Effects of frequency on fatigue behavior of type 316 low-carbon, nitrogen-added stainless steel in air and mercury for the spallation neutron source

The high-cycle fatigue behavior of type 316 low-carbon, nitrogen-added (LN) stainless steel (SS), the prime-candidate target-container material for the spallation neutron source (SNS), was investigated in air and mercury. Test frequencies ranged from 0.2 to 10 Hz with an R ratio of −1, and 10 to 700 Hz with an R ratio of 0.1. During tension-compression fatigue studies, a significant increase in the specimen temperature was observed at 10 Hz in air, which decreased the fatigue life of the 316 LN SS relative to that at 0.2 Hz. Companion tests in air were carried out, while cooling the specimen with nitrogen gas at 10 Hz in air. In these experiments, fatigue lives were comparable at 10 Hz in air with nitrogen cooling and at 0.2 Hz in air. During tension-tension fatigue studies, a higher specimen temperature was observed at 700 than at 10 Hz. After cooling the specimen, comparable fatigue lives were found at 10 and at 700 Hz. The frequency effect on the fatigue life in mercury was found to be much less than that in air, due to the fact that mercury acts as an effective coolant during the fatigue experiment. Striation spacing on the fracture surface at different test frequencies was closely examined, relative to calculated ΔK values, during fatigue of the 316 LN SS. Specimen self-heating has to be considered in understanding fatigue characteristics of 316 LN SS in air and mercury.     

Effect of surface roughness on low-cycle fatigue behavior of type 304 stainless steel

The effects of surface roughness on the low-cycle fatigue life of Type 304 stainless steel at 593°C in air have been investigated. It is observed that, at a strain rate of 4 × 10⁻³ s⁻¹ and a total strain range of 1 pct, the fatigue life (N _f cycles) decreases with an increase in surface roughness. Information on crack growthvs strain cycles has been generated, as a function of surface roughness, by the measurement of striation spacing on fractured surfaces of specimens tested to failure. Crack propagation follows the Ina ∞N (wherea is the crack length afterN strain cycles) relation for longer specimen fatigue lives (N_f > 2700 cycles) and departs from Ina ∞N for shorter fatigue lives. A quantitative estimate is made of the number of cycles N_o(r) to generate a crack length equal to 0.1 mm (≈ 1 grain diam). The initial surface roughness significantly affects only the initiation component of specimen life time. The effect of roughness on crack initiation is described byN ₀ (R) = 1012R^−0.21, whereR is the surface roughness (root-mean-square value) in microns.     

Modeling creep and fatigue of copper alloys

This article reviews expressions to quantify the thermal creep and fatigue lifetime for four copper alloys: Cu-Ag-P, Cu-Cr-Zr, Cu-Ni-Be, and Cu-Al₂O₃. These property models are needed to simulate the mechanical behavior of structures with copper components, which are subjected to high heat-flux and fatigue loading conditions, such as molds for the continuous casting of steel and the first wall in a fusion reactor. Then, measurements of four-point bending fatigue tests were conducted on two-layered specimens of copper alloy and stainless steel, and thermal ratchetting behavior was observed at 250 °C. The test specimens were modeled with a two-dimensional elastic-plastic-creep finite-element model using the ABAQUS software. To match the measurements, a primary thermal-creep law was developed for Cu-0.28 pct Al₂O₃ for stress levels up to 500 MPa and strain rates from 10⁻⁸ to 10⁻² s⁻¹. Specifically, (s⁻¹)=1.43×10¹⁰ exp (−197,000/8.31 T(K)) (σ(MPa))^2.5 (t(s))^−0.9.     

The debonding and fracture of Si particles during the fatigue of a cast Al-Si alloy

