Thermomechanical Fatigue Crack Propagation in an Anisotropic (Directionally Solidified) Nickel-Base Superalloy

Abstract

Strain-controlled thermomechanical fatigue tests were performed on two nickel-base superalloys, directionally solidified (DS) Mar-M200 plus hafnium and conventionally cast B-1900 plus hafnium. The DS alloy was tested at various angles (θ) to the direction of grain growth. It was found that crack growth rates were a minimum for the DS alloy tested parallel to the direction of grain growth (crack propagating normal to the direction of grain growth). The crack growth rate increased for other orientations in the sequence θ = 15, 30 or 90, and 45 deg until equivalence was reached with rates for the conventionally cast alloy, B-1900 plus hafnium. A linear elastic fracture mechanics approach was extended to strain-controlled crack growth as a function of θ for the DS alloy. Prediction of crack growth rates as a function of ϑ was achieved by normalizing with the elastic modulus. The sequencing of crack growth rates with θ and a smooth-to-rough fracture surface transition are explained on the basis of the orientation dependence of crack opening displacement for a given strain intensity range.

Thermal-Mechanical Fatigue Crack Growth in Inconel X-750

“…First, it can be seen that the crack growth rates are higher for TMF cycling than for the equivalent isothermal condition (650°C) which is in agreement with the results obtained on other nickel-base alloys [5][6][7]9, 131. Secondly, it is observed (Figure 4) that the crack growth rates are higher for R = -1 than for R = 0.05 which indicates that compressive stresses play an important role in the mechanics of TMFCG.…”

“…In fact, the study of fatigue has generally bypassed real thermal fatigue loading partly because isothermal tests are relatively simple to perform, but also because it has often been felt that such tests carried out at the maximum service temperature would give worst case results. However, several studies which have compared fatigue resistance under thermal cycling conditions with that in isothermal tests have shown that in many cases, the latter, rather than giving a worst case situation, can seriously overestimate the real fatigue life [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15].…”

Hold-time effect on the thermo-mechanical fatigue crack growth behaviour of Inconel 718

“…The induction coil was designed in such a way that during heating, the main part of the heat input occurs at the shoulders of the specimen outside the gauge section of the specimen. This is illustrated in Figure [ 3] where one can see the dynamic temperature distribution in the mid-section of the specimen at a temperature of approximately 360C and a constant heating rate of 1C/s as recorded using a thermal imaging system from Flir. Similar measurements were also performed under static conditions in order to measure the temperature distribution during a hold time at 550C.…”

“…The pioneering work in this field where conducted by Leverant and co-workers at Pratt & Whitney Aircraft in the 70'ies [1][2][3] when the influence of oxidation-resistant protective coatings on fatigue crack initiation as well as on propagation of coating-initiated cracks into the superalloy substrate were studied. The approach was based on the combination of a specimen design and testing method that could simulate the various straintemperature phase relationships experienced in turbine airfoils, including measurement of thermo mechanical fatigue crack growth.…”

A review of thermo-mechanical fatigue behaviour in polycrystalline nickel superalloys for turbine disc applications

“…However, pyrometers are highly sensitive to any changes in the surface emissivity of the specimen which may result from oxidation [13]. Furthermore the TMF Code of Practice [7] does not recommend using this method of temperature measurement but pyrometry has still been used on several occasions [9,21,42], provided a stable emissivity value can be established for the duration of the test, via pre-oxidation at a suitable temperature, or other methods. Codrington et al [35] chose to use thermocouples over optical measurement methods such as pyrometry, based simply on cost and ease of use.…”