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.     

Inclusion size effect on the fatigue crack propagation mechanism and fracture mechanics of a superalloy

Low cycle fatigue life of nickel-base superalloys is enhanced as a consequence of inclusion reduction in the melt process; however, the functional dependencies between fatigue characteristics and inclusions have not been well investigated. In this study, the propagation mechanism of the fatigue crack initiated from inclusions is examined in fine-grained IN718, which is a representative turbine disc material for jet engines. There is a faceted-striated crack transition on the fracture surfaces. This faceted-striated transition also appears in theda/dN vs crack length curves. It is observed that the faceted crack propagation time can be more than 50 pct of total lifetime in the low cycle fatigue test. The significance of inclusion size effect is explained on the premise that the faceted fatigue crack propagation time scales with the inclusion size, which is taken as the initial crack length. A predictive protocol for determining inclusion size effect is given.     

Corrosion Fatigue Crack Initiation in a Mode II Notch Specimen

The stress intensityK _II of a Mode II specimen was calculated using a finite element methodvia theJ integral. The site, direction, and the threshold value for crack initiation from the notch under cyclic Mode II loading in air, in water, and under dynamic charging with hydrogen were investigated. The results showed that the Mode II fatigue crack in a high strength steel initiated at or close to the site of the maximum principal stress, rather than at the site of the maximum shear stress, and the subsequent crack growth was oriented approximately normal to the direction of the maximum principal stress. The site and direction of crack initiation in water and under dynamic charging with hydrogen were similar but different from that in air. The threshold values for crack initiation in air, in water, and under dynamic charging were 28.8, 12.3, and 10.2 MPa m^1/2, respectively. The fracture surface of a corrosion fatigue crack in water and under dynamic charging consisted of intergranular facets at low ΔK _II values but of quasi-cleavage at higher ΔK _II values and were different from those in air.     

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.     

Fatigue Fracture of Concrete Subjected to Biaxial Stresses in the Tensile C–T Region

In this paper cyclic quasi-static and constant amplitude fatigue responses of concrete subjected tensile compression–tension (C–T) biaxial stress are presented. In the tensile C–T region within the biaxial stress space, magnitude of the principal tensile stress is larger than or equal to that of the principal compressive stress. An experimental program consisted of subjecting hollow, cylindrical concrete specimens to torsional loading. Failure in both quasi-static and fatigue is due to crack propagation. It is shown that the crack propagation resulting from the biaxial loading can be predicted using Mode I fracture parameters. The fatigue crack growth is observed to be a two-phase process: an acceleration stage that follows a deceleration stage. The crack length where the rate of crack growth changes from deceleration to acceleration is shown to be equal to the crack length at the quasi-static peak load. Analytical expressions for crack growth in the deceleration and acceleration stages are developed in terms of the mechanisms that influence quasi-static crack growth. The model parameters obtained from uniaxial fatigue tests are shown to be sufficient for predicting the biaxial fatigue response. Finally, a fracture-based fatigue-failure criterion is proposed, wherein the fatigue failure can be predicted using the critical Mode I stress intensity factor.     

Modeling the texture dependence of environmentally assisted growth of long and short cracks

The deflection of long cracks subject to far field Mode I loading is modeled for simply kinked and zig-zag configurations representing local Mode I/Mode II loading and for simply twisted and doubly twisted configurations representing local Mode I/Mode III loading. Twisted crack geometries are considered in both plane stress and plane strain conditions. Crystallographic texture is incorporated explicitly into the model through the texture parameter,f, and model predictions express a normalized CTOD as a function off. Data for the propagation threshold in fracture mechanics specimens of ZIRCALOY* exposed to an iodine environment at 573 K agree well with model predictions. Discrepancies are explained by plastic processes complementing the transgranular cleavage. An analogous model predicts the threshold stress for propagation of short cracks as a function of texture using the normalized CTOD from the long crack data. Model predictions are correlated with threshold stress data for ZIRCALOY tubing exposed to an iodine environment at 593 K. Model assumptions are discussed relative to the applicability of LEFM to short cracks. The model predicts a low threshold stress in materials with a texture well oriented for transgranular cleavage.     

