The nature of the physical processes and harsh environments associated with erosion and wear in propulsion environments makes their measurement and real-time rate quantification difficult. A fiber optic sensor capable of determining the wear (regression, erosion, ablation) associated with these environments has been developed and tested in a number of different applications to validate the technique. The sensor consists of two fiber optics that have differing attenuation coefficients and transmit light to detectors. The ratio of the two measured intensities can be correlated to the lengths of the fiber optic lines, and if the fibers and the host parent material in which they are embedded wear at the same rate the remaining length of fiber provides a real-time measure of the wear process. Testing in several disparate situations has been performed, with the data exhibiting excellent qualitative agreement with the theoretical description of the process and when a separate calibrated regression measurement is available good quantitative agreement is obtained as well. The light collected by the fibers can also be used to optically obtain the spectra and measure the internal temperature of the wear layer

Korman, Valentin

Polzin, Kurt A.

NASA Technical Reports Server

Statement A: Approved for public release. Distribution is unlimited. 
JANNAF paper 2011-0015FV - 8th MSS / 6th LPS / 5th SPS Joint Army-Navy-NASA-Air Force (JANNAF) Meeting. 
Dec. 2012. 
TESTING OF A FIBER OPTIC WEAR, EROSION AND REGRESSION SENSOR 
 
Valentin Korman 
K Sciences GP, LLC, Huntsville, AL 35805 
 
Kurt A. Polzin 
NASA - Marshall Space Flight Center, Huntsville, AL 35812 
 
ABSTRACT 
The nature of the physical processes and harsh environments associated with erosion and wear 
in propulsion environments makes their measurement and real-time rate quantification difficult. A 
fiber optic sensor capable of determining the wear (regression, erosion, ablation) associated with 
these environments has been developed and tested in a number of different applications to 
validate the technique. The sensor consists of two fiber optics that have differing attenuation 
coefficients and transmit light to detectors.  The ratio of the two measured intensities can be 
correlated to the lengths of the fiber optic lines, and if the fibers and the host parent material in 
which they are embedded wear at the same rate the remaining length of fiber provides a real-time 
measure of the wear process. Testing in several disparate situations has been performed, with 
the data exhibiting excellent qualitative agreement with the theoretical description of the process 
and when a separate calibrated regression measurement is available good quantitative 
agreement is obtained as well.  The light collected by the fibers can also be used to optically 
obtain the spectra and measure the internal temperature of the wear layer. 
INTRODUCTION 
Harsh environments, by their very nature, adversely affect materials commonly associated with 
propulsion systems. These materials may be subject to high temperature, high energy particle 
impingement, exposure to ionized gases, or surface-to-surface frictional interaction. These 
different environments can all wear the exposed surface in some manner (erosion, regression, 
etc).  In propulsion applications quantification of the wearing processes and rates is critical to 
propulsion component development and qualification, but it can be difficult to experimentally 
quantify these wearing mechanisms. 
Wear metrics (including regression and erosion rates) are typically based on numerical 
models [1]. Few sensing devices or methods are capable of performing intrinsic wear 
measurements, particularly in real-time and in situ. Commonly, non-real-time extrinsic methods 
are instead used to measure wear, with most of these involving interruption of a test to permit 
physical measurements using more traditional methods [2]. Available real-time measurement 
methods rely on either surface profiling (e.g. laser profilometers) [3] or eroding ‘sensors’ which 
are capable of monitoring the temperature within a wearing environment but do not themselves 
have the same in situ response to the actual wear mechanisms affecting the material under test 
[4-5].  
Recently, a fiber optic sensor was presented that is capable of measuring real-time in situ 
regression, erosion and wear in a variety of materials under numerous conditions [6]. The 
Regression, Erosion and Ablation Sensor Technology (REAST) [7] was initially developed to 
perform real-time measurement of the regression of solid rocket propellants, but it has since been 
demonstrated in a wide range of applications, including but not exclusively limited to propulsion.  
In this paper, we briefly review the REAST sensing technique and then present wear data 
obtained for many different applications in a number of different environments demonstrating the 
wide applicability of the method. 
https://ntrs.nasa.gov/search.jsp?R=20120002945 2019-08-30T19:28:12+00:00Z
SENSING PRINCIPLE 
Fundamental to the operation of the fiber optic wear sensor is the transmission of light through an 
attenuating waveguide (e.g. optical fiber) as described mathematically by Beer’s law [8] and 
shown schematically in Fig. 1. The attenuation coefficient, α, carries units of loss per unit length 
(e.g. dB/cm) and is an intrinsic property of the particular waveguide material. For high quality fiber 
optic glasses attenuation values are on order of a single dB/km. The attenuation of the fiber may 
be altered by a variety of methods to achieve values as high as 10 dB/mm. The REAST sensor, 
shown schematically in Fig. 2, is formed using two optical fibers with differing attenuation 
coefficients. The fibers are embedded in a parent material and both it and the fibers are worn or 
eroded over time by the same process. In the simplest case, the thermal environment responsible 
for the wear mechanism also provides the light collected by the sensor.   
 
