Elevated Temperature Fatigue Research Articles

An inspection of elevated temperature fatigue crack extension data in low alloy steels

The present paper describes a reasonable review of various high temperature fatigue crack growth data obtained from a range of low alloy steels under both continuous cycle conditions and those where a dwell time at maximum load had been imposed. The continuous cycle data all resided below the 1/2CTOD line and were found to be primarily frequency and Δ K 2 dependant with all crack extensions occurring in a transgranular fashion. The dwell time fatigue crack growth data could be reasonably portrayed in terms of maximum stress intensity K max, prevalent at the dwell portion of the fatigue cycle. Furthermore, it was established that the dwell time data fell roughly into two groups, viz., those for dwell times of 30 min or less and data with dwell times greater than this value. A comparison of similar dwell time results from various literature sources indicated reasonable commonality and it was observed that in the vast majority of cases: (1) fatigue crack growth rates resided above the 1/2CTOD line; (2) exhibited an approximately Δ K 10 relationship; and (3) fatigue crack growth was wholly intergranular in nature. Finally the existence of some critical stress intensity condition and/or cavity damage condition was discussed to account for the observed abrupt switch from continuum controlled fatigue crack growth with a Δ K 2 dependence to super-continuum growth characteristics which obeyed a Δ K 10 dependence.

A Tapered Double-Cantilever-Beam Specimen Designed for Constant-KTesting at Elevated Temperatures

AbstractA compact, constant stress intensity factor, side grooved, tapered double-cantilever-beam (T-DCB) specimen has been designed for measuring elevated temperature creep and fatigue crack growth rates. This specimen is much smaller than the standard DCB specimen generally used for studying corrosion fatigue in airframe materials and it can be easily accommodated in standard furnaces generally used in elevated-temperature fracture mechanics testing. The specimen possesses a constant-K region of 30 mm. This constant-K crack length range was first established analytically through detailed two- and three-dimensional finite-element analyses and the finite-element calculations were further verified experimentally through quantitative fractographic analysis using the striation measurement technique.

Elevated temperature fatigue crack growth under dwell conditions in Waspaloy

Growth of corner cracks at temperatures up to 700°C under cyclic (0.25 Hz), 300 s dwell at peak load, both at R = 0.1, and constant load conditions has been studied in the nickel base turbine disc alloy Waspaloy. No significant effect of dwell on fatigue crack growth rate (d a/d N) is found at 550°C, where transgranular fatigue cracking predominated. At 600 and 650°C, under dwell cycling a transition to entirely intergranular cracking is accompanied by a severe increase in cyclic crack growth rate. Contrary to the trend for 600–650°C, dwell and constant load data at 700°C shows lower crack growth rates than for 650°C, indicating that a time dependent microstructural change occurs during prolonged exposure at 700°C generating an increased resistance to intergranular cracking. Using the stress intensity range (†K) as a correlating parameter, fatigue crack growth rates are successfully predicted by linear summation of pure fatigue and time dependent data.

Mechanisms governing deformation and damage during elevated-temperature fatigue of an aluminum-magnesium-silicon alloy

The cyclic deformation and fracture characteristics of aluminum alloy 6061 are presented and discussed. The specimens were cyclically deformed using fully reversed tension-compression loading under total strain-amplitude control, over a range of temperatures. The alloy showed evidence of softening to failure at all test temperatures. The degree of softening during fully reversed deformation increased with test temperature. The presence of shearable matrix precipitates results in a microstructure that offers a local decrease in resistance to dislocation movement, causing a progressive loss of strengthening contributions to the matrix. At elevated temperatures, localized oxidation and embrittlement at the grain boundaries are exacerbated by the applied cyclic stress and play an important role in accelerating crack initiation and subsequent crack propagation. The fracture behavior of the alloy is discussed in light of competing influences of intrinsic microstructural effects, deformation characteristics arising from a combination of mechanical and microstructural contributions, cyclic plastic strain amplitude and concomitant response stress, and the test temperature.

