Mark Woike

Glenn Research Center, Cleveland, Ohio

Ali Abdul-Aziz

Cleveland State University, Cleveland, Ohio

Michelle Clem and Gustave Fralick

Glenn Research Center, Cleveland, Ohio

Optical Strain and Crack-Detection Measurements

on a Rotating Disk

NASA/TM—2013-216523

June 2013

SPIE 8693-23
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Mark Woike

Glenn Research Center, Cleveland, Ohio

Ali Abdul-Aziz

Cleveland State University, Cleveland, Ohio

Michelle Clem and Gustave Fralick

Glenn Research Center, Cleveland, Ohio

Optical Strain and Crack-Detection Measurements

on a Rotating Disk

NASA/TM—2013-216523

June 2013

SPIE 8693-23

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

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subject to revision as analysis proceeds.
NASA/TM—2013-216523 1

Optical Strain and Crack-Detection Measurements

on a Rotating Disk

Mark Woike

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Ali Abdul-Aziz

Cleveland State University

Cleveland, Ohio 44115

Michelle Clem and Gustave Fralick

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract

The development of techniques for the in-situ measurement and structural health monitoring of the 

rotating components in gas turbine engines is of major interest to NASA. As part of this on-going effort, 

several experiments have been undertaken to develop methods for detecting cracks and measuring strain 

on rotating turbine engine like disks. Previous methods investigated have included the use of blade tip 

clearance sensors to detect the presence of cracks by monitoring the change in measured blade tip 

clearance and analyzing the combined disk-rotor system’s vibration response. More recently, an 

experiment utilizing a novel optical Moiré based concept has been conducted on a subscale turbine engine 

disk to demonstrate a potential strain measurement and crack detection technique. Moiré patterns result

from the overlap of two repetitive patterns with slightly different spacing. When this technique is applied 

to a rotating disk, it has the potential to allow for the detection of very small changes in spacing and radial 

growth in a rotating disk due to a flaw such as a crack. This investigation was a continuation of previous 

efforts undertaken in 2011 to 2012 to validate this optical concept. The initial demonstration attempted on 

a subscale turbine engine disk was inconclusive due to the minimal radial growth experienced by the disk

during operation. For the present experiment a new subscale Aluminum disk was fabricated and 

improvements were made to the experimental setup to better demonstrate the technique. A circular 

reference pattern was laser etched onto a subscale engine disk and the disk was operated at speeds up to 

12 000 rpm as a means of optically monitoring the Moiré created by the shift in patterns created by the 

radial growth due the presence of the simulated crack. Testing was first accomplished on a clean defect 

free disk as a means of acquiring baseline reference data. A notch was then machined in to the disk to 

simulate a crack and testing was repeated for the purposes of demonstrating the concept. Displacement 

data was acquired using external blade tip clearance and shaft displacement sensors as a means of 

confirming the optical data and for validating other sensor based crack detection techniques.

Introduction

The development of in-situ measurement technologies and fault-detection techniques for the 

structural health monitoring of gas turbine engines is of high interest to NASA’s Aviation Safety and 

Fundamental Aeronautics Programs. The rotating components of modern gas turbine engines operate in 

severe environmental conditions and are exposed to high thermal and mechanical loads. The cumulative 

effects of these loads can lead to high stresses, structural deformities, cracks, and eventual component 

failure. Current detection practices involve periodic inspections and schedule-based maintenance of 
NASA/TM—2013-216523 2

engine components to ensure their integrity over the lifetime of the engine. However, these methods have 

their limitations, and failures are experienced leading to unscheduled maintenance and unplanned engine 

shutdowns. To prevent these failures and enhance aviation safety, the NASA Glenn Research Center has 

investigated several new techniques and sensor technologies for the in-situ structural health monitoring 

and detection of flaws in gas turbine engines.

Over the last several years research has been conducted in the NASA Glenn Research Center’s Rotor 

Dynamics Laboratory to develop both global and local approaches for monitoring critical rotor components (Refs. 1 to 8). This paper will present data from a sensor based technique that was previously 

investigated along with the results of a recently conducted novel optical Moiré based strain measurement 

and crack detection experiment. The sensor based anomaly detection method involved introducing a notch 

on a simulated turbine engine disk and monitoring its vibration response as the disk was rotated at speeds 

up to 12 000 rpm. The vibration response was characterized by using externally mounted capacitive blade 

tip clearance sensors to measure the combined disk-rotor system’s whirl amplitude and phase during 

operation. Testing was performed on a clean undamaged baseline disk and a disk with a 50.8 mm (2 in.) 

long notch machined into the disk to simulate a crack. The responses were compared and evaluated 

against the theoretical models to investigate the applicability and success of detecting the notch.