Fatigue crack propagation of titanium alloys under dwell-time conditions

The effect of dwell-time at peak load on the fatigue crack propagation of a near-α alloy (Ti-11) and on α + β alloy (Ti-6A1-4V) was investigated. Several composition, microstructure and texture conditions were studied under fatigue cycling with a 5 min dwelltime. Three different product forms of Ti-6A1-4V were used: cross-rolled plate, highly textured plate and highly textured low interstitial plate. No deleterious effect on fatigue crack growth rate was observed for the 5 min swell cycling when compared to a 6 Hz baseline data for any of the material variables studied. In fact, for some of the microstructures studied, the dwell cycling resulted in a significantly lower fatigue crack growth rate. This deceleration can be explained on the basis of the observed increase in crack path tortuosity associated with the dwell cycling. This increased tortuosity may be the result of a crack tip blunting process which occurs during the dwell period of the load cycle. Formerly with the Metals and Ceramics Division, Wright Patterson AFB     

Crack deflection during fatigue and monotonic fracture of a SiC whisker reinforced 2009 aluminium alloy

Fatigue crack growth under Mode I loading, and static fracture in both symmetrical and asymmetrical notched four point bend specimens, have been examined in SiC whisker reinforced 2009 aluminium alloy. In the fatigue tests a range of orientations of the starter notch, with respect to the extrusion direction, L, was examined. Slanted crack propagation was observed in all specimens except that in the T-L configuration. For the monotonic tests the specimen orientation (L-T) remained constant whilst the ratio of Mode I to Mode II loading was varied. Again crack deflection was observed in all cases apart from the L-T specimen under pure Mode I loading. Whisker debonding was found to be the dominant factor controlling crack deflection, independent of the mixity of the loading mode. Under mixed-mode static loading, the deflection angle was controlled by the average orientation of the whiskers subject to the asymmetrically distributed maximum principal stress. In fatigue loading, however, the crack tended to follow the most frequently observed whisker orientation. These contrasting observations are interpreted in terms of the different sampling volumes at the crack tip in monotonic fracture and in fatigue crack growth.     

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.     

Simulation of Microstructurally Controlled Short Crack Propagation

A model for the simulation of stage I fatigue crack propagation is presented. The model considers the barrier effect of grain boundaries and phase boundaries on short crack propagation rate and the crack path deflection due to the microstructure. The plastic zones in front of the crack tip are modelled as yield strips. The mechanical boundary value problem is solved numerically by a boundary element method. The method was applied based on a statistical model of duplex microstructures, generated by a special Voronoi‐algorithm, so that the influence of the distribution of phases on crack propagation can be analysed.     

Q345R疲劳裂纹扩展过程的声发射研究

为了建立疲劳过程与声发射参数之间的关系,对压力容器常用钢材的典型代表——Q345R的疲劳裂纹扩展过程的声发射信号进行详细研究.结果表明:Q345R疲劳裂纹扩展的声发射过程分为三个明显的阶段,累积计数值和累积能量值可以很好地表征整个疲劳裂纹扩展过程;声发射参数在第2阶段到第3阶段的转折点比线弹性断裂力学定义下的要提前,表明声发射技术对疲劳进入失稳扩展阶段更加敏感;建立了Q345R声发射计数率和能量率与疲劳裂纹扩展速率的关系,它可以为Q345R剩余寿命的预测提供依据.      

