The importance of surface layer on fatigue behavior of a Ti- 6AI- 4V alloy

The effects of precycling and surface removal on the fatigue life and fatigue limit of a Ti-6A1-4V alloy were investigated. It was shown that both the fatigue life and fatigue limit were strongly dependent on the severity of precycling. The fatigue limit lost its significance if the alloy was subjected to a precycling treatment with a high stress amplitude. Cycling with stress amplitude below the fatigue limit after precycling showed a dependence of the logarithmic number of cycles to failure on the fraction of prefatigue damage. The interdependence of fatigue life and fatigue limit to precycling history was attributed to microcrack formation, principally restricted to a surface layer of less than 100 μm. Depending on the severity of precycling and on the magnitude of the applied cyclic stress, the fatigue damage could be either partially or totally eliminated by surface removal. The α/β interphase region of the surface layer appeared to offer preferred sites for dislocation pile-ups and crack initiation.     

Effects of microstructure on the strength and fatigue behavior of a silicon carbide fiber-reinforced titanium matrix composite and its constituents

The results of a systematic study of the effects of microstructure on the strength and fatigue behavior of a symmetric [0/90]_2s Ti-15Al-3Cr-3Al-3Sn/SiC (SCS-6) composite are presented along with relevant information on failnure mechanisms in the composite constituents, i.e., the interface, fiber, and matrix materials. Damage micromechanisms are elucidated via optical microscopy, scanning electron microscopy (SEM), and nondestructive acoustic emission (AE) and ultrasonic techniques. Composite damage is shown to initiate early under cyclic loading conditions and is dominated by longitudinal and transverse interfacial cracking. Subsequent fatigue damage occurs by matrix slip band formation, matrix and fiber cracking, and crack coalescence, prior to the onset of catastrophic failure. However, the sequence of the damage is different in material annealed above or below the β solvus of the Ti-15-3 matrix material. Mechanistically based micromechanics models are applied to the prediction of the changes in modulus induced by fatigue damage. Idealized fracture mechanics models are also employed in the prediction of the fatigue lives of smooth specimens deformed to failure at room temperature. The article highlights the potential to develop mechanistically based predictive models based on simplified mechanics idealizations of experimental observations.     

Creep-fatigue life prediction of in situ composite solders

Eutectic tin-lead solder alloys subjected to cyclic loading at room temperature experience creep-fatigue interactions due to high homologous temperature. Intermetallic reinforcements of Ni₃Sn₄ and Cu₆Sn₅ are incorporated into eutectic tin-lead alloy by rapid solidification processes to formin situ composite solders. In this study, thein situ composite solders were subjected to combined creep and fatigue deformation at room temperature. Under cyclic deformation, the dominant damage mechanism ofin situ composite solders is proposed to be growth of cavities. A constrained cavity growth model is applied to predict creep-fatigue life by taking into account the tensile loading component as well as the compressive loading component when reversed processes can occur. An algorithm to calculate cavity growth in each fatigue cycle is used to predict the number of fatigue cycles to failure, based on a critical cavity size of failure. Calculated lives are compared to experimental data under several fatigue histories, which include fully reversed stress-controlled fatigue, zero-tension stress-controlled fatigue, stress-controlled fatigue with tension hold time, fully reversed strain-controlled fatigue, and zero-tension straincontrolled fatigue. The model predicts the creep-fatigue lives within a factor of 2 with the incorporation of an appropriate compressive healing factor in most cases. Discrepancy between calculated lives and experimental results is discussed. This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the joint TMS-SMD/ASM-MSD Composite Materials Committee.     

Air fatigue of notched 1018 steel in the endurance limit range

Reverse bend, constant deflection fatigue tests were performed in ambient temperature air on notched 1018 steel specimens. Endurance limit for these exhibited scatter in excess of 25 pct, and this was correlated with concentration of manganese sulfide inclusions at the notch root vicinity. For specimens designated as “high inclusion” the endurance limit was 95 MN/m² (14 ksi); and for “low inclusion” ones it was 130 MN/m² (19 ksi). By monitoring crack length as a function of number of cycles it was observed that unbroken specimens tested above approximately 85 MN/m² (12.5 ksi) contained nonpropagating cracks. Also, high inclusion specimens fatigued at stresses above their endurance limit, but less than about 140 MN/m² (20 ksi) exhibited fatigue crack hesitation; that is, cracks initiated and grew to microscopic size but then arrested for a finite number of subsequent cycles. The endurance limit for high inclusion specimens was then a stress above which arrested cracks repropagate and below which they do not. Repropagation of hesitant fatigue cracks is projected to result from inclusion related cyclic stress damage near the crack tip, which was possibly stimulated by atmospheric interactions.     

