modified alloys exhibit a 10 to 20 pct higher fatigue crack growth threshold compared to an unmodified alloy for testing at a load ratio of 0.5. No difference was observed for testing at a load ratio of 0.1. Formerly Research Project Engineer, Briggs & Stratton Corporation, Milwaukee, WI 53222     

Mixed-Mode I and II fatigue threshold and crack closure in dual-phase steels

Fatigue threshold under mixed-mode I and II loading and elastic plane-strain conditions has been studied in dual-phase steels (DPS) of two types of volume fraction of martensite (V_m) in laboratory air at room temperature. Near-threshold mixed-mode (I and II) fatigue crack growth occurs mainly by two mechanisms: shear mode, and tensile mode. Particular emphasis was placed on the influence of the mode II component. The mixed-mode threshold is controlled not only by mode I displacement but also by the mode II component. Apparent- and effective-bound curves (corrected closure) are obtained for the threshold condition and discussed in terms of the shape and size of the plastic region of crack tip; crack surface rubbing; and especially, roughnessinduced closure and shear resistance of crack surface that resulted in an extremely high extrinsictoughening contribution to the mixed-mode fatigue threshold values. The ratio of the threshold value of pure mode II to that of pure mode I (ΔK _thII/ΔK_ththI) attained highly to 1.9 times; the maximum hoop direction stress-intensity factor range of pure mode II branch crack tip is 2.2 times that of pure mode I. Obviously, the resistance of pure mode II crack growth here is far larger than that of pure mode I; the former is just to introduce the shear resistance of crack surface, the latter, to reduce the driving force of crack tip for crack closure. It is proposed that the apparent- and effective-bound curves are nonconservative risky and too conservative for design purposes, respectively. So, the threshold data should be obtained under the specific conditions found by concrete mechanical, microstructural, and environmental factors. Y.S. ZHENG, Formerly Ph.D. Student, State Key Laboratory for Fatigue and Fracture of Materials, Institute of Metal Research, Academia Sinica, Shenyang, 110015, People’s Republic of China     

Jumplike fatigue crack growth in compressor blades

It is shown that power relations between the two main fractographic characteristics of fracture surfaces forming during jumplike fatigue crack growth, namely, the crack depth and the corresponding crack front length, can be used to estimate the fracture stress during vibration tests of the compressor blades of an aviation gas turbine engine, which are made of VT3-1 titanium alloy.