A disk crack detector was developed and applied to a small military engine for use as a flight-line turbine crack monitor. The system consists of an eddy current type sensor and its cables within the engine, external connecting cables, and a remotely located electrical capacitance-conductance bridge and signal analyzer. As the turbine spins, the rotor is monitored by the sensor for radial surface cracks emanating from the interblade region of the rotor

Barranger, J. P.

English

NASA Technical Reports Server

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APPLXCATION OF A FLIGHT-LINE DISK CRACK DETECTOR
TO A SMALL ENGINE
John P. Barranger
NASA L¢_2is Research Center, Cleveland, Ohio
A disk crack detector has been developed which is intended
to operate while in flight or at the flight line (ref.) The
detector is being applied to a small military engine for use as
a flight-line turbine crack monitor. The system consists of an
eddy current type sensor and its cables within the engine, ex-
ternal connecting cables, and a remotely located electrical +
capacitance-conductance bridge and signal analyzer. As the tur- !
bine spins, the rotor is monitored by the sensor for radial sur-
face cracks emanating from the interblade region of the rotor.
The sensor is a coil of insulated wize wound on a ceramic
bobbin mounted in the nozzle. It is located approximately
2 1/2 millimeters (3/32 inch) away from the face of the downstream
side of the first stage turbine wheel where experience has
shown cracks are likely to occur. The coil has I00 turns of
silver palladium ceramic coated wire with a coil inside diameter
/
of 3.18 millimeters (0.125 inch), an outside diameter of
12.7 millimeters (0.50 inch), and a length of 1.59 millimeters
(0.062 inch). The coil leads pass through cored nozzle vanes
: and are brazed to the sensor cables. The coll and toll leads
are cooled by air through the core passage in the vanes.
A commercial bridge is used in the monitoring system and is
F
designed to measure capacitance and conductance. By adding
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designed to measure capacitance and conductance. By adding
capacitance in series with the sensor coil, the combined react-
ance is made capacitive. The bridge cable length is limited to
3 3/4 meters (12 feet) because of the decreased bridge sensitivity
resulting from the combination of high carrier frequency (IMHz)%
and excessive cable capacitance. The capacitance-conductance
bridge is self-balancing, automatically adjusting to changes in
average coil inductance _nd resistance caused by temperature ef-
fects and variations in disk-to-sensor spacing.
A test cell at Lewis is being prepared to evaluate the mon-
itor system under full scale engine conditions. Disks that
have been removed from service because of time expiration will
be installed in the test engine. Bench tests indicate that the
system is able to detect a crack 3 millimeters (I/8 inch} long
in these disks. This length is considerably shorter than the
critical crack length.
John P.: Flight Monitor for Jet Engine iReference: Barranger,
Disk Cracks and the Use of Critical Length Criterion of Fracture
Mechanics, NASA TN D-7483, November 1973
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1978002125-382
!DISCUSSION
H. Garten, GE-Lynn
How big a crack do you think you can detect? Also must it be on the
surface?
%
J. Barranger, NASA-Lewis
The crack must be a minimum of I/8-inch long for this system. The
crack must be on the surface because this is a high frequency eddy current
type detector.
I have a co_ent stemming from a number of inquiries. Everyone's trying
to find that elusive crack that's always under the bolt head. This system
cannot detect the disk crack until it propagates ovt beyond the boundaries of
the bolt edge.
B.L. Koff, GE-Cincinnati
Is your plan to keep working until you can detect the crack that is i
under the bolt hole? What's your plan? |
J. Barran@er, NASA-Lewis
- The present plan is to finish this program and turn the results over to
the military. We do not plan to go any further beyond this program.
J
B.L. Koff, GE-Cincinnati
Do you plan to run this detector full time in the engine? Also, what
kin_ of aerodynamics losses do you ha_e with the step that you put in the
flow path? IJ. Barran_er, NASA-Lewis
I'll answer the second question first. If we talk only about cross-
sectional area, it's very small. For the total passage area, that step pro-
duced only a smalt change in the area. Concerning other aerodynamic situations,
we really have not looked at it very hard. The detector may be monitored full
time or may be monitoredperiodically. With regard to the testing program
/ since we're putting a disk with a crack in it, which is verboten in most test
_ programs, a high level of control will be exercised with continuous monLtoring. :
Unknown_uestioner
Why was this engine used for this crack detector study?
J. Barran_@r, NASA-Lewls
.% We are looking at this particular engine because of the average disk llfe
. aspects of it. The disks have beam taken out of service because someone says,
after so many hours this disk comes out, regardless, cracked or uncracked --
this is the standard procedure. We looked at the disks that were taken out
and found that there are cracks in some of the disks at the "end of llfe".
_5
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1978002125-383
These are not critical cracks: that is, they are at least four times smaller
than critical and, according to the manufacturer, are in a low stress region.
What will be done with it in the future is again a matter of decision for the
people who are the users, that is, the military. If they want to extend the
program to its ultimate, one of these detectors would be installed on the
engines that they have in existence. The electronics equipment would be
ground based: it would be plugged in the side of the aircraft at the end of
a flight or every month or whenever the testing interval might be. Then the
