Acoustic Emission Lifetime Estimation for Carbon ...

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Fiber/Epoxy Composite Overwrapped Pressure Vessels. Charles T. Nichols,1 Jess M. Waller,2 and Regor L. Saulsberry3. NASA-JSC White Sands Test Facility, ...
NASA USRP – Internship Final Report

Acoustic Emission Lifetime Estimation for Carbon Fiber/Epoxy Composite Overwrapped Pressure Vessels Charles T. Nichols,1 Jess M. Waller,2 and Regor L. Saulsberry3 NASA-JSC White Sands Test Facility, 12600 NASA Rd., Las Cruces, NM, 88012

Acoustic emission (AE) data acquired during intermittent load-hold (ILH) pressure tests of cylindrical 6.3-in. diameter carbon fiber-reinforced epoxy (C/Ep) Composite Overwrapped Pressure Vessels (COPVs) were analyzed using Felicity ratio (FR) methods to determine the characteristic FR at failure (FR*). FR trends and the extrapolated FR* values calculated for a representative C/Ep COPV were found to be in agreement with analogous C/Ep strand data. A new method was developed for calculating FRs based on how the onset of significant AE was defined. This method yielded FRs that decreased more linearly with applied load in strand and COPV tests than previously obtained using conventional FR determination methods. COPV tests revealed that vessels instrumented with six AE sensors gave higher fidelity FR results (R2≈0.9) than vessels instrumented with one to three AE sensors (R2≈0.7). This observation suggests that substantial attenuation effects exist in C/Ep COPVs, such that AE events contributing progressive damage are neglected, hence masking the linear decrease in the FR when an insufficient number of sensors are used. Last, fast Fourier transforms were analyzed to assess frequency and micromechanical damage progression during ILH tests of COPVs. Waveform signatures commonly seen in COPVs are categorized and AE noise-filtering criteria are presented. Frequency analysis of the AE waveforms showed that progressive damage in a representative C/Ep COPV was similar to that observed in C/Ep strands, and that the events contributing to the FR upon loading are indicative of a co-operative micromechanical damage mechanism.

Nomenclature AE C/Ep COPV E-wave F-wave FFT FR FR* FRn FRn% ILH in-situ K/Ep LR NDE R2 SR vf

= = = = = = = = = = = = = = = = = = =

acoustic emission carbon-fiber impregnated epoxy composite overwrapped pressure vessel extensional waveform component flexural waveform component fast Fourier transform Felicity ratio extrapolated Felicity ratio at rupture (strand) or burst (COPV) FR based on the nth AE event in a load ramp FR based on events located n% into the event cluster during loading FR based on the mean of the first n Felicity ratios during loading intermittent load hold real-time structural health monitoring Kevlar® 49-impregnated epoxy load ratio relative to rupture (strand) or burst pressure (COPV) nondestructive evaluation linear least squares coefficient of determination stress rupture volume fraction

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USRP Intern, Department of Mechanical and Aerospace Engineering, MSC 3450, PO Box 30001, Las Cruces, New Mexico 88003.

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Materials Scientist, NASA-JSC White Sands Test Facility, Laboratories Department, MS 200LD, Las Cruces, New Mexico 88004.

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NDE Projects Manager, NASA-JSC White Sands Test Facility, RF, 12600 NASA Rd., Las Cruces, New Mexico 88012.

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I. Introduction

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OMPOSITE Overwrapped Pressure Vessels (COPVs) (Figure 1) are widely used in launch vehicles and satellites, where the strong drive to reduce weight has pushed designers to adopt high performance and high specific strength composite materials with a relatively high volume fraction (vf 0.6 to 0.7) of fiber. Composite materials used in COPV designs have typically consisted of aramid or carbon fiber embedded in a thermoset matrix such as epoxy. The role of the matrix and vessel liner is to transfer pressurization load to the fiber, whereas the role of the fiber is to withstand the majority of the load over time under the environmental exposure conditions encountered in service. Pressurizations of 35 to 70 MPa (5,000 to 10,000 psi) are common for COPVs. This has necessitated the common use of high load bearing composite overwraps wound around a thin-walled metal liner.

Figure 1. Typical carbon-fiber composite pressure vessels used by NASA in the ISS.

NASA has been faced with recertification and life extension issues for both epoxy-impregnated Kevlar® 49 (K/Ep) and epoxy-impregnated carbon (C/Ep) COPVs distributed throughout various systems including the Space Shuttle and International Space Station (ISS). Spacecraft COPVs have varying criticality, usage histories, damage and repair histories, time at pressure, and number of pressure cycles. Also, C/Ep and K/Ep COPVs are of particular concern due to the insidious and catastrophic “burst-before-leak” failure mode caused by the stress rupture (SR) of the composite overwrap in K/Ep COPVs and “leak before burst” failure mode seen in C/Ep COPVs. Stress rupture life has been defined by the American Institute for Aeronautics and Astronautics (AIAA) Aerospace Pressure Vessels Standards Working Group as “the minimum time during which the composite maintains structural integrity considering the combined effects of stress level(s), time at stress level(s), and associated environment”. 1 Stress rupture mechanisms in COPVs have none of the predictable features associated with those of metal pressure vessels, such as crack geometry, growth rate and size, or other features that lend themselves to currently established nondestructive evaluation (NDE) methods. The variability or “surprise factor” associated with SR in COPVs cannot be eliminated. C/Ep COPVs are also highly susceptible to impact damage which can lead to reduced burst pressure even when the amount of damage to the COPV is below the visual detection threshold, thus necessitating implementation of a mechanical damage control plan. 2 3 4 5 6 7 8 9 10 11 12 13 For these reasons, NASA has developed NDE methods that can be used during post-manufacture qualification, in-service inspection, and in-situ structural health monitoring. One of the more promising NDE techniques for detecting and monitoring, in real-time, the strain energy release and corresponding stress-wave propagation produced by actively growing flaws and defects in composite materials is acoustic emission (AE).3-10 Procedures described in this paper build on previous findings 11-13 on modal acoustic emission (mAE) of K/Ep and C/Ep strand specimens, and lay the groundwork for establishing critical thresholds for accumulated damage in composite structures so preemptive engineering steps can be implemented to minimize or obviate the risk of catastrophic failure.