Figure 1 – Schematic description of Beer’s law governing light propagation through a waveguide 
of given an attenuation α and length L [8]. 
 
In the limit where the cross-sectional area of the fiber is small compared to the exposed surface 
of the host material, we can assume that the amplitude of the light collected from the wearing 
process is equal for the two fibers. This permits algebraic manipulation to solve the equations 
shown in Fig. 2 for length. Here length refers to the fiber length, which will provide a measure of 
the host material wear rate if the two wear at the same rate. In practice, the tips of the two 
sensing fibers need to have differing attenuation coefficients only over the distance of expected 
wearing. The probes are typically formed from fusion splicing the attenuating regions upon a 
length of standard low-loss optical fiber. The low-loss fibers act as light conduits permitting the 
detector electronics to be located away from the wearing zone.  
Detection of the optical signal is done through photoelectric conversion within a photodiode. 
Silicon photodiodes have shown a sufficient response when the temperatures at the wearing 
layer surpass 350 
o
C. For cooler wear processes, two alternative options have been 
demonstrated. A photodiode sensitive to longer wavelengths (and thereby cooler blackbody 
temperatures) may be used for detection if the temperature isn’t too cool. Alternatively, a third 
fiber may be embedded into the parent material  to backward propagate an exterior source of 
light (provided by a light emitting diode, laser, etc) that can be re-collected by the fibers 
comprising the sensor. 
 
 Figure 2 – Operational schematic of the fiber optic wear sensor.  
 
RESULTS AND DISCUSSION 
The REAST sensor was originally developed to monitor the propellant regression within a solid 
rocket motor. Tests were conducted on various small-scale rocket motors or in “motor-like” 
environments. These included tests on perchlorate-based road flares (2800 
o
C) and Estes-brand 
model rocket motors.  In both these tests the fibers were introduced longitudinally into the 
components to monitor the regression of the propellant rate. The data obtained using the flare are 
presented in Fig. 3.  These data exhibited reasonable agreement with the highly unstable burning 
rate of the flare, which was observed to chugs, crackle and pop as it burned.  Unlike the longer-
lasting flare, the Estes rocket motor burned very quickly and in a more stable manner as 
demonstrated in the data presented in Fig 4.  
 
 
Figure 3 – Length sensing on a road flare. Burn instabilities are shown on a faster timescale. 
 
 Figure 4 – Regression in fast burning Estes-brand rocket motor. 
In addition to the regression measurements in combustion applications, testing was performed 
using a fluted-end cutter on an automated milling machine to slowly cut away an aluminum 
substrate that was instrumented with a REAST sensor.. The wearing process closely resembles a 
fracturing or chipping mechanism.  The use of the mill, with its computer controller, permitted real-
time feedback of the length eroded as the cutting tool was advanced into the substrate. In this 
application, the temperature is relatively low so an additional fiber was used in the sensor to 
propagate light from a white light LED source in the manner described in the previous section. 
The test setup and data obtained for two separate measurements of the cutting process are 
shown Fig. 5. The ‘cleaner’ test data were acquired in between passes of the cutter.  The 
aluminum test article was capped by an aluminum cap for these data creating a simulated “target” 
for the emitted LED signal. The ‘coarser’ or noisier raw data plot was obtained real-time during 
the cutting operation.. During the cutting operation, it was discovered that the sensor scavenged 
light from the facility overhead lighting (fluorescent lights), providing a means to perform the 
measurement real-time. Of particular note is the improvement in the signal-to-noise as the fiber is 
worn. When the probe is longest, the difference in the two signals transmitted by the sensing 
fibers is greatest and the amount of noise in the ratio of the two intensities is greatest. This noise 
decreases as the probe shortens and the difference in the two signals decreases. 
 
 Figure 5 – Wear simulated by the cutting of an aluminum substrate. 
 
Testing was also carried out through metal-to-metal high temperature friction-type erosion. A 
thermally isolated relatively hard stainless steel alloy (type 316) was held by the rotating part of a 
machine lathe. While rotating, a softer (type 304) stainless steel test article with an embedded 
wear sensor was forced into the rotating part. The position of the probe relative to the harder 
wearing drum was monitored using a position gauge. During testing a molten zone was quickly 
formed as well as spallation of the softer alloy onto the harder. The formation of the melt zone of 
the 304 alloy signifies a temperature exceeding 1400 
o
C [9]. The test fixture and resulting wear 
measurements are presented in Fig. 6. 
The REAST sensor has been tested at NASA Marshall Space Flight Center’s Hyperthermal Test 
Facility. An arc-heated nitrogen flow is passed through a throat generally comprised of graphite. 
For wear sensor testing, the contoured graphite ‘shoe’  was modified so samples of differing 
materials would compromise the outer wall of the throat, ablating over time as she hot arc-heated 
gas passed through the system. Results obtained for an ethylene propylene diene Monomer 
(EPDM) rubber sample are presented in Fig. 7.  
 
