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.     

Local yielding attending fatigue crack growth

Fatigue crack growth rate measurements were performed at 100°C on an Fe-3Si steel in three thickness conditions and at different ΔK-levels. The test pieces were subsequently sectioned and etched to reveal the plastic deformation attending crack growth both on the surface and in the interior. Unlike preceding studies, the Fe-3Si steel displayed classical cyclic crack growth: well-defined fatigue striations with a spacing close to the per-cycle growth rate, and essentially the same growth rates that have been reported for low and medium strength steels. A highly strained region, approximately one-fifth the size of the monotonie plastic zone, is identified as the cyclic plastic zone. On this basis three regions with distinct cyclic strain histories that precede the crack are identified: a microstrain region wherein the material receives ∼10³ to 10⁴ strain cycles in the range 0 < Δε _P ≲ 10^-3; a cyclic plastic zone corresponding to ∼200 cycles in the range 10^-3 < Δ∈ _P ≲ 10^-1, and a COD-affected zone that receives ∼10 strain cycles in the range 10^-1 ≲ Δ∈ _P ≲ 1. It is suggested that the damage associated with the instabilities in the fatigue substructure to overstrain contribute to the growth mechanism.     

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.     

Use of the nanoindentation technique for studying microstructure/crack interactions in the fatigue of 4340 steel

The objectives of this research are to study the influence of microstructure on the fatigue crack growth behavior in 4340 steel and to explore the application of the nanoindentation technique for determining the plastic deformation zone at a fatigue crack tip. Two heat treatment conditions were chosen for the steel: annealed and quenched plus tempered. The annealed steel consists of coarse pearlite and proeutectoid ferrite, while the quenched and tempered steel consists of fine tempered martensite. Fatigue crack propagation tests were conducted on disklike compact (DCT) specimens. Subsequently, the nanoindentation technique was applied to quantitatively determine the plastic deformation zone at fatigue crack tips. The plastic deformation zone size determined by the nanoindentation test seems larger than the cyclic deformation zone calculated using the fracture mechanics equation, which involves many assumptions. The fatigue crack growth test results show that the annealed steel has a higher resistance to crack growth than the quenched and tempered steel. The fatigue crack in the annealed steel tends to grow along pearlite domain boundaries, or the cementite/ferrite interfaces within a pearlite domain. In contrast, the fatigue crack in the quenched and tempered steel tends to traverse the fine martensite laths. Consequently, the actual crack path in the annealed steel is rougher than in the quenched and tempered steel and more secondary cracks are observed in the annealed steel.     

The effects of temperature on fatigue crack propagation in linepipe steel

Fatigue crack propagation rates in an A.P.I. 5L Grade steel were investigated by means of constant deflection amplitude bending fatigue tests at 640 c.p.m. on single edge notched specimens at — 50, — 10, 20 and 70°C in argon. The data were evaluated in terms of the crack propagation rate (da/dN) as a function of the stress intensity range (Δ/K), according toda/dN = ΔK ^m . It was found that dynamic strain aging has a major influence on fatigue crack propagation, resulting in a maximum of the crack propagation rate at room temperature. Similarly, the cyclic plastic zone size is a maximum at room temperature. D. H. Andreasen, formerly with the Department of Mining and Metallurgy, University of Alberta, Edmonton, Alberta, Canada.     

The effect of phase continuity on the fatigue and crack closure behavior of a dual-phase steel

The effect of phase continuity on the low cycle fatigue and fatigue crack growth behavior of a Fe-C-Mn dual-phase steel has been investigated. Two microstructures, one consisting of continuous ferrite and the other continuous martensite, were examined. Although there was no difference in the low cycle fatigue lives between the two microstructures, the continuous martensite structure exhibited an extremely high fatigue threshold value of 20 MPa m^1/2, compared to 16 MPa m^1/2 for the continuous ferrite microstructure. A major effect of phase continuity has also been found in the crack closure levels during fatigue crack propagation studied over three decades of crack growth rates. The continuous martensite microstructure exhibited much higher closure levels due to the martensite constraining the plastic deformation in the ferrite and bearing a larger portion of the applied cyclic load. This effect is similar to the extrinsic toughening phenomenon cited in the literature. After accounting for the closure levels the intrinsic or effective fatigue crack growth rates are similar for the two microstructures. These intrinsic thresholds are predicted by employing experimentally obtained low cycle fatigue parameters and the ferrite grain size.     