The most recent investigation was to determine if an optical technique based on the Moiré principal

could be used to measure localized radial growth and strain due to high stresses caused by an anomaly

such as a crack in a rotating disk. The experiment involved laser etching a circular reference pattern on a 

subscale engine disk and introducing a 50.8 mm (2 in.) long notch machined into it to simulate a crack. 

The disk was operated at speeds up to 12 000 rpm and the Moiré pattern due to the shift with respect to 

the reference pattern was monitored as a means of detecting the localized radial growth of the disk due to 

the defect. In addition, displacement data sets were acquired using external blade tip clearance and shaft 

displacement sensors for comparison to the optical data and to further develop and validate other sensor 

based fault detection techniques.

Vibration Based Crack Detection Technique

This crack-detection methodology involved introducing a notch on a simulated turbine engine disk 

and monitoring its vibration response as the disk was rotated at speeds up to 12 000 rpm. The vibration 

response was characterized by monitoring the disk-rotor system’s whirl amplitude and phase during 

operation. The disk-rotor system’s whirl amplitude and phase were derived from the blade tip clearance 

profile which was measured using externally mounted capacitive blade tip clearance sensors. The testing 

was performed on a clean undamaged baseline disk and a disk with a 50.8 mm (2 in.) long notch 

machined into the disk to simulate a crack. The responses were compared to each other and the theoretical 

model to determine whether this technique could detect the presence of the notch. A description of the 

High Precision Spin Rig, the baseline theory behind this technique, the experimental setup, and the results 

are presented in the following sections.

High Precision Spin Rig Description 

Figure 1 shows the Rotordynamics Laboratory’s High Precision Spin Rig, which can accommodate 

simulated engine rotor disks of up to 235 mm (9.25 in.) in diameter. It has a stainless steel shaft with a 

length of 781 mm (30.75 in.) and diameter of 20 mm (0.79 in.). The shaft is supported by precision contact 

ball bearings on each end and has adjustable dampers that were positioned along the length of the shaft for 

these experiments. An encoder mounted on the end of the shaft was used by the control system to provide 

closed-loop control of rig speed. A secondary optical tachometer was used to record the speed into the data 

system and to synchronize the data to the rig’s rotation. A 12-hp custom-built, brushless direct-current (dc) 

motor was used to rotate the spin rig and the simulated engine disks at speeds up 12 000 rpm.
NASA/TM—2013-216523 3

Figure 1.—High precision spin rig. Figure 2.—Capacitive blade tip 

clearance probe.

The rig was set up to acquire two channels of radial blade-tip-clearance data from the simulated 

engine disk using capacitive displacement sensors (Fig. 2). Theses sensors were developed as part

of a NASA Small Business Innovation Research (SBIR) contract and were different from traditional 

capacitive sensors in that their operation was based on a dc offset technique instead of the typical 

modulation technique (Ref. 8). A National Instruments System was used to acquire data from the 

capacitive displacement probes at a fixed sampling rate of 1 MHz. This system and its application 

software were delivered as part of the SBIR contract for the capacitive blade-tip-clearance sensors.

The system used custom data acquisition and processing applications that were tailored for acquiring

and processing data from the capacitive sensors.

Experimental Theory and Setup

The theory behind the crack-detection methodology that was investigated was based on previous 

theoretical and experimental work performed by Abdul-Aziz et al., (Refs. 2 and 3), Gyekenyesi et al.,

(Refs. 7 and 8), and Haase and Drumm, (Ref. 9). The goal of this experiment was to determine if the 

crack-detection methodology investigated in these earlier studies could be validated by using the spin rig 

to conduct tests on simulated engine rotor disks with a notch introduced to replicate a crack.

The detection methodology is based on monitoring the vibration response of rotating disks to 

determine if a crack has developed. The theory implies that a defect, such as a crack, creates minute 

deformations in the disk as it is rotated. The deformation, in turn, creates a speed-dependent shift in the 

disk’s center of mass. It was theorized that this shift could be detected by analyzing the amplitude and 

phase of the vibration (whirl) response as measured by the tip clearance profile. The system’s behavior 

was modeled after a 2-degree-of-freedom Jeffcott rotor. The model predicts that the vibration amplitude 

peaks when a clean, undamaged disk goes through the first critical speed, but heads to a lower steadystate value as the speed is increased above the critical speed. Correspondingly, the phase shifts 180° when 

going through the first critical speed and then stabilizes to a steady-state value as the speed is increased 

past critical. At this point, the rotor is rotating about the combined system’s center of mass and has 

stabilized. However, as the speed of a cracked disk is increased, centrifugal forces open the crack. This, in 

turn, deforms the disk and shifts its center of mass. At speeds above critical, the crack-induced shift in the 

disk's center of mass starts to grow and dominate the overall system’s vibration response. The modeling 