Microstructural stability and mechanisms of fatigue and creep crack growth in Ti–24Al–11Nb

Titanium intermetallics are being developed for long term applications at elevated temperatures, particularly those alloys based on the alloys Ti3Al and TiAl. Typical approaches include the design of appropriate microstructures for room and elevated temperature fatigue and creep resistance. However, a little explored area is the stability of these microstructures at elevated temperature and its effect on fatigue crack growth. The present investigation documented the microstructural stability, fatigue crack behaviour, and stress rupture of Ti-24Al-11Nb, a Nb modified Ti3Al alloy. A coarse two phase α2+β Widmanstatten microstructure was found to exhibit the best resistance to fatigue crack growth. Microstructural stability and elemental segregation were studied as a function of exposure time for up to 500 h at 800°C using transmission electron microscopy (TEM). Results indicate that the Widmanstatten microstructure is metastable and the β phase breaks up into particles. The absence of a continuous β phase surrounding the α2 phase reduces the resistance of the microstructure to fatigue crack growth at room temperature. At elevated temperature the microstructure stability does not play a role in determining the fatigue resistance. A fine Widmanstatten microstructure has the best resistance to creep deformation. Stress rupture tests were conducted in vacuum and air at 649°C and 760°C. Two types of failure mechanisms were seen in stress rupture; these include transgranular and intergranular failure within prior β grains. When tested in air at 760°C a combination of transgranular and intergranular failure occurred. Specimens that exhibited a higher proportion of transgranular failure had longer lives. When tested in vacuum at 760°C the predominant failure mode was intergranular. At 760°C extensive microstructural changes like breakup and spherodization of the β phase occurred under stress while the rate of coarsening without any stress was much slower. At 649°C the specimens tested in vacuum consistently exhibited longer lives. The creep crack growth when tested in air at 649°C was always a brittle transgranular mode while the specimens tested in vacuum always failed by an intergranular mode.

A Study on Elevated Temperature Fatigue Crack Growth Using Round Bar Specimen with a Surface Crack

The compact tension specimen geometry has been widely used for measuring fatigue crack growth rates at elevated temperature when the fatigue load is under tension/tension condition. However, most of the elevated temperature components which have significant crack growth life experience fatigue load under tension/compression conditions. Thus test techniques are required since the compact tension specimen cannot be used for tension/compression loading. In this paper, a simplified test procedure for measureing fatigue crack growth rates is proposed, which employs a round bar specimen with a small surface crack. Fatigue crack growth rates under tension/ tension loading conditions at elevated temperature were measured according to the proposed procedure and compared with those previously measured by C/(T) specimens. Since both the measured crack growth rates were comparable, the fatigue crack growth rates under tension/ compression load can be reliably measured by the proposed procedure. For monitoring crack depth. DC electric potential method is employed and an optimal probe location and current input conditions were proposed.

Arrest Function of Elevated-Temperature Fatigue Short Crack Growth from Notch Root for Metallic Materials Containing Dispersed Y2O3 or Pb Particles.

An arrest function of fatigue short crack growth from notch root at 200 and 550°C in air was investigated for Fe-20Cr alloy containing 1.1 weight percent of dispersed Y2O3 particles and for SUS403 (Fe-12Cr) and SUS304 (Fe-18Cr-8Ni) stainless steels containing 0.2 weight percent of dispersed Pb particles. The main conclusions obtained are as follows. The fatigue Iimits at the high temperatures were 40∼60% higher for the alloy containing dispersed Y2O3 particles and the stainless steels containing dispersed Pb perticles than for their base metals. Above the fatigue limit, the fatigue life was longer for the materials containing dispersed Y2O3 or Pb particles than for their base metals, because the growth rate of a fatigue short crack, which was initiated from the notch root, was low. Those improvements in high-temperature fatigue properties were explained, considering that the presence of Y2O3 or Pb particles strengthened the oxide film at the notch root or at the crack tip of a short crack. Accordingly, we concluded that Y2O3 and Pb dispersed particles show an arrest function of fatigue short crack growth at elevated temperatures.

Fractographic interpretation of failure mechanisms in titanium matrix composites

Titanium matrix composites (TMC) offer a combination of good mechanical properties and high temperature durability that make them attractive candidate materials for advanced engine components and high temperature structural applications. In such applications the material will be subjected to changing mechanical loads and temperature fluctuations, resulting in complex stress states within the constituents of the composite. This study examines how the various loading conditions on the TMCs are reflected in the fracture behaviour to gain insight into the damage mechanisms active in these materials. A fractographic study was conducted on several different TMC specimens, fabricated from Ti-15-3 and Timetal 21S alloys and SCS-6 fibres, that have been subjected to various thermomechanical loading conditions. The analysis showed that the Ti-15-3 composites were more susceptible to damage during sustained load at elevated temperature than the Timetal 21S composites. For both materials, striations only appear during elevated temperature fatigue when the residual processing stresses are relieved. During the Generic Hypersonic Flight Profile (GHFP) tests, the higher temperatures of the Mission 1 profile induce more damage in the Timetal 21S composites. The striations appear in the composites containing centre holes, unlike the unnotched specimens, indicating that the combined effect of stress concentration due to the hole and an underdeveloped fibre bridging zone may have resulted in crack closure.