II. Background A. Stress Waves When a structure is subjected to an external stimulus such as a change in pressure, load, temperature, or in certain cases, is exposed to chemical attack or sufficiently energetic electromagnetic radiation, energy is released from newly created or actively growing flaws or defects in the form of transient elastic stress waves. These waves propagate to the surface and can then be recorded by sensors. These stress waves travel in a spherical path that is distorted by the orientation of the flaw or defect. In the case of microcracks, most energy is directed perpendicular to the crack face 14 (Figure 2. Acoustic emission energy angular dependence. NDT Resource Center ; the crack is oriented along the x1x2 plane).

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Figure 2. Acoustic emission energy angular dependence. NDT Resource Center 14

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Acoustic emission is the measurement of the transient elastic stress waves produced by active flaws and defects after they have propagated through the material between the event source and the sensor.

B. COPV Waveforms Waveforms are the directly measured signals, expressed in units of electric potential (V) vs. time (µs), from each event (Figure 3, Figure 4 left). Waveforms are composed of four main components: background noise, extensional waves (E-waves), flexural waves (F-waves), and reflections. 15 Although the AE system expresses amplitude in units of voltage, it is more desirable to express it in decibels using the 20 log rule,* since the decibel (dB) is a unit independent of gain settings: (1) 4

where, in C/Ep COPV tests AdB AV Gp Gsum Vmax Vref

= = = = = =

Figure 3. Typical acoustic emission waveforms.

system amplitude in dB, e.g. 108 dB system amplitude in V, e.g. 1 volt preamplifier gain in dB, e.g. 0 dB signal gain in dB, e.g. 12 dB , e.g. 0.25 volts reference voltage, e.g. 10-6 volts

The direct wave, composed of an E-wave and an F-wave with 80-300 µs durations, is the closest form of a pure, non-reflected wave that can be obtained from AE testing. 15 The signal duration section of Figure 3 and Figure 4, right contains the direct wave. In C/Ep COPVs the E-wave occupies the rise time, is composed of moderate frequencies (90-150 kHz) that increase over time, may have continuous amplitude (< 250 mV, < 96 dB), and is attenuated first from the direct wave leaving only noise, reflections, and eruptive emissions in the waveform at large source displacements. F-waves are an eruptive emission composed of very high amplitudes (500-1,000 mV, 102-108 dB) and low frequencies (80-90 kHz) that decrease over time.

100 mV Threshold

Figure 4. A typical waveform seen in testing (left) and its direct wave (right). Units are volts vs. microseconds. From 6.3 inch diameter HyPerComp IM-7 carbon vessel SN070908-02 tested on 1/6/2010, event #17,624.

*

This equation was provided by Digital Wave Corp., the AE software and hardware developer of the equipment used in these tests.

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NASA USRP – Internship Final Report Background noise (Figure 5 left) is a low amplitude (< 100 mV, < 88 dB in these tests), low energy (< 2.0 V2·µs) continuous emission resulting from fretting or artifacts originating from within the signal conditioning unit or other electrical equipment. EMI spikes, a form of background noise seldom seen in our testing, but nevertheless observed, are single-cycle waveforms characterized by short signal duration (< 80 µs), low amplitude (< 200 mV, < 94 dB), and low energy (< 0.30 V2·µs). Reflections (Figure 5 right) are disrupted echo signals that directly follow the F-wave 15 and are characterized by very high amplitudes, jagged waveforms, decreasing frequencies, and irregular, non-periodic behavior. Since the strength of AE events is determined by the energy contained within the waveform through integration, it is important that waveforms are filtered for noise and reflection content prior to analyzing data. Otherwise, event energies will be much higher and frequency spectra will have a distorted, jagged appearance and will favor low frequency ranges, since the reflections contain proportionately more low frequency content.

Figure 5. Typical noise (left) and severe reflections (right) seen in vessel testing. Units are volts vs. microseconds. From 6.3 inch dia. HyPerComp IM-7 carbon vessels 13 & 16 batch tested on 5/19/2010 (Chan. /Event: 15/4050, 21/4053).

Felicity Ratio

C. Felicity Ratios Compared to all other AE techniques including event count, energy, and frequency correlations, Felicity ratios (FRs) have shown the most promise as an analytical parameter for evaluating progressive damage in C/Ep and K/Ep strand specimens and for predicting failure loads (Figure 6 left). 12 In COPV tests, the Felicity ratio is defined as the ratio of the pressure at onset of significant AE during loading to the maximum pressure observed prior to the event. 16, 17 For example, inspection of the intermittent load hold (Figure 6, right) shows the first valid FR determination zone. With significant AE occurring at 15.93 MPa (2,311 psi) and an average pressure on the previous maximum pressure of 13.86 MPa (2,010 psi) the Felicity ratio in this pressurization ramp was found to be 1.149. 1.3

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T1000 carbon/epoxy tow IM7 carbon/epoxy tow Kevlar 49/epoxy tow IM-7 carbon/epoxy vessel

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Figure 6. Felicity ratio trends (95% confidence interval lines) with IM-7 vessel data (left) and a Felicity ratio determination example (right). Felicity ratio curves were plotted against the actual load ratio (average previous highest load divided by failure load). Vessel data is from a 6.3-inch diameter IM-7 vessel (SN070908-02) which failed at 54.3 MPa (7,869 psi) instrumented with six acoustic emission sensors.

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Determining the onset of significant AE is based on a combination of statistical observations and experience and many methods have emerged. Methods of significance developed by ASTM International 16, 17 for fiberglass tanks/vessels and composite pipe rely heavily on counts, which is the number of times a waveform crosses the positive threshold in an event. 14 The software used in this experiment did not permit counts per events to be extracted from data in post processing, so alternate methods were pursued.