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.     

Influence of gaseous environments on rates of near-threshold fatigue crack propagation in nicrmov steel

The influence of hydrogen environment (448 kPa) on near-threshold fatigue crack propagation rates was examined in a 779 MPa yield strength NiCrMoV steel at 93 °C. An automatically decreasing and increasing stress intensity technique was employed to generate crack growth rates at three load ratios(R = 0.1, 0.5, and 0.8). Results show that the crack propagation rates in hydrogen are slower than those in air for levels of stress intensity range, ΔK, below about 12 MPa√m. The crack closure concept does not explain the slower crack growth rates in hydrogen than in air. Near-threshold growth rates appear to be controlled by the levels of residual moisture in the environments. In argon and air, the fracture morphology is transgranular, while in H₂ the amount of intergranularity varies with ΔK and achieves a maximum when the cyclic plastic zone is approximately equal to the prior austenite grain size.     

An Evaluation of the summation of cyclic plastic strain damage in two high strength aluminum alloys

A damage equation based upon the integration of low cycle fatigue plastic strain ranges was verified experimentally for two high strength aluminum alloys 2024-T4 and 7075-T651. The damage equation which has been used extensively for many fatigue crack propagation theories assumes cyclic damage under increasing plastic strain ranges. In order to verify the damage equation, low cycle fatigue specimens were subjected to a fully reversed strain cycle in which the total strain-range was increased linearly by a constant amount Δ[Δε_d] per cycle. An excellent agreement was obtained between the predicted and observed fatigue lifetimes. The stress-strain response of these alloys was also measured. The experimental results showed that these two alloys cyclically harden substantially and that the single strain increment stress-strain curve is a fair lower bound approximation of the cyclic stress-strain curve.     

Short fatigue crack growth behavior in a ferritic-bainitic steel

Short fatigue crack growth behavior was studied in a ferrite-bainite microstructure in C-Mn steel with respect to microstructural variations. Specimens were subjected to cyclic loading at three different stress levels: 559, 626, and 687 MPa. The crack propagation rates varied from 10^-4 to 10^-2 μm/cycle. Crack lengths were measured using a replication technique. The growth rates were systematically decreased at microstructural heterogeneities up to a length of 3 to 4 grain diameters. A two-stage short fatigue crack growth model previously developed by Hussain et al. was modified to predict the crack growth behavior. The calculated values were within 10 pct error of the experimentally determined results. The model was then used to present the effect of grain boundaries on cracks propagating at constant rates. It was shown that the mode of presenting of the fatigue data can help in understanding different practical problems in stage I. These include situations such as block loading and short-duration stress spikes in nonpropagating crack regimes.     

Fatigue of stainless steel in hydrogen

The fatigue crack growth rates of two austenitic stainless steel alloys, AISI 301 and 302, were compared in air, argon, and hydrogen environments at atmospheric pressure and room temperature. Under the stresses at the crack tip the austenite in type 301 steel transformed martensitically to a’ to a greater extent than in type 302 steel. The steels were also tested in the cold worked condition under hydrogen or argon. Hydrogen was found to have a deleterious effect on both steels, but the effect was stronger in the unstable than in the stable alloy. Cold work decreased fatigue crack growth rates in argon and hydrogen, but the decrease was less marked in hydrogen than in argon. Metallographic, fractographic, and microhardness surveys in the vicinity of the fatigue crack were used to try to understand the reasons for the observed fatigue behavior.     