12HP Motor

Simulated 

Engine Disk

Capacitive Tip 

Clearance Sensor

Shielding

Damper

Bearing 

Assembly

Encoder

Tachometer
NASA/TM—2013-216523 4

Figure 3.—Subscale turbine engine disk with notch. 

predicts that, instead of heading toward a steady-state value, the vibration amplitude will change as a 

second-order function of rotational speed as it is increased beyond the first critical speed. This can be 

detected by analyzing the vibration response of the disk-rotor system, particularly the amplitude and 

phase of the first harmonic, as the system is operated over a range of speeds. The testing approach used in 

this experiment was to spin a simulated engine disk over a speed range from 0 to 12 000 rpm and 

simultaneously record its vibration response. The vibration response was acquired as previously described 

using capacitive displacement probes to measure the blade tip clearance profile. Two subscale simulated 

engine turbine disks were tested. The first disk was undamaged and was used to acquire the baseline 

vibration response data. The disk had an outside diameter of 235 mm (9.25 in.), a bore thickness of

25.4 mm (1 in.), and an outside rim thickness of 31.75 mm (1.25 in.).

The thinnest portion of the disk’s web was 2.54 mm (0.10 in.). Thirty-two teeth to simulate blades 

were evenly spaced around the circumference of the disk. Each simulated blade had a cross section of

31.75 mm (1.25 in.) by 3.30 mm (0.13 in.) and a height of 8.38 mm (0.33 in.). The disk was made of a 

nickel base alloy, Haynes X-750 (Haynes International, Inc.) and had a weight of 4.88 kg (10.75 lb). 

The second disk is shown in Figure 3. It was identical to the baseline disk with the exception that a 

50.8 mm (2 in.) long notch had been introduced in its mid-span region to imitate a crack in the disk. The 

mid-span region was selected because prior finite-element analysis had shown that this area experienced 

high stress levels during operation. The data acquired from this disk were compared with the data from 

the undamaged baseline disk to determine if the simulated crack could be detected by analyzing the 

vibration response. Supportive analytical calculations were made using finite-element analysis to 

complement the experimental work and determine the expected radial growth of the disk. The finiteelement analysis predicted a radial growth on the order of ~0.075 mm (~0.003 in.) at the highest operating 

speed, 12 000 rpm. This was within the detection limit of the blade-tip-clearance instrumentation.

However, it should be noted that the major effect that was to be monitored was how the notch changed the 

combined center of mass of the disk-rotor system and its combined vibration amplitude and phase—not 

the radial blade tip growth of the disk.

Experimental Results

The experimentation consisted of operating the baseline and notched disks at several speed profiles 

and recording their vibration responses using the capacitive blade-tip-clearance sensors. The first harmonic component of the tip-clearance profile for each revolution was then analyzed to determine if the 

amplitude and phase exhibited the expected characteristics associated with the simulated crack in the disk.

Approximately 15 test runs were conducted for each disk. A typical test run or cycle consisted of ramping 

up the disk’s speed from 0 rpm to a predefined maximum speed, remaining on condition for 0 to 60 s, 
NASA/TM—2013-216523 5

Figure 4.—Simulated mission profile.

then ramping back down to 0 rpm. Blade-tip-clearance data were acquired over the entire cycle. Data sets 

were acquired at ramp rates of 60 and 100 rpm/s for peak operating speeds of 5000, 10 000, and 12 000 

rpm. In addition, complex mission profiles were conducted where the speed was ramped to various power 

levels in an attempt to simulate the rigorous loading conditions that an engine would experience during a 

typical mission. 

The run profile presented in this paper is shown in Figure 4. It is typical of the profiles that were used 

in the investigation. For this test case, the speed of the disk was increased from 0 to 10 000 rpm at a ramp 

rate of 100 rpm/s, cycled between 10 000 and 5000 rpm for three cycles, then ramped down from 10 000 

to 0 rpm. This was done to simulate a mission profile from cruise to full power and to observe the effects 

of cyclical loading on the disks. 

Figure 5 shows the results from this multiple-cycle test run for both the baseline and notched disks. 

The figure shows the analysis for the last cycle, starting at 10 000 rpm and ending at 0 rpm. As previously 

discussed, the presence of a crack is indicated if a second-order speed-dependent variation can be observed in the vibration amplitude as it is operated in the post critical speed regime. In this test case, a 

speed-dependent rise was observed in the amplitude data for the notched disk. The area of interest 

showing the speed-dependent rise is highlighted within the circle of Figure 5(b). A curve-fit analysis, 

shown in Figure 6, was conducted on the vibration data in this region and it was found to closely follow a 

second-order polynomial fit. Thus, possibly indicating the presence of the notch or simulated crack. Even 

though the vibration data looked promising in this region, it was also observed that the second-order 

speed-dependent variation was not consistent over the entire range and that it appeared to reset itself at a 