III. Experimental A. Load and Pressure Apparatuses 1. COPV Testing ILH pressure schedules were applied in batch tests using a 20-vessel test stand, referred to as the NASA NDE Working Group (NNWG) Carbon Stress Rupture Test System (CSRTS) (Figure 7). The CSRTS provides a test bed for NDE and structural health monitoring (SHM) development and verification. The design is unique because it uses a technique called active pressure, which uses computer control to maintain the bottle pressure within ± 14 kPa (± 2 psi) regardless of temperature variation without the use of accumulators. The CSRTS is housed in a protective Lexan® enclosure so viewing and visual inspection can be safely performed while vessels maintain full test pressure. In addition, pressure vessels are automatically isolated as they rupture using computer controlled solenoid valves, reducing pressure variations in adjacent vessels. Pre-test NDE (shearography and thermography) was performed on vessels to identify preexisting defects or flaws introduced during manufacturing or handling prior to installation in the test system. After pre-test NDE, the vessels were instrumented in lots of 20 and installed in the test system. 18 The single vessel tests were conducted in Test Cell (TC) 860 of the Figure 7. NASA NDE Working Group’s Hazardous Fluids Test Area. The system uses a HydroPac Inc. Li'l Carbon Stress Rupture Test System. 18 Critter high pressure hydraulic pump which has a ten-to-one doubleended intensifier to produce up to 207 MPa (30,000 psi) of pressure; however, the maximum output of the pump allowed by WSTF is 103 MPa (15,000 psi). The pressure rates typically produced by the pump are between 69-689 kPa·s-1 (10-100 psi·s-1) depending on the size of the test article. Depressurization rates can also be controlled through the use of a motor operated needle valve. The pressurant media, or process fluid, is a 95% water 5% soluble oil solution. The oil lubricates the seals and packing in the pump. The process fluid supply barrel is set atop a high precision weight scale (±0.004 N, ±0.001 lbf) so that the mass of fluid pumped into the system can be measured. The test article (TA) is installed within blast containment on the blast pad outside TC 860. The blast containment consists of a Lexan® burst enclosure and tire mats. The burst enclosure sits within the tire mats and is considered primary containment. The tire mats are secondary containment. TA pressure is measured using pressure transducers located just upstream of the test article. The control system software, which enables the user to perform burst tests, cycle tests, and intermittent ILH stress schedule tests was developed and written by WSTF using C program language. 19 2. Strand Testing ILH stress schedules were applied using an Instron Corp. 5569 Series Electromechanical Test Instrument equipped with a 50 kN (11,200 lbf) capacity load cell. Other features included self-tightening 25 mm 51 mm (1 in. 2 in.) wedge action mechanical grips with knurled faces, and Instron Bluehill data acquisition software (version 1.8.289). 12

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B. Materials 1. COPVs COPVs were fabricated by HyPerComp Eng. Inc. (Westlake, CA) using 3775 denier Hexcel Corp. HexTow™ (Southbury, CT) IM7-W-12K fiber pre-impregnated with UF3323-102 resin, vf ≈ 0.69 (ATK Space Systems, Odgen, UT). Cylindrical vessels were fabricated from a 16.0 cm (6.29 in.) outer dia., 22 N (4.9 lb f) Al liners with 10 N (2.3 lbf) of IM-7 strands to an OD of 16.6 cm (6.55 in.) and total weight of 32 N (7.2 lbf). Wrap pattern 3H/15C was used to overlay the liner. The helical angles, indicated by “H” representing 2 plys (both a + and -) of helical wraps, were oriented at 13.8° and 17.1° respectively with an average angle of 14.9° relative to the vessel’s axial direction. The cirque or “hoop” wrap was one ply. The theoretical thicknesses of the helicals and hoops respectively based on 65% fiber volume are 1.1 mm (0.045 in.) and 1.7 mm (0.067 in.). Each COPV was wrapped with a nylon peel ply prior and cured at 149 °C (300 °F) for 12 h. Temperature rates were maintained below 1 °C·min-1 (2 °F·min-1) on ramps up and 6 °C·min-1 (10 °F·min-1) on ramps down. The peel ply was removed after curing. Strand coupons were fabricated from this fiber by wrapping them over a steel mandrel. Two representative COPVs out of this lot were hydro tested to failure pressure which averaged 51.91 MPa (7,529 psi) (std. dev. = 1.014 MPa, 147.1 psi).20 2. Strands Tensile tests were conducted on unidirectional 3817 denier Torayca ® T1000G and 3775 denier HexTow™ IM-7 (5000) 12,000-filament composite strands with ultimate tensile strength values of 1,268 ± 307 N (285 ± 69 lbf) and 1,005 ± 111 N (226 ± 25 lbf), respectively. Each tensile specimen had ribbon-like geometry with irregular width and thickness. Calculated cross-sectional areas were 0.236 mm2 (365 mil2) for T1000, and 0.255 mm2 (395 mil2) for IM-7. The nominal thickness and width of T1000 tow specimens was 0.36 mm (0.014 in.) and 1.4 mm (0.057 in.), respectively; while that of IM-7 tow specimens was 0.28 mm (0.011 in.) and 1.8 mm (0.071 in.), respectively. The strands were cured using the same oven program as the COPVs (see above), but were not vacuum bagged during curing. A gage length of 25 cm (10 in.) was used in each test. Each specimen had cardboard end tabs with an l × w of 25 mm × 51 mm (1 in. × 2 in.). Tow ends were secured to the cardboard with a 3.5 gr. (0.12 oz) bead of 1:1 Hardman® Extra-Fast Setting Epoxy (Ellsworth Adhesives, Germantown, WI) which cured for at least 24 h prior to testing.12 C. Stress Schedules 1. COPV Testing Both batch and single-vessel tests were stressed following an ILH stress schedule which was scaled from strand tests on the basis of equivalent load ratios. IM-7 COPVs were aged in lots of 20 and stress rupture tested using an ILH stress schedule which lasted for 10 weeks. These bottles were configured with 1, 2, or 3 AE sensors. Details of the first 6 hours (360 min) of the ILH pressure schedule (Figure 8 left), based on a pressure tank examination stressing sequence from ASTM E 1118 17 guidelines, used on these vessels is presented in Table 1. A similar schedule (Figure 8 right) was used on an individual bottle with 6 AE sensors. All tests experienced little background noise and low EMI.

Figure 8. Pressure profiles used in batched vessel (left) and single-vessel (right) tests. Left: The initial dwell was skipped in batch tests since background noise was previously characterized. The stepped shape of the pressure profiles is due to hydraulic-cylinder recharge points. The final plateau was lengthened in this batch test to characterize creep rupture and is not pictured. Right: Single-vessel tests were conducted at accelerated rates (0.1 LR/min) for accelerated testing and lower initial pressurizations (0.25 LR) for more Felicity ratio data collection points. Profiles are from Batch 3 and SN070908-02 vessel tests, respectively.