Thermomechanical Fatigue Behavior of the Intermetallic <Emphasis Type="Italic">γ</Emphasis>-TiAl Alloy TNB-V5 with Different Microstructures

The cyclic deformation and fatigue behavior of the γ-TiAl alloy TNB-V5 is evaluated under thermomechanical load for three different microstructures. For this purpose, strain-controlled thermomechanical fatigue (TMF) tests were carried out with different temperature-strain cycles, different temperature ranges from 400 °C to 800 °C (673 K to 1073 K), and with two different strain ranges to set a fatigue-life relation. Cyclic deformation curves, stress-strain hysteresis loops, and fatigue lives of the tests are presented. The microstructures near-gamma (NG) and duplex (DP) show comparable fatigue lives under all test parameters. The microstructure fully-lamellar (FL) offers longer fatigue lives at the same loading conditions. For a general life prediction, the damage parameter of Smith, Watson, and Topper, P _SWT vs fatigue life, is well suitable, if the testing and the application temperature ranges, respectively, include temperatures above the ductile-brittle transition (approximately 750 °C). In the completely brittle material behavior regime the quality of the lifetime prediction is unacceptable. The damage parameter P _HL by Haibach and Lehrke shows a comparable correlation to the fatigue life as P _SWT. The results are discussed with microstructural investigations.     

Effect of grain size, cold work, loading frequency and temperature on fatigue limit of mild steel

Fatigue tests are performed on mild steel (0.07 pct ‡C) having different grain sizes or 5 and 10 pct cold work at frequencies of 83.3 Hz (LF) and 23 kHz (HF) at room and elevated temperatures. The fatigue limit was found to be a function ofd ^−1/2, whered is grain size, for HF loading at different temperature conditions. The ratio between fatigue limits determined at various frequencies and different temperatures during testing was found not to be strongly dependent on grain size. Prior cold work increased the HF fatigue limit atRT. HF tests at room temperature displayed higher fatigue limits than for LF loading. Increases in temperature during HF tests significantly decreased the fatigue limit for different grain sizes and various amounts of cold work. These results are explained by physical-metallurgical models.     

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.     

An analysis of non-propagating fatigue cracks

An analysis for the formation of nonpropagating fatigue cracks at (the base of V-shaped) notch roots, based on the considerations of the extent of the critically stressed region ahead of a notch or a crack tip, and the resulting volumetric strength effect, is developed. Assuming that the minimum local cyclic stress required for crack initiation from a notch root is equal to the unnotched fatigue limit, σ_e, and that the minimum local cyclic stress required for the propagation of the crack is equal to the theoretical strength of the material, σ_e, a model of notch fatigue limit is proposed that shows that nonpropagating cracks should form at the notch base if ρ≤ ρ⁰, a critical root radius, provided the notch is sufficiently deep,i.e. d ≥ ρ₀. The radius ρ₀ is a material constant and can be estimated from known material properties. The estimated values of ρ₀ are in fairly good agreement with available experimental values for steels and pure copper. For stresses near the notch fatigue limit it is suggested that p₀ be regarded as a radius above which notch fatigue limit is essentially initiation controlled and below which essentially propagation controlled. The notch fatigue limit based on complete fracture can then be estimated more accurately with mild as well as sharp notches. D. N. LAL, formerly a Graduate Assistant in Materials Science, Syracuse University     

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.     

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.     

The Study of Fatigue Behaviors and Dislocation Structures in Interstitial-Free Steel

There are three types of cyclic hardening for cyclically deformed interstitial-free (IF) steels. The magnitude of cyclic hardening was unobvious and dislocation cells smaller than 2 μm were very hard to find when total strain amplitude (Δε/2) was controlled to within 0.1 pct. When Δε/2 is increased to 0.125 to 0.3 pct, secondary cyclic hardening takes place prior to fatigue failure. Δε/2 = 0.6 pct, following an initial rapid-hardening stage. Dislocation cells smaller than 2 μm tend to develop near grain boundaries and triple junction of the grains while cycling just above Δε/2 = 0.125 pct. Such dislocation development results in secondary hardening. However, no failure occurs if cycling just below Δε/2 = 0.1 pct; hence, the fatigue limit for IF steel should be very close to Δε/2 = 0.1 pct.     

The effect of ballistic impacts on the high-cycle fatigue properties of Ti-48Al-2Nb-2Cr (atomic percent)

The ability of γ-TiAl to withstand potential foreign object damage (FOD) and/or domestic object damage (DOD) is a technical risk to the implementation of γ-TiAl in low-pressure turbine (LPT) blade applications. The overall purpose of the present study was to determine the influence of ballistic impact damage on the high-cycle fatigue strength of γ-TiAl-simulated LPT blades. Impact and specimen variables included ballistic impact energy, projectile hardness, impact temperature, impact location, and leading-edge thickness. The level of damage induced by the ballistic impacting was studied and quantified on both the impact (front) and backside of the specimens. Multiple linear regression was used to model the cracking and fatigue response as a function of the impact variables. Of the impact variables studied, impact energy, had the largest influence on the response of γ-TiAl to ballistic impacting. Backside crack length (BSCL) was the best predictor of remnant fatigue strength for low-energy impacts (<0.74 J), whereas Hertzian crack length (HCL) (impact side damage) was the best predictor for higher-energy impacts. The impacted γ-TiAl samples displayed a classical mean stress dependence on the fatigue strength. For the fatigue design stresses of a sixth-stage LPT blade in a GE90 engine, a Ti-48Al-2Nb-2Cr LPT blade would survive an impact of normal service conditions.     

