I am a PhD Researcher that focuses on characterising the microstructural behaviour of superalloys.
Pinboard providing a brief overview of TMF of high temperature materials in turbine engines
In 10 Seconds? As a result, of the continual drive towards more efficient turbine engines - components within the engine must be able to withstand much higher operating temperatures than ever before. Therefore, thermo-mechanical-fatigue (TMF) is currently the subject of an abundance of research within the field of materials science.
One interesting article, which has been recently published “Thermo-Mechanical-Fatigue of RR1000” (see pinboard), highlights how testing for TMF has at to undergo development to keep up with the development of turbine engines. The research currently being investigated is vitally important to ensure that the safety of these highly critical components are never pushed beyond their limits, therefore making air-travel as safer than ever before!
Why is it so critical? TMF is a critical consideration when designing a turbine engine because TMF cracking can lead to a life-limiting degradation mode – for components such as vanes and blades. Therefore, could result in drastic failure if not found beforehand and consequently cost the airline a substantial amount of money in the replacement of said damaged component. TMF is a combination of creep and fatigue.
• Creep: is a process that involves the gradual plastic deformation of a material overtime – it is slow, temperature aided and time dependent.
• Fatigue: weakening of a material due to being the subject of repeating applied loads
What is being done to prevent TMF? One particular research group has focused on creating computational prediction models for TMF of high temperature alloys, see pinned article “Thermo-Mechanical-Fatigue-Damage Mechanisms and Mechanism-Based Life Prediction Methods”. Therefore, this would prevent any drastic mechanical failure of a critical component – consequently making air travel much safer.
Throwing Coins into a jet engine…Cracking idea!
Now, had this not been reported by a fellow passenger this could have caused damage to the engine – particularly, the tips of the fan blades which could lead to imbalance and as a consequence could cause vibration. This is known as an FOD within the aviation industry – which stands for Foreign Object Damage, which can be defined as debris, substances or articles that have potential to cause damage. There are four groups of FOD: metal, stone miscellaneous and birds.
Abstract: The safety-critical rotating parts of aircraft engines are mainly designed using experimental material data, based on standard specimens and procedures, while few data are available on the effect of manufacturing anomalies on fatigue life. In this context, the paper investigates the effects of different machining parameters on the high-temperature fatigue resistance of Inconel 718 superalloy specimens, cut from engine disk forgings, machined by turning on a vertical lathe. An unconventional specimen was designed in order to have the machining marks aligned with the fatigue loading axis, so to reproduce the hoop stresses in engine disks. For the test campaign, three machining parameters were chosen (depth of cut, cutting speed and insert wear) that typically may generate non-geometrical anomalies. A correlation has been found between the machining parameters, the residual stresses, the surface roughness, and the distorted and amorphous layer thicknesses. Correlations of such data with fatigue life are also presented and discussed.
Pub.: 23 May '17, Pinned: 08 Jun '17
Abstract: Publication date: April 2017 Source:International Journal of Fatigue, Volume 97 Author(s): M. Esmaeilzadeh, F. Qods, H. Arabi, B.M. Sadeghi In this research, fatigue and induction heating thermo-mechanical fatigue (TMF) were performed on Hastelloy X superalloy in the small and large scale yielding in plane stresses mode. The crack growth rates were measured and formulated by fracture mechanics parameters. Furthermore, the fatigue life was predicted by employing resistance curves technique. The TMF behavior of this superalloy was investigated. The results demonstrated that in-phase loading TMF conditions lead to short fatigue life (more crack growth rate) at high strain amplitudes and temperatures up to 600°C. For higher temperatures, the predominant damage was due to creep. A model based on damage contributions due to pure fatigue and cyclic creep has been presented for predicting TMF crack growth rates. Fracture mechanic method was used to suggest a model for fatigue part of TMF crack growth rate, while the temperature effects during TMF crack growth rate was considered to be due to cyclic creep. In addition the TMF crack healing or crack closure occur during application of induced eddy currents were investigated explicitly, as environmental effects of induction heating. The results show the higher current density in the crack tip area produced more heat and resulted in a significant rise in temperature. So it was concluded that the compressive thermal stress due to change of thermal expansion causes crack healing.
Pub.: 06 Jan '17, Pinned: 11 Jun '17
Abstract: Non-isothermal conditions during flight cycles have long led to the requirement for thermo-mechanical fatigue (TMF) evaluation of aerospace materials. However, the increased temperatures within the gas turbine engine have meant that the requirements for TMF testing now extend to disc alloys along with blade materials. As such, fatigue crack growth rates are required to be evaluated under non-isothermal conditions along with the development of a detailed understanding of related failure mechanisms. In the current work, a TMF crack growth testing method has been developed utilising induction heating and direct current potential drop techniques for polycrystalline nickel-based superalloys, such as RR1000. Results have shown that in-phase (IP) testing produces accelerated crack growth rates compared with out-of-phase (OOP) due to increased temperature at peak stress and therefore increased time dependent crack growth. The ordering of the crack growth rates is supported by detailed fractographic analysis which shows intergranular crack growth in IP test specimens, and transgranular crack growth in 90° OOP and 180° OOP tests. Isothermal tests have also been carried out for comparison of crack growth rates at the point of peak stress in the TMF cycles.
Pub.: 04 Jan '17, Pinned: 11 Jun '17
Abstract: An investigation was made on the strain-controlled low-cycle fatigue (LCF) of K40S cobalt-base superalloy at 900 °C in ambient atmosphere. The results show that K40S alloy possesses high LCF resistance in comparison with X-40 alloy. Under the testing conditions in this study, K40S alloy exhibits a cyclic stress response of initial hardening followed by softening. The cyclic stress response behavior has been attributed to dislocation-dislocation interactions and dislocation-precipitate interactions. The high response stress can lead to a large stress concentration at locations where inelastic strains of high amplitude accumulate, which account for the decreasing fatigue life with increasing strain rate. The well-distributed carbide particles are the “secondary” crack initiation sites. The secondary crack initiation relaxes the stress concentration at the crack tip, reducing the driving force of crack propagation. High-temperature LCF failure of K40S alloy results from the interaction of the mechanical fatigue and environmental oxidation.
