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.     

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     

Initiation and growth of small fatigue cracks in a Ni-base superalloy

This article reports research on the initiation and growth of small fatigue cracks in a nickel-base superalloy (produced commercially by INCO as INCOLOY* 908) at 298 and 77 K. The experimental samples were square-bar specimens with polished surfaces, loaded in fourpoint bending. The crack initiation sites, crack growth rates, and microstructural crack paths were determined, as was the large-crack growth behavior, both at constant load ratio (R) and at constant maximum stress intensity (K _max). Small surface cracks initiated predominantly at (Nb,Ti)_xC_y, inclusion particles, and, less frequently, at grain boundaries. Small cracks grew predominantly along {111} planes in individual grains and were perturbed or arrested at grain boundaries. For values of ΔK above the large-crack threshold, ΔK _th, the average rate of smallcrack growth was reasonably close to that of large cracks tested under closure-free conditions. However, short-crack growth rates varied widely, reflecting the local heterogeneity of the microstructure. The threshold cyclic stress (Δσ_th) and the threshold cyclic stress intensity (ΔKσ_th) for small surface cracks were measured as functions of the crack size, 2c. The results suggest that a combination of the fatigue endurance limit and the threshold stress intensity for closure-free growth of large cracks can be used to define a fatigue-safe load regime. formerly with Lawrence Berkeley Laboratory     

The effect of microstructure and environment on fatigue crack closure of 7475 aluminum alloy

The effects of slip character and grain size on the intrinsic material and extrinsic closure contributions to fatigue crack growth resistance have been studied for a 7475 aluminum alloy. The alloy was tested in the underaged and overaged conditions with grain sizes of 18 μm and 80 μm. The fracture surface exhibited increased irregularity and planar facet formation with increased grain size, underaging, and tests in vacuum. These changes were accompanied by an increased resistance to fatigue crack growth. In air the 18 μm grain size overaged material exhibited relatively poor resistance to fatigue crack growth compared with other microstructural variants, and this was associated with a lower stress intensity for closure. All materials exhibited a marked improvement in fatigue crack growth resistance when tested in vacuum, with the most significant difference being ˜1000× at a ΔK of 10 MPa m^1/2 for the 80 μm grain size underaged alloy. This improvement could not be accounted for by either an increase in closure or increased crack deflection and is most likely due to increased slip reversibility in the vacuum environment. The intrinsic resistance of the alloy to fatigue crack growth was microstructurally dependent in vacuum, with large grains and planar slip providing the better fatigue performance.     

Microcrack closure phenomena for AI 2219-T851

The effect of maximum cyclic stress, σ_max, on the closure behavior of surface microcracks is studied for Al 2219-T851 subjected to fatigue for fully reversed cyclic loading. The relationship between σ_max and the crack closure stress σ_cc, is found to change as the noncrystallographic transgranular microcracks grow from intermetallic particles at the alloy surface (where they initiate) to the grain boundaries. After crack initiation, with crack lengths just slightly greater than the width of the intermetallics from which they have propagated, σ_cc is found to be independent of σ_max· For longer microcracks with lengths of approximately the grain size, σ_cc∞ σ_max. Crack closure mechanisms which can explain this behavior are discussed.     

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.     

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.     

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     

Growth behavior of microstructurally short cracks in the 6061 aluminum alloy with and without 22 Vol Pct SiC whiskers

Short crack growth behavior of the 6061 Al alloy with and without SiC whiskers was investigated. Fluctuations in the growth rate of short cracks converge with growth of the cracks and become substantially constant between 25 and 40 μm in the metal matrix composite (MMC) and 110 and 183 μm in the unreinforced alloy. This is attributed to the release of the short cracks from the microstructural effects,i.e., the interaction with reinforcement structure in the MMC and grain boundaries in the unreinforced alloy. Furthermore, there exists slowing down of short crack advance in the MMC, and this was explained from rapid development of crack closure obtained in this study.     

The influence of crack tip plasticity in the growth of small fatigue cracks

In order to rationalize observed differences in the growth behavior of large and small cracks, local crack tip opening micromechanics have been characterized for both crack size regimes in a high strength aluminum alloy. It is found that crack tip opening displacement, crack tip opening load, and crack opening mode all differ widely for large and small cracks at equivalent cyclic stress intensities (ΔK). High crack tip opening displacements and relatively low, approximately constant, crack opening loads for microcracks account both for their rapid rate of growth relative to large cracks and the absence of a microcrack threshold stress intensity. Crack tip plastic zone sizes also were measured, and it was found that the ratio of plastic zone size to crack length for small cracks is ∼ 1.0, while for large cracks the same ratio is ≪ 1. Simple empirical corrections to ΔK are found inadequate to correlate the growth of large and small cracks. It is concluded that for small cracks, linear elastic fracture mechanics similitude does not apply, and that an alternative crack driving force must be formulated.     

