Role of foreign-object damage on thresholds for high-cycle fatigue in Ti-6Al-4V

The increasing incidence of military aircraft engine failures that can be traced to high-cycle fatigue (HCF) has prompted a reassessment of the design methodologies for HCF-critical components, such as turbine blades and disks. Because of the high-frequency vibratory loading involved, damagetolerant design methodologies based on a threshold for no crack growth offer a preferred approach. As impact damage from ingested debris is a prime source of HCF-related failures, the current study is focused on the role of such foreign-object damage (FOD) in influencing fatigue crack-growth thresholds and early crack growth of both large and small cracks in a fan blade alloy, Ti-6Al-4V. FOD, which was simulated by the high-velocity (200 to 300 m/s) impact of steel spheres on a flat surface, was found to reduce markedly the fatigue strength, primarily due to earlier crack initiation. This is discussed in terms of four salient factors: (1) the stress concentration associated with the FOD indentation, (2) the presence of small microcracks in the damaged zone, (3) the localized presence of tensile residual hoop stresses at the base and rim of the indent sites, and (4) microstructural damage from FOD-induced plastic deformation. It was found that no crack growth occurred from FOD impact sites in this alloy at ΔK values below ∼ 2.9 MPa √m, i.e., over 50 pct higher than the “closure-free”, worst-case threshold value of ΔK _TH = 1.9 MPa √m, defined for large cracks in bimodal Ti-6Al-4V alloys at the highest possible load ratio. It is, therefore, concluded that such worst-case, large fatigue crack thresholds can, thus, be used as a practical lower-bound to FOD-initiated cracking in this alloy.     

A critical assessment of the mechanistic aspects in HAYNES 188 during low-cycle fatigue in the range 25 °C to 1000 °C

The low-cycle fatigue (LCF) behavior of a wrought cobalt-base superalloy, Haynes 188, has been investigated over a range of temperatures between 25 °C and 1000 °C employing a triangular waveform and a constant strain amplitude of ±0.4 pct. Correlations between macroscopic cyclic deformation and fatigue life with the various microstructural phenomena were enabled through scanning electron microscopy (SEM) and transmission electron microscopy (TEM), detailing the crack initiation and propagation modes, deformation substructure, and carbide precipitation. Cyclic stress response varied as a complex function of temperature. Dynamic strain aging (DSA) was found to occur over a wide temperature range between 300 °C and 750 °C. In the DSA domain, the alloy exhibited marked cyclic hardening with a pronounced maximum at 650 °C. Dynamic strain aging has been documented through the occurrence of serrated yielding, inverse temperature dependence of maximum cyclic stress, and cyclic inelastic strain developed at half of the fatigue life. Additionally, the alloy also displayed a negative strain rate sensitivity of cyclic stress in the DSA regime. These macroscopic features in the DSA domain were accompanied by the substructure comprised of coplanar distribution of dislocations associated with the formation of pileups, stacking faults, and very high dislocation density. Toward the end of the DSA domain, dislocation pinning by M₂₃C₆ precipitates occurred predominantly. The deformation behavior below and above the DSA domain has also been investigated in detail. The temperature dependence of LCF life showed a maximum at ≈300 °C. The drastic reduction in life between 300 °C and 850 °C has been ascribed primarily to the deleterious effects of DSA on crack initiation and propagation, while the lower life at temperatures less than 200 °C has been attributed to the combined influence of low ductility and larger cyclic response stress.     

Mechanisms of fatigue in mill-annealed Ti-6Al-4V at room temperature and 600°F

The mechanisms of the fatigue of mill-annealed Ti-6A1-4V were studied at 600°F and room temperature. Early crack initiationN ₀/N _f< 0.14) was found to occur in hcpα-grains by a slip-band mechanism under all but the least severe conditions of cyclic stress. Under cyclic stresses near the fatigue limit at room temperature, fatigue cracks began much laterN ₀/N _f ∼ 0.4) at the interface between hcpα and bccβ grains without detectable slip. Under all conditions, Stage I fatigue crack growth occupied 50 to 80 pct of the total life. Although mechanical twins were produced in profusion near the growing Stage II fatigue cracks, they appeared to play no role in crack initiation or Stage I crack growth; nor did they facilitate Stage II growth. None of the observations could be interpreted as evidence for a metallurgical instability or strain-induced phase transformation which might be harmful to the fatigue resistance of the alloy. J. C. GROSSKREUTZ, formerly with Midwest Research Institute     

Fracture and fatigue behavior of sintered steel at elevated temperatures: Part II. Fatigue crack propagation

Fatigue cracking resistance of sintered steel as a function of temperature is characterized by crack growth rate vs the stress intensity range, ΔK. The stress ratio effects on fatigue crack propagation (FCP) are investigated from room temperature to 300 °C. The crack closure effects on FCP are evaluated by both theoretical and experimental approaches. We found that the crack closure cannot be fully responsible for the observed increase of fatigue resistance with low stress ratio. Experimental results support that both K _max and ΔK control near-threshold crack growth. Fatigue crack resistance at high ΔK regime decreases with temperature. The apparent increase of fatigue resistance at the near-threshold regime at elevated temperatures might be attributed to microcrack toughening.     