speed of ~9500 rpm. Moreover, this amplitude variation was not consistently observed in other test runs, 

which cast some doubt on whether the effect was entirely due to the notch or to some other factor that is 

yet to be determined. However, this case yielded the best data so far and showed positive indications of 

being able to detect a defect such as a crack by analyzing the disk’s vibration response, specifically the 

vibration amplitude as a function of disk speed. It is theorized that what made this case different, and 

more promising than the other test runs, is that this test was conducted towards the end of a test series and 

that many cycles had been placed on the disk prior to this run. In addition, the disk experienced more 

loading because of the cyclical nature of the test profile. Overall, the results were promising and plans are 

in place to further investigate and develop this crack-detection technique as part of future studies.
NASA/TM—2013-216523 6

(a) (b)

Figure 5.—10 000-rpm multiple-cycle run comparison. (a) Baseline disk. (b) Notched disk.

Figure 6.—Detailed vibration amplitude plot for 10 000-rpm multiple-cycle run.
NASA/TM—2013-216523 7

Moiré Optical Based Crack Detection/Strain Measurement Technique

The most recent investigation undertaken was to determine if an optical technique based on the Moiré 

principal could be used to observe localized radial growth due to high stresses caused by an anomaly such 

as a crack in a rotating disk. This optical strain measurement technique is being developed in conjunction 

with NASA’s Aeronautical Science Project—Innovative Measurements task. The experiment involved 

applying a circular reference pattern onto a subscale engine disk that had a notch machined into it to 

simulate a crack. The disk was operated at speeds up to 12 000 rpm as a means of optically monitoring 

the Moiré created by the shift in patterns created by the radial growth of the disk due the presence of the

simulated crack. In addition, displacement data sets were acquired using external blade tip clearance and 

shaft displacement sensors for comparison to the optical data. This investigation is a continuation of 

previous efforts undertaken in 2012 (Ref. 12) for the demonstration of this optical concept. The initial 

demonstration attempted on a subscale turbine engine disk and was inconclusive due to the lack of

minimal detectable radial growth experienced by the disk during operation. For the present experiment a 

new subscale turbine engine disk was fabricated out of Aluminum and improvements were made to the 

experimental setup to better demonstrate the technique. Testing was first accomplished on a clean defect 

free disk as a means of acquiring baseline reference data. A notch was then machined on to the disk to 

simulate a crack and testing was repeated for the purposes of demonstrating the Moiré optical detection 

concept. The results from the previous investigation in 2012, and the partial results from the current 

baseline “clean” disk testing will be presented. 

Theory

Moiré patterns are quite common in everyday occurrences and result from the overlap of figures with 

periodic spacing (Refs. 10 and 11). For this experiment it was desired to use the geometrical relationship 

associated with Moiré patterns as a means of optically measuring localized radial growth or strain in a 

rotating disk. The concept behind this technique is as follows.

Figure 7 shows the overlap of two vertical patterns. The first pattern has a vertical line spacing of p, 

while the second pattern has been “stretched” and has a vertical line spacing of q, which is related to p by 

the following equation:

0 1

,

< ∆ <

q = p + ∆p (1)

Figure 7.—Example of a Moiré pattern.
NASA/TM—2013-216523 8

If the patterns are lined up starting on the left side, the shift increases going to the right. Initially the lines

of the patterns coincide and match up. This represents a light zone. After a given number of lines, the

lines of the second pattern fall between the lines of the first. This represents a dark zone. This appearance 

alternates depending on whether the lines coincide (light zone) or are opposed (dark zone). The middle of 

the first dark zone is when shift in spacing between the first and second pattern is p/2. The n

th line of the

second pattern is shifted by n∙∆p when compared to the n

th line of the first. Thus, at the middle of the first 

dark zone the following relationships can be established:

2

p n∆p = (2)

n = p /(2∆p) (3)

Where n in Equation (3) is the number of lines on the second pattern that it takes to get to the middle of 

the dark zone. The distance, D, between the initial start point or the middle of a light zone, and the middle 

of the dark zone is then defined by the following:

D = n( p + ∆p) (4)

Substituting Equation (3) for n at this point yields:

2 2

2

p

p

p D +

∆ = (5)

The distance between two dark zones or two light zones would be 2D. Finally, rearranging Equation (5)

to solve for ∆p yields the following:

(2 )

2

D p

p p − ∆ = (6)

If the first pattern is as a baseline image of a test article in its initial state and the second pattern is

obtained while the article is being loaded or stretched. Strain, ε, the test article is undergoing would be 

given by the following equations:

p

∆p ε = (7)

(2D p)

p

− ε = (8)

Thus, by knowing the initial line spacing, p, and optically observing the distance between two dark zones

or two light zones, 2D, or the distance between a light and dark zone, D, the difference in spacing, ∆p, 

and hence strain, ε, can be calculated.
NASA/TM—2013-216523 9

Figure 8.—Moiré concept applied to a rotating disk.