Fatigue crack initiation and early propagation in Al 2219-T851

Fatigue crack initiation in Al 2219-T851 for fully reversed loading(R = σ/σ_max =−1) parallel to the material rolling direction is found to occur at intermetallic inclusions at the specimen surface. The inclusions are not involved in crack initiation for fatigue perpendicular to the rolling direction, and for this orientation crack initiation is at grain boundaries and specimens have an increased fatigue life. Except for fatigue at low peak stress, multiple numbers of microcracks are formed and for selected failed specimens the number of cracks has been determined as a function of crack length. Such crack length distribution measurements show that there is significant retardation of microcracks by interaction with grain boundaries. Furthermore it is found that the coalescence of microcracks provides a mechanism for cracking to “jump“ grain boundaries and reduce fatigue lifetime. The effect of relative humidity on this process is to increase the observed mean crack length, and decrease the number of crack initiations apparently due to weakening of the matrix-intermetallic interface at potential initiation sites. The overall result is that no significant dependence of fatigue life on relative humidity is found. Formerly with the Science Center, Rock-well International     

Effect of cyclic stress wave form on corrosion fatigue crack propagation in al-zn-mg alloys

Fatigue crack growth rates of a 7075 type aluminum alloy were measured as a function of environment, frequency, stress wave form, alloy chemistry, and thermomechanical treatment. At low ΔK values (belowK _ISCC ), the crack growth rates in a 3.5 pct sodium chloride solution were ten times greater than those in a reference argon environment. Comparison of the effects of a square wave, a negative-sawtooth wave, and a positivesawtooth wave at different frequencies indicates that the synergistic interaction with the environment occurs during the loading part of each cycle. Overaging the alloy and limiting the alloy impurity content results in a reduced corrosion fatigue crack growth rate, but a thermomechanical treatment leading to a grain size refinement increases it.     

Effects of crack aspect ratio on the behavior of small surface cracks in fatigue: Part I. Simulation

A simple simulation of alternate growth of a small surface crack in the surface and depth directions was performed to illustrate the changes in crack aspect ratio, induced by grain boundaries, as a function of crack size. It is shown that at small crack sizes, large variations in aspect ratio, a/c (a is the crack depth and c is the half-surface length), occur, due to local crack front perturbations induced by grains that are oriented for crack growth. At these crack sizes, the assumption of a semicircular crack shape (a/c=1.0) was found to cause errors in stress intensity range (ΔK) calculations. This, in turn, led to significant scatter or “anomaly” in small crack growth rates relative to large cracks. At large crack sizes, the effects of local crack front perturbations on crack aspect ratio and ΔK were found to be insignificant. As a result, the scatter in crack growth data was found to decrease to a negligible level at large crack sizes. It is suggested that the limiting crack size above which the small crack behaves as a large crack, l ₂=10d (d = grain size), proposed by Taylor and Knott, is related to the crack size above which the effects due to aspect ratio variations are small.     

Indentation loading studies of acoustic emission from temper and hydrogen embrittled A533B steel

Isothermal tempering at 500 °C (within the region rendering low alloy steels susceptible to reversible temper embrittlement) induced acoustic emission activity in A533B steel during indentation loading. Samples, when sectioned, were found to contain small (∼10 μm long) MnS inclusions, some of which had debonded from the matrix material when they were near the indentations. Hydrogen charging prior to testing greatly enhanced the acoustic emission activity. It also resulted in the formation of small (∼20 to 200 μm) microcracks in samples tempered at 500 °C. These microcracks, when examined by optical metallography, appear to have propagated along prior austenite grain boundaries, consistent with fractographic observations of temper embrittlement in other low alloy steels. Many were nucleated by MnS inclusion debonding and all were confined to within a few hundred micrometers of the sample surface and within two or three indenter diameters from the indent. It is proposed that trace impurities (P, As, Sb, Sn) diffuse during the 500 °C temper to both the MnS inclusion interfaces and the prior austenite grain boundaries, reducing local cohesive strength. The tensile field created by the indenter debonds inclusions to form crack nuclei. Moderate acoustic emission results. In the absence of hydrogen these void nuclei may grow but do not coalesce to form observable cracks. The prior austenite grain boundaries, which in contrast to the dispersed inclusions can provide continuous crack paths, are not sufficiently temper embrittled to fracture without the assistance of hydrogen at these stresses. Hydrogen charging induces a high hydrogen concentration in a surface layer of the sample. This reduces further the grain boundary cohesion, and cracks initiated at inclusions are able to propagate along continuous grain boundary paths, generating additional energetic acoustic emission signals. This process can continue after unloading the indenter due to hydrogen diffusion to the residual stress field.     