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 tempering temperature on near- threshold fatigue crack behavior in quenched and tempered 4140 steel

Fatigue crack growth in compact tension samples of high purity 4140 steel quenched and tempered to various strength levels was investigated. Tempering temperatures of 200, 400, 550, and 700 °C produced yield strengths from 1600 to 875 MPa, respectively. Crack propagation and crack closure were monitored inK-decreasing tests performed underR = 0.05 loading conditions in laboratory air. Results indicated that as the yield strength increased the crack growth rate increased at a given ΔK and ΔK_th decreased. Threshold values varied from 2.8 MPa m^1/2 (200 °C temper) to 9.5 MPa m_1/2 (700 °C temper). Cracks in the 200 °C tempered samples grew by an intergranular mechanism following prior austenite grain boundaries probably caused by hydrogen embrittlement or tempered martensite embrittlement. Tempering above 200 °C produced transgranular fatigue crack growth. The level of crack closure increased with tempering temperature and with crack propagation in a given tempered condition. Crack closure was caused by a combination of plasticity-induced and oxide-induced mechanisms. The use of an effective stress intensity range based on crack closure consolidated the fatigue crack growth curves and the threshold values for all tempering temperatures except 200 °C. Formerly Graduate Research Assistant, Department of Materials Science and Engineering, Stanford University, Stanford, CA. Formerly Professor, Department of Materials Science and Engineering, Stanford University, Stanford, CA.     

Corrosion Fatigue Crack Initiation in a Mode II Notch Specimen

The stress intensityK _II of a Mode II specimen was calculated using a finite element methodvia theJ integral. The site, direction, and the threshold value for crack initiation from the notch under cyclic Mode II loading in air, in water, and under dynamic charging with hydrogen were investigated. The results showed that the Mode II fatigue crack in a high strength steel initiated at or close to the site of the maximum principal stress, rather than at the site of the maximum shear stress, and the subsequent crack growth was oriented approximately normal to the direction of the maximum principal stress. The site and direction of crack initiation in water and under dynamic charging with hydrogen were similar but different from that in air. The threshold values for crack initiation in air, in water, and under dynamic charging were 28.8, 12.3, and 10.2 MPa m^1/2, respectively. The fracture surface of a corrosion fatigue crack in water and under dynamic charging consisted of intergranular facets at low ΔK _II values but of quasi-cleavage at higher ΔK _II values and were different from those in air.     

Elevated-temperature fatigue crack growth

The fatigue crack growth behavior of MAR-M200 single crystals was examined at 982 °C. Using tubular specimens, fatigue crack growth rates were determined as functions of crystallographic orientation and the stress state by varying the applied shear stress range-to-normal stress range ratio. Neither crystallographic orientation nor stress state was found to have a significant effect on crack growth rate when correlated with an effective ΔK which accounted for mixed-mode loading and elastic anisotropy. For both uniaxial and multiaxial fatigue, crack growth generally occurred normal to the principal stress direction and in a direction along which ΔK _II vanished. Consequently, the effective ΔK was reduced to ΔK_I and the rate of propagation was controlled by ΔK _I only. The through-thickness fatigue cracks were generally noncrystallographic with fracture surfaces exhibiting striations in the [010], [011], and [111] crystals, but striation-covered ridges in the [211] specimen. These fracture modes are contrasted to crystallographic cracking along slip bands observed at ambient temperature. The difference in cracking behavior at 25 and 982 °C is explained on the basis of the propensity for homogeneous, multiple slip at the crack tip at 982 °C. The overall fracture mechanism is discussed in conjunction with Koss and Chan’s coplanar slip model.     