decision would be made either to take a disk out earlier than its "end of life"
because a crack shows up, or as an alternative, to continue to run the disk
after the "end of life" (which is more risky) until a crack did show up. How-
ever, whether the average life would be less or greater than what it is now is
hard to say. But that part of the program is uncertain at this particular time.
G.J. Man_ano, NAPTC
Was that detector developed specifically for _hat application, or was it
a general program, and you're using this _articular engine as a test vehicle?
J. Barran_er, NASA-Lewis
It was primarily a study program. The particular engine being used just
happens to fit the test program needs.
S. Weiss, NASA-Lewis
: This has been a concept study which is being further investigated by the
Army.
W. Springer, Allison-GMC
The top of the disk is exposed to the gas flow pattern and a severe
thermal stress concentration exists. After rapid crack propagation, that crack
may become benign. Its growth rate drop off tremendously once the crack tip
gets below the high stress field.
For the future do you think that you will ever get this device working
for smaller cracks than an eighth of an inch and have it farther away from
the disk than ninety mils?
/ J. Barran_er, NASA-Lewis
• '_ The signal that I showed was a raw data signal, so without any further
processing, it was pretty clear the crack was there. I've not tried to increase
the sensitivity. I found that the blade root provides an undesirable signal,
and a crack very often looks like an extension of one of the blade signals.
To distinguish one from the other might be very difficult. In answer to the
first part of your question, I do not think that we can get a substantial
" improvement in the small crack sensitivity unless the sensor is positioned
very close to the disk. For the second part, the farther away you get, the
less sensitive it is. More sensitivity with distance implies making the coil
larger. However, as it becomes larger, a smaller fraction of the sensor area
is exposed to the crack region so it becomes less sensitive to the crack. Thus,
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1. I
JL
1978002125-384
it is a balance between those two situations. I do not think that we will do
a whole lot better than this. But, I have not looked at the problem hard
enough to determine what the answer to that question really is.
J. Doherty, P&W
Just a general comment. It would seem from an operational point of view,
that you've chosen a very convenient problem: you know where the crack will be
before you start looking. If you don't know where the crack is beforehand, you
must have the engine full of sensors -- in every conceivable location. Many
of us who have engines in the field know that when we have cracks in components,
we must get those cracked parts out as fast as we can; we really don't have a
lot of time to go around and find out where the next crack might be.
J. Barranger, NASA-Lewis
Yes, you're right. In this particular example, the cracks are chronic,
which means that it is amenable to this sort of solution. If they're random,
that's a much more difficult situation.
i
J. Morellf, TWA
I am going to summarize some comments tomorrow morning to put the meeting
in perspective from an airline point of view. Quite honestly I would tend to
with the gentleman who just spoke here this morning from Pratt & Whitney,agree
that flying that kind of equipment is not the right way to go. But the thing
that is important to us, I would like to point out tomorrow, is the ability of
detecting cracks of any size installed in an engine. Because we quite honestly
# fall heir to problems that occur overnight and we're faced with a large fleet of .
engines and are faced with the problem of trying to segregate from those engines
• which are the ones we should worry about and which are all right. So I'm very
happy to see that work is being done in this area, because I feel it's extremely
important. But, perhaps, the fliqnt application is not the one that we would
choose as an airline, but instead something that could be done with the engine
installed on the airplane (and again to help isolate, because we have had
extremely good success in some applications), and I'd like to point that out
as we talk tomorrow.
S. Weiss, NASA-Lewis
When we first got into the rotor burst protection problem, we set up a
4 three-prong effort. The first thing that occurred to us was that you build
a better wheel, but if you succeed, _he designers concerned with increased
_ / performance will load it to its maximum and negate any safety benefits. Another
concept was also pursu_. That was to divert rotor fragments away from any vital
" parts of the airplane. Yet a third concept was to develop a system that would
warn the operator that he has a wheel that is going to fail, before it actually
does fail. This is the crack detector and it is not being developed as an NDE
device for inspection. We had hoped to lay the groundwork for development of
a sensor system that could be flluht certified for installation on an engine.
r Dr. Barringer suggested the idea of trying to modify eddy current devices
for installation in flight engines which would detect a crack of some reasonable
size. On the basis of fracture mechanics i_spection, a critical crack length
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1978002125-385
criterion might be established. Continuous monitoring of the growth of a
detected crack, with such a device, would permit re_val of the wheel before
the crack length grew to a danger threshold.
B.L. Koff, GE-Cinc.
In industry, we need to have fundamentally sound ideas in order to obtain
funding and I doubt that such a program could gain support. We learn by doing
but the scheme must be (a) practical in the end result and (b) one that will be
accepted.
/
J
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1978002125-386


Application of a flight-line disk crack detector to a small engine

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