Technical Approach

The Moiré effect amplifies small changes and has the potential to make these changes optically 

measureable by observing the spacing between the dark or light zones of two patterns overlaid on each 

other. This characteristic is the basis of the experiment to measure radial growth or strain on a rotating 

disk due to high stresses caused by operation or an anomaly such as a crack. Figure 8 shows the overall 

concept behind the experiment. The technical approach was to apply a pattern of concentric equally 

spaced rings at a known spacing on a subscale turbine engine disk and acquire images of it at both static, 

0 rpm, and at a loaded condition of 12 000 rpm. The image that was taken on-condition was then overlaid 

on the static reference image using image processing software. Registration marks that were placed on the 

disk were used for sub-pixel alignment. The composite image was then analyzed for the appearance of the 

dark and light zones due to the radial growth of the disk. Measuring the distance between the dark zones, 

2D, or between a light and dark zone, D, and knowing the initial spacing, p, would allow the calculation 

of the strain and radial growth that the disk was subject to during operation. To enhance the localized 

stress that the disk would experience during operation, a 50.8 mm (2 in.) long notch was machined into it 

to simulate a crack.

Experimental Setup Phase I Test Entry (2012)

The subscale simulated turbine engine disk that was used in the initial experiment was the same disk 

that was used in the previously discussed Vibration Based Crack Detection experiments and is shown in 

Figure 9. The disk is 235 mm (9.25 in.) in diameter and has 32 evenly spaced teeth to simulate blades. Its 

geometry is representative of that encountered in turbine engine disks. The notch and image pattern that 

was machined onto the disk is also shown in Figure 9. The notch was machined into the disk prior to the 

experiment and was used for previous engine fault and crack detection experiments. The notch is located 

mid-span on the disk in the thinnest region. It is 50.8 mm (2.00 in.) long, 0.38 mm (0.15 in.) wide, and 

fully penetrates the disk with a depth of 2.54 mm (0.10 in.). The notch’s purpose was to simulate a crack

and enhance the localized stress and growth in the area around the crack. While considered large for 

realistic applications, it has served as an ideal test case in previous proof-of-concept fault and crack 

detection studies similar to the Moiré concept being investigated in this paper. The pattern that was 

applied to the disk in order to view the Moiré shift consisted of concentric circles that were 0.254 mm 

4

Reference Image 

Acquired at Static 

Conditions (0 RPM)

Image Acquired On 

Condition at 12000 RPM

Overlay of “On Condition” 

image on “Reference” 

image to get disk growth

2D
NASA/TM—2013-216523 10

Figure 9.—Subscale engine disk with notch and 

pattern applied.

Figure 10.—Setup for Moiré crack detection experiment.

(0.010 in.) wide and uniformly spaced 0.508 mm (0.020 in.) apart. The pattern was machined on to the

disk using a precision lathe. The circles were made just deep enough, ~0.08 mm (~3 mils), so that a 

material could be applied to the grooves to provide contrast when imaging. A blue dye used in machining 

applications was applied to the disk then lightly sanded off, leaving the dye in the grooves to provide the 

needed contrast for viewing the circular pattern. Registration marks were also added to the disk to 

facilitate the alignment of the images acquired statically and on-condition.

One issue that was encountered with the application of the patterns is that, due to the curvature of the 

disk’s surface, only the flat portion of the disk from a radius beginning at 47.2 mm (1.86 in.) and ending 

at 71.6 mm (2.82 in.) was able to have the circles applied as opposed to covering the desired hub-to-blade 

region of the disk, which began at the shaft and ended at a radius of ~109 mm (4.275 in.). It was also 

intended for the circle width and spacing to be on the order of 0.13 mm (0.005 in.). However, this was not 

able to be done in a consistent manner using the precision machining method that was employed, and 

resulted in the use of the larger 0.254 mm (0.010 in.) width and 0.508 mm (0.020 in.) spacing for the 

experiment. Both of these issues combined, limited the ability to observe the Moiré dark and light zones

set up in the pattern. Even with these limitations, it was decided to still proceed with the experiment to 

determine what could be observed and use it as a learning opportunity for further refining this technique 

for future use.

As was done for the vibration based crack detection experiment, the NASA Glenn Research Center’s 

High Precision Spin Rig was used to operate the disk for this investigation. The setup for the experiment 

is shown in Figure 10. A five (5) Megapixel miniature machine vision digital camera with a 12 mm and 

25 lens was used to acquire images of the disk statically and when it was being operated at 12 000 rpm. 

