Limiting Creep in Aerospace Materials
The Larson-Miller parameter was first used in the 1950s as a model to predict the lifetime of a material vs. time and temperature. Without close attention to materials selection, many products are not fit for purpose due to mechanical stress and natural creep. That’s why creep resistant materials are so important in the aerospace industry.
Materials under service conditions are required to sustain steady loads for prolonged periods of time and often undergo a time dependent deformation that is referred to as creep. Creep is the natural tendency of a material to gradually move or permanently deform as a result of mechanical stress or strain.
From steam turbines to nuclear power plants, creep is a key factor in design decision making. It especially applies to materials that are frequently exposed to elevated or extreme temperatures, subject to the material’s melting point. For example, creep can be a significant issue for materials operating in high-performance systems, such as jet engines, that often reach extreme temperatures surpassing 1200 degrees Celsius.
This makes creep an especially important consideration for engineers designing jet engine turbines, as the high temperatures and revolutions per minute (RPM) can accelerate the onset of creep in turbine blades. For turbines, creep can cause the blades to elongate, which can ultimately lead to the blade beginning to damage or pierce the turbine casing. Creep is typically measured in units of millionths-of-an-inch per hour, and a standard TF33 engine turbine operating at an exhaust gas temperature (EGT) of 555 degrees Celsius and 100 percent RPM can experience at least 50 units of creep per hour. Design engineers must account for such potential elongation during design.
While time-dependent deformation commonly occurs because of high temperatures, it’s also important to note that for materials like polymers, this can also happen at room temperature due to their low melting temperature.
The type of deformation depends on the material and structure. Generally, creep deformation occurs by grain boundary sliding, so that the adjacent grains or crystals within a material move as a unit relative to each other. This means that the greater the grain boundary area, the easier it is for creep deformation to occur. Therefore, using a single crystal material with larger grain size can improve creep strength, and this depends on the processing of a material.
At high temperatures, the atomic bond between molecules of the materials can start failing, resulting in the movement of atoms and atomic planes within the materials. This movement of atomic bonds causes the restructuring of atoms, generating movements of dislocations and diffusion of the bonds that leads to permanent deformation of the materials, even with high tensile strength.
To plot how much stress and strain a material can withstand against temperature or other loading variables, design engineers can conduct tests with a creep-testing machine. The device generates a curve graph that can then be assessed to pinpoint the temperature and time interval for the various stages of creep. It permits the calculation of the equivalent times necessary for stress rupture to occur at different temperatures, so design engineers can design components that are fit for purpose.
There are three main stages of creep. The first is primary creep. This is when creep rises rapidly over a brief period and then slowly decreases. This is followed by secondary creep, which creeps at a relatively steady rate. Finally, there’s tertiary creep. This is when the rate is accelerated until the material breaks.
Increased performance in aircraft engines and land-based power generators require the development of a new generation of high-temperature structural materials that are resistant to creep. Creep resistance is the ability of the material to resist any kind of distortion when subjected to prolonged compressive loads over an extended period of time.
As the demand for high performance materials increases across the globe, particularly in Japan and North America, we are seeing more creep-free materials that feature performance enhancing additives enter the market. These materials are becoming particularly popular in the defense, aerospace and nuclear sectors. Predominantly made of carbon-carbon (C/C) composites, they are suitable for applications exposed to heat up to 1600 degrees Celsius like turbine blades and jet engines.
Nickel superalloys have good creep resistance at up to 80 per cent of their melting point. Carbon fiber reinforced with titanium alloys is another creep resistant material used in jet engines and turbine blades. It is one of the hardest high-performance creep resistant materials available and is 50 percent harder than tungsten carbide.
Some materials have demonstrated improved behavior against creep with increased fracture toughness, oxidation resistance, combine strength and mechanical and microstructural stability when combined with carbon fiber and titanium alloys.
Molecular density of carbon fiber reinforced titanium is in excess of 95 percent of most materials, which can be used in high-purity applications. Certain grades of stainless steel used in weld metals have shown higher resistance to creep than that of others at high temperature and can be used to create stronger shafts and wear resistant bearings.
In modern aircraft engines and gas turbines in power plants, a cooling stream of air protects the metal components from the hot gas. This however decreases the efficiency of the turbines. Higher efficiency is preferable as it uses less energy, saving fossil fuel usage and reducing emissions of carbon dioxide. Increased performance in aircraft engines requires the development of a new generation of creep resistant materials that can operate at these high temperatures, thus saving energy. Studies have suggested that a turbine blade made from a ceramic composite would weigh only one third of one of today’s blades made of high-alloy steel. Lighter aircraft engines would further save energy.
Jet engines must be recalled from service well before they fail, so a suitable safety margin is built in. A good understanding of how a material behaves when exposed to these conditions for an extended period of time would allow jet engines to run closer to their actual operating limits. Today, we have a greater understanding of safety and mechanical stress. With this knowledge, design engineers can mitigate the chances of any problems occurring and extend the service life of their application, by understanding the properties of any materials sourced.
While the benefits of exploring alternative material selection is clear, metals age in a way that engineers are familiar with, therefore engine manufacturers can state how long a part can safely remain in operation before it must be replaced. Design engineers must find the balance between safety and innovation. They can mitigate problems or product failure by consulting creep test data during materials selection and setting regular inspections, relative to the life expectancy of the material. Electric motors are being explored as an alternative propulsion method, however, gas turbine engines are here for the foreseeable future and design engineers must have clear standardized information available to them to create components fit for purpose.
This article was written by Ben Smye, Head of Growth, Matmatch (Munich, Germany). For more information, visit here .
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