Stephen D. Antolovich

Low cycle fatigue, fatigue crack propagation and substructures in a series of polycrystalline Cu-Al alloys

Low Cycle Fatigue (LCF) on smooth hour glass specimens and Fatigue Crack Propagation (FCP) studies on Single Edge Notch (SEN) specimens were carried out at room temperature on four Cu-Al polycrystalline alloys to investigate the effects of Stacking Fault Energy (SFE) and mechanical property variations on fatigue characteristics. Significant improvements in fatigue properties were observed for alloys of low SFE. A microhardness technique was used to delineate the fatigue plastic zone ahead of stopped cracks at several stress intensity ranges for all the alloys. Planar slip was associated with a less than a second power dependence of plastic zone size on the stress intensity range. Transmission Electron Microscopy (TEM) was used to observe the substructures that developed both in LCF at different strain ranges and also ahead of fatigue cracks at different stress intensity ranges. Fractography was carried out to study the micromechanisms of crack propagation using a two stage replication technique. The experimental results were in good agreement with a theoretical model for FCP developed previously by the authors which incorporates mechanical and microstructural variables.

Low cycle fatigue behavior of René 80 at elevated temperature

Low cycle fatigue of René 80 was studied at 871 and 982 °C. It was found that when the data were represented on the basis of plastic strain, the life increased with decreasing frequency and imposition of a 90 s hold at maximum strain. Transmission electron microscopy studies showed that the ′ coarsened and an interfacial array of edge dislocations developed. The density of dislocations in the matrix was very low. Light optical microscopy revealed that cracks generally inititiated at oxide spikes in surface connected grain boundaries. A crack initiation criterion based on the maximum stress and oxide depth at the time of crack initiation was found to represent the data very well. Based on that representation, an expression for the initiation fatigue life was developed. That expression includes temperature, frequency and cyclic stress strain parameters as variables.

A study of fatigue damage mechanisms in Waspaloy from 25 to 800 C

Abstract The low cycle fatigue behavior of Waspaloy was studied as a function of heat treatment and temperature. When Waspaloy was heat treated to contain coarse grains (ASTM size 3) and small γ′ particles (50–80A), deformation took place as a result of shearing of the precipitates by dislocations whereas, when Waspaloy was heat treated to contain fine grains (ASTM size 9) and large γ′ particles (about 900 A), deformation occurred by Orowan looping. At temperatures up to and including 500°C, cracks initiated transgranularly. For the coarse-grained material, cracks formed along the well-defined slip bands on {111} planes. At 500°C there was an anomalously high dislocation density for both heat treatments and continual hardening during testing which is indicative of either dynamic strain aging or precipitation on dislocation lines. At 700 and 800°C the coarse-grained small-γ′-particle material exhibited carbide precipitation on grain boundaries, twin boundaries and slip bands. The carbide precipitation, aggressive environment and planar slip all combined to give an increasing amount of intergranular fracture until at 800°C the fracture surface was predominantly intergranular and the life was substantially reduced. The fine-grained large-γ′-particle alloys were stable at all temperatures and contained grain boundary carbides as a result of the initial heat treatment. At high temperatures, failure also occurred by an intergranular mechanism for this heat treatment and the life was correspondingly reduced. The total life at a given strain range was independent of heat treatment. This was interpreted as being due to differences in the initiation and propagation lives. Based on our results, suggestions are made for improving both the low and the high temperature fatigue lives.

A model for fatigue crack propagation

Abstract A model for fatigue crack propagation is presented which incorporates low cycle fatigue, mechanical properties and a microstructurally-associated process zone. Comparison of the model to published date for 4340 (hard and soft), a series of TRIP steels, Ti-6A1-4V, 2024-T6 and 300 grade maraging steel shows good agreement.

Yielding and deformation behavior of the single crystal superalloy PWA 1480

Interrupted tensile tests were conducted to fixed plastic strain levels on (001) oriented single crystals of the nickel-base superalloy PWA 1480. Testing was done in the range from 20 to 1093 °C, at strain rates of 0.5 and 50 pct/min. The yield strength was constant from 20 to 760 °C, above which the strength dropped rapidly and became a strong function of strain rate. The data could be represented very well by an Arrhenius-type equation, which resulted in three distinct temperature regimes. The deformation substructures could also be grouped in the same three regimes, indicating that there was a fundamental relationship between the deformation mechanisms and the activation energies. At low temperatures, the activation energy for yielding was zero, and the deformation was dominated by γ′ shearing by pairs of 111a/2(110) dislocations. At high temperatures, the true activation energy for yielding was calculated to be 500 kJ/mol, which is indicative of a diffusion-controlled process, and deformation was dominated by γ′ by-pass. Intermediate temperatures exhibited transitional behavior. No currently available precipitation hardening model could adequately describe the behavior observed in the low temperature regime, due to the observation that penetration into the precipitate was not rate-limiting at all temperatures. In the high temperature regime, the functional form of the Brown-Ham by-pass model fit the data fairly well. The results of this study also demonstrated that the initial deformation mechanism was frequently different from that which would be inferred by examination of specimens which had been tested to failure.

On the toughness increment associated with the austenite to martensite phase transformation in TRIP steels

AbstractThe fracture toughness of a high carbon TRIP alloy, deformed approximately 75 pct at 460°C was investigated over a range of temperatures from −196°C to 200°C. Two distinct temperature regimes were present: a low temperature regime where martensite formed during fracturing and a high temperature regime where no martensite formed. The toughness values of the low temperature regime were higher than the extrapolated values of the high temperature regime indicating that the transformation makes a positive contribution to the fracture toughness of TRIP alloys. For the alloys used in this investigation the room temperature plane strain fracture toughnessKIC was on the order of 95 ksi

Low cycle fatigue of René 77 at elevated temperatures

\sqrt {in}

Increased fracture toughness in a 300 grade maraging steel as a result of thermal cycling

or in terms of the crack extension forceGIC, 274 in.-lb per sq in. The fracture mode was cleavage and the extraordinary toughness for this mode of crack extension is attributed to the energy absorbed by the simultaneous phase transformation. The contribution due to the phase transformation was determined to be in the range 37 to 57