An approach to fatigue life modeling in titanium-matrix composites

A review of the procedures developed by the author and his colleagues over the last several years for predicting elevated-temperature fatigue life of metal-matrix composites is presented. Modeling approaches involve concepts of both linear and non-linear summation of damage from cycle-dependent as well as time-dependent mechanisms. The analyses, further, treat the micromechanical stresses in the constituents as parameters in the life prediction models. The material characterized is SCS-6/Timetal®21S, a metastable beta titanium alloy reinforced with continuous SiC fibers. Modeling is applied to isothermal fatigue at different frequencies and temperatures, and thermomechanical fatigue (TMF) under both in-phase and out-of-phase loading conditions at different temperature ranges and maximum temperatures. Experimental data are used as the basis for determining the parameters embedded in the models. The numerical results, in turn, provide insight into the dominant mechanisms controlling fatigue life under a given condition. The capability to correlate experimental data from a wide variety of test conditions for several versions of a damage summation model is demonstrated.

Arrest Function in Elevated-Temperature Fatigue Crack Propagation for Stainless Steels with Dispersed Lead Particles.

The arrest function in fatigue crack propagation at 25∼550°C in air was investigated, using SUS 304 (18 Cr-8 Ni) and SUS 403 (12 Cr) stainless steels containing 0.2 weight percent dispersed lead particles. The Pmax-constant ΔK-decreasing test, where the minimum load, Pmin, increased with increasing crack length while the maximum load, Pmax, remained constant, was employed in order to avoid crack closure. The main conclusions obtained are as follows : (1) Fatigue crack propagation properties including the threshold were independent of the material at room temperature under the closure-free condition. Fatigue threshold values at temperatures higher than the load melting point of 327°C were about 40% higher for lead-particle-dispersed steels than for host steels. (2) The increase in threshold level at elevated temperatures was explained, considering that the oxide layer spalled at the fatigue crack tip was repaired by the molten lead. Accordingly, we concluded that dispersed lead particles could show the arrest function of the fatigue crack propagation at elevated temperatures.

Observation of Elevated-Temperature Fatigue Surfaces for Stainless Steel with Dispersed Lead Particles by an AFM/STM Hybrid System.

Fatigue threshold for SUS304 stainless steel containing 0.2 weight percent dispersed lead particles was about 40% higher for the host steel at test temperatures above 200°C. This was explained by considering that dispersed lead particles could control or arrest the fatigue crack growth. In this study, using an AFM/STM hybrid system which enables one to obtain topographic and current images at the same time, fatigue surfaces for SUS304 steel with lead particles were examined to clarify the controlling mechanisms in the fatigue crack growth. Topographic and current images showed that the oxide layer spalled at the fatigue crack tip was repaired by the molten lead at test temperatures higher than 200°C. Accordingly, we concluded that dispersed lead particles controlled the fatigue crack growth at elevated temperatures.

High‐Temperature Crack Growth in Monolithic and SiCw‐Reinforced Silicon Nitride under Static and Cyclic Loads

Experimental results are presented on subcritical crack growth under sustained and cyclic loads in a HIPed Si3N4 at 1450°C and a hot–pressed Si3N4–10 vol% SiCw composite in the temperature range 1300°–1400°C. Static and cyclic crack growth rates are obtained from the threshold for the onset of stable fracture with different cyclic frequencies and load ratios. Fatigue crack growth rates for both the monolithic and SiCw‐reinforced Si3N4 are generally higher than the crack growth velocities predicted using static crack growth data. However, the threshold stress intensity factor ranges for the onset of crack growth are always higher under cyclic loads than for sustained load fracture. Electron microscopy of crack wake contact and crack–tip damage illustrate the mechanisms of subcritical crack growth under static and cyclic loading. Critical experiments have been conducted systematically to measure the fracture initiation toughness at room temperature, after advancing the crack subcritically by a controlled amount under static or cyclic loads at elevated temperatures. Results of these experiments quantify the extent of degradation in crack–wake bridging due to cyclically varying loads. The effects of preexisting glass phase on elevated temperature fatigue and fracture are examined, and the creep crack growth behavior of Si3N4–based ceramics is compared with that of oxide‐based ceramics.

