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 of metastable beta titanium-vanadium alloys

The fatigue crack propagation behavior of three titanium-vanadium alloys (24, 28, and 32 wt pct V) which have tensile deformation modes ranging from coarse twinning to wavy and planar slip has been measured in laboratory air and correlated with their low cycle fatigue properties and microstructure. The fatigue crack growth rate of alloys with similar microstructures but different deformation modes, and of alloys with similar deformation modes but different microstructures have been compared. Increasing the deformation barrier mean free path and improving low cycle fatigue properties has been observed to reduce the fatigue crack growth rate at low and inter mediate ΔK levels. The fatigue crack growth data have been compared with that calculated from equations which use microstructure and low cycle fatigue parameters. The predictive capability of these equations which contain only measurable parameters has been found to be quite adequate.     

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 two classes of directionally solidified eutectic alloys

Fatigue crack propagation rates were measured in two classes of directionally solidified eutectic alloys under isothermal, stress-controlled cycling at temperatures of 298 to 1311 K. Alloy 73C, a cobalt-base material reinforced by fibers of Cr₇C₃, and γ/γ′ + δ, a nickel-base alloy reinforced by lamellae (platelets) of Ni₃Cb, were grown at solidification rates of 1 and 25 cm/h to achieve significant differences in interfiber and interlamellar spacing (λ). No influence of the spacing of the reinforcing phase on crack growth rates were found for either alloy. In addition, chromium level and perfection of the microstructure had a minimal effect on propagation rates for γ/γ′ + δ. The independence of the fatigue crack growth rates on λ may be associated with the ratio of the cyclic plastic zone diameter at the crack tip to λ. In all instances, this ratio was estimated to be greater than one for the test conditions employed. At the lower temperatures, crack propagation rates in γ/γ′ + δ were up to two orders of magnitude lower than those in Alloy 73C due to crack deflection at interlamellar interfaces and grain boundaries which lowered the effective stress intensity range for opening mode cracking.     

Fatigue crack propagation in two classes of directionally solidified eutectic alloys

Fatigue crack propagation rates were measured in two classes of directionally solidified eutectic alloys under isothermal, stress-controlled cycling at temperatures of 298 to 1311 K. Alloy 73C, a cobalt-base material reinforced by fibers of Cr₇C₃, and γ/γ′ + δ, a nickel-base alloy reinforced by lamellae (platelets) of Ni₃Cb, were grown at solidification rates of 1 and 25 cm/h to achieve significant differences in interfiber and interlamellar spacing (λ). No influence of the spacing of the reinforcing phase on crack growth rates were found for either alloy. In addition, chromium level and perfection of the microstructure had a minimal effect on propagation rates for γ/γ′ + δ. The independence of the fatigue crack growth rates on λ may be associated with the ratio of the cyclic plastic zone diameter at the crack tip to λ. In all instances, this ratio was estimated to be greater than one for the test conditions employed. At the lower temperatures, crack propagation rates in γ/γ′ + δ were up to two orders of magnitude lower than those in Alloy 73C due to crack deflection at interlamellar interfaces and grain boundaries which lowered the effective stress intensity range for opening mode cracking. Formerly of Pratt & Whitney Aircraft     

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 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 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.     

Discontinuous precipitation in aluminum-base zinc alloys

The kinetics of discontinuous precipitation in Al-21.6 at. pct. Zn and Al-22 at. pct. Zn-0.01 at. pct Sn alloys was determined by quantitative metallographic measurements on solution-treated and aged specimens. The growth rate of the discontinuous precipitate appears to be controlled by interfacial diffusion. It was found that the addition of a small amount of tin reduced the overall reaction rate. Nucleation of the discontinuous precipitate was confined to grain edges or boundaries, filling the available sites (site saturation) at a very early stage in the reaction. It was observed that a variation in quench rate markedly affected the growth rate of the precipitate and the site-saturating-dimensionality during a room-temperature aging treatment.     

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 growth behavior in niobium-hydrogen alloys

Near-threshold fatigue crack growth behavior has been investigated in niobium-hydrogen alloys. Compact tension specimens (CTS) with three hydrogen conditions are used: hydrogen-free, hydrogen in solid solution, and hydride alloy. The specimens are fatigued at a temperature of 296 K and load ratios of 0.05, 0.4, and 0.75. The results at load ratios of 0.05 and 0.4 show that the threshold stress intensity range (ΔK _th ) decreases as hydrogen is added to niobium. It reaches a minimum at the critical hydrogen concentration (C _cr ), where maximum embrittlement occurs. The critical hydrogen concentration is approximately equal to the solubility limit of hydrogen in niobium. As the hydrogen concentration exceeds C _cr , ΔK _th increases slowly as more hydrogen is added to the specimen. At load ratio 0.75, ΔK _th decreases continuously as the hydrogen concentration is increased. The results provide evidence that two mechanisms are responsible for fatigue crack growth behavior in niobium-hydrogen alloys. First, embrittlement is retarded by hydride transformation-induced and plasticity-induced crack closures. Second, embrittlement is enhanced by the presence of hydrogen and hydride.     

