www.testdevices.
com
571 Main Street, Hudson MA, 01749-3035 /
Tel: 978-562-6017 / Fax: 978-562-7939
Turbine Blade Testing
To validate the line on the Goodman diagram, Test Devices conducts multiple tests of blades at various
speeds and alternating stress amplitudes.
There are two types of stress happening to a turbine blade: an alternating, or vibration, stress and a centrifugal
static stress. As blades rotate, they experience natural resonant frequencies at different rpms. While vibrating,
the blade is also under a centrifugal load as if it is being pulled from root to tip. For a complete stress test, all
stresses should be tested for simultaneously.
There have been attempts to pull on the end of a vibrating blade to test the interaction of both static
and dynamic stresses which has proven unreliable due to failure of the blades at the tip before reaching
adequate loading or cycles. Pulling on the blade also creates a constant stress from root to tip. Actual blades
experience a stress gradient (high at the root and zero at the top) since one end of the blade is free. This fact
also adds to experimental error.
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www.testdevices.com
571 Main Street, Hudson MA, 01749-3035 /
Tel: 978-562-6017 / Fax: 978-562-7939
Other than using dynamic spin testing, the only alternative to test for both accurate static and
dynamic stress is to run the blade in an actual jet engine. This is not only extremely expensive,
but to test the part to the point of failure will ruin the engine. What we developed was a way to
induce customer-spec fed vibrational modes and associated amplitudes in a spinning environment
that require very tight speed control and adequate excitation force to produce the right test
conditions for those blades.
The process basically imparts dynamic (vibrational) and static (centrifugal) stresses in jet turbine engine
blades to validate the predicted blade life. The interaction between static and dynamic stresses is often
depicted in a Goodman diagram, where the vertical axis represents dynamic stress and the horizontal axis
represents static stress. The Goodman line typically intersects the vertical axis at 107 reverse bending
cycles (static stress=0) and the horizontal axis at the yield strength of the given material under test
(dynamic stress=0). The area under the Goodman line represents safe life for the blade, and the area
above the Goodman line represents potential failure.
Instrumentation typically includes a non-intrusive stress-measurement system (NSMS) and strain gauges.
If the customer desires a heated test, thermocouples may also be involved. An NSMS plot of the excitation
mode, stress amplitude, frequency, and rpm is generated. Some tests require exceptionally tight speed control
(±0.25 rpm) to hold on resonance for 3-4 Hz wide and Q-factor (poorly damped) mode. Conditions inside
the spin rig change during the test, which produces slight variations in the natural blade frequency(s). These
variations require altering speed slightly during the test to stay on the resonance.
Test Devices’s dynamic test method is well suited for blade crack growth studies. Understanding how a crack
progresses, specifically the time it takes to reach a critical length, in a specific type of blade drives the frequency
of the inspection intervals which drives the cost to maintain a particuar turbine engine.
Testing various blade damping methodologies and anti-wear coatings, such as anti-fretting coatings, are good
applications for this test method as well. Also, testing to determine foreign-object damage (FOD), the effect on
blade Iife and helping to design blades that are more FOD tolerant are continuing interest.