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Table 1. ILH pressure schedule used in batched COPV tests. 1. 2. 3. 4. 5. 6. 7.

Dwell at ambient pressure for 10 minutes to establish noise characteristics Pressurize1 to 25.5 MPa gage2 (3,700 psig, 0.49 LR) Dwell at plateau for 30 minutes to allow stress to equally distribute throughout the overwrap Depressurize -2.8 MPa (-400 psi, -0.05 LR) Dwell at saddle for 30 minutes to simulate the vessel’s unstressed state Pressurize +6.9 MPa (+1,000 psi, +0.13 LR) Repeat steps 3-6 until the pressure target (0.99-1.5 LR) is reached. If targeted pressure does not result in failure and failure is not a requirement, hold at this stress for at least 30 minutes then depressurize to ambient. 1 Rates are 2.5 MPa/min (360 psi/min, 0.05 LR/min). 2 Dwell pressure variations are within ± 0.5% of target.

2. Strand Testing The stress schedule used in strand tests was based on the pressure tank examination procedure described in ASTM E 1067 16 and similarly referred to as the manufacturer’s qualification test in ASTM E 1118. 17 The load sequence began with an initial hold period of 30 min at 20 N (5 lbf) to determine the level of spurious events attributable to background noise as the specimen was held at a constant-stress state. The strand is loaded to 534 N (120 lbf) at a rate of 4.5 lb/min., held at that load for 10 min., then unloaded by 89 N (20 lb f) at which load the specimen remains for 10 minutes. This cycle is repeated until the strand fractures. This ILH stress schedule encompassed lower hold stresses between 445 N (100 lbf) and rupture, that were increased sequentially in 133 N (30 lbf) increments with a 25:10 load to unload ratio. 12 D. Acoustic Emission 1. COPV Testing Acoustic emission testing was Table 2. Typical acoustic emission sensor placement in single-vessel tests. Sensor System Channel Sensor Sensor Location conducted using Digital Wave No. No. S/N Rotary Angle, Displacement Corporation (DWC, Centennial, 1 095032 0˚, 11.4 cm (4.5 in.) from edge of top boss 1 CO) equipment to mirror 2 095019 180˚, 11.4 cm (4.5 in.) from edge of top boss 2 previously conducted strand tests 3 095033 270˚, at equator 3 11 12 13 that showed promising AE 4 033036 90˚, at equator 4 5 095037 180˚, 11.4 cm (4.5 in.) from edge of bottom boss 5 results. In the 20-COPV batch 6 095021 0˚, 11.4 cm (4.5 in.) from edge of bottom boss 6 test, 29 B-1025 AE microdot sensors (9.3 mm, (0.375 in.) diameter), were connected to a 32-channel, 20 MHz, 16-bit FM-1 signal conditioning unit through 0 dB preamplifiers. These broadband, high fidelity piezoelectric sensors had a frequency range of 100 kHz to 3.0 MHz (output signals were noisy and nonlinear below 50 kHz). Sensors were coupled to vessels with 10:1 Lord® 202 acrylic adhesive to Lord® 17 accelerant (Lord Corp, Cary, NC) which set within 2 mins and cured for at least 30 min prior to testing. For individual tests, six sensors were placed on the COPVs in a zigzag pattern (Table 4). A parametric channel was connected to the FM-1 to ensure that pressure and AE data were synchronized in batch tests by equating pressure transducer voltage to psi. In single COPV tests, six B-1025 sensors were connected to an 8-channel, 20 MHz, 16-bit FM-1 signal conditioning unit through 0 dB preamplifiers. The AE system settings are shown in Error! Reference source not found.. Sensor response was qualitatively assessed using pencil lead breaks performed midway between adjacent sensors, following the guidelines described in ASTM E 976. 21 In the 20-COPV batch test, 1, 2, or 3 sensors were placed along the equator of each vessel. Since fewer sensors were used, the effect of lower sensor coverage on the fidelity of the FR data could be evaluated. Individual vessel tests utilized a time-synced system (Znet, version 06.23.2010) developed by WSTF

Table 3. Typical acoustic emission settings for carbon pressure vessels. WaveExplorerTM Configuration COPV860 Setup File: 71 # of Channels: 2 MHz Sampling Rate: 4096 Number of Points: 512 Pre-trigger Points: +/-1 V Voltage Range:

FM-1 Test Configuration Line Driver Switch: +V/75 Ω Preamp. Gain: 18 dB Signal Gain: 12 dB Signal H.P. Filter: 20 kHz Trigger Gain: 21 dB Trigger H.P. Filter: 50 K Trigger L.P. Filter: 1.5 MHz FM-1 Lead Break Configuration2 Preamp. Gain: 24 dB

Parametric Setup Parameters 1Hz Parametric Sampling Rate: +/-10 V Signal Gain: 12 dB Parametric Voltage Range: 1 Trigger Gain: 18 dB Total Parametric Channels: 1 The first channel was defective, so channels 2-7 were used. 2 A pre-installation sensor check was performed on a ¼” Al plate with 0.2mm lead break 1/8 in. from the sensor edge (Preamp=35, Signal=24, trigger=18).

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to record manifold pressure data from a calibrated pressure transducer (PT-101). Acoustic emission data acquisition was provided in all tests by a DWC Lunchbox Computer equipped with DWC WaveExplorer™ software, version 7.2. This software allowed arrival time, event energy, event location, and event frequency band data collection for all events that triggered the FM-1. WaveExplorer™ was also used in post processing to filter the data for noise, reflections, and frequency content. A possible deficiency in the software is the inability to extract count data for individual events, which is an important factor for event significance in many ASTM Felicity ratio methods involving fiber reinforced vessels 16 and pipe. 17 Once collected, the data were filtered before analysis to reduce reflection and noise (Table 4). Table 4. Steps involved in filtering post-test data in WaveExplorer™ by order of application. Filter Mitigate Reflections

Enforce Frequency Restrictions

Eliminate Background Noise

Eliminate System Noise

Data Reduction Rationale and Associated Restrictions Define the time centered on threshold tolerance low enough to remove most of the reflections by observing the typical flexural wave duration: Threshold ≥ 100 mV (88 dB) TCOTTM ≤ 125 µs Optional – Define the frequency range over which the data will be analyzed. For example, de Groot 5 has defined fiber breakage events occur within the 300-600 kHz regime, so the waveform data would be filtered to exclude frequencies outside of this range: Frequency ≥ 300 kHz Frequency ≤ 600 kHz Define the waveform threshold value to exceed the typical background noise amplitude on at least one channel (takes awhile to process): Must pass on at least 1 channel Threshold ≥ 100 mV (88 dB) Remove all events from exported text files and spreadsheets that first arrive on the internal channel -1.