An overview of elevated temperature damage mechanisms and fatigue behavior of a unidirectional SCS-6/Ti-15-3 composite: Castelli, M.G. and Gayda J. (NASA Lewis Research Center) NASA Technical Memorandum TM-106131 (1993) 16p

A review of the procedures developed by the author and his colleagues over the last several years for predicting elevated-temperature fatigue life of metal-matrix composites is presented. Modeling approaches involve concepts of both linear and non-linear summation of damage from cycle-dependent as well as time-dependent mechanisms. The analyses, further, treat the micromechanical stresses in the constituents as parameters in the life prediction models. The material characterized is SCS-6/Timetal®21S, a metastable beta titanium alloy reinforced with continuous SiC fibers. Modeling is applied to isothermal fatigue at different frequencies and temperatures, and thermomechanical fatigue (TMF) under both in-phase and out-of-phase loading conditions at different temperature ranges and maximum temperatures. Experimental data are used as the basis for determining the parameters embedded in the models. The numerical results, in turn, provide insight into the dominant mechanisms controlling fatigue life under a given condition. The capability to correlate experimental data from a wide variety of test conditions for several versions of a damage summation model is demonstrated.

Elevated temperature fatigue crack growth in alloy 718. I. Effects of mechanical variables: Ghonem, H., Nicholas, T. and Pineau, A. Fatigue Fract Eng mater Struct (May 1993) 16 (5), 565–576

The experiments reported here were conducted using the framework of the strain-space formulation of plasticity. A servohydraulic multiaxial testing machine was used to perform combined axial-torsional experiments on a thin-walled cylindrical aluminum specimen. The specimen was machined from extruded aluminum 1100 tubing, and annealed at 650 °F for 1 h. Conical epoxy sections were molded on the ends of the specimen for gripping. The specimen was loaded at strain rates under 20×10−6 per minute. An offset strain of 5×10−6 was used to define yield. A novel technique for identifying yield surfaces is introduced, which makes use of an experimentally measured focal point through which all yield probes are made to pass. Yield surfaces were obtained both in stress space and in strain space at various stages of plastic deformation. Although all strains were under 1%, the inelastic states included situations in which the origin in stress space lies outside the elastic region. The yield surfaces in both spaces were found to change shape considerably with deformation, usually developing a region of high curvature near the preload point and a region of low curvature on the opposite side of the yield surface. Experiments were also designed and conducted to test the validity of a prescription for identifying plastic strain in the context of the strain-space formulation. Changes in plastic strain were induced under incremental loading conditions and measured using the prescription. To within experimental error, the measured plastic strain increments satisfy the normality condition. The plastic strain increments are consistent with a flow rule of the type postulated in the strain-space formulation of the theory. The experimental results lend support to the constitutive theory and to the prescription for identifying plastic strain.

Effect of prior deformation on the elevated temperature fatigue behaviour in nimonic PE16 alloy

It is well known that prior deformation (either monotonic or cyclic) affects the subsequent fatigue behavior of some metals and alloys. A study of this history dependence of cyclic deformation behavior is important, because in engineering application, components quite often experience varying loads and temperatures. Results reported so far indicate that the fatigue behavior of materials, with low/intermediate stacking fault energy, depends very much on the prior history of the sample whereas the behavior of the wavy slip mode materials is not so dependent. The authors chose for the present investigation, a nickel base superalloy, Nimonic PE16, strengthened by the precipitation of [gamma][prime] particles (Ni[sub 3]Al,Ti), having spherical morphology, low volume fraction and uniform distribution in the austenite matrix. This paper reports the results obtained on the effect on prior room temperature cyclic deformation on the elevated temperature fatigue behavior of this alloy, corresponding to two initial microstructural conditions. The results are interpreted in the light of the present understanding of the mechanism of fatigue deformation and the results of transmission electron microscopic (TEM) examination of fatigued samples.

Development of Tial-Based Automotive Engine Valves

AbstractThe application of TiAl-based alloys as an exhaust valve material would allow automotive engines to operate at higher temperatures with increased efficiencies. Development of these materials at Ford initially concentrated on the Ti-48A1-1V (at%) system. This included; 1) room and elevated temperature fatigue, 2) creep and 3) tensile ductility optimization. Results from this test program in conjunction with other available data, previous ceramic experience and limited engine testing led to the conclusion that the major roadblock to implementation was not optimizing mechanical properties, but rather low cost and reliable valve manufacturing technology. When a cost effective manufacturing technology is developed, then the emphasis will shift to ensuring the product durability. Hence, the focus of the current program is the development of valve manufacturing technologies, in particular valve blank fabrication. Currently, casting appears to be the lowest cost alternative for valve blank fabrication. This paper reviews the technology development process as it pertains to TiAl-based valves.