 Figure 6 – Stainless steel alloy friction wear (304 stainless melting against 316 stainless). 
 
 
Figure 7 – Current results from EPDM erosion in NASA MSFC’s Hyperthermal Test Facility, 
shown in the lower right part of the figure. 
 
In addition to the regression, erosion and wear data presented here, the sensor has also 
demonstrated the capability of capturing optical spectra permitting the optical measurement of 
temperature at the wearing interface. As seen in Fig. 2, both fibers are in physical contact with the 
wearing layer, providing a window on the wearing event. By splitting the optical signal and 
measuring the spectrum using a spectrometer a black or gray body temperature may be 
determined. The visible optical spectrum from the 
burning process is presented in Fig. 8
coupled to the fiber) the intensity of the light for an integrated waveband, such as that of a 
photodiode, will be proportional to temperature
taken. One, a blackbody or graybody 
as a function of temperature and wavelength
wear physics can be calibrated using some other method and then
application of interest. The light transmission response as a function of temperature is shown Fig. 
9 where the optical fiber was responding to 
create a ‘fit’ function for the particular scenario 
A thermocouple located inside the oven 
expected the fit function to calibrate the output of the fiber had the form of 
fourth power. The test was repeated
accurate results were obtained.
 
Figure 8 – Optical spectrum from the
Figure 9 – Raw optical signal versus temperature 
(left) and the fit function for that particular system derived from the raw data
 
interior of a road flare obtained 
.  For a set of fixed conditions (e.g., thermal layer is close
 to the fourth power. Two approaches may be 
temperature approximation using estimates of the emissivity 
.  Similarly the thermal response of the 
 assumed to be similar in 
a calibration thermal oven. Initial data were used to 
of a fiber pointed at the interior of a ceramic oven. 
was used as the calibration benchmark
temperature to the 
 in the oven using the derived calibration function
 
 
 (internal) burning layer in a road flare.
 
obtained using a thermal calibration source 
 (right). 
during the 
ly-
probe to the 
the 
 source. As 
, and highly 
 
 
SUMMARY AND CONCLUSIONS 
The REAST sensor operates using two fibers of different attenuation constants, with Beer’s law 
providing the link between the ratio of light intensity transmitted by each fiber and the remaining 
length of fiber.  This measurement technology has been demonstrated to a variety of wearing 
conditions and environments. The embedded fiber sensor provides a reliable measure of length 
and when properly matched and embedded in a host material can provide real-time in situ 
measurement of regression, burning, erosion, ablation or wear of that material. Additionally, the 
optical signals may be repurposed to obtain optical spectra of the wearing layer and, with 
calibration, the internal temperature of the wearing layer can be found. 
ACKNOWLEDGEMENTS 
This work was supported in part by NASA's Advanced In Space Propulsion Program managed by 
Dr. Michael LaPointe.   
REFERENCES 
[1]  Patnaik, A., et al, Solid Particle Erosion Wear Characteristics of Fiber and Particulate 
Filled Polymer Composites: A Review, Wear, Volume 268 (January 2010). 
[2]  Polk, J., et al, An Overview of the Results from an 8200 Hour Wear Test of the NSTAR 
Ion Thruster, Joint Propulsion Conference, (June 1999). 
[3]  Polk, J., et al, In Situ Time-Resolved Accelerator Grid Erosion Measurements in the 
NSTAR 8000 Hour Ion Engine Wear Test, JPL Technical Report 22603 (August 1997). 
[4]  Crookstan, Jr., et al, Instantaneous Heat Transfer, Pressure & Surface temperature 
Characteristics of Solid Rocket Igniters, U.S. Naval Propellant Tech. Memo. Report 178, 
(April 1960). 
[5]  Xu, Q. Rocket Plume Temperature Measurement by Wire-Welded Thermocouples, 
Proc. SPIE 6222, (April 2006).  
[6]  Korman, V., Dhote, N., and Polzin, K. A., Demonstration of a Fiber Optic Regression 
Probe: REAST – Regression, Erosion and Ablation Sensor Technology, AIAA Joint 
Propulsion Conference (July 2010). 
[7] Korman, V., Regression and Erosion Measurement System and Method, U.S. Patent 
7,720,319, May 2010. 
[8]  Born, M. and Wolf, E., Principles of Optics, Cambridge Press, (1999). 
[9]  Lide, D. R., Handbook of Chemistry and Physics, 71
st
 ed., CRC Press, (1991). 


Testing of a Fiber Optic Wear, Erosion and Regression Sensor

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