2. Strand Testing Acoustic emission measurements were taken using a DWC FM-1 system equipped with 8-channel capability. Each of the four channels used in strand studies was connected to a DWC PA-0, 0 dB gain preamplifier, and then to a broadband, high fidelity DWC B-1080 piezoelectric sensor with a frequency range of 50 kHz to 2.0 MHz (output signals were noisy and nonlinear below 50 kHz). Four sensors were coupled to the same side of the tow specimen using the same Lord® 202 acrylic adhesive to Lord® 17 accelerant mixture ratio described above. The AE system was supported with a lunch box computer equipped with WaveExplorer™ software (version 6.2). The software allowed arrival time, event energy, event location, event frequency band, and event time to be acquired for all events. The FM-1 data acquisition parameters were as follows: Preamp: 5x6 dB (preamplifier gain), signal: 24 dB (gain), signal: 20 kHz (HP), trigger: 3x3 dB (system gain), trigger: 20 dB (gain), trigger: 50 kHz (HP filter), trigger: 1.5 MHz (LP filter). Sensor sensitivity was checked using pencil lead breaks performed midway between adjacent sensors, according to the guidelines described in ASTM E 976. 21 The default velocity for the propagation of elastic stress waves in graphite (4,600 m·s-1) was used in all IM-7 and T1000 tests. This velocity was verified by conducting PLBs on a T1000 tow specimen, which gave a measured velocity of 4,356 m·s-1. 12 Use of the (higher) default velocity value resulted in a worst case error of 2.8 mm (0.11 in.) for events occurring at the tab edges relative to the top and bottom sensors, which was considered to be insignificant for purposes of source location given the comparatively small size of gage length (ca. 250 mm, 9.84 in.) used in this study. Post processing the data was accomplished by filtering against energy level to remove background noise. Application of the filtering criteria did not remove all the AE events that were not source locatable, only the events that were less than 0.22 V2· s. 12

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E. Source Location Source location was not used in this study; however, the feasibility of making source location measurements was examined and some general comments are made here and offered as future guidance. AE events are source located based on event arrival time (Δt) data from nearby sensors. If the sensor distance from the event is small, attenuation effects can be neglected and asymmetric pressure wave propagation can be simplified and circular dissipation awayfrom the source assumed. Since wave propagation velocity differs with frequency, 14 traditional kinematics cannot be adequately applied to determine accurately an event location without the aid of a neural network although 4,600 m·s-1 (5,030 yd·s-1) may be used to approximate locations in carbon-composite materials. 12 The two common methods used for source locating are onedimensional (1D) and zone (2D and 3D) models. When onedimensional specimens (e.g., uniaxial strand, single fiber, etc.) are tested, and the event is known to lie between two sensors onedimensional models (Figure 9) are effective. The DWC algorithms used to determine AE event source location are proprietary; however, for simple 1D geometries source locations can be calculated solely on arrival time data and results verified using the software. An equation for 1D source displacement from sensor 1 relative to arrival times and sensor positions, but does not require knowledge of the propagation velocity, is presented below: (2)

Figure 9. Acoustic emission source location example for a one-dimensional specimen.

For more complex 2D and 3D geometries, source locations is not as simple, but is thought to proceed through three steps: 1) proximity (zone) determination to identify the nearby sensors, 2) 1D source location using the adjacent sensors to determine the location an event occurs along a line of sight between sensors, and 3) triangulation methods. Geometrically, the circular wave propagation assumption may be used to approximate event locations by extending perpendicular lines from the point at which 2D source location assumes the event occurred at between two sensors (Figure 10). The intersection of these lines can then be used to approximate the position of an AE source within the lowest-arrival time zone.

Figure 10. Two-dimensional acoustic emission source location diagram.

The most important factor that needs to be taken into account to insure accurate source location in 3D specimens, such as COPVs, is knowledge about how the transient elastic stress waves are transmitted. In isotropic lay-ups, these waves are expected to be transmitted equally in different directions; however, in the COPVs tested in this study that may not be the case. This can be determined by performing an attenuation characterization on the COPV using pencil lead breaks at fixed distances from a given sensor along fixed angles corresponding to ply directions (e.g., 0 , ±45 , ±90 ).16-17 Understanding the attenuation characteristics can also be used to determine the minimum number of sensors17 that need to be used to characterize a particular specimen or test article.

F. FFT Composition Analysis Frequency band shifts are common in other materials 22 and may one day facilitate failure prediction in COPVs. However, in this study, it was more convenient to determine the relative amount of certain characteristic frequency bands corresponding to known types of composite damage. 5 To perform this type of frequency composition analysis, waveforms are filtered (Table 4) and frequencies were segmented into failure mechanism bands. For an event to pass the screening, the 100 mV (88 dB) threshold had to be exceeded by a waveform composed of only frequencies lying within the desired frequency range. The filtered event vs. time data is exported from the AE software into a text file. Each text file, representing a range of frequencies, is then imported into a Microsoft® Excel 2007 spreadsheet.

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Figure 11. Frequency composition analysis example (Microsoft® Excel).