To capture the image of the disk as it was being spun a 10 W white light emitting diode (LED) with 

pulsed power supply was used as a strobe. A laser 1/rev was used to precisely drive the strobe as a 

function of rotor location and speed. In-house designed and fabricated pulse delay and pulse width control 

circuitry was used to condition the 1/rev signal for control of the strobe. The pulse delay was varied in 

order to image different regions of the disk while it was spinning. The pulse width was adjusted to obtain 

the crispest image of the disk while it was on test condition. In addition to the optical instrumentation 

listed above, microwave and capacitive blade tip clearance (BTC) sensors were used to acquire blade tip 

clearance data. These sensors, shown in Figure 10, were located 180° across from each other on the disk. 

The purpose of using this additional instrumentation was to compare any radial growth observed optically 

using the Moiré principle with the radial growth measured by these external sensors.

Circular Patterns 2” Long Notch Registration Marks

LED Strobe

Camera

Pattern on 

Disk w Notch

Capacitive BTC Sensor Microwave BTC Sensor
NASA/TM—2013-216523 11

Experimental Results Phase I Test Entry (2012)

Optical Data

As previously stated the methodology that was used to conduct the experiment was to acquire 

reference images of the disk at various locations at the static, 0 rpm condition, then spin the disk up to

12 000 rpm and acquire images of the disk at full speed operation in the same the locations that the 

reference images were acquired at. The two locations that were chosen were the region located by the 

notch and a clean, defect free region, located 180° across from it. These were chosen as a means of 

viewing the radial growth associated with normal loading of the disk and the localized radial growth due 

to the simulated crack. The images were then post processed by first aligning them using the registration

marks located on the inside radius of the disk and then overlaying the image that was acquired at

12 000 rpm onto the image that was taken statically. Imaging processing software was used to accomplish 

the sub-pixel alignment and overlay of the images. The composite image was then analyzed for the 

appearance of the Moiré light and dark zones due to the radial growth of the disk.

Figure 11 shows the images that were acquired for the clean, or baseline region, of the disk using a

12 mm lens. Figure 11(a) shows the reference image acquired at static conditions, Figure 11(b) shows the 

on-condition image acquired at 12 000 rpm and Figure 11(c) shows the resulting composite image 

obtained by overlaying the two images. For the baseline region of the disk the two images lined up 

exactly on top of each other and the light and dark zones due to the Moiré shift were not observed. Thus, 

indicating that very little radial growth was observed during high speed operation, which was expected for 

this side of the disk.

(a) (b)

(c)

Figure 11.—Baseline region. (a) Static reference image. (b) On-condition image acquired at 12 000 rpm.

(c) Composite image.
NASA/TM—2013-216523 12

Figure 12 shows the resulting composite image obtained by overlaying the on-condition and reference 

images that were acquired for the region of the disk where the notched was located. A 25 mm lens was 

used to magnify the region of interest. Again, as previously observed, the anticipated light and dark zones 

due to the Moiré shift were not observed. The static reference image patterns lined up almost perfectly on 

the patterns of the image taken at 12 000 rpm. Thus, further verifying that the disk was experiencing very 

little if any growth due to the notch and the loads induced on it during operation. It should be noted that 

the same results were also observed when using the 12 mm lens on this area of the disk. This was 

unexpected, as finite element analysis showed that up to a ~0.075 mm (~0.003 in.) localized radial growth 

of the disk could be observed in the region of the notch under these loading conditions. The expectation 

was that at least the initiation of a pattern shift would be observable, especially in the rings beginning at 

the notch region and heading towards the outside edge of the disk. To further verify the lack of radial 

growth a line intensity profile analysis was conducted over the last 10 rings on the composite image to 

determine if there was a shift due to radial growth of the disk as this would represent the best region to 

detect any growth.

The region that was used for the intensity profile is indicated by the yellow line in Figure 12. The 

intensity profile is shown in Figure 13. The green line indicates the intensity for the image that was 

acquired at 12 000 rpm and the blue line indicates the intensity of the static reference image. The valleys 

on the chart indicate the location of the grooves or dark lines of the pattern. The peaks indicate the space 

between the grooves or the light lines of the pattern. From visually analyzing the profile it is evident that 

the peaks and valleys of the intensity profiles match each other in the x-axis or radial direction on the 

disk. Again, this verifies that the disk experienced very little if any radial growth during operation, or that 

the growth is under the detection limits using this optical technique. 

Figure 12.—Composite image of notch region at

12 000 rpm using 25 mm lens. 

Figure 13.—Intensity profile.

Static Reference Image

12000 RPM Image
NASA/TM—2013-216523 13

Figure 14.—Blade tip clearance profile.

