NASA/TM-2008-215105 Eddy Current COPV Overwrap and Liner Thickness Measurement System and Data Analysis for 40-Inch Kevlar COPVs SN002 and SN027 Russell A. Wincheski Langley Research Center, Hampton, Virginia January 2008 The NASA STI Program Office . . . in Profile Since its founding, NASA has been dedicated to the • CONFERENCE PUBLICATION. Collected advancement of aeronautics and space science. The papers from scientific and technical NASA Scientific and Technical Information (STI) conferences, symposia, seminars, or other Program Office plays a key part in helping NASA meetings sponsored or co-sponsored by NASA. maintain this important role. • SPECIAL PUBLICATION. Scientific, The NASA STI Program Office is operated by technical, or historical information from NASA Langley Research Center, the lead center for NASA’s programs, projects, and missions, often scientific and technical information. 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Wincheski Langley Research Center, Hampton, Virginia National Aeronautics and Space Administration Langley Research Center Hampton, Virginia 23681-2199 January 2008 The use of trademarks or names of manufacturers in this report is for accurate reporting and does not constitute an official endorsement, either expressed or implied, of such products or manufacturers by the National Aeronautics and Space Administration. Available from: NASA Center for AeroSpace Information (CASI) National Technical Information Service (NTIS) 7115 Standard Drive 5285 Port Royal Road Hanover, MD 21076-1320 Springfield, VA 22161-2171 (301) 621-0390 (703) 605-6000 Eddy Current COPV Overwrap and Liner Thickness Measurement System and Data Analysis for 40-Inch Kevlar COPVs SN002 and SN027 Buzz Wincheski NASA Langley Research Center Introduction As part of the health assessment of flight spare 40 inch diameter Kevlar composite overwrapped pressure vessels (COPVs) SN002 and SN027 an eddy current characterization of the composite and liner thickness change during pressurization was requested under WSTF-TP-1085-07.A, “Space Shuttle Orbiter Main Propulsion System P/N MC282-0082-0101 S/N 002 and Orbital Maneuvering System P/N MC282-0082-001 S/N 027 COPV Health Assessment” [1]. The through the thickness strains have been determined to be an important parameter in the analysis of the reliability and likelihood of stress rupture failure [2]. Eddy current techniques provide a means to measure these thickness changes based upon the change in impedance of an eddy current sensor mounted on the exterior of the vessel [3-6]. Careful probe and technique design have resulted in the capability to independently measure the liner and overwrap thickness changes to better than +/- 0.0005 in. at each sensor location. Descriptions of the inspection system and test results are discussed below. Instrumentation The inspection system was designed using a commercially available eddy current impedance plane instrument with custom wound eddy current sensors. In addition, a probe mounting fixture was designed and fabricated to meet compatibility requirements of the flight spare hardware. A list of the critical components of the system includes: 1. ZETEC MIZ-27-SI eddy current Tester, ECN#3022157 2. Dell Laptop Computer 3. Four COPV eddy current probes, custom wound 4. Four Interface Boxes with matching circuitry 5. Four eddy current sensor mounts 6. Timing Trigger Circuit Engineering drawings of the COPV eddy current probe and Eddy current sensor mount designs are given in Figs. 1 and 2. All dimensions are in inches. A 20 foot cable connects each of the sensors to the back of the eddy current instrument. The corresponding channel number on the Miz-27 displays the output for each sensor. Cables are labeled at all ends. During testing a 20 kHz, 5.0V drive is supplied to the test coils and a 40dB gain is applied across the measurement bridge. A 34dB gain is used on the trigger channel. All system settings are stored in nonvolatile memory on the Miz-27 instrument. Fig. 3 show the front panel boot up screen of the eddy current instrument. The system is configured to display sensors 1-4 on ‘Page(1)’, and trigger channel 5 along with sensors 2,3 and 4 on ‘Page(2)’. Labeled buttons on the bottom of the instrument are used to change the page being viewed in the display. During data acquisition all five channels of data are stored to mass storage at a rate of 100 Hz. Fig. 1. Eddy current sensor design. 2 Fig. 2. Eddy current probe mount and sensor mounting configuration. 