1. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Fiber Optic Strain Gage FDR 1-24-07 K Freudenberg Jim Tsai David Williamson

2. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Topics  Fiber Optic Gage overview and previous work  Glue search  Test setup  Gage Testing results  FEA modeling (gage angle and Stycast thickness)  Fiso conversations  Displacement gage (a quick look)  Placement of gages on mod coils (Williamson)  Recommendations

3. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY What is a fiber optic strain gage?  There are several types of fiber-optic strain gages, two of which comprise the majority of commercially available strain sensors. The most popular manufacturing method produces the Fabry-Perot strain sensor with the second most popular type being the Bragg-grating strain sensor.  The Fabry-Perot type (sometimes referred to as interferometer) of sensor uses a phase difference or “shift” between reference and sensing reflections of the fibers for making strain measurements  FISO gages under consideration are Fabry-Perot type Definition: These sensors consist of a multimode optical fiber that transports white light, with the sensing element at the tip. The sensing element is defined by a micro capillary tube that holds the end of the fiber close to another small piece of the same fiber, leaving a cavity in between. The fiber-ends that define the cavity are deposited with mirrors, so that the white light entering the cavity is reflected, and hence frequency- modulated in accordance to this length. When the sensor is bonded to a surface, the length of the cavity in the micro capillary expands or contracts exactly by the same amount of strain experienced by the surface,

4. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Advantages of Fiber Optic Strain Gages? The fiber sensor system has many advantages over conventional electrical strain gauges, such as: Absence of electromagnetic interference Absence of voltage interference Extended operation time (before becoming uncalibrated) High resolution Small size

5. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Prior work done on Fiber Optic Strain Gages (Fabry-Perot type) Experimental Evaluation of White Light Fabry-Perot Interferometry Fiber-Optic Strain Gages when Measuring Small Strains St. Cyr, William; Figueroa, Fernando; VanDyke, David; McVay, Greg; Mitchell, Mark; January 10, 2002; 17 pp.; In English 20040000044 NASA Stennis Space Center, Bay Saint Louis, MS, USA Report No.(s): SE-2002-01-00001-SSC; No Copyright; Avail: CASI; A03, Hardcopy “In conclusion, FOSG are ‘better’ than foil gages as long as the application allows calibration of individual units as installed for operation.” ***We will revisit this in a minute*** Fiber-Optic Strain-Gage Tank Level Measurement System for Cryogenic Propellants Figueroa, Fernando; Mitchell, Mark; Langford, Lester; May 18, 2004; 5 pp.; In English 20040050782 NASA Stennis Space Center, Bay Saint Louis, MS, USA Report No.(s): SE-2003-12-00109-SSC; No Copyright; Avail: CASI; A01, Hardcopy “Experimental results also show that the use of fiber optic strain gages (FOSG) over classic foil strain gages extends the operation time (before the system becomes uncalibrated), and increases accuracy. “ A Fiber-optic Strain Measurement and Quench Localization System for Use in Superconducting Accelerator Dipole Magnets Johannes M. van Oort, Ronald M. Scanlan and Herman H. J. ten Kate IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 5, NO. 2, JUNE 1995 “The ability of the fiber optic sensor system to survive in a cryogenic environment under high pressure has been demonstrated.”

6. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY FISO Pricing

7. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Adhesive Testing The sst samples were sanded to 60 and 220 grit surfaces along with the mirror like surface in the middle of the plate. After reviewing several papers and talking to several adhesive companies: six glues were chosen. They are: Emerson and Cumming Stycast 2859 FT, Vishay MBond AE- 10, CTD 540, Norland Optical UV Adhesive, Loctite 1C Hysol, and Epoxy Technologies EPO-TEK 301-2 The procedure for these first adhesive tests is to immerse the specimen for 75 mins with about 14.5 lbs of load applied to it, which results in around 800 ue on the center of the specimen in the three point bend fixture. M bond AE-10 Specimen in LN2

8. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Stycast and AE-10 after test M Bond AE-10 Cracking and one spot became unbonded on the AE-10 specimen. There is no degradation of the stycast specimen There was no discernable difference with the type of surface roughness used. Stycast 2850 FT

9. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Glue Adhesive Summary

10. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Best Adhesive of the group.  Stycast 2850 FT was chosen as the glue of choice after these rounds of cryogenic dunking.  Stycast 2850 FT bonds well to a variety of surfaces including G-11 and Stainless steel.  Stycast 2850 FT shows no cracking or debonding even after repeated thermal and mechanical strain cycling.

11. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Stycast Strain Curve on MTS Machine (ASTM D695) Test Procedure: The specimen is placed between compressive plates parallel to the surface. The specimen is then compressed at a uniform rate. An extensometer attached to the front of the fixture is used to determine modulus. Specimen Size: Specimens were both blocks or cylinders. For ASTM, the blocks were 12.7 x 12.7 x 25.4mm (½ by ½ by 1 in) and the cylinders were 12.7mm (½ in) in diameter and 25.4mm (1 in) long.

12. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 3 Point Bend Testing of the Fiso gages Overriding equations: sst plate with Gages in bending G-11 plate with Gages Static testing consisted of placing known weights on the beam and measuring deflection.

13. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY LN2 Cyclic three point bend tester 30 RPM motor (approximately same period as a coil pulse) and cam have been assembled. This provides for approximately.12 in of vertical deflection of the rod. Dial Indicator

14. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY FOS-N Type Fiber Optic Strain gage Sensor Portion Wire Diameter ≈.03” (.75 mm) Senor Diameter ≈.01” (.3 mm)

15. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Cyclic response of Gages in LN2 This data sampling rate is the best we can for two channels with the UMI-8 Response of gages is very similar for all types tested. Gages provide repeatable results. 1st 10 Cycles Approx 1200 cycles Last few Cycles, gages return to zero.

16. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Preliminary test results of Fiso Gages After Initial Gage error was discovered, the remaining gages were examined under a microscope and glue problems were discovered. FISO Was Contacted. Microscope investigation is highly speculative.

17. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Pictures (gage S6456) It is extremely difficult to determine where the edges of the glue are and thus the gage factor has some error associated with it. These images show that glue has run the length of the sensor but where it is attached to the glass capillary tube remains unclear.

18. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Fiso Conversation (November 2006)  Errors of greater than 20 % and up to 45% were noted at room temp with 5 of 12 Fiso Gages attached with various glues Stycast, CTD 540 and Vishay AE-10.  Examination of three gages under a microscope showed glue problems and possible inaccurate gage factors.  Fiso was contacted and made aware of the situation.  They agreed with our assessment and our photographic evidence of possible faulty gages.  They agreed to supply us with an additional 10 gages free of charge. These gages will be temp compensated to LN2. (+-2500 ue)  5 gages will have the gage glued to the quartz tube as before.  5 gages will have the gage laser sintered to the quartz tube. (makes for a more delicate gage)  Additionally, Fiso will perform an internal investigation to determine why they shipped faulty gages and they will furnish us with a copy to guarantee that this problem will not occur again.

19. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY SFO-W Type Weldable Strain gages  Two weldable gage were applied to a sst sample plate and tested.  26 spot welds are recommended for installation of each gage.  Gage tip is magnetic, permeability < 1.1u from Severn Gage.  Weld plate is approx 0.005” thick  Internally, SFO-W is the same as the FOS-N  Weldable gages can be applied much faster than FOS-N. 10 mins/gage Welded Strain gage in LN2 (look close to see spot welds)

20. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Weldable Gage Static Performance

21. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 1 st Minute 51 st Minute Over 50 minutes there is only a 20 ue movement in strain (most likely due to wear of the Teflon follower). The delta strain is the same. Weldable Gage Cyclic Performance

22. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Finite element model of simple beam with Stycast layer Sst beam Stycast layers Model has 8 in span and two layers of stycat with variable thickness No Stycast Layer: benchmark

23. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Stycast thickness and gage angle Gage angles less than 5% produce little error. Does not account for the errors from the gages. Stycast thickness shown up to the diameter of the fiber optic wire casing does not produce double digit errors either. Thus, application of glue or small gage angles do not account for the large errors seen in the FISO gages.

24. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY New Gages arrive in December 06  This represents a typical static test, 5 of these tests are averaged to make up the previous and upcoming error plots.  They produce the same results as before with errors at room up to 30%.  Gages are again repeatable having been run on the cyclic tester for 8 fours (14,400 cycles) with only a few percent change in strain and the gage returned to zero once motor was discontinued. Gage number

25. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY ALL GAGES Error (Room Temperature)

26. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Application Procedures (1) Fiso Base Procedure uses tape to secure the fiber optical cable, the incoming optical fiber, and finally the tip of capillary tube in succession. The mixed Stycast glue is then used to attach the sensor onto the test plate. (1A)Same as (1) except Loctite glue is used to secure the lead optical cable on the test plate. (sensor portion not compressed during curing) (1B) Same as (1) except uses non-stick tape over the glue attachment of the main gage section of the capillary tube. This step may reduce Stycast glue diffusing into the region between the capillary tube and the test plate during curing. (2) Uses Loctite glue attachment on the incoming fiber at 3 mm from the capillary tube. This step should improve the quality of sensor installation over the Procedure 1B. (3) The application of Stycast adhesive is performed in 2 phases. Phase 1 glues the main gage section of the capillary tube and the incoming optical fiber onto the test plate, except the taped spots. Phase 2 removes the tape pieces on the capillary tube tip and the incoming optical fiber, and applies Stycast glue to these spots.

27. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Another talk with Fiso Here are questions we sent FIso and their responses back from emails. 1. Glue gages always read less than theory (- 5-20%) Fiso ans: 1. “This is expected results.” ORNL Comment: 20% seems high to me as far as accuracy goes, To clarify of the 4 glue gages we tested one read 5% below theory but the other three read 20% below theory. Is within 20% the best the gages can do? [Support *-* ] “We recommend you make your own calibration. This will provide you with better accuracy, especially because the sensors are repeatable. The offset can be explained by the current method used by FISO to calibrate the FOS sensors. The current method is based on visual aspect of the inside of the sensor. The attachment point can be as large as 30% of the total length of the sensitive portion of the sensors.” 2. Laser Sintered gages read greater than theory (+ 20-30%) Fiso Ans: “They seem to be more sensitive than expected. However, it is possible.” ORNL Comment: Please explain how this is possible. [Support *-* ] “This sounds great. We cannot visually accurately evaluate the gage factor determinants (attachment points). We only can base ourselves on worst case scenario to evaluate the gage factors.”

28. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Another talk with Fiso… Continued 3. We have tested two Weldable gages (SFO-W) but I am unsure if I can conclude that they are performing better than the epoxy attached gages especially if they are constructed the same. [Support *-* ] “FISO cannot conclude regular FOS being more accurate then SFO-W. The only difference is that gluing standard FOS sensors is less controllable than gluing SFO-W. Furthermore, some customers have reported calibrating SFO-W prior to installation. This calibration provides better accuracy that using provide gage factors. When possible, the best solution for calibrating FISO strain gages is to also install standard electric strain gage and run a test of strain. You could then either destroy or simply leave the electric sensor on the DUT. Doing such calibration might not come naturally.”

29. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Going back to the NASA study "The laboratory test results obtained from phase 1, shown in table 1.1, indicated that the measured strain between the resistance and fiber-optic strain gages differed by 25 to 30 percent. The resistance and fiber-optic gages were tested on the same test articles by using the same installation techniques and adhesives. Based on the three fiber-optic gages that were tested, the repeatability of the gages varied between ±0.025 and ±0.057 percent, corresponding to <±0.3 and <±0.7 micro-strain, respectively.“ It is our experience too that the gages are very repeatable as shown from the cyclic testing. "The manufacturer was notified of the findings pertaining to the potential error in their stated gage factor. The manufacturer then responded by indicating that they were considering incorporating an improvement into their manufacturing processes that would enable them to furnish the user with a more accurate stated gage factor. This improvement would provide a more accurate measurement of the distance between the two attachment points for the fibers at either end of the capillary tube. It is this distance that defines the gage factor, that is, the sensitivity of the sensor.“ This is exactly what Fiso did for us. Here's what they say after they tested they improved gages. "The indication is that even though the manufacturer of these gages has made improvements, the gage factors are still not precisely determined for their fiber-optic gages." Based on what appear to be inaccuracies in the stated gage factor, which affects the overall performance of this particular type of fiber-optic gage, it would be advantageous to determine the gage factor of each gage, that is, calibrate each gage for sensitivity to strain before use.”

30. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Summary of the last three slides  The gages are only as accurate as the stated gage factor.  If the gages can be calibrated after installation they will provide accurate results.  Fiso, so far, has been unable to provide us with accurate gage factors.  Given that NASA experienced the same problem from another company who makes fabry-perot gages, I can only conclude that the accuracy errors are a limitation of the technology, out of the box.

31. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Possible ways to calibrate gages  Weldable types could be welded to another support plate and tested in a bend test. The new support plate with the calibrated gage could then be welded to a casting.  Glue on gages could have a traditional foil gage placed next to it on the machine and calibrated during cool-down with the conventional gage. Conventional foil gages would not be used during operation. Does C1 still have foil gages attached?  Glue on gages could be glued with an easily dissolvable epoxy to a sample plate. They would be tested to get the correct gage factor, the glue would be dissolved and the gage could be then attached the modular coils. **we have tried this and have found that the agent used to dissolve the adhesive also reacts with the internal glue of the gage.

32. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Recommendations  Use Stycast 2850 FT with catalyst 23lv to attach gages and wires.  All places where we can weld gages to the castings, use the weldable gages. They may cost more (by 2x’s) the “glue on” type but they are far easier and quicker to install.  To calibrate the gages before operation, recommend testing the weldable gages before installation. For the glue on gages, place a standard foil gage next to the fiber optic gage and calibrate during cooldown.

33. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Follow Up with Fiso  Apparently, the future lies with the laser fused gages.  Per Email on 1-22-07  “FISO will proceed to the qualification of the FOS Laser Sintered. Nevertheless, the timeframe is about 8 weeks before we could move resources on the qualification of this design… Therefore, what I can suggest to you is:  Option A: -You purchase some more FOS Laser Sintered and you test them the same way than the three previous one. If they still give a -25% of error with a small variation, it will demonstrate that we have a predictable behaviour.  Option B: -You can wait ~8 weeks that we do internally the testing of the Laser Sintered gage and develop the method to provide you an accurate gage factor…”

34. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Fiber Optic Deflection Gage (quick look)  One Fiber Optic Deflection gage was Procured from Fiso.  Such a gage could be mounted in the cryogenic environment and provide deflection feedback during operation.

35. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Frozen Deflection Gage Initial test showed that the gage sticking while measuring the plate deflection. Tip was touching the LN2 pool. gap Gage was then positioned at the top of the cryo box completely out of the LN2 pool. Gage was removed from cryogenic box, depressed and became stuck, until it warmed 1. 2. 3.