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     

Critical assessment of the local cleavage stress σ ƒ * in notch specimens of C-Mn steel

An elastic-plastic finite element method (FEM) was used to calculate the stress and strain distributions ahead of notches with various root radii in a bending specimen of C-Mn steel with grain sizes of 10 and 30 μm. By accurately measuring the distance of the cleavage initiation site from the root of the notch, the local cleavage stress σ _ƒ ^* was measured. When the notch radius increased from 0.25 to 1.0 mm, the distribution of high stress had a definite variation but the σ _ƒ ^* remained relatively constant. In notch specimens with different root radii, the critical fracture event is identical,i.e., propagation of a ferrite grain-sized crack into the neighboring matrix. Therefore, the σ _ƒ ^* is mainly determined by the length of the critical microcrack, here, the size of ferrite grain instead of the high stress volume for finding an eligible brittle particle. The critical strain for initiating a crack was about 1 pct. The cleavage site ahead of a notch was related to the relative distributions of stress and strain and the random distribution of the weakest grains. The higher fracture load of the fine-grain material can be attributed to its higher value of σ _ƒ ^* /σ_o as compared with the coarse-grain. The σ _ƒ ^* /σ_o is a potential engineering parameter for toughness assessment in notch specimens.     

Microstructural effects on high-cycle fatigue-crack initiation in A356.2 casting alloy

The effects of various microconstituents on crack initiation and propagation in high-cycle fatigue (HCF) were investigated in an aluminum casting alloy (A356.2). Fatigue cracking was induced in both axial and bending loading conditions at strain/stress ratios of −1, 0.1, and 0.2. The secondary dendrite arm spacing (SDAS) and porosity (maximum size and density distribution) were quantified in the directionally solidified casting alloy. Using scanning electron microscopy, we observed that cracks initiate at near-surface porosity, at oxides, and within the eutectic microconstituents, depending on the SDAS. When the SDAS is greater than ∼ 25 to 28 μm, the fatigue cracks initiate from surface and subsurface porosity. When the SDAS is less than ∼ 25 to 28 μm, the fatigue cracks initiate from the interdendritic eutectic constituents, where the silicon particles are segregated. Fatigue cracks initiated at oxide inclusions whenever they were near the surface, regardless of the SDAS. The fatigue life of a specimen whose crack initiated at a large eutectic constituent was about equal to that when the crack initiated at a pore or oxide of comparable size.     

Growth of small fatigue cracks in PH 13-8 Mo stainless steel

The growth of small fatigue cracks in PH 13-8 Mo (H1050) stainless steel under constant amplitude loading at different mean stresses (R=0.1 and −1) under generally high cycle fatigue conditions was investigated. Small cracks were allowed to initiate naturally at the root of a single edge notch specimen and were monitored using a surface replicating technique. It was found that the initiation and growth of surface cracks up to 100 μm encompassed 70 to 90 pct of the total fatigue life at stress amplitudes just above the fatigue limit. Cracks of length less than 100 μm were subject to strong influences of the microstructure and exhibited stage I (shear-dominated) growth, which was manifested in oscillatory crack growth rates. The oscillations diminished as the crack transitioned to stage II growth. The higher stress ratio (R=0.1) resulted in a more rapid transition from stage I to stage II growth in comparison to R=−1. After transitioning to stage II, the crack growth could be well characterized by conventional long crack tools even when the crack was still physically small. The small crack growth behavior is shown to be similar to that of a quenched and tempered AISI 4340 steel having a comparable strength.     

Short crack growth behavior in a particulate-reinforced aluminum alloy composite

Short and long crack propagation behaviors in a coarse A1₂O₃ particulate-reinforced 6061 aluminum alloy composite (Al₂O₃/6061 Al) are investigated and compared under different ranges of tensile-compressive cyclic stress. It is found that short cracks up to 400 μm in length propagate in a shear-dominant mode at maximum cyclic stress level below the fatigue limit until they are permanently trapped by the surrounding particles. The microstructure sensitivity of short crack growth in the composite decreases as the short crack length and/or applied stress range increase. The characteristics of short cracks and the mechanisms of short crack trapping by particles in the material are discussed. leave from Taiynan University of Technology This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee.     

The Small Fatigue Crack Growth Behavior of an AM60 Magnesium Alloy

The effects of thermomechanical processing and subsequent heat treatment on the small fatigue crack growth (FCG) behavior of an AM60 (Mg-6.29Al-0.28Mn wt pct) alloy were evaluated. The effects of mechanical loading parameters, such as maximum stress and load-ratio, on the small FCG behavior were also determined. Maximum stress did not appear to affect the crack propagation rate of small cracks in the stress and crack size ranges considered. Materials with different microstructures and yield stresses, introduced by different processing conditions, showed similar crack growth rates at equivalent stress intensity factor ranges. The effect of load ratio on small crack growth rates was recorded. Fracture surface characterization suggested that the fatigue crack propagation mechanism was a mixture of transgranular and intergranular cracking. Porosity and other material defects played respective important roles in determining the fatigue crack initiation and propagation behavior.