Influence of heat treatment on the fatigue crack growth rates of a secondary hardening steel

The relationships between microstructure and fatigue crack propagation behavior were studied in a 5Mo-0.3C steel. Microstructural differences were achieved by varying the tempering treatment. The amounts, distribution, and types of carbides present were influenced by the tempering temperature. Optical metallography and transmission electron microscopy were used to characterize the microstructures. Fatigue fracture surfaces were studied by scanning electron microscopy. For each heat treatment the fatigue crack growth properties were measured under plane strain conditions using a compact tension fracture toughness specimen. The properties were reported using the empirical relation of Paris [da/dN = C_oΔK^m]. It was found that secondary hardening did influence the fatigue crack growth rates. In particular, intergranular modes of fracture during fatigue led to exaggerated fatigue crack growth rates for the tempering treatment producing peak hardness. Limited testing in a dry argon atmosphere showed that the sensitivity of fatigue crack growth rates to environment changed with heat treatment.     

Intrinsic fatigue crack growth

The influences of microstructure and deformation mode on inert environment intrinsic fatigue crack propagation were investigated for Al-Li-Cu-Mg alloys AA2090, AA8090, and X2095 compared to AA2024. The amount of coherent shearable δ (Al₃Li) precipitates and extent of localized planar slip deformation were reduced by composition (increased Cu/Li in X2095) and heat treatment (double aging of AA8090). Intrinsic growth rates, obtained at high constantK _max to minimize crack closure and in vacuum to eliminate any environmental effect, were alloy dependent;da/dN varied up to tenfold based on applied ΔK or ΔK/E. When compared based on a crack tip cyclic strain or opening displacement parameter (ΔK/(σ_ys E)^1/2), growth rates were equivalent for all alloys except X2095-T8 which exhibited unique fatigue crack growth resistance. Tortuous fatigue crack profiles and large fracture surface facets were observed for each Al-Li alloy independent of the precipitates present, particularly δ, and the localized slip deformation structure. Reduced fatigue crack propagation rates for X2095 in vacuum are not explained by either residual crack closure or slip reversibility arguments; the origin of apparent slip band facets in a homogeneous slip alloy is unclear. Better understanding of crack tip damage accumulation and fracture surface facet crystallography is required for Al-Li alloys with varying slip localization.     

Crack paths,microstructure, and fatigue crack growth in annealed and cold-rolled AISI 304 stainless steels

To assist in the understanding of micromechanisms for corrosion fatigue crack growth in metastable austenitic steels, the relationships between the crack paths and the underlying microstructure were investigated for annealed and cold-rolled (CR) 304 stainless steels that had been tested in a deaerated 3.5 pct NaCl solution, air, and vacuum. Corrosion fatigue in the deleterious environments (3.5 pct NaCl and air) was brittle and occurred primarily by {001}_γ and other unidentified, quasi-cleavage (QC), accompanied by preferential cracking along {111}_γ twin and grain boundaries. In contrast, fatigue cracking in vacuum was ductile, fully transgranular, and noncrystallographic. Transformation to alpha prime (α′-) martensite by fatigue was found to be essentially complete in the CR steel, which contained ε-martensite, and in the annealed steel tested in vacuum, but was substantially less in the annealed steel tested in air and 3.5 pct NaCl solution. These results, taken in conjunction with the crack growth and electrochemical reaction data, support hydrogen embrittlement (HE) as the mechanism for corrosion fatigue crack growth in 304 stainless steels in 3.5 pct NaCl solution. Martensitic transformation appears not to be the only responsible factor for embrittlement. Other microstructural components, such as twin and grain boundaries, slip bands, and cold work-induced lattice defects, may play more important roles in enhancing crack growth rates.     

Influence of inclusion content on fatigue crack

Fatigue crack growth rates were measured at room temperature in dry air for three 7075-T6 aluminum alloys with different inclusion content. Volume fractions of inclusions were determined for each alloy by the point count method with two different automated systems. Plots of the fatigue crack growth rate (da/dN) vs the stress-intensity-factor range (ΔK) show a well defined change of slope at the transition between plane strain and plane stress fracture. This transition is associated with a marked increase in the amount of fracture by void growth around inclusions. The volume fraction and mean spacing of voids within the cyclic plastic zone have been determined as a function of ΔK by quantitative fractography. Fracture by voids is important when the mean spacing of such voids is approximately equal to the width of the cyclic plastic zone in the plane of the crack. It is concluded that the inclusion content increases the fatigue-crack growth rates only within the plane stress range, that is for values of the stress-intensity-factor range ΔK \s> 20 kpsi√in.     