Pub.: 01 Apr '03, Pinned: 11 Jun '17
Abstract: In this study, the conjoint effect of a broaching operation, similar to that used for machining fir-tree slots on turbine discs, and subsequent heat treatments at 550 °C and 650 °C on the fatigue performance and corresponding crack initiation behavior of forged Inconel 718 has been investigated. Four-point bending fatigue tests were conducted under load control on specimens of two groups, i.e. a polished group and a broached group, with totally six different surface conditions. Compared to the as-polished specimens, a beneficial effect of the broaching operation was found on the fatigue life due to the high compressive residual stresses on the broached surface which transfer the fatigue crack initiation from the surface into the sub-surface region. Introducing a heat treatment generally deteriorated the fatigue performance of the alloy because of the oxidation assisted crack initiation, while the reduction in fatigue life was found to be more remarkable for the broached specimens, in particular when heat treated at 650 °C, as the thermal impact also led to a great relaxation of the compressive residual stresses; the combined effect, together with the substantial anomalies created by broaching on the surface, such as cracked carbides and machining grooves, caused an increased propensity to surface cracking in fatigue and consequently a loss of the lifetime. Furthermore, it was found that the occurrence of surface recrystallization at elevated temperatures in machined Inconel 718 could lead to intergranular oxidation, creating micro-notches as preferable sites for the fatigue crack initiation.
Pub.: 14 Jun '16, Pinned: 11 Jun '17
Abstract: An existing extensive database on the isothermal and thermomechanical fatigue behaviour of high-temperature titanium alloy EVII 834 and dispersoid-strengthened aluminum alloy X8019 in SiC particle-reinforced as well as unreinv conditions was used to evaluate both the adaptability of fracture mechanics approaches to TMF and the resulting predictive capabilities of determining material life by crack propagation consideration. Selection of the correct microstructural concepts was emphasised and these concepts were, then adjusted by using data from independent experiments in order to avoid any sort of fitting. It is shown that the cyclic /-integral (δJeff concept) is suitable to predict the cyclic lifetime for conditions where the total crack propagation rate is approximately identical to pure fatigue crack growth velocity. In the case that crack propagation is strongly affected by creep, the creep-fatigue damage parameter δCF introduced by Riedel can be successfully applied. If environmental effects are very pronounced, the accelerating influence of corrosion on fatigue crack propagation can no longer implicitly be taken into account in the fatigue crack growth law. Instead, a linear combination of the crack growth rate contributions from plain fatigue (determined in vacuum) and from environmental attack is assumed and found to yield a satisfactory prediction, if the relevant corrosion process is taken into account.
Pub.: 01 Feb '03, Pinned: 11 Jun '17
Abstract: A transversely isotropic continuum elasto-viscoplasticity model, which was developed from Chaboche’s unified constitutive model, was formulated to capture the thermal mechanical creep fatigue deformation behavior of a directionally solidified nickel-based superalloy. A fourth-order tensor was introduced to model material anisotropy. In order to model the tertiary creep behavior, the Kachanov damage evolution equation was coupled into the stress tensor. Based on the test results of uniaxial tensile, fatigue, and creep loadings at isothermal temperature conditions, the material parameters are obtained. Thermal mechanical fatigue (TMF) and creep–fatigue interaction test results were used to verify the robustness of the model. Additionally, strain–temperature-dependent stress–strain responses under TMF loadings were analyzed using the present model. Under strain-controlled conditions, both of the stress ranges and mean stresses are strongly influenced by the strain–temperature phases, a key parameter for TMF tests.
Pub.: 28 Apr '16, Pinned: 11 Jun '17
Abstract: The principal factors in the performance of aerospace materials are strength-to-weight ratio, fatigue life, fracture toughness, survivability and of course reliability. Machining processes, and in particular grinding under adverse conditions, have been found to cause damage to surface integrity and affect the residual stresses distribution in the surface and sub-surface region. These effects have a direct bearing on the fatigue life. In this investigation the effect of grinding conditions on the fatigue life of titanium 5Al-2.5Sn was studied. This alloy is used in ground form in the manufacturing of some critical components in the space shuttle's main engine. It is essential that materials for such applications be properly characterized for use in severe service conditions. Flat sub-size specimens 0.1 in. (2.5 mm) thick were ground on a surface grinding machine equipped with a variable-speed motor at speeds of 2000 to 6000 fpm (10 to 30 m sec−1) using SiC wheels of grit sizes 60 and 120. The grinding parameters used in this investigation were chosen from a separate study. The ground specimens were then fatigued at a selected stress and the resulting lives were compared with that of the virgin material. The surfaces of the specimens were examined under a scanning electron microscope and the roughness and hardness were measured using a standard profilometer and microhardness tester, respectively. The fatigue life of the ground specimens was found to decrease with an increase in speed for both dry and wet conditions. For both the grit sizes, the fatigue life was lower than that of the virgin material for the dry condition. The fatigue life of specimens ground under wet conditions showed a significant increase at the wheel speed of 2000 fpm (10 msec−1) for both grit sizes, and thereafter decreased with increase in speed to below that of the virgin material. The results of the investigation are explained using profilometry, microhardness measurements and scanning electron microscopic examination.
Pub.: 01 May '91, Pinned: 11 Jun '17
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