System can be utilized in flight or at flight line. It monitors disk rim for surface cracks emanating from blade root interface. System consists of eddy-current sensor, mounted approximately 1 1/2 mm (1/16 in) away from face of disk, and remotely located electrical capacitance-conductance bridge and signal analyzer

Barranger, J. P.

English

NASA Technical Reports Server

•	 Aril 1975
	
B75-10012 
O
	
NASA TECH BRIEF 
Lewis Research Center 
NASA Tech Briefs announce now technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questons relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
In-Service Turbine Wheel Crack Monitor 
A turbine wheel crack monitor has been developed 
which can be utilized in flight or at the flight line. It 
monitors the disk rim for surface cracks emanating from 
the blade root interface. 
Rotating parts present the most critical design and 
maintenance problems in an aircraft jet engine. Modes of 
failure depend not only on the material, but also on the 
operating stresses, vibration, temperature, and environ-
ment. Compressor, fan, and turbine disks are subject to 
fatigue-type failures because of the cyclic nature of 
stresses caused by temperature and centrifugal forces. 
Maintenance reports indicate that radial crack propagation 
from the blade root area of the disk rim has been 
frequently encountered in service. Further, some disks are 
designed with abrupt changes in web thickness that result 
in circumferential fatigue cracks. If not detected, these 
cracks can ultimately lead to engine failures. 
The usual nondestructive tests for turbine disk surface 
cracks are the liquid penetrant, ultrasonic, and eddy 
current methods. These procedures usually require engine 
disassembly. Therefore, the inspections can be performed 
only when the engine is being overhauled. 
This crack monitoring system consists of an eddy-
current sensor, mounted approximately 16 mm (1/16 in) 
away from the face of the disk, and a remotely located 
electrical capacitance-conductance bridge and signal 
interface 
Fig ure 1. - Typical turbine wheel segment showing location of a radial surface 
crack and position of sensor.
analyzer. The sensor is a coil of wire wound on a bobbin 
of glass ceramic insulator material. The sensor can 
withstand operating temperatures of 811 K (10000F), 
permitting its use in the high-temperature environment of 
the turbine section. 
As the rotor spins, the disk is monitored by the sensor 
for surface or near-surface cracks emanating from the 
blade root interface. Figure 1 illustrates a typical turbine 
wheel segment, showing the location of a radial surface 
crack and the position of the sensor. The sensor can be 
made an integral part of the stator or other appropriately 
located supporting strut. 
Figure 2 is a simplified block diagram of the crack 
monitor. The sensor coil and cables are located inside the 
engine, while the automatic capacitance-conductance 
bridge and the crack signal analyzer are located remotely. 
The sensor and the bridge are connected by series 
capacitances and connecting cables. 
Engine
-I 
I Sensor 
:coil
Q----.Q U Q-1 
Shielded	 Cs Shielded	 II 
sensor cables bridge cables	 I 
L__ O.6m(2ft) 3	 m(12.ft)
C rack	 Automatic To warning	 capacitance-
analyzer indicator
signal	
-'-e-	 conductance 
bridge
CS-64556 
Figure 2. - Simplified block diagram of crack monitor. 
The sensor cables, 3.18 mm (0.125 in) in diameter, 
pass through hollow stator blades or structural support 
members. The cables are made of thermocouple-type 
materials that can withstand inside-the-blade temperatures 
of 1200 K(1700°F). 
The bridge used in the monitoring system is designed 
to measure capacitance and conductance. By adding 
capacitances in series with the sensor coil, the combined 
reactance can be made capacitive. 
Radial 
surtac
(continued overleaf) 
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https://ntrs.nasa.gov/search.jsp?R=19750000012 2020-03-19T19:58:45+00:00Z
Eddy current devices are sensitive to temperature and 
displacements between the coil and the surface, as well as 
to all surface discontinuities. The impedance of the coil 
varies with temperature because of changes in the resist-
ance of the coil and of the disk. The capacitance-
conductance bridge is self-balancing, automatically 
adjusting ,
 to changes in average coil inductance and 
resistance caused by temperature effects and variations in 
disk-to-sensor spacing. The bridge has two phase 
detectors, one whose output voltage is proportional to the 
unbalanced conductance. 
The bridge takes about ½ second to balance, after 
which time the output of the conductance phase detector 
is monitored for out-of-balance voltages. The output of 
the conductance phase detector is used in the system since 
it is less sensitive to variations in disk-to-sensor spacing, 
during the monitoring time, than the capacitance phase 
detector. The sequence, balance then monitor, is repeated 
every 10 seconds. As the blade root region passes the 
sensor during the monitoring time, the conductance phase 
detector senses the change in effective conductance of the 
sensor coil. 
The crack signal analyzer compares the crack voltage 
and the noise voltage to determine whether a crack exists. 
The noise voltage can be determined by measuring the 
negative peak amplitude of the circuit noise and blade 
root signal. The crack voltage is determined by measuring 
the positive peak voltage of the sum of the crack signal, 
the circuit noise, and the blade root signal. The two 
voltages are compared, and a crack is indicated if the 
crack voltage is greater than 1.3 times the noise voltage. 
The quantity 1.3, which was derived experimentally, 
prevents random pulses from exceeding the noise voltage 
and causing false crack indications.
Notes: 
I. Tests indicate that the system is useful' at disk rim 
velocities to 460 rn/sec ( 15 00 ft/sec). 
2. The disk-crack detector should be of interest to 
manufacturers or users of electric ground based power 
systems. 
3. Further information is available in the following 
report: 
NASA TN-D.7483 (N74-12187), Flight Monitor 
for Jet Engine Disk Cracks and the Use of Critical 
Length Criterion of Fracture Mechanics 
Copies may be obtained at cost from: 
Aerospace Research Applications Center 
Indiana University 
400 East Seventh Street 
Bloomington, Indiana 47401 
Telephone: 812-337-7833 
Reference: B75-10012 
4. Specific technical questions may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B75-10012 
Patent Status: 
NASA has decided not to apply for a patent. 
Source: John P. Barranger

Lewis Research Center

(LEW-1 2422)
. 
a 
S 
B75-10012	 Category 02


In-service turbine wheel crack monitor

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