Constant-amplitude high-cycle fatigue tests (σ_max=133 MPa, σmax/σy=0.55, and R=0.1) were conducted on cylindrical samples machined from a cast A356-T6 aluminum plate: The fracture surface of the sample with the smallest fatigue-crack nucleating defect was examined using a scanning electron microscope (SEM). For low crack-tip driving forces (fatigue-crack growth rates of da/dN<1 × 10⁻⁷ m/cycle), we discovered that a small semicircular surface fatigue crack propagated primarily through the Al-1 pct Si dendrite cells. The silicon particles in the eutectic remained intact and served as barriers at low fatigue-crack propagation rates. When the semicircular fatigue crack inevitably crossed the three-dimensional Al-Si eutectic network, it propagated primarily along the interface between the silicon particles and the Al-1 pct Si matrix. Furthermore, nearly all of the silicon particles were progressively debonded by the fatigue cracks propagating at low rates, with the exception of elongated particles with a major axis perpendicular to the crack plane, which were fractured. As the fatigue crack grew with a high crack-tip driving force (fatigue-crack growth rates of da/dN>1 × 10⁻⁶ m/cycle), silicon particles ahead of the crack tip were fractured, and the crack subsequently propagated through the weakest distribution of prefractured particles in the Al-Si eutectic. Only small rounded silicon particles were observed to debond while the fatigue crack grew at high rates. Using fracture-surface markings and fracture mechanics, a macroscopic measure of the maximum critical driving force between particle debonding vs fracture during fatigue-crack growth was calculated to be approximately K _max ^tr ≈6.0 MPa √m for the present cast A356 alloy.     

Near-threshold fatigue crack growth behavior in copper

Near-threshold fatigue crack growth rate data were developed in annealed, quarter-hard, and full-hard copper at various load ratios, (R = σ_min/σ_max). Increasing theR value decreases the resistance to threshold crack growth. At a fixed value ofR, annealed copper has the slowest near-threshold crack propagation rate while full-hard copper has the fastest crack growth rate. Waveform (sine and triangle) and specimen geometry (WOL, CT, and CCT) do not appear to affect the rates of near-threshold crack propagation. The influences of load ratio and material strength on threshold crack growth behavior can be rationalized by crack closure.     

Quantitative characterization of the substructure of AISI 316 stainless steel resulting from creep

The substructure of AISI 316 stainless steel resulting from creep deformation has been quantitatively characterized using transmission electron microscopy. The specimens were tested at temperatures and stresses ranging from 593° to 816°C and 8000 to 35,000 psi, respectively. Subgrains whose boundaries are predominantly (111) twist boundaries were formed in all tests at and above 704°C but were observed very infrequently at 650°C and were completely absent after creep at 593°C. The subgrain diameter,d, and the mobile dislocation density, ρ, were found to vary with the applied stress, σ_a, according to:d =kσ_a ^-1 and ρα σ_a ². Subgrain misorientation varys from less than 0.1 to 1 deg in each specimen seemingly independent of all parameters evaluated. A double triple node dislocation configuration was frequently observed in all specimens. Its relation to the deformation process is discussed in a mechanism involving the breaking of attractive dislocation nodes. Formerly Graduate Student, Materials, Science and Metallurgical Engineering Department, University of Cincinnati, Cincinnati, Ohio 45221     

The low-cycle fatigue and fatigue-crack-growth behavior of HAYNES® HR-120 alloy

The low-cycle fatigue and fatigue-crack-growth behavior of the HAYNES HR-120 alloy was investigated over the temperature range of 24°C to 980°C in laboratory air. The result showed that increasing the temperature usually led to a substantial decrease in the low-cycle fatigue life. The reduction of fatigue life could be attributed to oxidation and dynamic strain-aging (DSA) processes. The strain vs fatigue-life data obtained at different temperatures were analyzed. It was also found that the fatigue-crack-growth rate per cycle generally increased with increasing temperature and R ratio (R=σ _min/σ _max, where σ _min and σ _max are the applied minimum and maximum stresses, respectively). The relationship between the stress-intensity-factor range and fatigue-crack-growth rate was determined. Scanning-electron-microscopy (SEM) examinations of the fracture surfaces revealed that the fatigue cracks initiated and propagated predominantly in a transgranular mode.     

Correlation of the hot-hardness with the tensile strength of 304 stainless steel to temperatures of 1200°C

It is shown that the ultimate tensile strength, σ, of 304 stainless steel can be correlated with hot-hardness measurements at temperatures up to 1200°C using the expression σ = (H/3.0)(n/0.2n) ⁿ whereH is the diamond pyramid hardness number andn is the strain hardening coefficient. The strain hardening coefficient was obtained from a Meyer’s hardness coefficient at the room temperature test condition and for test temperatures up to about 0.5T _m , from the empirical relationshipn =k/λ. Herek is a constant equal to approximately 0.2 microns and λ is the subgrain dimension of the deformed specimen as obtained by transmission electron microscopy. Formerly with General Electric Company, Cincinnati, Ohio     

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.     