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Relative percentages for each type of failure mechanism were determined for each AE event, after which selected AE events corresponding to a particular part of the ILH profile (i.e., the loading part) could be summed in a new worksheet, thus yielding the percentage of each type of failure mechanism that occurred during that interval (Figure 11). This table was then sorted by event time (Column C, Fig. 11) in descending order. To accurately count the sum from the filtered list, the equation =ROW(C10)-9 was used in cell E10 to produce the total sum of events at that time in the 90-600 kHz range and replicated throughout the column. To determine the percentage of events in the 90-600 kHz range that are classifiable as matrix cracking events the formula =COUNTIF($B$10:B10,"Matrix")/E10 is used in cell F10 and copied throughout the column. This command counts all events with the Matrix label in the range and divides it by the total number of events at that point in time. In other words, the previous equation is (percentage matrix cracking events at time) = (total matrix cracking events at time) / (total events at that time). More specifically, for unidirectional carbon-epoxy plates 5 which simulate the uniaxial strand tested in this study, the following frequency ranges were adopted: 90-180 kHz (matrix cracking), 180-240 kHz (fiber pullout), 240-300 kHz (debonding), and 300-600 kHz (fiber breakage). It has also been suggested 13 that frequencies above 600 kHz correspond to fiber breakage.

IV. Discussion A. Felicity Ratio 1. FR Determination Methods The two FR determination methods that were used for IM-7 and T1000 C/Ep strand tests 12 13 and preliminary IM-7 C/Ep COPV tests 12 23 were the mean FR and nth FR methods, denoted , and FRn, respectively. The FRn method assumes that statistical outliers occur at events less than the nth event in a load ramp and uses the nth AE event to determine the FR for each load ramp. This method was first applied in strand tests, but gave poor linearity (R2≈0.7); however, preliminary vessel results looked promising for n = 15 and 20 (R2≈0.8). On the other hand, the method minimizes the effect of early AE by averaging of the first n AE events during loading. This method has been effective in strand 12 13 and preliminary COPV tests 23 using n = 10 and 15. A new FR determination method, denoted FRn% or the FR percentage method was developed and applied in this study, The method is the most successful (R2≈0.9) method used to date in evaluating strand and vessel data using the intermittent load hold (ILH) cycle. The FRn% technique provides a method that is independent of the number of events that occur during each loading, which is important given the large and nonlinear increase in the number of AE events during successive ILH load cycles that occur as failure is approached. This method calculates the onset of significant AE by using the first n% of the AE events recorded during a given loading ramp (Eq. (3)). The optimal percentage has not been determined, but n = 5 has shown excellent linearity in strand and vessel tests, but the optimal percentage is likely to lie at higher values of n judging from the high event rate linearity deeper into the AE event cluster during load ramps. (3) where FR(x) = Felicity ratio at event x n = percentage of the total number of events in the pressurization ramp N = total number of events in the pressurization ramp Since all of the previously mentioned FR determination methods depend on AE sensor sensitivity, threshold level, and gain settings, it is imperative to use consistent settings between tests and calibrated test equipment. 2. COPV Lifetime Estimation Predicting a COPV’s burst pressure has proven to be difficult as evidenced by the wide Weibull variability of C/Ep COPVs. 24 If FR has a good or better linear dependence on load ratio with an R2 > 0.8, the FR* may be used to predict the burst pressure at failure of vessels having the same design and materials-of-construction (Eq. (4)). For example, in theIM-7 C/Ep COPV test that was instrumented with six AE sensors (Figure 12), the FR* was found to be 0.961 ± 0.018 using Eq. (5). Notation in Eqs. 4 and 5 is based on the linear least squares fits where m is the slope and b is the hypothetical zero load FR. Assuming that COPV FR*s exhibit the same scatter as strand FR*s, 12 the

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FR5% method would have predicted the burst pressure to within several percent if the slope, m, of the COPV FR versus LR plot was known. Furthermore, the data show, that for strand tests at least, that the steepness of m is a great indicator of a COPV’s susceptibility to fail (a steeper slope would suggest a higher propensity to fail).

(4) (5) 3. FR Linearity Dependencies The source of FR linearity (or lack thereof) was revealed after statistically analyzing each load ramp taken from two consecutive tests conducted on the same IM-7 C/Ep COPV that had been instrumented with six AE sensors (2). The distribution of AE events versus time was constructed for each loading ramp. These distribution was skewed for each ramp, indicating the majority of AE events occur later in the load ramps (due to new flaw sites being created). Also, the AE event rate didn’t stabilize and reach a steady-state value until after a finite amount of time had elapsed into the loading ramp. FR standard deviations on these ramps started at 0.032 at low LR/pressure and decreased to 0.016 at high LR/pressure later in the COPV’s life (these value would correspond to the standard deviation from the mean FR values given by the black diamonds). The first and last AE events that occurred in each load ramp were used to calculate the first (open green squares) and last (open red triangles) FRs, and were plotted (Figure 12) along with the 95% confidence interval for the FRs (dashed black lines). The FRs calculated from the first AE event in each load ramp typically lie outside of the 95% FR confidence interval indicating that these points are outliers, which illustrates why these cumbersome FR determination methods were necessary.

Figure 12. Felicity ratio statistics. IM-7 carbon vessel SN070908-02 was tested 1st on 1/6/2010, but was stopped prior to failure. The test was continued in a 2nd test until failure at 54.3 MPa (7,869 psi) on 3/11/2010.

The FR5% data (FR 5%, 1st Test, Figure 12, open blue circles) exhibited very good linearity (R2≈0.91). Good linearity was obtained (R2≈0.86) after merging the results from the second test with the first (Fig. 12, crosses). The FR5% method takes advantage of the Mean FR's linearity. Future work is needed to determine the optimal n for the FRn% method, and if FRn% leads to consistently better linearity compared to the mean FR method The averaging methods are independent of the sensitivity of the AE equipment used as long as all events register on the equipment

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with high enough amplitude to allow them to be distinguished from noise. These methods should therefore be highly repeatable between COPVs of similar construction and material.

B. Fast Fourier Transforms The fast Fourier transform (FFT) algorithm separates waveform data into energy vs. frequency spectra having the same total energy level as the waveform. For example, the FFT algorithm was applied to the waveform presented in Figure 4 to yields a frequency spectrum (Figure 1, left, FFT of raw waveform), which can be smoothed by removing reflections (Figure 1, right, FFT of only the direct wave). Aside from cleaning up the data and making it easier to interpret, using the raw versus direct wave may lead to misidentification of the most prominent peak in the FFT spectrum (28 kHz in raw waveform FFT, versus 52 kHz in direct wave FFT). Since the linear response of the sensors used in this study (DWC B-1025) is above 100 kHz, the true peak position of this event is taken as 210 kHz which, according to de Groot 5 and other researchers 4-(10),(13) corresponds to debonding in C/Ep laminates. FFT composition analysis results on the IM-7 C/Ep COPV instrumented with six AE sensors (Figure and Figure ) revealed some interesting trends, yet in other ways were inconclusive. Note that the predominance of fiber breakage over matrix cracking in the first test (Fig. 14, left) was transposed in the final test (Fig. 14, right). Further testing on additional IM-7 C/Ep COPVs is required to determine if the observed trends are real. Since the two tests depicted in Fig. 14 were conducted at two separate times over 2 months apart, it is possible a systematic error was made, such as

Figure 13. Fast Fourier transform before (left) and after (right) reflection filtering. Units are amplitude (volts) vs. frequency (kHz). From 6.3 inch diameter HyPerComp IM-7 vessel SN070908-02 tested on 1/6/2010, event #17,624.