Effect of testing frequency on the corrosion fatigue of a squeeze-cast aluminum alloy

The corrosion fatigue crack propagation behavior of a squeeze-cast Al-Si-Mg-Cu aluminum alloy (AC8A-T6), which had been precracked in air, was investigated at testing frequencies of 0.1, 1, 5, and 10 Hz under a stress ratio (R) of 0.1. Compact-toughness specimens were precracked about 6 mm in air prior to the corrosion fatigue test in a 3 pct saline solution. At some near-threshold conditions, these cracks propagated faster than would be predicted by the mechanical driving force. This anomalous corrosion fatigue crack growth was affected by the initial stress-intensity-factor range (ΔK _i), the precracking conditions, and the testing frequency. The initial crack propagation rate was as much as one order of magnitude higher than the rate for the same conditions in air. This rapid rate was associated with preferential propagation along the interphase interface in the eutectic structure. It is believed that a chemical reaction at the crack tip and/or hydrogen-assisted cracking produced the phenomenon. Eventual retardation and complete arrest of crack growth after this initial rapid growth occurred within a short period at low ΔK values, when the testing frequency was low (0.1 and 1 Hz). This retardation was accompanied by corrosion product-induced crack closure and could be better explained by the contributory stress-intensity-factor range (ΔK _cont) than by the effective stress-intensity-factor range (ΔK _eff).     

Environmentally assisted fatigue crack propagation in steel

The influence of various gasenous environments on fatigue crack propagation has been determined for three quenched and tempered steels with yield strength levels of 800 to 1400 MN/m². The crack growth rate was increased by an order of magnitude in low pressure (13 KPa) hydrogen, and by a factor of two in most mildly aggressive environments relative to the growth rate in vacuum. The gases oxygen, acetylene, carbon monoxide, and nitrous oxide were dominant in a combined environment with hydrogen while methane and carbon dioxide had only a small effect on crack propagation when added to hydrogen. The crack propagation in acetylene was intermediate between that in hydrogen and the mildly aggressive environments. The increase in fatigue crack propagation rate in the hydrogen environment was dependent on the temperature and the cyclic stress intensity. The fracture mode was transgranular for all conditions except the hydrogen influenced HP-9-4-20 fractures. These results are discussed relative to various stages of the hydrogen embrittlement mechanisms. In pacticular, the results are discussed with respect to the adsorption-dissociation of the environment, transport of the gaseous specie within the plasticly deformed zone by mobile dislocations and interaction with segregated impurities within the metal. H. L. MARCUS, formerly with Science center, Rockwell International, Thousand Oaks, CA     

Some considerations on fatigue crack closure at near-threshold stress intensities due to fracture surface morphology

Summary It is noted that at near-threshold levels, in addition to the role of plasticity-and oxide-induced crackclosure, fracture surface roughness or morphology may promote significant closure effects in plane strain, as similarly noted by Minakawa and McEvily.This is considered to result from the fact that, where maximum plastic zones sizes are small compared to the grain size, fatigue crack growth proceeds by a single shear decohesion mechanism (Stage I) with associated Mode II+I displacements. The resulting serrated or faceted fracture surfaces (“microstructurally-sensitive growth”) coupled with Mode II crack tip displacements thus induce high closure loads (i.e., K _cl/K _max ~0.5) by wedging the crack open at discrete contact points. At higher growth rates where the plastic zone encompasses many grains, striation growthvia alternating or simultaneous shear mechanisms (Stage II) produces a more planar fracture surface, with pure Mode I displacements, and a corresponding reduction in closure loads. Such concepts of roughness-induced closure are shown to be consistent with observations of the role of coarse grain sizes in reducing near-threshold crack growth rates at low load ratios and of the absence of this effect at high load ratios. R. O. RITCHIE and S. SURESH, both formerly with Massachusetts Institute of Technology     

