Turbine blade


A turbine blade is a radial aerofoil mounted in the rim of a turbine disc and which produces a tangential force which rotates a turbine rotor. Each turbine disc has many blades. As such they are used in gas turbine engines and steam turbines. The blades are responsible for extracting energy from the high temperature, high pressure gas produced by the combustor. The turbine blades are often the limiting component of gas turbines. To survive in this difficult environment, turbine blades often use exotic materials like superalloys and many different methods of cooling that can be categorized as internal and external cooling, and thermal barrier coatings. Blade fatigue is a major source of failure in steam turbines and gas turbines. Fatigue is caused by the stress induced by vibration and resonance within the operating range of machinery. To protect blades from these high dynamic stresses, friction dampers are used.
Blades of wind turbines and water turbines are designed to operate in different conditions, which typically involve lower rotational speeds and temperatures.

Introduction

In a gas turbine engine, a single turbine stage is made up of a rotating disk that holds many turbine blades and a stationary ring of nozzle guide vanes in front of the blades. The turbine is connected to a compressor using a shaft. Air is compressed, raising the pressure and temperature, as it passes through the compressor. The temperature is then increased by combustion of fuel inside the combustor which is located between the compressor and the turbine. The high-temperature, high-pressure gas then passes through the turbine. The turbine stages extract energy from this flow, lowering the pressure and temperature of the gas and transfer the kinetic energy to the compressor. The way the turbine works is similar to how the compressor works, only in reverse, in so far as energy exchange between the gas and the machine is concerned, for example. There is a direct relationship between how much the gas temperature changes and the shaft power input or output.
For a turbofan engine the number of turbine stages required to drive the fan increases with the bypass-ratio unless the turbine speed can be increased by adding a gearbox between the turbine and fan in which case fewer stages are required. The number of turbine stages can have a great effect on how the turbine blades are designed for each stage. Many gas turbine engines are twin-spool designs, meaning that there is a high-pressure spool and a low-pressure spool. Other gas turbines use three spools, adding an intermediate-pressure spool between the high- and low-pressure spool. The high-pressure turbine is exposed to the hottest, highest-pressure air, and the low-pressure turbine is subjected to cooler, lower-pressure air. The difference in conditions leads to the design of high-pressure and low-pressure turbine blades that are significantly different in material and cooling choices even though the aerodynamic and thermodynamic principles are the same.
Under these severe operating conditions inside the gas and steam turbines, the blades face high temperature, high stresses, and potentially high vibrations. Steam turbine blades are critical components in power plants which convert the linear motion of high-temperature and high-pressure steam flowing down a pressure gradient into a rotary motion of the turbine shaft.

Environment and failure modes

Turbine blades are subjected to very strenuous environments inside a gas turbine. They face high temperatures, high stresses, and a potential environment of high vibration. All three of these factors can lead to blade failures, potentially destroying the engine, therefore turbine blades are carefully designed to resist these conditions.
Turbine blades are subjected to stress from centrifugal force and fluid forces that can cause fracture, yielding, or creep failures. Additionally, the first stage of a modern gas turbine faces temperatures around, up from temperatures around in early gas turbines. Modern military jet engines, like the Snecma M88, can see turbine temperatures of. Those high temperatures can weaken the blades and make them more susceptible to creep failures. The high temperatures can also make the blades susceptible to corrosion failures. Finally, vibrations from the engine and the turbine itself can cause fatigue failures.
Higher temperatures lead to better cycle efficiency in a turbine, calling for a need for materials innovation that continually pushes the previous benchmark. As the most common modes of failure in SC and polycrystalline blades are intrinsically different, SC blades, thanks to their lack of grain boundaries, are ideal for first and second stage turbine blade applications. Turbine blades are typically made of Ni-Cr superalloys, which present a unique microstructure.
Single crystal superalloys are typically cast with the ⟨001⟩ crystallographic direction along the turbine blade's axis, and often with a controlled secondary orientation, which has been found to be ideal for reducing localized stresses. Their microstructure consists of cuboidal γ′ precipitates aligned along ⟨001⟩ within a γ matrix. These alloys exhibit anisotropic mechanical properties due to the FCC structures of γ and γ′, also known as orthotropic properties. These include tensile and creep properties, which are also influenced by temperature alloy composition. This anisotropy strongly affects deformation and damage during thermo-mechanical fatigue.