Figure 14. Carbon-epoxy pressure vessel frequency distribution analysis. Note how the distribution of fiber breakage and matrix cracking shift from high percentages of breakage events to high percentages of matrix cracking. Pressure vessel failure mechanism propagation is based on de Groot’s frequency ranges 5 and cumulative events from 6-sensor IM-7 carbon-composite pressure vessel data of SN070908-02 from 1/6/2010 (left) and 3/11/2010 (right).

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different sensors were used, or were placed at different locations, or were transposed when they were reattached. However, if one focuses just on the trends occurring during the first test (for example, Fig. 14, left), it is evident that the same or similar co-operative damage hierarchy is governing progressive damage and that this damage hierarchy remains more or less invariant with respect to pressure (Fig. 15). It should also be noted that the Load Ramp Number 1 data was based on only one AE event, and therefore, skewed by the failure mechanism (matrix cracking) operative during that event. The FFT compositions shown for Load Ramp Numbers 2 (approx. 20 events were used) through 9 (approx. several hundred events were used) were, therefore, not influenced by solitary AE events.

Figure 15. Early-life frequency band shifts in an IM-7 composite vessel approaching failure. Fiber breakage dominates in early and mid life, but is surpassed by matrix cracking in late life. Pressure vessel failure mechanism propagation is based on de Groot’s frequency ranges 5 and cumulative events from 6-sensor IM-7 carboncomposite pressure vessel data (SN070908-02 from 1/6/2010). Frequencies below 90 kHz are ignored.

V. Conclusion Batch test data on COPVs instrumented with 1-3 AE sensors showed poorer FR linearity, presumably due to a much lower number of events than was observed in the individual COPV test instrumented with six AE sensors. This was indicative of insufficient AE sensor coverage in the batch test and correspondingly higher attenuation than when more sensor were used. The FRn and methods produced FRs with poor to good linearity (R2 = 0.5 to 0.9, respectively). The FR5% method, which showed promise using available strand data, gave excellent results for available COPV data(R2s ≈0.9 in both cases were obtained). FR results for the six-sensor individual IM-7 COPV test were in agreement with IM-7 strand tests(Figure 6, left). This agreement suggests that the characteristic FR* will not vary dramatically depending on test specimen type (strand versus COPV) provided that the sensor coverage is adequate(. More work is needed to determine how the characteristic FR* varies for IM-7 COPVs. If a consistent FR* is observed in a significant number of vessel tests, the FR5% method shows great promise for determining a COPV’s health in-situ and for predicting its burst pressure.

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Appendix Pressure schedule used in batched pressure vessel creep rupture tests. Date Time (min) Dwell Pressure (psi) Dwell Time (min) Delta (psi) Load Ratio1 5/17/2010 0 0 30 0.00 5/17/20102 30 3700 30 3700 0.49 5/19/20103 0 0 30 -3700 0.00 5/19/2010 30 3700 30 3700 0.49 5/19/2010 60 3300 30 -400 0.44 5/19/2010 90 4300 30 1000 0.57 5/19/2010 120 3900 30 -400 0.52 5/19/2010 150 4900 30 1000 0.68 5/19/2010 180 4500 30 -400 0.60 5/19/2010 210 5500 30 1000 0.73 5/19/2010 240 5100 30 -400 0.68 5/19/2010 270 6100 30 1000 0.81 5/19/2010 300 5700 30 -400 0.76 6/2/2010 20460 6700 20160 (2 weeks) 1000 0.90 6/2/2010 20490 6300 30 -400 0.84 6/16/2010 40650 6800 20160 (2 weeks) 500 0.91 6/16/2010 40680 6400 30 -400 0.85 6/30/2010 60840 6900 20160 (2 weeks) 500 0.92 6/30/2010 60870 6500 30 -400 0.87 7/14/2010 81030 7000 20160 (2 weeks) 500 0.93 7/14/2010 81060 6600 30 -400 0.88 7/28/2010 101220 7100 20160 (2 weeks) 500 0.95 1 Load ratio calculations are based on the designed burst pressure of 51.7 MPa (7,500 psi). 2 The test was paused and reset using the same batch of IM-7 COPVs. 3 Pressure rates were approximately 4.5 MPa/min (650 psi/min, 0.09 LR/min).

Accelerated pressure schedule used in individual pressure vessel tests. Time (min) Dwell Pressure (psi) Dwell Time (min) Delta (psi) Load Ratio1 0 0 30 0.00 332 2000 30 2000 0.27 63 1600 30 -400 0.21 95 2600 30 1000 0.35 125 2200 30 -400 0.29 157 3200 30 1000 0.43 187 2800 30 -400 0.37 219 3800 30 1000 0.51 249 3400 30 -400 0.45 280 4400 30 1000 0.59 310 4000 30 -400 0.53 342 5000 30 1000 0.67 372 4600 30 -400 0.61 404 5600 30 1000 0.75 434 5200 30 -400 0.69 465 6200 30 1000 0.83 495 5800 30 -400 0.77 527 6800 30 1000 0.91 557 6400 30 -400 0.85 5883 7400 30 1000 0.99 1 Load ratio calculations are based on the designed burst pressure of 51.7 MPa (7,500 psi). 2 Pressure rates were approximately 5.2 MPa/min (750 psi/min, 0.10 LR/min). 3 This row is approximate since the vessel burst at 54.3 MPa (7,869 psi, 0.89 LR). 4

Dwell pressure variations are within ± 1.5% of target.