Sensor Data

As part of this experiment displacement data sets were acquired using external blade tip clearance and 

shaft displacement sensors for comparison to the data obtained from the optical technique. Both microwave and capacitive based blade tip clearance sensors were used to measure the radial clearance for each 

blade and hence the radial growth of the disk. The variation in blade tip clearance as measured by the

microwave blade tip clearance sensor over 1 revolution, 32 blades, at a rotational speed of 12 000 rpm is 

shown in Figure 14. The red line indicates the region of the disk that contains the notch. From analyzing 

the graph a variation of +0.03 mm (+0.001 in.) to –0.04 mm (–0.0016 in.) is observed during operation of 

the disk. This variation observed is due to the whirl amplitude caused from Rotordynamics of the diskrotor system rather than radial growth of the disk. A shaft whirl amplitude on the order of ±0.03 mm 

(0.001 in.) has been measured and observed at 12 000 rpm on the eddy current shaft displacement sensors. 

In addition, in the area of the notch, the blade tip clearance is observed to be stable or actual slightly

increasing, which is counter to what would be expected if the disk experienced localized growth in this 

region. Hence, the external sensor data verified what was observed on the optical data regarding the

minimal blade tip radial growth on the disk during this experiment.

Experimental Setup Phase II Test Entry (2013)

Since the previous experiment was inconclusive due to the minimal radial growth experienced in the 

Haynes X-750 disk, several improvements were attempted for the continued investigation of this technique in 2013. First, a new simplified subscale disk was fabricated out of Aluminum (Fig. 15). This disk 

was made of a simple flat geometry which would better allow for the application of the circular Moiré 

pattern over the entire surface of the disk. This disk was 12.7 mm (0.50 in.) thick, 235 mm (9.25 in.) in 

diameter and had 32 evenly spaced teeth to simulate blades, each 12.7 mm (0.50 in.) long and 3.18 mm 

(0.125 in.) thick. Aluminum was chosen because it properties would allow for greater radial growth at the 

test conditions that it would be operated at. Thus, allowing for a better demonstration of this novel 

measurement concept. As in the previous experiment a notch was machined into the disk. The notch was

located mid-span on the disk. It was 50.8 mm (2.00 in.) long, 0.30 mm (0.015 in.) wide, and fully 

penetrates the disk with a depth of 12.7 mm (0.50 in.). Again, the notch’s purpose was to simulate a crack

and enhance the localized stress and growth in the area around the crack. The second improvement that 

Notch Region
NASA/TM—2013-216523 14

Figure 15.—Aluminum rotor disk with laser 

etched circular pattern.

was attempted was to apply the circular pattern to the disk using a laser etching process. This process 

yielded concentric circles that were 0.127 mm (0.005 in.) wide and uniformly spaced 0.254 mm

(0.010 in.) apart. This was one half of the previous width and spacing achieved using the precision lathe

application technique, thus doubling the pattern’s measurement sensitivity. The pattern was able to be 

nearly applied nearly over the entire surface of the disk, starting at a radius of 24.9 mm (0.98 in.) and

ending at a radius of 95.5 mm (3.76 in.). Registration marks were also added to the disk to facilitate the 

alignment of the images acquired statically and on-condition. The test setup in the High Precision Spin 

Rig was nearly identical to the setup discussed previously. The only difference this time was the use of a 

different light source controller which would allow for pulse width as low as 1 μs, in order to capture 

crisper images of the disk while it was rotating.

Experimental Results Phase II Test Entry (2013)

Baseline Optical Data

For this test entry it was decided to first test on a clean disk without the notch installed in order 

toobtain baseline data for demonstrating both the optical strain measurement crack-detection technique 

and the previously described vibration based crack detection technique. As in the phase I test entry, 

reference images were first acquired at the static, 0 rpm condition, then after the disk was spun up to

12 000 rpm, images were acquired on condition at the same locations as the reference images.

Figure 16 shows the composite images that were acquired for the region where it is intended to locate

the notch at. The composite images were made by overlaying the images acquired at 12 000 rpm over the 

reference images. Figure 16(a) shows the composite image acquired using the 25 mm lens, Figure 16(b) 

shows the same composite image zoomed in to focus on the outer rings of the disk. From analyzing these 

images, no apparent shift was observed. Which was expected on this baseline disk as minimal growth was 

expected on the clean disk as it was operated at 12 000 rpm. This was confirmed by running a line 

intensity profile over the last 10 rings on the composite image as this area would yield the largest 

detectable radial growth. The region that was used for the intensity profile is indicated by the yellow line 

in Figure 16(b).