Fatigue crack growth through

Fatigue cracks were grown at 25 °C and 800 °C in a titanium aluminide alloy heat-treated to give a γ+ α ₂ lamellar microstructure. These lamellae, having widths of =0.5 to 2 μm, were in colonies approximately 1.2 mm across. Crack growth was observed and photographed under high resolution conditions using a loading and heating cyclic stage for the scanning electron microscope. Stereoimaging was used to measure displacements around crack tips, from which crack opening displacements and strains were derived. Cracks were found to grow about 10 times faster at 25 °C than at 800 °C, and the threshold stress intensity for fatigue crack growth was lower at 25 °C. Strain to fracture the lamellae was determined as ≈0.08, while fatigue crack tips could sustain up to 0.3 strain at 25 °C and 0.5 strain at 800 °C. The lamellar micro- structure was found to have a strong influence on crack tip behavior.     

Thermomechanical fatigue of particulate-reinforced aluminum 2xxx-T4

Thermomechanical fatigue (TMF) and isothermal fatigue of unreinforced and SiC_p-reinforced aluminum 2xxx-T4 alloy were examined. Thermomechanical fatigue experiments were conducted underT _min = 100 °C,T _max = 300 °C andT _min = 100 °C,T _max = 200 °C conditions, and isothermal experiments were conducted at 200 °C and 300 °C. Based on stress range, substantial improvements in fatigue life were observed with reinforcement under both isothermal and thermomechanical loading conditions. Based on strain range, the TMF lives of the reinforced material increased in out-of-phase (OP) loading and remained unchanged in in-phase (IP) loading. A decrease in isothermal fatigue lives of the reinforced material compared to those of unreinforced material was observed in both 3 × 10⁻³ s⁻¹ and 3 × 10⁻⁵ s⁻¹ experiments at 200 °C and in 3 × 10⁻³ s⁻¹ experiments at 300 °C. Crack growth mechanism maps were constructed to identify crack growth behavior of the unreinforced and the reinforced materials. The TMF OP conditions were more favorable to transgranular cracking. Mixed (transgranular and intergranular) crack growth occurred in TMF IP experiments. Evidence of void formation at grain boundaries, crack deflection due to particles, and oxide penetration at the crack tips is demonstrated using scanning electron microscopy (SEM) and Auger spectroscopy analysis.     

Effects of oxidation on the fatigue crack growth resistance and the interfacial properties of a Ti-MMC laminate composite

Fatigue crack growth rates in a [0/90]_2s Ti-6Al-4V/SCS-6 cross-ply laminate, correlated with push-out tests, have been measured to assess the effects of varying test temperature, environment, load ratio (R), and initial stress intensity factor range (ΔK). The fatigue crack growth resistance is degraded in tests at 450 °C in air, but tests carried out at test temperatures of up to 450 °C under vacuum, both at R=0.1 and R=0.5, have shown crack arrest/catastrophic failure transitions (CA/CF), which are similar to those observed for specimens studied at room temperature and at 300 °C in air. Moreover, for such [0/90] composites, the critical role of intact 0 deg fibers bridging in the crack wake, in promoting fatigue crack growth resistance, has been confirmed. Sudden increases of fatigue crack growth rate can be attributed to individual fiber failure(s), which were detected by acoustic emission techniques. The effect of the experimental conditions (environment, test temperature, and duration) on the mechanical behavior (fatigue crack growth rate, push-out tests, and broken fibers pull-out lengths) of this laminate may be explained by the modification of the interfacial zone (decrease in the carbon layer thickness due to oxidation and formation of TiO₂).     

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.     

Fatigue crack initiation and microcrack growth in 4140 steel

Initiation and growth of fatigue microcracks were studied in vacuum degassed 4140 steel in three conditions: as-quenched, tempered at 400°C, and tempered at 650°C. Micro-scopic examinations were made of specimens with metallographically polished notches using a 400 times long working distance microscope with an x-y micrometer base mounted directly on an MTS machine. Following Barsom and McNicol, the cycles to fatigue crack initiationN _i were plottedvs ΔK/√p and threshhold values of gDK√p were determined. The data of logN _i vs log [ΔK/√p-ΔK/√p|_th] fit on a straight line. Microcracks grew most rapidly in as-quenched specimens and least rapidly in 650°C tempered specimens at the same ΔK/√p. In as-quenched specimens, fatigue cracks initiated at grain boundaries but in the 400 and 650°C tempered specimens they initiated at intrusions-extrusions. They are also associated with Northwestern’s Materials Research Center.     

