Fatigue crack propagation behavior of a single crystalline superalloy

Crack propagation experiments were performed as a function of temperature on the single crystalline superalloy RENé N4. Crack growth rates at 21 °C, 704 °C, 927 °C, 1038 °C, and 1093 °C were measured on specimens with [001] and [110] directions parallel to the load axis and the machined notch, respectively. Fracture surfaces at each temperature were examined and discussed in terms of the crack tip stress field and the crystallographic orientation of the specimens.     

Fatigue crack propagation in copper and Cu−Al single crystals

The effect of crystallographic orientation, temperature, and stacking fault energy on the rate of fatigue crack propagation was studied in polycrystalline copper, copper single crystals, and Cu−Al single crystals at room temperature and at liquid nitrogen temperature. A stress intensity factor was used to normalize the crack propagation data. It was found that dislocation cross slip plays a critical role on the rate of fatigue crack propagation. Existing mathematical crack propagation formulae could not explain the data on single crystals. A new fatigue crack propagation model to explain the observed results is proposed. H. ISHII, formerly Research Assistant at Materials Science Department, Northwestern University, Evanston, Ill. This paper is based on a thesis submitted by H. ISHII in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Northwestern University.     

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.     

Fatigue crack growth in MAR-M200 single crystals

The effects of crystallographic orientation on the fatigue crack growth behavior of MAR-M200* single crystals were examined. Using compact-tension specimens tested at 20 Hz, fatigue crack growth rates were determined at ambient temperature at minimum stress to maximum stress ratios,R, of 0.1 and 0.5. In most cases, subcritical crack growth occurred either along a single {111} slip plane or a combination of {111} planes. The mode of cracking was generally mixed and contained mode I, II, and III components. Considerable crack deflection and branching were also observed. Some fracture surfaces were found to contain a significant amount of asperities and, in some specimens, black debris. Based on Auger spectroscopic analyses and the fracture surface appearance, it appears that the black debris represented oxides formed due to rubbing of the fracture surfaces. Using stress intensity solutions obtained based on the Boundary-Integral-Equation technique, an effective ΔK was successfully used for correlating the crack growth rate data. The results indicate that the effect of crystallographic orientation on crack growth rate can be explained on the basis of crack deflection, branching, and roughness-induced crack closure. Formerly with Southwest Research Institute     

Fatigue crack propagation under varied mean stress conditions

Measurements of fatigue crack growth rates in copper monocrystalline and polycrystalline sheet specimens have been made at 295 K and 77 K to determine mean stress effects on growth rates. When load conditions remained unchanged throughout the period of crack growth, the rate of fatigue crack growth is independent of the level of mean stress and depends only on the cyclic stress amplitude. When the mean stress is changed during the crack growth period, a reduction of mean stress under plane strain conditions causes complete cessation of growth. A similar effect was not observed in plane stress crack growth, presumably due to reduced elastic constraint in narrow specimens containing large cracks. No change in growth rates occurs if the mean load is increased. In the event of crack growth stoppage, either restoration of the full previous mean load or crack re-nucleation under continued cycling at the reduced load levels is sufficient to restore the prior growth rate. A simple model is adapted to explain these observations which emphasizes the interaction of the growth rate with compressive residual stresses generated at the tip of the propagating crack. R. A. Yeske, formerly Research Assistant at Materials Science Department, Northwestern University, Evanston, III. This paper is based on a portion of a thesis submitted by R. A. Yeske in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Northwestern University.     

Fatigue hardening of lif single crystals in push-pull cyclic deformation

The fatigue hardening response of LiF single crystals subjected to uniaxial cyclic deformation, at various strain amplitudes and strain-rates, has been related to the monotonic hardening rate. Cumulative strain in fatigue is obtained through Kocks' flow stress theory on a statistical distribution of obstacles. The difference between monotonie and cyclic deformation is in the relative rate of accumulation of short and long range internal stresses. In the case of LiF at room temperature, fatigue hardening is essentially due to the increase in short range stresses. LiF crystals which are susceptible to cleavage fracture, can withstand large tensile stresses in fatigue when compared to monotonie deformation. Activation volume analysis indicates that vacancy and jog concentrations determine the dislocation mobility and fatigue hardening of LiF single crystals. Microscopic examination reveals the existence of very fine slip bands. In the case of crystals subjected to large number of fatigue cycles, surface irregularities and dislocation banding is observed. Optical and electron microscopy suggests dynamic recovery in the crystal studied.     

Behavior of nickel-base superalloy single crystals under thermal-mechanical fatigue

The thermal-mechanical fatigue behavior of AM1 nickel-base superalloy single crystals is studied using a cycle from 600 °C to 1100 °C. It is found to be strongly dependent on crystallo-graphic orientation, which leads to different shapes of the stress-strain hysteresis loops. The cyclic stress-strain response is influenced by variation in Young’s modulus, flow stress, and cyclic hardening with temperature for every crystallographic orientation. The thermalmechanical fatigue life is mainly spent in crack growth. Two main crack-initiation mechanisms occur, depending on the mechanical strain range. Oxidation-induced cracking is the dominant damage mechanism in the lifetime of interest for turbine blades.     

