Time-Dependent Fatigue Crack Propagation Behavior of Two Solid-Solution-Strengthened Ni-Based Superalloys—INCONEL 617 and HAYNES 230

The fatigue crack propagation (FCP) as well as the sustained loading crack growth (SLCG) behavior of two solid-solution-strengthened Ni-based superalloys, INCONEL 617 (Special Metals Corporation Family of Companies) and HAYNES 230 (Haynes International, Inc., Kokomo, IN), were studied at increased temperatures in laboratory air under a constant stress-intensity- factor (K) condition. The crack propagation tests were conducted using a baseline cyclic triangular waveform with a frequency of 1 3 Hz. Various hold times were imposed at the maximum load of a fatigue cycle to study the hold time effect. The results show that a linear elastic fracture mechanics (LEFM) parameter, stress intensity factor (K), is sufficient to describe the FCP and SLCG behavior at the testing temperatures ranging from 873 K to 1073 K (600 C to 800 C). As observed in the precipitation-strengthened superalloys, both INCONEL 617 and HAYNES 230 exhibited the time-dependent FCP, steady SLCG behavior, and existence of a damage zone ahead of crack tip. A thermodynamic equation was adapted to correlate the SLCG rates to determine thermal activation energy. The fracture modes associated with crack propagation behavior were discussed, and the mechanism of time-dependent FCP as well as SLCG was identified. Compared with INCONEL 617, the lower crack propagation rates of HAYNES 230 under the time-dependent condition were ascribed to the different fracture mode and the presence of numerous W-rich M6C-type and Cr-rich M23C6-type carbides. Toward the end, a phenomenological model was employed to correlate the FCP rates at cycle/time-dependent FCP domain. All the results suggest that an environmental factor, the stress assisted grain boundary oxygen embrittlement (SAGBOE) mechanism, is mainly responsible for the accelerated time dependent FCP rates of INCONEL 617 and HAYNES 230

High temperature fatigue crack growth in titanium microstructures

Titanium alloys are interesting to several industrial applications including transportation and biotechnology’s, aerospace designers having been particularly interested because of the combination of their fatigue and time-dependent mechanical properties, good formability, high specific properties due to a relatively low density, and high resistance to aggressive environmental and impact loads. Titanium based alloys represent 30% and 40% of the total mass in commercial and military engines, respectively, a considerable amount of this mass being devoted to high temperature rotating disc and blade components

Bridging fiber stress in metal matrix composites - An analytical model

A modified shear-lag model suitable for describing bridging fiber stress in metal matrix composites subjected to fatigue loadings has been developed. The model considers the influence of the stress field in the fiber/matrix bonded zone as well as the ratio of the reversed to non-reversed sliding length within the interface debonding region of the fiber. Parameters representing the post-processing residual stress field and the load ratio of the applied loading cycle are also considered

Constant-probability crack growth curves

This paper details a stochastic, time-inhomogeneous model that serves as a theoretical basis for the prediction of crack growth and its variability under constant-amplitude loading. Crack evolution is described as a set of constant probability curves, each of whose points possess equal probability of advancing from one position to another forward position. This probability is governed by a transition intensity parameter for which two mathematical interpretations are examined. A simplified crack growth rate equation, employing one of the definitions, is derived and applied to A17075-T6 material for different loading conditions. Results of this application are compared with those experimentally obtained. © 1988

Frequency effects of fatigue crack growth behaviour in a near- alpha titanium alloy

The loading frequency effects on elevated temperature fatigue crack growth behaviour in a new near- alpha, beta-processed titanium alloy were examined in both air and vacuum environments. The individual influences of environment and creep are identified in terms of both the macroscopic crack growth rates and detailed fractographic observations as a function of the applied DELTAK. Results indicate the existence of two regions of transgranular crack growth: microstructurally sensitive and insensitive. (from authors\u27 abstract

Depth of intergranular oxygen diffusion during environment-dependent fatigue crack growth in alloy 718

The elevated-temperture fatigue crack growth behavior in alloy 718, when subjected to a loading frequency lower than the transitional frequency of this alloy, is viewed as fully environment dependent. In this process, the crack growth increment per loading cycle is assumed to be equal to the intergranular oxygen diffusion depth at the crack tip during the cycle effective oxidation time. In order to identify the trend of this diffusion depth an experimental program was carried out on compact tension specimens made of alloy 718 at 650 °C in which fatigue crack growth measurements were made for cyclic load conditions with and without hold time periods at minimum load level. This work resulted in establishing a relationship correlating the intergranular oxygen diffusion depth and the value of the stress intensity factor range ΔK. This relationship, when integrated over the cycle effective oxidation time, results in a closed-form solution describing the environment-dependent fatigue crack growth rate. A comparison is made between the results of this solution when applied to different loading frequencies and the corresponding experimental results. This comparison shows good agreement between the two sets of results. Furthermore, by combining the parabolic rate law of diffusion and the equation for the intergranular oxygen diffusion depth, an explicit expression for the oxygen diffusivity of grain boundaries is derived. It is found that this diffusivity is both a ΔK- and a frequency-dependent parameter. © 1992

A quantitative model to estimate rail surface failure

This paper is concerned with the formulation of an approximate analytical model describing the total wear on both high and low rails as a function of the angle of attack ψ. The rail life γ(ψ) is obtained in this model as follows as: γ(ψ) = l(ψ-m)2+ nψ≤ 0.0058 radγ(ψ)=Pψ+q ψ ≥ 0.0058 rad where l,m, n, p and q are constants which can be determined for a specific set of boundary conditions. The results of applying this equation to various values of ψ are compared with field data and good agreement is observed. © 1984

Dynamic flow stress of shock loaded low carbon steel

This study identifies the role of twin volume fraction in the dynamic stress-strain relationship of shock loaded low carbon steel. This relationship is modeled using thermal activation concepts based on dislocations interactions with thermal and athermal stress barriers. Model parameters are determined by first subjecting a set of as-received and 3% pre-strained steel specimens to shock loads of 6 GPa, and 11 GPa using a gas gun. These specimens, as well as ones with no prior shock loading, were subjected to dynamic compression tests at temperatures in the range 293–923 K and at strain rates between 1E+ 3 s −1 and 5E+ 3 s −1 using a split Hopkinson pressure bar. The resulting stress-strain curves were partitioned into thermal and athermal stress components in which the role of twin boundaries is discussed in terms of their relation with the interaction of dislocation motion and grain subdivision. Results of this work show that the thermal stress component of both the as-received and pre-strained materials is insensitive to the impact history. The athermal stress component for as-received specimens which have been previously shock loaded is shown to be proportional to the shock loading pressure. For specimens, however, which have been pre-strained before shock loading, the athermal stress did not vary for the two shock load levels. This difference in athermal stress between as received and pre-strained materials is explained in terms of the twin volume fraction generated as a function of the impact load. Model results are presented and discussed in relation to the proposed stress-strain formulations