Effect of hydrogen on fatigue crack propagation in vanadium

The influence of hydrogen on fatigue crack propagation in unalloyed vanadium and several hydrogen-charged vanadium alloys has been investigated. The Paris-Erdogan equation,da/dN =C(Δ.K)^m, was approximately obeyed for all alloys. Crack growth rates were lowest in vanadium and dilute vanadium-hydrogen alloys, and were not very sensitive to volume fraction of hydrides in more concentrated alloys. The crack growth exponent,m, is inversely proportional to the cyclic strain hardening rate,n′, and the rate constantC is inversely proportional to the square of the ultimate tensile stress, σ_UTS: Metallographic examination showed hydride reorientation and growth in the originally hydrided alloys. No stress-induced hydrides were observed in V-H solid-solution alloys. Fractures in hydrided materials exhibited cleavage-like features, while striations were noted in unalloyed vanadium and dilute solid-solution alloys.     

Effect of environment on crack growth behavior in austenitic stainless steels under creep and fatigue conditions

Crack growth behavior at high temperatures under cyclic, static, and combined loads was studied in annealed and 20 pct cold-worked Type 316 and 20 pct cold-worked Type 304 austenitic stainless steels in air and vacuum. Under cyclic load, crack growth rates in annealed Type 316 steel are slightly lower in vacuum than in air, but this difference decreases with increase in crack growth rate. Most importantly, the effect of temperature on crack growth is present even in vacuum and arises mostly from the variation of elastic modulus with temperature. In the cold-worked Type 316 steel, the pronounced hold-time effects on fatigue crack growth in air reported in the literature persists even in vacuum. This implies that at high crack growth rates these hold-time effects arise mostly from creep-fatigue interaction rather than environment fatigue interaction. Environment has a negligible effect also on crack growth under static load. Thus, time dependent crack growth in these steels is due to creep processes. Crack growth behavior in annealed and cold-worked materials are compared and reasons for the enhanced time dependent crack growth in cold-worked material are discussed in detail.     

A notch analysis of fracture approach to fatigue crack propagation

A model of fatigue crack growth is proposed that utilizes the recent developments in notch analysis of fracture and a concept of size effect that results from the changes in the critically stressed volume ahead of a crack tip. Accordingly, the fatigue crack growth mechanism involves local stresses reaching the theoretical cohesive strength and causing brittle fracture of atomic bonds at nominal stresses near the threshold, whereas slip-plane decohesion and plastic blunting and resharpening of the crack tip process may occur at stresses above the threshold range. The model contains three material parameters σFF nF, and ρF, that conveniently extend continuum analysis to situations where inhomogeneity of the material structure can influence the behavior appreciably. The analytical expression from the model was found to correlate fatigue crack growth data reasonably well in the low and intermediate stress ranges in Al 2024-T3, Al 7075-T6 and 250 grade maraging steel. The fracture modes observed are in agreement with the predictions from the model. The same fatigue crack growth model can be extended to estimating the threshold stress intensity factor range, ΔK_o and fatigue notch sensitivity of different materials.     

Low cycle fatigue crack propagation in hastelloy-X at 25 and 760 °C

Fatigue crack propagation tests were conducted on Hastelloy-X in air, at 25 °C and at 760 °C under controlled plastic strain amplitudes in the fully plastic low cycle fatigue regime. The crack growth rate data for different strain levels were correlated with the range of theJ integral ΔJ. The ΔJ values were calculated from finite element numerical solutions. It was found that the assumption thatda/dN =A(Δε _ρ ) ^α a is only an approximation of the more general equationda/dN =B(ΔJ) ^α in a narrow range of crack lengths. It is shown that theoretical models predicting low cycle fatigue lives by integrating the fully plastic crack growth rates will be in error if the (da/dN, ΔJ) relationship is not used.