The intensity profile is shown in Figure 17. The green line indicates the intensity for the image that was 

acquired at 12 000 rpm and the blue line indicates the intensity of the static reference image. The valleys on 

the chart indicate the location of the dark lines of the pattern. The peaks indicate the light lines of the 

pattern. From analyzing the profile it is evident that the peaks and valleys of the intensity profiles match 

each other in the x-axis or radial direction on the disk. Again, this verifies that the baseline, undamaged disk 

experienced very little if any radial growth during operation. Which again was expected as the purpose of 

this test was to get baseline data for comparison to the notched disk data. However, a decrease in average 

blade tip clearance on the order of 0.04 mm (0.0016 in.) was observed in the capacitive displacement sensor 
NASA/TM—2013-216523 15

data during the time interval that the disk was operated at 12 000 rpm. It is not known if this was real or due 

to some temperature related phenomena on the sensor. One possible explanation is that the Aluminum 

disk grew due to the rig’s temperature increasing during operation. With a thermal expansion coefficient 

of 12.3E-6 in./in. per °F for Aluminum this would translate to an increase in disk temperature of 36 °F. 

While not out of the realm of possibilities, as the rig’s motor does heat up the rig environment when 

operated at 12 000 rpm, it is unlikely that the disk heated up that much in the 8 min that it was operated at 

for the test run. In addition, the radial growth of 0.04 mm (0.0016 in.) should have also been detectable in 

the optical data as this translates to a 2 to 3 pixel shift, which would have been observable in the outer 

rings of the disks pattern. A more likely scenario is that the sensitivity of the capacitive was affected by 

the increase of rig temperature, causing the apparent shift in measurement. This discrepancy is in the 

process of being reviewed and addressed in the next phase of testing with the notched disk.

(a) (b)

Figure 16.—Aluminum baseline disk at 12 000 rpm. (a) Composite image. (b) Zoomed in to outer edge.

Figure 17.—Intensity profile Aluminum baseline disk.
NASA/TM—2013-216523 16

Notched Disk Optical Data

The disk with the notch is currently in the process of being tested and data was not yet ready to be

presented at this time. It is planned to incorporate the additional of temperature monitoring to assess the

increase in rig temperature and its effect on the blade tip clearance sensor readings and Aluminum rotor disk 

growth. In addition, a higher power lens and higher resolution camera will be used to maximize optical

sensitivity. Current plans are to complete this testing and present the results as part of a future paper.

Conclusion

The results of two crack detection experiments were presented in this paper. The first experiment

involved monitoring the vibration response on a disk as it was operated at speeds up to 12 000 rpm. The 

vibration response was measured using externally mounted capacitive blade tip clearance sensors. Testing 

was performed on a clean undamaged baseline disk and a disk with a 50.8-mm- (2-in.-) long notch

machined into the disk to simulate a crack. Test runs were conducted over several operational speeds to

determine if a simulated crack could be detected by analyzing the combined disk-rotor system’s vibration 

response (whirl amplitude and phase) as measured by the blade-tip-clearance sensors. The data acquired 

from the 10 000 rpm multiple-cycle test run yielded the best results. It showed a speed-dependent rise in 

the vibration amplitude, indicating that a crack-induced shift in the disk’s center of mass was detectable at 

least for this particular test scenario. In the other test cases, it was noted that the system was on the verge 

of being able to detect the simulated crack using this technique, but not all of the expected trends were 

consistently observed. However, this technique does look promising and appears to have the sensitivity

to detect cracks in a rotating disk. Additional investigations will be conducted in the future to further

validate and refine this crack detection concept.

The second investigation was to determine if an optical technique based on the Moiré principal could 

be used to measure localized radial growth and strain due to high stresses caused by an anomaly such as a 

crack in a rotating disk. The experiment involved etching a circular reference pattern on a subscale engine 

disk and introducing a 50.8 mm (2 in.) long notch machined into it to simulate a crack. The disk was

operated at speeds up to 12 000 rpm and the Moiré pattern due to the shift with respect to the reference 

pattern was monitored as a means of detecting the localized radial growth of the disk due to the defect. 

The initial phase I investigation under taken in 2012 was found to be inconclusive as the test disk did not 

experience the radial growth that was expected in the notched area during operation. For the present

phase II experiment (2013) a new subscale turbine engine was fabricated out of Aluminum and improvements were made to the pattern application to better demonstrate the technique. Testing was first accomplished on a clean defect free disk as a means of acquiring baseline reference data. This baseline data 

confirmed that the clean, undamaged disk experienced little or detectable radial growth as was predicted 

and expected by the finite element analysis model. A notch was then machined on to the disk to simulate 

a crack and testing was to be repeated for the purposes of demonstrating the technique. The optical data 

for the notched disk is currently in the process of being acquired and is not ready to be reported on at this 

time. However, this novel optical measurement technique does look promising and will continue to be 

investigated along with other optical techniques. In addition, displacement data that was acquired using 

blade tip clearance and shaft displacement sensors on the clean and notched disk during this test entry will 

be used to further validate the previously discussed vibration based crack detection technique. Plans are to 

report on these validation results and the optical data acquired on the notched disk in future papers.
NASA/TM—2013-216523 17

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