Hi everyone, my name is Max Holliday,
and I'm Peter Knapp and I'm Andrew Dianetti,
We are part of the 2013 NASA Aeronautics Academy at Glenn Research Center , where we are
characterizing and investigating the failure modes of Nickel-based superalloys in jet turbine disks.
This exciting research can help improve the thermal capabilities of these alloys which
can directly increase the fuel efficiency of commercial aircraft by lowering the amount
of fuel used during operation
Turbine engines are used in virtually all commercial aircraft.
In these engines, air is compressed into the combustion chamber, where fuel is mixed and ignited.
A turbine is used to extract energy from this combustion to power the compressor, and
the remaining energy is used for thrust.
The turbine disk is one of the most critical components in engine design, as it is
subjected to a high velocity stream of hot gas.
The maximum temperature that can be withstood by the turbine disc limits the amount of
energy that can be extracted from the fuel, and thus impacts the engine s performance.
Materials that can withstand higher temperatures under the harsh operating conditions of
the engine are sought to improve engine performance.
The advanced powder metallurgy disk alloy ME3 was developed in the NASA High Speed
Research/Enabling Propulsion Materials program in cooperation with GE Aviation and Pratt
& Whitney Aircraft Engines.
This alloy was designed to have extended durability at temperatures up to 700 C in large disks.
The higher temperature capability of ME3 significantly improved fuel efficiency in jet
turbine engines and is considered a major advancement in disk alloys.
The team we are working with at NASA, along with GE and Pratt & Whitney won a 2004 R&D
100 award for the development of ME3.
Superalloys such as these are being utilized in compressor and turbine disks in current
and emerging aircraft such as the Boeing 787 and Airbus A380.
A turbine disk is a fracture critical structural engine component.
Failures are typically uncontained, and can result in the loss of an engine,
considerable airframe damage, or loss of the entire aircraft.
These advanced alloys are susceptible to surface processing defects that have been known
to cause failures.
The FAA frequently requires enhanced inspections to detect disk cracking, in order to
ensure no uncontainable failures occur.
The durability of these material systems was assessed during material and engine
development, however, issues can emerge as these new components spend more time in service.
It would be too expensive and time consuming to produce hundreds of disks to test, so we
use tensile specimens to observe corrosion effects on alloy life and predict fracture initiation.
The specimens are corroded using different techniques to help simulate similar
engine-like conditions and analyzed on the Alicona 3D imaging microscope to look at
corrosion pit depth, width, and overlap.
We use these metrics to predict exactly where the specimen will fail.
The specimens are then Fatigue tested until failure.
Then we use a high scanning electron microscope to observe the fracture surface and
determine the points of failure in a process known as Fractography.
The data collected during fractography can be used to determine what type of pit
initiated crack growth and which type lead to failure.
So far we are 85% successful at predicting which pit will cause fatigue failure, but we
are improving with every new set of data.
These facets of failure analysis not only help characterize the ME3 superalloy, but
allow materials scientists to evaluate failed disks and determine exactly where the
failure occurred and why.
A key goal of this project is to understand how physical and microstructural factors
control the physical properties of nickel-based superalloys.
Specifically, the machining of disk features can profoundly affect the fatigue like of
these alloys by imparting cold work or surface defects.
Additionally, changes in the distribution of phases within these alloys can cause order
of magnitude reductions in fatigue life.
It is possible to quantify the effects of these processes using a technique called
Vickers microhardness testing.
In Vickers mcrohardness testing a square oyramidal diamond identer is pressed into a
surface at a given load, the hardness is then determined from the dimensions of the
remaining indent.
We can use this testing procedure to create a map of surface hardness to determine the
effects of machining.
This summer we are examining broached specimens of NASA s Low Solvus High Refractory
alloy that have been cycles to the point of failure.
Following hardness testing we will etch these samples to expose the microstructure, i.e.
grain and phase distribution, inorder to correlate changes in hardness with changes in phase.
One method that can be used to resist the growth of fatigue cracks is to create a
residual stress in the surface of the disk.
A process known as shot peening, where small pellets are fired at the material surface,
is used to create a compressive stress layer near the surface that resists external
tension and suppresses the growth of fatigue cracks.
Residual stress can be measured using a technique known as X-Ray Diffraction.
In XRD, the strain of the lattice is measured to determine the stresses present.
By removing different amounts of surface material using a process known as electropolishing
the residual stress profile, as a function of depth, can be determined.
Understanding these stresses will allow us to better understand how to resist crack growth.
This summer, we are working to characterize the effect of different shot peening
conditions on the residual stresses present in these alloys, as well as to characterize
the effect of cyclic loading and elevated temperature conditions on the residual stresses
throughout the life of a component.
Our projects have involved characterizing aspects related to the failure modes of turbine disks.
Current and future work involving these superalloys will help to create more efficient,
safer aircraft engines.
We would like to thank NASA for the opportunity to work on such an important and
exciting program and we encourage any and all prospective engineers to consider NASA as
outlet for conducting meaningful research. Thank you!