3 Fig. 3. Front panel of eddy current instrument after startup. Laboratory Sensor Calibration The system is calibrated such that overwrap thickness changes produce a vertical signal response, and designed such that changes in the liner thickness produce an orthogonal, horizontal, output voltage. The calibration factors for each of the sensors were determined in the laboratory by recording the sensor response for known changes in lift-off and metal thickness near the 40in diameter COPV nominal values. Table 1 displays the calibration data for each of the four sensors. The lift-off distance, simulating the thickness of the Kevlar overwrap, was varied through the use of non- Table 1. Calibration Data for Eddy Current Sensors. Lift-off Titanium S1_H S1_V S2_H S2_V S3_H S3_V S4_H S4_V (in) (in) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) 0.855 0.112 -0.959 -1.004 -0.931 -0.937 - 0.85 0.112 -0.096 -1.628 -0.01 -1.525 -0.113 -1.55 0.0429 -1.57 0.845 0.112 -2.239 -2.15 -2.181 -2.187 0.84 0.112 -2.875 -2.67 -2.816 -2.833 0.835 0.112 -3.551 -3.454 -3.477 -3.451 0.83 0.112 -4.247 -4.169 -4.17 -4.15 0.825 0.112 -4.869 -4.839 -4.89 -4.826 0.82 0.112 -5.554 -5.53 -5.558 -5.547 0.815 0.112 -6.237 -6.193 -6.25 -6.187 0.85 0.095 1.999 2.114 2.023 2.053 4 conducting spacers. The titanium thickness was varied by either placing the probe over a single titanium plate of 0.095” or combining two plates with a total thickness of 0.112”. A plot of the vertical output voltage of the sensors versus changing stand-off (simulating changes in overwrap thickness) for each of the sensors is given in Fig. 4. A linear fit to the data is also shown in the plot, with the results indicating a sensitivity of approximately 130 mV/mil for each of the sensors. Sensor Installation and Field Calibration Following testing and calibration, the eddy current system was transferred to NASA White Sands Test Facility for sensor installation, field calibration, and testing. Eddy current sensors were mounted using the probe mount shown in Fig. 2. A circumferential O-ring was used to hold the probe mount stationary on the COPV and a second O-ring fastened to the probe mount maintained a constant force on the eddy current sensor normal to the COPV surface. The mounting location of the sensors on the COPV is described in WSTF-TP-1085-07.A, “Space Shuttle Orbiter Main Propulsion System P/N MC282-0082-0101 S/N 002 and Orbital Maneuvering System P/N MC282- 0082-001 S/N 027 COPV Health Assessment” [1]. As shown in Fig. 2, a small access notch was machined into the probe mount to enable a field calibration of the sensors to changes in the overwrap thickness. Nonconducting shims of 5, 10, 15 and 20 mils were sequentially inserted through this notch and placed between the probe and the outer layer of the COPV overwrap. Fig. 5 displays the field calibration data acquired for sensor 3 mounted on COPV SN002. A large spike in the data is observed as the probe is pulled away from the surface to enable each of the four successive shims to be placed between the probe and the COPV. Each shim was kept in place for approximately 10 seconds before being pulled out. The Flat Plate Calibration -- Overwrap Thickness 0 0.81 0.815 0.82 0.825 0.83 0.835 0.84 0.845 0.85 0.855 0.86 -1 V) -2 e ( g a olt -3 Sensor4_V ut V Sensor3_V p Sensor2_V Out -4 Sensor1_V cal Linear (Sensor4_V) y = 131.75x - 113.53 Verti -5 Linear (Sensor3_V) y = 133.57x - 115.07 Linear (Sensor2_V) y = 132.16x - 113.86 Linear (Sensor1_V) y = 131.74x - 113.58 -6 -7 Overwrap Thickness (In) Fig. 4. Laboratory calibration data for system sensitivity to overwrap thickness changes. 5 8 7 6 ) V ( ut 5 p ut O 4 r o s 3 n e S 2 1 0 10 20 30 40 50 60 70 Time (s) Fig. 5. Field calibration data for sensor 3 mounted on COPV SN002. Data corresponds to 5,10,15 and 20 mil shims placed between COPV and sensor at approximately 10, 25, 40, and 55 seconds. resulting drop in voltage for each shim thickness was measured and the calibration factor determined by fitting a straight line to the output voltage versus shim thickness plots. Table 2 lists the calculated calibration factors for each sensor mounted on both COPV SNs 002 and 027. Table2. Sensor Output versus Coating Thickness Sensor1 Sensor2 Sensor3 Sensor4 SN002(mV/mil) 145 130 133 129 SN027(mV/mil) 149 144 129 156 Test Results for SN002 and SN027 Pressurizations Health assessment pressurization runs on flight spare COPV SNs 002 and 027 were performed from ambient pressure to approximately 4000 psi using a hydraulic pressurization according to the White Sands Test Facility test plan [1]. During each pressurization run, eddy current data was acquired and synchronized with pressurization data. Previously calculated calibration data was then applied to convert the measured sensor voltages into thickness changes of the liner and overwrap. Figs. 6 and 8 display the measured overwrap thickness changes and pressurization as a function of time for COPV SNs 002 and 027 respectively. Figs. 7 and 9 display the overwrap thickness changes versus pressurization for these data sets. Both sets of data 6