Fatigue crack path prediction in UDIMET 720 nickel-based alloy single crystals

The effect of orientation, applied stress state, environment, and temperature on the crack growth and crack-path behavior of single-crystal specimens of UDIMET 720 has been examined. Stage I cracking has been promoted by mixed-mode loading, plane stress, and vacuum conditions; increasing the test temperature to 600 °C does not suppress stage I crack growth in vacuum. Consideration of the local resolved shear-stress intensity and local resolved normal-stress intensity for each slip system as it intersects the nominal crack-growth plane allows the prediction of stage I crack paths, clarifying the importance of secondary single-crystal testing orientation (i.e., nominal crack-growth direction as well as effective tensile axis). A combination of both opening and shearing are found to promote stage I crack growth, and boundary conditions have been established within which stage I cracking is promoted. Highly deflected stage I cracking gives rise to significant shielding effects, but under suitable mixed-mode loading, highly oriented, coplanar stage I crack growth can be produced. Intrinsic stage I cracking under mixed-mode loading appears to be greatly accelerated compared with mode I-dominated stage II crack growth for comparable stress-intensity levels.     

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     

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 crack propagation in 4140 steel

The fatigue crack propagation rates, da/dN, of 4140 steel were measured in dry argonvs tempering temperature. In specimens 3.2 mm thick at a given ΔK between 15 and 30 MN/ m^3/2, da/dN decreases with increasing tempering temperature, reaches a shallow minimum for tempering at 400°C. The rate for as-quenched specimens increases withR ratio; this is not the case for the 400, 550 and 650°C tempers. Reducing the specimen thickness to 1.3 mm has little effect on the 650°C temper but causes a large decrease in da/dN for the asquenched condition and 200°C temper. Edge notch specimens tempered at 550 and 650°C are subject to crack arrest from cycling prior to crack initiation. The results are discussed in terms of the metallurgical structures and various fatigue crack propagation equations which have been proposed. The results cannot be explained on the basis of da/dN being determined only by Young’s modulus andK _c.     

Fatigue crack propagation in aluminum-base copper alloys

Fatigue crack propagation ratesda/dN in binary Al alloys with 3.6 wt pct Cu and 6.3 wt pct Cu and commercial 2024 aged at 21°C were compared with 99.95⁺ wt pct aluminum. Omitting an anomalous region at lowΔK, the extrapolated rates for “pure” aluminum are more than 100 times greater than those in the three alloys at the same ΔK. The data for the alloys fit into a single scatter band of a factor of three. It was suggested thatda/dN varies inversely with the square of the strength of the alloy but that another parameter related to the fatigue crack propagation energy per unit area is also important. Theda/dN vs ΔK curves were determined for 3.6 wt pct Cu single crystals aged seven days at 21°C which containGP zones and two and seven days at 160°C which contain mixtures ofθ′ andθ′’. No systematic variation of (da/dN _Δ with crystallographic orientation was discerned, but the naturally aged specimen had a strong orientation dependence on crack initiation. At low ΔK 21°C aged specimens gave the lowestda/dN while at high ΔK the warm aged specimens gave the lower values ofda/dN. Measurement ofda/dN vs ΔK curves were conducted on specimens of 3.6 wt pct Cu with 1 mm equiaxed grains aged for various times at 130°C, 160°C, and 190°C. All warm aged specimens experienced brittle intergranular fracture at sufficiently high ΔK. The transition ΔK where intergranular fracture first appears is inversely proportional to the aging temperature. The change of fracture mode from intra to intergranular occurs gradually over a broad range of ΔK which shifts to lower ΔK with increase in aging temperature. This research was supportd by U.S. Air Force Office of Scientific Research, Office of Aerospace REsearch, Grant No. AF-AFOSR-73-2431.     

Fatigue crack propagation in some PM steels

Abstract

Mechanisms of fatigue crack growth have been studied for a range of PM steels at relative densities of 0·90 and 1·0, for which strength, fracture toughness, and microstructural information was also available. It is shown that the Paris exponents for steady state crack growth are between 8 and 18 when ρ_r is ～0·9 but when ρ_r is ～1·0 the exponents are between 2·6 and 4·0, i.e in the range typical of wrought steels (2–4). At both densities, threshold stress intensities are between 5·5 and 10·8 MPa m_1/2 when R = 0·1. Combinations of these thresholds and yield strengths are comparable with those for wrought steels. When R = 0·8, reductions in threshold to between 2·7 and 5 MPa m_1/2 are attributed to crack closure effects. At ρ_r = 0·90, Fe–0·5C fails by progressive rupture of sinter necks. Astaloy A, with 0·2%C and 0·6%C, and Distaloy AB–0·6C have smaller plastic zone sizes and the cracks follow more difficult paths through particles as well as necks. When ρ_r is ～1·0, fracture is partially by true fatigue modes and partly by cleavage, the bursts of cleavage being more noticeable when K_maxis high.     