Influence of foreign particles on fatigue behavior of Ti-6Al-4V prealloyed powder compacts

The effect of three types of contaminants on the fatigue life of prealloyed Plasma Rotating Electrode Process (PREP) Ti-6A1-4V hot isostatically pressed (HIP’d) powder compacts was studied. Ultraclean powder was seeded separately with SiO₂, A1₂O₃, and 316 stainless steel (SS) contaminants of 50 μm, 150 μ, and 350 μm nominal size, a total of nine conditions. Seeded compacts were fatigue tested at room temperature and the results were compared to those of an unseeded baseline compact. It was found that a substantial loss in fatigue life occurred even at the smallest seeded contaminants used in this work. Angular nonmetallic SiO₂ and A1₂O₃ contaminants were found to be more detrimental to fatigue strength than spherical metallic 316 SS contaminants of the same size range also indicating a shape effect. The loss of fatigue life suggests that at the high stress levels there is almost no crack initiation period and fatigue lives are controlled mainly by crack growth.     

Internal stresses and structures developed during creep

Specimens of 304 stainless steel subjected to different thermomechanical histories develop different internal stresses, σ _i , and different substructures. Creep rate is uniquely related not to the applied stress, σ _A , but to the effective stress, σ^*=(σ _A −σ _i ). Values of σ^* are determined from experimental results and σ _i calculated from σ _i =(σ _A −σ^*). Results show σ _i increases with the applied stress according to σ _i ∝σ _A ^1.7 . Transmission electron microscopic observations show that the density of dislocations within subgrains, ϱ _D , and the subgrain diameter,D, vary with applied stress according to: ϱ _D ∝σ _A ^K ,D ∝ σ _A ^−0.8 , whereK=1.4 to 2.0. Subgrain misorientation is independent of creep stress, strain, or temperature. The contributions of these structural variables to the internal stress are discussed.     

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     

Fatigue-crack growth in Ti-6Al-4V-0.1Ru in air and seawater: Part I. Design of experiments, assessment, and crack-grown-rate curves

Fatigue-crack growth rates for different simulated ocean environments and loading conditions have been investigated for beta-annealed Ti-6Al-4V-0.1Ru (extra-low interstitials, (ELI)), a candidate material for oil production risers; the focus was on uncovering whether certain combinations of conditions could produce unexpectedly high crack growth rates. A two-level, one-quarter-fraction factorial design-of-experiments (DOE) approach was used to ascertain which testing variables and environmental conditions warranted further study. This study used eight different combinations of variables: parent/deformed material, 27 °C/85 °C temperature, 2 Hz/20 Hz loading frequency, 0.1/0.6 load ratio (R=σ _min/σ _max, where σ _min is the minimum and σ _max is the maximum stress during a fatigue cycle), and aerated/deaerated seawater. Comparisons were based on crack growth rates at ΔK=17 MPa , roughly the middle of the Paris portion of the da/dN vs ΔK curves. The da/dN vs Δ_K curves were also examined, and conclusions based upon these data were compared with those from the DOE. Consideration of the microstructure’s influence on the crack path is postponed until Part II of this article. Samples tested at the higher load ratio showed a statistically significant increase in the crack propagation rate compared to those tested at R=0.1; the same was true of specimens tested at 20 Hz vs those tested at 2 Hz, but the level of significance was lower. The parent material had somewhat higher crack growth rates than the deformed samples. Changes in environmental conditions other than frequency produced little effect on the crack growth rate. Comparison of crack growth rates over the ΔK range measured revealed details that would have not been uncovered in comparisons at a single ΔK value. The Paris exponent ranged between 3.7 and 6.7, and the only systematic variation observed was an increase in the exponent with increasing test frequency. In seawater, cold work (a 5 pct reduction in thickness by rolling) reduced fatigue-crack growth rates by a factor of 2 (compared to the parent material) at intermediate and high ΔK values. There was a crossover of crack growth rates for low ΔK values: below 10 MPa , growth rates were lower for the parent material than for the cold-rolled material, suggesting a higher ΔK _th for the parent material, while above this value, fatigue cracks grew more rapidly in the parent material than in the cold-rolled material. Crack growth rates were slightly higher in seawater than in air, but only slightly more than the sample-to-sample variation of crack growth rates, and cold work reduced fatigue-crack growth rates in air by about the same amount as in seawater. Somewhat more scatter was observed for the R=0.1 tests than for the R=0.6 tests. Differences in temperature (27 °C, 53 °C, and 85 °C) do not appear to affect fatigue-crack growth rates. For ΔK<20 MPa , crack growth rates were similar for 0.2 and 2 Hz but were higher for 20 Hz; above 20 MPa , the crack growth rates were similar for all three frequencies. One explanation for the unusual frequency dependence relies on the possibility that the environment produces different amounts of closure for different test frequencies. According to this view, closure is effective in air and in seawater at 0.2 and 2 Hz but not at 20 Hz: perhaps the higher loading rate breeches the passive layer at a rate more rapid than it can reform. Because the crack growth rate appeared independent of temperature, it is unlikely that there is a significant influence of thermally activated corrosion-fatigue mechanisms for the conditions tested. The results demonstrate that beta-annealed Ti-6V-4Al-0.1Ru (ELI) possesses a robust response to the combinations of environment and loading expected in oil production riser service. The value of the DOE approach was clear, and supplementary tests verified the main effects predicted by the DOE results. Comparison of the single-value DOE results with the da/dN curves reveals a limitation in the former: different slopes of the Paris curves and crossover effects are or would be missed for DOE comparing crack growth rates derived from constant ΔK tests. The use of constant Δσ data and a second level of interrogation, following DOE analysis and based on the da/dN curves, addressed this limitation effectively. A DOE comparison based, for example, on three ΔK values (the lower, middle, and upper portions of the Paris regime) might be another way of proceeding.     