Fatigue crack initiation and microcrack propagation in X7091 type aluminum P/M alloys

Fatigu crack initiation in extruded X7091 RSP-P/M aluminum type alloys o°Curs at grain boundaries at both low and high stresses. By a process of elimination this grain boundary embrittlement was attributed to Al₂O₃ particles formed mainly during atomization and segregated to some grain boundaries. It is not due to the small grain size, to Co₂Al₉, to η precipitates at grain boundaries, nor to a precipitate free zone. Thermomechanical processing after extrusion of X7091 with 0.8 pct Co was done by Alcoa to produce large recrystallized grains. This resulted in initiation of fatigue cracks at slip bands, and the resistance to initiation of fatigue cracks at low stresses was much greater. Microcrack growth is, however, much faster in the thermomechanically treated samples, as well as in ingot alloys, than in extruded and aged X7091.     

Fatigue crack propagation in iron and Fe-Mo solid solution alloys (77 to 296 K)

Fatigue crack propagation data were obtained for iron and Fe-Mo solid solution alloys through the ductile-brittle transition temperature (77 to 296 K). The fatigue crack growth rates were found to decrease with decreasing temperature for each alloy and to decrease with increasing solute content at 296 K but to increase with solute content at 77 K. These results are discussed in terms of the wide range of mechanical properties observed for the materials. It is shown that the fatigue crack growth rates are inversely proportional to a plastic work term and the square of a characteristic stress approximated by the yield or ultimate stress. Formerly Research Assistant at Materials Science and Engineering Department and Materials Research Center, Northwestern University, Evanston, Ill. This paper is based on a thesis submitted by L. H. Burck in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Northwestern University.     

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.     

Fatigue crack propagation in austenitic stainless steel weldments

The fatigue crack propagation rate (FCPR) in 316L austenitic stainless steel (ASS) and its weldments was investigated, at two loading amplitudes, 7 and 8.5 kN, under tension-tension mode. Two welding techniques, submerged arc welding (SAW) and manual arc welding (MAW), have been used. Magnetic δ-ferrite, depending upon Ni and Cr content in the metal, in the weld zone upon solidification was considered. The ferrite number (FN) of δ-ferrite formed in the SAW zone was much higher (maximum 9.6) compared to the corresponding value (maximum 0.75) in the MAW zone. A fatigue starter notch was positioned at different positions and directions with respect to the weld zone, in addition to the heat-affected zone (HAZ). Regions of high and low FCPRs as the fatigue crack propagated through and across the weld zone have been noticed. This is related to the direction of the tensile residual stresses present in weld zone, resulting from solidification of the weld metal. The FCPR was higher along through the HAZ and weld zone because of the microstructural change and direction and distribution of tensile residual stresses. The FCPR was much lower when crack propagated perpendicular to the weld zone, particularly in the case of SAW in which higher δ-ferrite volume fraction was noticed. A lower FCPR found across the weld zone, in both SAW and MAW, was accompanied by rubbed areas in their fractures.     

Fatigue crack propagation in rapidly solidified aluminum alloys at 25 °C and 300 °C

The fatigue crack propagation performance of two rapidly solidified aluminum alloys was investigated in air at 25°C and 300°C. The results show that the crack propagation rates for continuous cycling tests of Al-8Fe-4Ce and Al-4.7-Fe-4.7Ni-0.2Cr alloys were similar at 25°C. Although the crack propagation rates of both alloys were increased at 300°C, the Al-Fe-Ce alloy exhibited the greater resistance to crack propagation. The inclusion of a tensile hold time in the fatigue loading cycle at 300°C produced an increase in the crack propagation rates for both alloys over the rates for continuous cycling. The fatigue crack propagation performance of the rapidly solidified alloys was not found to be superior when compared with the fatigue crack propagation performance of a wrought aluminum alloy tested under the same conditions. Transmission and scanning electron microscopy study of the tested specimens revealed that the crack propagation mode was primarily transgranular, with the metastable dispersoid particles providing impenetrable barriers to dislocation motion.