The effect of impact damage on the room-temperature fatigue behavior of γ-TiAl

The relationship between impact damage and the fatigue behavior of γ-TiAl has been examined. Axial fatigue specimens fabricated from cast Ti-47.9Al-2.0Cr-1.9Nb (to be referred to as 48-2-2) and Ti-47.3Al-2.2Nb-0.5Mn-0.4W-0.4Mo-0.23Si (to be referred to as WMS) alloys were damaged by impact under controlled conditions with a 60 deg wedge-shaped indenter to simulate assembly-related damage in low-pressure turbine blades. The level of damage produced was quantified and found to correlate well with the peak load of the impact event. The WMS alloy exhibited a greater resistance to impact damage due to its higher yield strength and lamellar microstructure. A measure of the ambient-temperature fatigue failure stress in the alloys was obtained by standard fatigue testing employing a step-loading approach. The failure stress of the WMS alloy was greater than that of the 48-2-2 alloy in the undamaged state. The relationship between impact damage and failure stress was examined using a threshold-based approach. These studies indicate that, for damage levels below a transitional flaw size, the failure stress is near that for undamaged specimens. At damage levels greater than the transitional flaw size, the failure stress can be adequately approximated using the threshold stress-intensity range (ΔK _TH ) from long-crack growth testing. Fractographic studies were performed to investigate impact damage and crack-advance mechanisms, which match those observed in other alloys tested at room temperature.     

The influence of aqueous environments on low δK and high ΔK fatigue crack propagation behavior in low carbon structural steels

The influence of aqueous environments on fatigue crack propagation behavior was investigated for two types of structural steel (SB42 and HT80) in pure and 3 pct NaCl water under freely corroding conditions. In the intermediate to high ΔK region, fatigue crack propagation rates were higher in both aqueous environments and in 1 atm hydrogen than in air for both types of steel, and the acceleration effect increased power functionally with decreasing frequency from 5 to 0.0005 Hz. Such a crack growth acceleration property was explained by the mechanism of cyclically induced hydrogen embrittlement, as shown by the brittle striations formed on the fracture surfaces. On the other hand, in the lower ΔK region, both aqueous environments inversely suppressed crack growth and enhanced the threshold stress intensity factor range ΔK _th just above the ΔK _th in air, while only in aerated 3 pct NaCl water was the crack observed to grow even under the condition below the ΔK _th in air, not showing the threshold. Probable mechanisms for such fairly complex environmental effects were also suggested.     

Low-cycle fatigue behavior of INCONEL 718 superalloy with different concentrations of boron at room temperature

Symmetrical push-pull low-cycle fatigue (LCF) tests were performed on INCONEL 718 superalloy containing 12, 29, 60, and 100 ppm boron (B) at room temperature (RT). The results showed that all four of these alloys experienced a relatively short period of initial cyclic hardening, followed by a regime of softening to fracture at higher cyclic strain amplitudes (Δɛ _t /2≥0.8 pct). As the cyclic strain amplitude decreased to Δɛ _t /2≤0.6 pct, a continuous cyclic softening occurred without the initial cyclic hardening, and a nearly stable cyclic stress amplitude was observed at Δɛ _t /2=0.4 pct. At the same total cyclic strain amplitude, the cyclic saturation stress amplitude among the four alloys was highest in the alloy with 60 ppm B and lowest in the alloy with 29 ppm B. The fatigue lifetime of the alloy at RT was found to be enhanced by an increase in B concentration from 12 to 29 ppm. However, the improvement in fatigue lifetime was moderate when the B concentration exceeded 29 ppm B. A linear relationship between the fatigue life and cyclic total strain amplitude was observed, while a “two-slope” relationship between the fatigue life and cyclic plastic strain amplitude was observed with an inflection point at about Δɛ _p /2=0.40 pct. The fractographic analyses suggested that fatigue cracks initiated from specimen surfaces, and transgranular fracture, with well-developed fatigue striations, was the predominant fracture mode. The number of secondary cracks was higher in the alloys with 12 and 100 ppm B than in the alloys with 29 and 60 ppm B. Transmission electron microscopy (TEM) examination revealed that typical deformation microstructures consisted of a regularly spaced array of planar deformation bands on {111} slip planes in all four alloys. Plastic deformation was observed to be concentrated in localized regions in the fatigued alloy with 12 ppm B. In all of the alloys, γ″ precipitate particles were observed to be sheared, and continued cyclic deformation reduced their size. The observed cyclic deformation softening was associated with the reduction in the size of γ″ precipitate particles. The effect of B concentration on the cyclic deformation mechanism and fatigue lifetime of IN 718 was discussed.     