Elevated Temperature Fatigue Behavior of Tungsten Fiber Reinforced Superalloy Composites

The low-and high-cycle fatigue behavior of three different fiber-reinforced superalloy (FRS) composite materials were evaluated. Each composite material contained 40 vol% unidirectionally aligned continuous length tungsten-1.5 wt% ThO2 fibers. The metal matrices were Waspaloy, Type 316L stainless steel, and Incology 907. Fatigue tests were conducted at 871°C in a helium atmosphere under load control with a load ratio (minimum stress/maximum stress) of 0.2. The composites were found to have excellent elevated temperature fatigue strength. The fibers played a major role in controlling the fatigue strength of the composites. Fatigue crack fronts were found to be retarded by the fibers. The cracks tended to branch at the fiber/matrix interface and grew by a sliding mode along the interface. Matrix surface cracks induced by thermal shock damage had little influence on the fatigue strength of the FRS composites.

Environmentally assisted crack extension during fatigue cycling in steels—Some fractographic observations

The present paper describes the relationships that existed between the extent of environmentally assisted crack (EAC) growth and the fatigue fractographic features prevalent at ambient and elevated temperatures in steels. At both ambient and elevated temperatures the extent of EAC growth was uniquely related to the amount of static failure on the fatigue fracture surface. Under ambient conditions mainly intergranular decohesion and transgranular cleavage were recorded, while at elevated temperatures the static failure modes were fan-shaped facet formation and, to a much lesser extent, microvoid coalescence. A selection of models dealing with EAC growth effects was assessed and it was established that Congleton's model could best describe both the crack growth effects and the fractographic feature recorded under both ambient and elevated temperature fatigue conditions, viz.: da dn ENV da dn MECH = 1 (1-ƒc) m where c is the area fraction of static fracture mode; and f and m are constant. Finally, the various mechanisms which promote EAC growth were assessed and it was observed that the degree of EAC growth was only dependent upon the extent of static failure on the fatigue surfaces and not on the type of static failure mode.

Arrest Function in Elevated Temperature Fatigue Crack Propagation for Y2O3-Dispersed Alloys.

The arrest function in fatigue crack propagation at 25 and 550°C in air was investigated for Fe-20Cr alloys containing dispersed oxide particles of Al2O3, ZrO2 and Y2O3 in weight percents from 0.3 to 1.2. The Pmax-constant ΔK-decreasing test, where the minimum load, Pmin, increased with increasing crack length while the maximum load, Pmax, remained constant, was employed in order to avoid crack closure. The main conclusions obtained are as follows : (1) Fatigue crack propagation properties including the threshold were independent of the material at room temperature under the closure-free condition. The fatigue threshold values at 550°C were higher than those at room temperature for all materials. In addition, the threshold values at 500°C were about 50% higher for Y2O3-dispersed alloys than those for the base alloy and Al2O3-or ZrO2-dispersed alloys. (2) The increase of threshold level at 550°C for Y2O3-dispersed alloys was explained in terms of suppression of the spalling of the oxide layer formed at the fatigue crack tip by dispersed Y2O3 particles. Accordingly, we concluded that Y2O3 dispersion could show the arrest function of the fatigue crack propagation at elevated temperatures.

The effect of temperature on the behaviour of short fatigue cracks in Waspaloy using an in situ SEM fatigue apparatus

Short fatigue crack growth behaviour was studied in Waspaloy at 25°C, 500°C and 700°C under cyclic loading conditions at a stress ratio of 0.1. Specimens were tested within an elevated temperature fatigue apparatus coupled to a scanning electron microscope which allowed in situ observations to be made of the fatigue process. Crack formation was dominated by slip band cracking at 25°C and 500°C, while at 700°C cracks formed along slip bands and twin boundaries. Crack propagation proceeded by means of slip band cracking at 25°C and 500°C, while at 700°C crack growth was primarily stage II-mode I type growth. Short fatigue crack growth rates were faster at 500°C than at 25°C for an equivalent stress intensity range, ΔK 1, owing to a change in slip character. Short crack growth rates tended to be lower at 700°C than at 500°C, owing to changes in the material leading to precipitate coarsening. As the temperature was increased, microstructural contributions to crack growth decreased, resulting in a reduced microstructural short crack effect.