Effects of load ratio and temperature on the near-threshold fatigue crack propagation behavior in a CrMoV steel

The influence of temperature in the range of 24 to 260 °C and load ratio on the near-threshold fatigue crack growth rate behavior of a CrMoV steel was characterized. At all temperatures investigated, the threshold stress intensity range, ΔK _th, for fatigue crack growth decreased with increasing load ratio. The near-threshold crack growth rates increased significantly at 149 °C when compared with the rates at room temperature. However, the crack growth rates at 260 °C were comparable to those at 149 °C. These observations are rationalized in terms of the concepts of roughness and oxide-induced crack closure. Extensive fracture surface characterization using SEM, oxide thickness measurements by Auger spectroscopy, and roughness measurements by light-section-microscopy were conducted to substantiate the explanations.     

Crack Propagation During Sustained-Load Cracking of Al-Zn-Mg-Cu Aluminum Alloys Exposed to Moist Air or Distilled Water

Intergranular sustained-load cracking of Al-Zn-Mg-Cu (AA7xxx series) aluminum alloys exposed to moist air or distilled water at temperatures in the range 283 K to 353 K (10 °C to 80 °C) has been reviewed in detail, paying particular attention to local processes occurring in the crack-tip region during crack propagation. Distinct crack-arrest markings formed on intergranular fracture faces generated under fixed-displacement loading conditions are not generated under monotonic rising-load conditions, but can form under cyclic-loading conditions if loading frequencies are sufficiently low. The observed crack-arrest markings are insensitive to applied stress intensity factor, alloy copper content and temper, but are temperature sensitive, increasing from ~150 nm at room temperature to ~400 nm at 313 K (40 °C). A re-evaluation of published data reveals the apparent activation energy, E _a for crack propagation in Al-Zn-Mg(-Cu) alloys is consistently ~35 kJ/mol for temperatures above ~313 K (40 °C), independent of copper content or the applied stress intensity factor, unless the alloy contains a significant volume fraction of S-phase, Al₂CuMg where E _a is ~80 kJ/mol. For temperatures below ~313 K (40 °C) E _a is independent of copper content for stress intensity factors below ~14 MNm^−3/2, with a value ~80 kJ/mol but is sensitive to copper content for stress intensity factors above ~14 MNm^−3/2, with E _a , ranging from ~35 kJ/mol for copper-free alloys to ~80 kJ/mol for alloys containing 1.5 pct Cu. The apparent activation energy for intergranular sustained-load crack initiation is consistently ~110 kJ/mol for both notched and un-notched samples. Mechanistic implications are discussed and processes controlling crack growth, as a function of temperature, alloy copper content, and loading conditions are proposed that are consistent with the calculated apparent activation energies and known characteristics of intergranular sustained-load cracking. It is suggested, depending on the circumstances, that intergranular crack propagation in humid air and distilled water can be enhanced by the generation of aluminum hydride, AlH_3, ahead of a propagating crack and/or its decomposition after formation within the confines of the nanoscale volumes available after increments of crack growth, defined by the crack arrest markings on intergranular fracture surfaces.     

Tensile and fatigue properties of 17-4 PH stainless steel at high temperatures

The tensile and high-cycle fatigue properties for 17-4 PH* stainless steels in three different conditions were investigated at temperatures ranging from room temperature to 400 °C. Results indicated that the yield strength and fatigue strength for the three conditions at a given temperature took the following order: condition H900 > condition A> condition H1150. The yield strength of each condition decreased with increasing temperature except for condition A, which was tested at 400 °C with longer hold times, where a precipitation-hardening effect took place. The S-N curves showed that the fatigue strengths of each condition in the short-life regime were decreased with an increase in temperature. In the long-life regime, the fatigue strengths of condition A at 400 °C were greater than those at lower temperatures as a result of an in-situ precipitation-hardening effect. The fatigue strengths of condition H900 in the long life regime at 300 °C were superior to those at lower temperatures, due to the mechanisms of surface oxidation and thermal activation of dislocations. Fractography observations indicated that a shift of fatigue fracture from surface to internal crack initiation occurred at higher temperatures (300 °C and 400 °C) with long fatigue lives.     

The crystallography of fatigue crack initiation in coarse grained astroloy at 20°C

The effects of crystallographic orientation on fatigue crack initiation has been examined for coarse-grained Astroloy at 20°C. Specimens were cycled by three-point bending at stress ranges between 5 and 95% of the proportional limit until fatigue cracks were detected. The crystallographic orientation of individual grains within which fatigue cracks initiated was determined by use of selected area electron channeling. Grains forming cracks were found to have surface normals near the 〈100〉, 〈011〉, and 〈113〉 directions. Conversely, grains which did not initiate cracks were not similarly grouped in orientation. Calculations of the Taylor factor using the Bishop-Hill approach revealed that fatigue crack initiation in Astroloy occurred at grains with low values of the Taylor factor.