The Effects of Fatigue on the Atomic Structure with Cyclic Loading in Zr50Cu40Al10 and Zr60Cu30Al10 Glasses

The potential damage effect from fatigue on Zr bulk metallic glass alloys of Zr₅₀Cu₄₀Al₁₀ at the eutectic point and Zr₆₀Cu₃₀Al₁₀ away from the eutectic point (in atomic percent) is examined via the local atomic structure, which was obtained from the pair density function analysis of the synchrotron X-ray radiation and neutron data. Samples cut from the same rods were subjected to 10⁴, 10⁵, and 10⁶ compression cycles ex situ, and the evidence for fatigue damage was investigated by comparing alloys before and after cyclic loading. Bond orientation was observed particularly in Zr₅₀Cu₄₀Al₁₀, suggesting that fatigue damage occurs even in the elastic range, below the yield point, and during cyclic loading. The initiation of fatigue changes is observed first within small localized atomic regions.     

Microstructural examination of

Fatigue tests were performed to examine how microstructural conditioning influences crack initiation and propagation in SA508 class 3 low-carbon steel. A 3-mm-long crack was introduced in compact tension (CT) fatigue test specimens under four different loads in order to obtain crack tip plastic zones at different stress intensity factor ranges, ΔK = 18, 36, 54, and 72 MPa√m. The microstructure of the plastic zones around the crack tip were examined by trans- mission electron microscopy (TEM) and selected area electron diffraction (SAD). Micro- orientation of the dislocation cells in the plastic zones of all of the CT samples increased to 4 deg from the level of an as-received sample. Four-point bending fatigue tests were performed for plate shape samples with a large cyclic strain range. The SAD value of the bending samples was also 4 deg in the damaged area where cracks already initiated at an early stage of the fatigue process. These test results indicate that the microstructural conditioning is a prerequisite for the fatigue crack initiation and propagation in SA508. These observations may lead to better under- standing of how fatigue initiation processes transit to cracks.     

Effects of composition and microstructure on fatigue of γ/γ′-δ type eutectic alloys

Room temperature tension-tension fatigue tests were performed on two lamellar γ/γ′-δ alloys, one with 0 pct Cr and one with 6 pct Cr. The 6 pct Cr alloy was solidified at 3 cmJh while the 0 pct Cr alloy was solidified at 3 cm/h and 5.7 cm/h. Fatigue testing was done on both alloys in the as-directionally solidified condition and on the 0 pct Cr alloy after heat treatment. Increasing the growth speed of the 0 pct Cr alloy increased the fatigue life of the material at stresses above the 10⁷ cycle fatigue limit. Partial solution treating and aging of the 0 pct Cr alloy,R = 3 cm/h, increased the fatigue life relative to the as-directionally solidified material at high stresses, to the same extent as increasing the growth speed. Full solution treatment and aging of the 0 pct Cr alloy,R = 5.7 cm/ h, caused a reduction in the fatigue life relative to the as-directionally solidified material. Fatigue cracking tended to be faceted in the 6 pct Cr alloy as opposed to the more ductile failure of the 0 pct Cr alloy. Microstructural perfection, grain size and shape, interlamellar spacing, longitudinal cracking, and longitudinal and transverse ductility all are believed to have influenced the fatigue resistance of the alloys.     

Evaluation of a threshold-based model of the elevated-temperature fatigue of impact-damaged γ-TiAl

Step-loading fatigue tests have been conducted on two γ-TiAl alloys with differing microstructures following quasi-static indentations intended to simulate assembly-related impact damage to low-pressure turbine blades. Fatigue tests were conducted at 600 °C using computer-controlled servohydraulic loading at a frequency of 20 Hz. Reasonably good agreement was achieved between the fatigue data and calculated fatigue strength based on the fatigue threshold and measured impact severity. In certain cases, the fatigue threshold model fails to completely describe the data. These discrepancies may be related to residual stresses, variations in crack-shape morphology, and small-crack effects. Residual stresses could not be directly measured, given the small size of the damage zones. However, a comparison of fatigue threshold approximations based on a through-thickness crack geometry and a corner-crack geometry suggests that these two models may represent the upper and lower bounds of the actual fatigue behavior. In addition, the behavior of small cracks was examined by modeling the stress-lifetime response of lightly damaged specimens of the duplex alloy. This effort indicates the need for small-crack fatigue threshold values when designing fatigue-critical γ-TiAl components.