Construction of constant fatigue life diagram for a carbon fiber composite

Using the constant amplitude fatigue data at various stress ratios, the constant fatigue life (CFL) diagram was constructed for the CFC material which is useful in prediction of fatigue life under variable amplitude fatigue loads. A quasi-isotropic lay-up sequenced carbon fiber-epoxy composite (CFC) laminate was fabricated by resin infusion technique. The tensile and compression tests were carried out to determine the static strength of the material. About 175 mm long, constant rectangular cross-sectioned fatigue test specimens were cut and prepared from the laminate. The stress-controlled, constant-amplitude fatigue tests were conducted in a 100 kN servo-hydraulic test machine, at room temperature and in lab air atmosphere. All the fatigue tests were performed with a sinusoidal waveform and a frequency of 1–3 Hz. The fatigue tests were conducted at four different stress ratio, R = σ_min/σ_max, Viz., 0.7,0.5 (tension-tension), −1.0 (tension-compression), and 4.0 (compression-compression). Anti-buckling guide was employed during the fatigue tests which involved compressive load cycles. The CFL Diagrams are material specific with predictive capability for any designed levels from selective experimental data.     

High frequency stage I corrosion fatigue of austenitic stainless steel (316L)

High frequency (123 Hz) fatigue crack propagation studies were conducted under rising ΔK conditions (R-ratio = 0.22) on single edge notch specimens of austenitic stainless steel (type 316L) that contained an annealed precrack. Tests were conducted in near neutral (pH 5.5) solutions of 1 M NaCl and 1 M NaCl + 0.01 M Na₂S₂O₃ under potentiostatically controlled conditions and in desiccated air. Attention was directed primarily to the near threshold behavior and the stage I (crystallographic) region of cracking. Good mixing between the crack solution and bulk solution was obtained and crack retardation and arrest effects, due to surface roughness induced closure, were minimized at high anodic potentials by electrochemical erosion. Thermodynamic considerations showed that hydrogen played no role in fatigue crack propagation. Analysis of the results in terms of the estimated effective cyclic stress intensity, ΔK _eff, showed a systematic effect of potential on the average crack growth increment per cycle,da/dN. Anodic dissolution processes were considered to make an insignificant contribution toda/dN. A model was proposed for stage I fatigue cracking based on the effect of oxide nucleation rate on restricted slip reversal. The essential features of the model were considered to be relevant to cracking in aqueous environments and in desiccated air.     

Study of mechanism of cleavage fracture at low temperature

In this investigation, a series of crack opening displacement (COD) tests were carried out at several low temperatures for C-Mn weld steel. Some of the specimens were loaded until fracture, and the mechanical properties and microscopic parameters on fracture surfaces were measured. Other specimens were unloaded before fracture at different applied loads. The distributions of the elongated cavities and the cleavage microcracks ahead of fatigue crack tips were observed in detail. Based on the experimental results, the combined criterion of a critical strainε _p ≥ ε_c) for initiating a crack nucleus, a critical stress triaxiality(σ _m/σ ≥ t_c) for preventing it from blunting, and a critical normal stress(σ _yy/σ_f) for the cleavage extension was proposed again, and the critical values of ε_p and σ_m/−σ for the C-Mn weld steel were measured. The reason why the minimum COD value could not be zero is explained. The mechanism of generation of the lower limit COD value on the lower shelf of the toughness transition curve is proposed.     