Phase Angle

  • For an in-phase TMF cycle, maximum tensile loading occurs at the maximum temperature of the cycle. The associated deformation modes are thus high-temperature creep at this tensile loading state, which can be intensified by internal stress and centrifugal force, even at moderate temperatures. Conversely, low temperatures correspond to a compressive state characterized by low-temperature plasticity, typically occurring near or within cooling channels designed into the blade.
  • In certain areas, such as the blade platform, with insufficient cooling, they can become “hot spots”. In areas like this, out-of-phase TMF cycle can be observed. It is characterized by creep relaxation caused by compressive stress at high temperatures, and plastic deformation by tensile stress at low temperatures. Thus, while running and hot, the area will be under compressive stress and will experience creep relaxation, that will translate into tension when returned to room temperature. Oxide formation at high temperatures causes low ductility in SC alloys, which can lead to cracking in tensile stress, so areas experiencing OP oftentimes have shorter lifespans, despite experiencing lower average stress and inelastic strain than IP.
  • In comparing the effects of IP vs OP, it is found that maximum tensile stress is the largest contributor to lifetime, resulting in OP having a shorter lifespan.
  • TMG is a very damaging process, and its fatigue lifespan for a SC superalloy at any given orientation is 70-90% lower than isothermal cyclic loading conditions at peak temperature.

    Deformation and Cracking

  • Deformation in SC alloys tends to be localized due to the lack of grain boundaries and anisotropy of the crystal. Deformation twinning is a common occurrence in SC alloys, which causes crack growth localized to along the twins, leading to failure. It is a leading cause of TMF failure, especially of second-generation SC alloys that contain Re. However, under conditions of temperature or stress gradients along these deformation bands, it can be difficult to interpret mechanics.
Fatigue crack paths in SC initiate with Mode I tensile opening and are subsequently propagated by crystallographic shearing along slip planes. This is in opposition to the behavior of conventional crack growth that follows the Stage I/Stage II pattern of crack nucleation and steady growth.
  • In OP-TMF, oxidation-fatigue interactions, and γ′ coarsening at high temperatures are the main causes of crack nucleation. Cracks tend to form in spots with oxide growth, the process of which is exacerbated by high temperature loading conditions, leading to well-defined, planar cracks-a key feature of brittle materials. It has been observed that localized clusters of twinned plates form ahead of the crack tip on the plane, also known as deformation twinning. This is caused by partial dislocation movement in the corresponding slip system. When big enough, they enable crack propagation, preferentially localized to along the twins.
  • Without the temperature cycling, in low cycle fatigue, the deformation mode is dominated by diffusion-based mechanisms.
  • While most propagation takes place at the low temperature part of the OP-TMF cycle, the high temperature and compressive state is responsible for damage caused by dissolution and recrystallization of the γ′ phase at the crack tip. Study of crack closure shows that crack growth rate is independent of maximum cycle temperature and dwell time as a function of change in effective stress intensity factor, indicating that the driving force for crack propagation is mitigated by stress relief at the crack tip as a result of closure.
  • For IP-TMF, cracks tend to form at weak interfaces. There is no observable difference between crack growth rate as a function of ΔKeff, showing a similar outcome to the corresponding LCF conditions. At prolonged hold times a reduction in crack growth closure and increase in K result in crack growth. Thus, IP-TMF and LCF tests are also described by crack growth rate vs ΔKeff.
  • Under stress-controlled conditions, IP has the shortest fatigue life, followed by LCF, then OP.

    Dwell Time

The addition of a compressive hold time impacts the direction of crack propagation as observed in OP-TMF. Without a dwell time, cracks initiate at the surface, and grow perpendicular to the direction of applied stress, before the growth along twin plates. Upon extension of dwell time beyond 10 minutes, the crack directly grows along the twin plates until failure. Application of dwell time decreases fatigue lifespan, even at high temperatures, likely as a result of stress relaxation, failure mode, increased inelastic strain, and high tensile mean stress. For IP-TMF, the effects of dwell time on crack initiation and propagation are accurately predicted by a combined model of fatigue and creep.
The yield strength anomaly in the pure γ′ phase describes a state in which the material has a higher yield strength at higher temperatures, so the impressive mechanical properties of SC superalloys is a result of the interaction between the γ and γ′ phases’ cuboidal structure. However, this state is unstable, and rafting formed as a result of high temperature and long dwell times in TMF tests will weaken the material, leading to a lower yield stress. This effect is thus exacerbated by lower values for minimum temperature in TMF cycles. As the yield stress decreases, the inelastic strain range increases, and cracks form sooner.