A pinboard by
this curator

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.


An investigation on crack growth rate of fatigue and induction heating thermo-mechanical fatigue (TMF) in Hastelloy X superalloy via LEFM, EPFM and integration models

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

On the Conjoint Influence of Broaching and Heat Treatment on Bending Fatigue Behavior of Inconel 718

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

Thermomechanical fatigue—damage mechanisms and mechanism-based life prediction methods

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

Effect of grinding conditions on the fatigue life of titanium 5Al-2.5Sn alloy

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