Mechanical properties of As-cast and directionally solidified Nb-Mo-W-Ti-Si <Emphasis Type="Italic">in-situ</Emphasis> composites at high temperatures

To improve the high-temperature strength of Nb-Mo-Ti-Si in-situ composites, alloying with W and a directional solidification technique were employed. The alloy composition of Nb-xMo-10Ti-18Si (x=10 or 20) was used as the base, and Nb was further replaced by 0, 5, 10 and 15 mol pct W. For samples without W, the as-cast microstructure was a eutectic mixture of fine Nb solid solution (Nb _SS ) and (Nb, Me)₅ Si₃ silicide (Me = Mo, W, or Ti), while large primary Nb _SS particles appeared besides the eutectic mixture as a result of replacing Nb by W. The directionally solidified samples consisted of coarse Nb _SS and (Nb,Me)₅ Si₃ silicides, and the microstructure showed a slight orientation in the direction of growth. The maximum compressive ductility (ɛ _max) at room temperature decreased with increasing W content and was in the range of 0.8 to 2.3 pct, in contrast to the Vickers hardness (HV), which increased with W content. The 0.2 pct yield compressive strength (σ _0.2) and the specific 0.2 pct yield compressive strength (σ _0.2S ) (σ _0.2 divided by the density of sample) at elevated temperatures were markedly improved by the W addition. The directionally solidified samples always showed higher σ _0.2 and σ _0.2S values than the as-cast samples. At elevated temperatures, the directionally solidified sample with 10 mol pct Mo and 15 mol pct W had the highest σ _0.2 and σ _0.2S values; even at 1770 K, σ _0.2 was as high as 650 MPa. The directionally solidified materials alloyed with W exhibited excellent compressive creep performance. The sample with 10 mol pct Mo and 15 mol pct W had a minimum creep rate of 1.4×10⁻⁷s⁻¹ and retained steady creep deformation at 1670 K and an initial stress of 200 MPa. Under compression, the damage and failure of these in-situ composites were dominated by decohesion of interfaces between the Nb _SS and silicide matrix.     

Tracer diffusion of63Ni in Fe-17 wt pct Cr-12 wt pct Ni

The tracer diffusion of⁶³Ni in Fe-17 Cr-12 Ni by both volume and grain boundary transport has been studied from 600° to 1250°C. The use of an RF sputtering technique for serial sectioning allowed the determination of very small volume diffusion coefficients at the lower temperatures. Volume diffusion of nickel in this alloy was observed to be much slower than in pure iron or austenitic stainless steel at comparable temperatures. The volume diffusion coefficient is described byD _v =8.8 exp (−60,000/RT) cm²/s and grain boundary diffusion is described by σD _gb =3.7×10⁻⁹ exp (−32,000/RT) cm³/s. R. A. PERKINS, formerly Presidential Intern, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak, Ridge, Tenn. 37830, is     

Comparative study of the impact response and microstructure of 304L stainless steel with and without prestrain

This study compares the dynamic plastic deformation behavior and microstructural evolution of 304L stainless steel with and without metal-forming prestrain, using the compressive split Hopkinson pressure-bar technique and transmission electron microscopy (TEM) under strain rates ranging from 8 × 10² to 5 × 10³ s⁻¹ at room temperature, with true strains varying from yield to 0.3. Results show that the flow stress of unprestrained and prestrained 304L stainless steel is sensitive to applied strain rate, but the prestrained material exhibits greater strength. A higher work-hardening rate and higher strain-rate sensitivity are also found in the prestrained material, while an inverse tendency exists for the activation volume. A constitutive equation with our experimentally determined specific material parameters successfully describes both unprestrained and prestrained dynamic behavior. Microstructural observations reveal that the morphologies of dislocation substructure, mechanical twins, microshear bands, and α′ martensite formation are strongly influenced by prestrain, strain, and strain rate. The density of dislocations increases with increasing strain and strain rate for both materials. The dislocation cell size decreases with increasing strain, strain rate, and prestrain. An elongated cell structure appears in the prestrained material as heavy deformation is applied. Mechanical twins are found only in the prestrained material. Microshear bands and α′ martensite are more evident at large strains and strain rates, especially for the prestrained material. Quantitative analysis indicates that the amount of dislocations, mechanical twins, and α′ martensite varies as a function of work-hardening stress (σ−σ _y), reflecting different strengthening effects and degrees of microhardness.