Crack size effects on the chemical driving force for aqueous corrosion fatigue

Small crack size accelerates corrosion fatigue propagation through high strength 4130 steel in aqueous 3 pct NaCl. The size effect is attributed to crack geometry dependent mass transport and electrochemical reaction processes which govern embrittlement. For vacuum or moist air, growth rates are defined by stress intensity range independent of crack size (0.1 to 40 mm) and applied maximum stress (0.10 to 0.95 Φ_ys). In contrast small (0.1 to 2 mm) surface elliptical and edge cracks in saltwater grow up to 500 times faster than long (15 to 40 mm) cracks at constant δK. Small cracks grow along prior austenite grain boundaries, while long cracks propagate by a brittle transgranular mode associated with tempered martensite. The small crack acceleration is maximum at low δK levels and decreases with increasing crack length at constant stress, or with increasing stress at constant small crack size. Reductions in corrosion fatigue growth rate correlate with increased brittle transgranular cracking. Crack mouth opening, proportional to the crack solution volume to surface area ratio, determines the environmental enhancement of growth rate and the proportions of inter- and transgranular cracking. Small cracks grow at rapid rates because of enhanced hydrogen production, traceable to increased hydrolytic acidification and reduced oxygen inhibition within the occluded cell.     

A critical evaluation of the stress-corrosion cracking mechanism in high-strength aluminum alloys

Attempts have been made to elucidate the mechanism of stress-corrosion cracking (SCC) in high-strength Al-Zn-Mg and Al-Li-Zr alloys exposed to aqueous environments by considering the temperature dependence of SCC susceptibility based upon the anodic dissolution and hydrogen embrittlement models. A quantitative correlation which involves the change of threshold stress intensity,K _ISCC, with temperature on the basis of anodic dissolution has been developed with the aid of linear elastic fracture mechanics. From the derived correlation, it is concluded that the threshold stress intensity decreases as the test temperature increases. This suggestion is inconsistent with that predicted on the basis of hydrogen embrittlement. It is experimentally observed from the Al-Zn-Mg and Al-Li-Zr alloys that the threshold stress intensity,K,_ISCC, decreases and the crack propagation rate,da/dt, over the stress intensity increases with increasing test temperature. From considering the change in SCC susceptibility with temperature, it is suggested that a gradual transition in the mechanism for the stress-corrosion crack propagation occurs from anodic dissolution in stage I, where the crack propagation rate increases sharply with stress intensity, to hydrogen embrittlement in stage II, where the crack propagation rate is independent of stress intensity.     

Influence of microstructure on fatigue behavior and surface fatigue crack growth of fully pearlitic steels

This work examined the influence of microstructure on the surface fatigue crack propagation behavior of pearlitic steels. In addition to endurance limit or S(stress amplitude)-N(life) tests, measurements of crack initiation and growth rates of surface cracks were conducted on hourglass specimens at 10 Hz and with aR ratio of 0.1. The microstructures of the two steels used in this work were characterized as to prior austenite grain size and pearlite spacing. The endurance tests showed that the fatigue strength was inversely proportional to yield strength. In crack growth, cracks favorably oriented to the load axis were nucleated (stage I) with a crack length of about one grain diameter. Those cracks grew at low ΔK values, with a relatively high propagation rate which decreased as the crack became longer. After passing a minimum, the crack growth rate increased again as cracks entered stage II. Many of the cracks stopped growing in the transition stage between stages I and II. Microstructure influenced crack propagation rate; the rate was faster for microstructures with coarse lamellar spacing than for microstructures with fine lamellar spacing, although changing the prior austenite grain size from 30 to 130 jμm had no significant influence on crack growth rate. The best combination of resistance to crack initiation and growth of short cracks was exhibited by microstructures with both a fine prior austenite grain size and a fine lamellar spacing. Formerly with Carnegie Mellon University     

The effect of hold time on the fatigue properties of a β-processed titanium alloy

Several recent papers have demonstrated that dwell periods at peak stress can significantly reduce the number of cycles to failure in LCF tests on titanium alloys and can cause enhanced growth rates in fatigue crack propagation tests. In all cases cleavage or quasi-cleavage facet formation has been intimately linked with the dwell sensitive fatigue response. The present paper demonstrates that facets can also form during creep deformation at ambient temperatures and it proposes that the LCF dwell effect and facet formation under cyclic conditions is dependent on time dependent plastic strain accumulation. If hydrogen contributes to the failure process it is suggested this must be through an interaction with dislocations. The significance of the model for dwell sensitive fatigue crack propagation is discussed.     

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.