Fatigue crack propagation in aluminum-base copper alloys

Fatigue crack propagation ratesda/dN in binary Al alloys with 3.6 wt pct Cu and 6.3 wt pct Cu and commercial 2024 aged at 21°C were compared with 99.95⁺ wt pct aluminum. Omitting an anomalous region at lowΔK, the extrapolated rates for “pure” aluminum are more than 100 times greater than those in the three alloys at the same ΔK. The data for the alloys fit into a single scatter band of a factor of three. It was suggested thatda/dN varies inversely with the square of the strength of the alloy but that another parameter related to the fatigue crack propagation energy per unit area is also important. Theda/dN vs ΔK curves were determined for 3.6 wt pct Cu single crystals aged seven days at 21°C which containGP zones and two and seven days at 160°C which contain mixtures ofθ′ andθ′’. No systematic variation of (da/dN _Δ with crystallographic orientation was discerned, but the naturally aged specimen had a strong orientation dependence on crack initiation. At low ΔK 21°C aged specimens gave the lowestda/dN while at high ΔK the warm aged specimens gave the lower values ofda/dN. Measurement ofda/dN vs ΔK curves were conducted on specimens of 3.6 wt pct Cu with 1 mm equiaxed grains aged for various times at 130°C, 160°C, and 190°C. All warm aged specimens experienced brittle intergranular fracture at sufficiently high ΔK. The transition ΔK where intergranular fracture first appears is inversely proportional to the aging temperature. The change of fracture mode from intra to intergranular occurs gradually over a broad range of ΔK which shifts to lower ΔK with increase in aging temperature.     

Hydride formation in a Ni-base superalloy

X-ray diffraction, optical- and scanning-electron metallographic and microhardness studies have been conducted in pure Ni and a Ni-base alloy (HASTELLOY Alloy C-276) in an effort to determine if hydrides form in the alloy during cathodic charging in the presence of a strong H-recombination poison. The studies indicate that hydrides do form and that the extent of surface damage caused by them is a function of the alloy thermomechanical state. In the annealed alloy, hydrides produce a network of crystallographic cracks. In the 50 pct cold-worked alloy, extensive intergranular blistering and lamellar peeling are observed in addition to the crack network. In the cold-worked and aged alloy, damage similar to, but less extensive than, that of the cold-worked alloy is found. The average depth of damage penetration may be accounted for by the bulk diffusion of H in the alloy. It is not clear whether the observed susceptibility of the cold-worked and aged alloy to H embrittlement can be attributed to hydride formation. Formerly a Senior Postdoctoral Fellow, Department of Metallurgical Engineering & Materials Science at Notre Dame.     

Fatigue crack propagation in martensitic and austenitic steels

Fatigue crack growth rates were measured in an annealed and in an aged maraging steel and in three different austenitic steels. Microhardness measurements were used to determine the plane strain plastic zone sizes as a function of ΔK and to evaluate the cyclic flow stress of the material near the crack tip. The presence of a reversed cyclic plastic zone within the monotonic plastic zone was confirmed. The two maraging steels work soften near the tip of the crack while the three austenitic steels work harden. The fatigue crack growth rates of the maraging steels are independent of the monotonic yield stress and are typical of the growth rates of steels with a bcc crystal structure. The crack growth rates in the stainless steels are an order of magnitude lower than for maraging steels for ΔK< 30 ksi √in. The excellent fatigue crack growth resistance of austenitic stainless steels is related to the de-formation induced phase transformations taking place in the plastic zone and to the low stacking fault energy of the alloys.     

Fatigue crack propagation in a directionally-solidified nickel-base alloy

High cycle fatigue crack growth rates have been measured in the cast nickel-base alloy IN 738 LC in directionally-solidified form, at room and high temperature and for crack propagation both parallel and perpendicular to the solidification direction. The resistance of this material to crack propagation has been compared with conventionally-cast material of the same composition. The considerable differences in observed growth rates may be understood in terms of the effects of chemical segregation, crack branching and crystallographic fracture. In particular, the high-temperature cyclic fracture toughness for crack growth perpendicular to the solidification direction is higher than in conventionally-cast material as cracks tend to deviate along the segregated interdendritic regions. However the room temperature threshold stress intensity amplitude is low because fatigue crack growth occurs on definite crystallographic planes.     

Fatigue crack propagation in a directionally-solidified nickel-base alloy

High cycle fatigue crack growth rates have been measured in the cast nickel-base alloy IN 738 LC in directionally-solidified form, at room and high temperature and for crack propagation both parallel and perpendicular to the solidification direction. The resistance of this material to crack propagation has been compared with conventionally-cast material of the same composition. The considerable differences in observed growth rates may be understood in terms of the effects of chemical segregation, crack branching and crystallographic fracture. In particular, the high-temperature cyclic fracture toughness for crack growth perpendicular to the solidification direction is higher than in conventionally-cast material as cracks tend to deviate along the segregated interdendritic regions. However the room temperature threshold stress intensity amplitude is low because fatigue crack growth occurs on definite crystallographic planes.