By Heather Rock Woods
Aircraft turbine engines are prone to ingesting pebbles and other
debris that can damage jet engine turbine blades, dramatically reducing
the longevity of the components, sometimes catastrophically.
(Graphic Courtesy of
Failures associated with such ‘foreign object damage’ cost the
aerospace industry an estimated $4 billion a year. Studies at SSRL have
helped show how and why turbine blades—which normally experience
significant stresses during flying—will fatigue more than 100 times sooner
than expected from foreign object damage.
"The results of our study are important for people on the tarmac
inspecting the blades, looking for damage and deciding whether that
pinhead-size ding has damaged the blade catastrophically," said SSRL
scientist Apurva Mehta (ESRD).
Working with principal investigator Brad Boyce (Sandia), Mehta and
collaborators simulated the damage by firing small hardened steel balls
onto a titanium alloy commonly used in turbine blades. At LLNL, these ball
bearings were fired at 200 meters per second (m/s), or 450 miles/hour, and
at 300 m/s, the typical velocities at which runway debris encounters
turbine blades when planes take off and land.
At SSRL, the team examined the resulting damage with the unique
abilities of synchrotron mesodiffraction (x-ray diffraction in the
sub-millimeter scale); in this case, 0.3 mm to match the size scale of the
"We wanted to see how things fail, the physics behind what happens. At
300 m/s the mode of failure is qualitatively different, and we wanted to
understand why," Mehta said.
The x-ray images revealed the magnitude and distribution of residual
stresses in and around the craters made by the ball bearings’ impact. For
example, the material was under tension in the center of the crater, under
compression in a ring around the center, and under normal stress beyond
In 200 m/s craters, the actual stresses measured in the lab matched
calculations from the finite-element models that engineers use to estimate
residual stress. This shows that the models are dependable in predicting
how much longer the dinged blades should last for impacts at 200 m/s and
below. The team was surprised to find that the 300 m/s craters actually
showed lower residual stresses than the 200 m/s craters. "It didn’t match
our intuition or the calculations’ predictions at all," said Mehta.
Careful examination of the 300 m/s craters revealed large variations in
residual stresses. The regions of lower stress often had micro-cracks
created by the impact. Mehta and his colleagues believe these relieve the
majority of the residual stress. It also explains the models’ off-base
predictions at 300 m/s. Micro-cracks, however, are the weak point in the
material. Subsequent mechanical tests have shown the material invariably
fails (and fails faster) there.
The good news is that not all impacts are harmful. The team found that
residual stresses around a 200 m/s crater located in the central part of
the blade goes away after a few cycles. However, impacts near an edge or
an angle in the blades leads to unrelieved strain on sides, edges and
corners that are not relaxed on cycling and become sites of fatigue
"Our study tells maintenance crews to look not only for the size of the
impact crater, but for its location and for signs of micro-cracks," Mehta
The funders of the study, the Lufthansa Technik AG, the U.S. Air Force
and the U.S. Army, were excited about the results. Turbine blades are
expensive, and the new data give more clues about which craters are
dangerous and which might ‘self-heal.’
This new understanding has been incorporated into a mathematical model
of failure to help design new blades to prevent failures resulting from
foreign object damage.
Boyce shared the results in a workshop last month called "Probing
Mechanical Deformation and Failure via Synchrotron X-ray," during SSRL’s
30th Annual Users Meeting.
"This example illustrates the utility of a synchrotron x-ray source to
solve real-world engineering problems," said SSRL scientist Mike Toney.