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Acknowledgments Studies were performed by Charles Thomas Nichols at the White Sands Test Facility (WSTF) near Las Cruces, NM under the Undergraduate Student Research Program (USRP) and were financed by NASA and USRP. He would like to thank NASA for the opportunity to support agency programs in a meaningful way, USRP for their financial support, HyPerComp Engineering (Brigham City, Utah) for supplying C/Ep test specimens, J. M. Waller and R. L. Saulsberry (JTI, NASA) for their expert guidance, WSTF’s Laboratory Dept. for their encouragement, D. E. Weathers and E. E. Kowalski (USRP) for their help during data analysis, and S. Ziola (DWC) for his software advice. Ongoing efforts at WSTF to develop AE methods specific to K/Ep and C/Ep have been sponsored by the NASA Office of Safety and Mission Assurance (OSMA), Washington, DC.

References 1

AIAA, Space Systems – Composite Overwrapped Pressure Vessels (COPVs), AIAA S-081A-2006, Reston, VA, 2006. 2 Beeson, H. D., Davis, D. D., Ross Sr., W. L., and Tapphorn, R. M., “Composite Overwrapped Pressure Vessels,” NASA/TP2002-210769, January 2002. 3 Awerbuch, J., and Ghafari, S., “Monitoring Progression of Matrix Splitting During Fatigue Loading Through Acoustic Emission in Notched Unidirectional Graphite/Epoxy Composites,” J. Reinf. Plast. Compos., Vol. 7, 1988, pp. 245-263. 4 Ely, T., and Hill, E. V. K., “Longitudinal Splitting and Fiber Breakage Characterization in Graphite Epoxy Using Acoustic Emission Data,” Mater. Eval., Vol. 53, No. 2, 1995, pp. 288-294. 5 de Groot, P., Wijnen, P., and Janssen, R., “Real-time Frequency Determination of Acoustic Emission for Different Fracture Mechanisms in Carbon/Epoxy Composites,” Compos. Sci. Technol., Vol. 55, 1995, pp. 405-421. 6 Prosser, W. H., et al., “Advanced, Waveform Based Acoustic Emission Detection of Matrix Cracking in Composites,” Mater. Eval., Vol. 53, No. 9, 1995, pp. 1052-1058. 7 Shiwa, M., Carpenter, S., and Kishi, T., “Analysis of Acoustic Emission Signals Generated during the Fatigue Testing of GFRP,” J. Compos. Mater., Vol. 30, No. 18, 1996, pp. 2019-2041. 8 El Guerjouma, J. C. et al., “Non-Destructive Evaluation of Damage and Failure of Fibre Reinforced Polymer Composites Using Ultrasonic Waves and Acoustic Emission,” Adv. Eng. Mater., Vol. 3, No. 8, 2001, pp. 601-608. 9 Huguet, S., Godin, N., Gaertner, R., Salmon, L., and Villard, D., “Use of Acoustic Emission to Identify Damage Modes in Glass Fibre Reinforced Polyester,” Compos. Sci. Technol., Vol. 62, 2002, pp. 1433-1444. 10 Dzenzis, Y. A., and Qian, J., “Analysis of Microdamage Evolution Histories in Composites,” Int. J. Solids Struct., Vol. 38, 2001, pp.1831-1854. 11 Waller, J. M, Saulsberry, R. L., and Andrade, E., “Use of Acoustic Emission to Monitor Progressive Damage Accumulation in Kevlar® 49 Composites,” QNDE Conference, Providence, RI, July 2009. 12 Nichols, C. T., Waller, J. M., and Saulsberry, R. L., “Use of Acoustic Emission to Monitor Progressive Damage Accumulation in Carbon Composites,” USRP Final Report, NASA-JSC Whites Sands Test Facility, Las Cruces, NM, December 2009 (to be published). 13 Wentzel, D. J., Waller, J. M., and Saulsberry, R. L. “Progression of Micromechanical Damage in Carbon Fiber/Epoxy Tows,” USRP Final Report, Las Cruces, NM, May 2010 (unpublished). 14 Collaboration for Nondestructive Testing, “Theory - Acoustic Waves,” NDT Resource Center [online resource], URL: http://www.ndt-ed.org/EducationResources/CommunityCollege/Other%20Methods/AE/AE_Theory-Wave.htm [cited July 28 2010]. 15 Digital Wave Corp., “WaveExplorerTM Version 7.2 Users Manual,” Digital Wave Corp., Centennial, Co, 2008, pp. 69-82. 16 ASTM, “Standard Practice for Acoustic Emission Examination of Fiberglass Reinforced Plastic Resin (FRP) Tanks/Vessels,” ASTM E 1067, ASTM International, West Conshohocken, PA, 2001. 17 ASTM, “Standard Practice for Acoustic Emission Examination of Reinforced Thermosetting Resin Pipe (RTRP),” ASTM E 1118, ASTM International, West Conshohocken, PA, 2000. 18 Saulsberry, R. L., and Hernandez, L., “Task: Composite Stress Rupture NDE Research and Development Project (Kevlar ® and Carbon),” NASA Technical Reports Server (NTRS) [online database], URL: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100017653_2010019595.pdf [cited July 28 2010]. 19 Lucero, R., Personal Correspondence, NASA-JSC Whites Sands Test Facility, Las Cruces, NM, August 12 2010. 20 Olson, M., “Certificate of Conformance for IM7 & T1000 COPVs and Strands,” HyPerComp Eng. Inc., Brigham City, UT, 2008. 21 ASTM, “Standard Guide for Determining the Reproducibility of Acoustic Emission Sensor Response,” ASTM E 976, ASTM International, West Conshohocken, PA, 2005. 22 Trimm, M. W., Droulliard, T. F., Miller, R. K., and Streckert, H. H., Acoustic Emission Testing, 3rd ed., ASNT, Columbus, OH, 2005, pp. 92. 23 Lucero, R., Carden, A., Unpublished Data, NASA-JSC Whites Sands Test Facility, Las Cruces, NM, January 2010. 24 Grimes-Ledesma, L., Phoenix, S. L., Beeson, H., Yoder, T., and Greene, N., “Testing of Carbon Fiber Composite Overwrapped Pressure Vessel,” NASA-Jet Propulsion Laboratory, Pasadena, CA, 2006.

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