Hypervelocity impact testing of a pressurized composite overwrapped pressure vessel and comparison to numerical analysis

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Procedia Engineering 204 (2017) 476–483

14th Hypervelocity Impact Symposium 2017, HVIS2017, 24-28 April 2017, Canterbury, Kent, UK

Hypervelocity impact testing of a pressurized composite overwrapped pressure vessel and comparison to numerical analysis M.A. Garciaa,*, B.A. Davisb, J.E. Millera a Unversity of Texas at El Paso, 500 W. University Ave., El Paso, TX 79912 Jacobs, NASA Johnson Space Center JETS contract, 2224 Bay Area Blvd., Houston, TX, 77058

b

Abstract As the outlook for space exploration becomes more ambitious and spacecraft travel deeper into space, it is increasingly important that systems with energetic material storage perform reliably within the space environment. Because hardware mass is limited, the use of high strength-to-weight Composite Overwrap Pressure Vessels (COPV) has enabled designers to store energy at a reduced mass penalty; however, because the composite overwrap carries a high fraction of the stress of pressurization, there is significant concern that damage to the composite overwrap can lead to vessel failure and potentially cause loss-of-vehicle/loss-of-crew. One of the greatest external threats to the integrity of a spacecraft’s COPV is an impact from the meteoroid and orbital debris environments (MMOD). Because the limited research regarding this problem cannot be generalized, the capacity for these vessels to tolerate such impacts is not well understood. This report details some of the early experimental efforts undertaken by the Hypervelocity Impact Technology (HVIT) group in the support of a NASA Engineering and Safety Center (NESC) assessment to understand how hypervelocity impact conditions comparable to MMOD impacts initiate catastrophic COPV failure. In addition to this experimental work, this publication also reports on the development of a numerical method to guide the interpretation of the experimental data and extension to other flight relevant configurations. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 14th Hypervelocity Impact Symposium 2017. Keywords: hypervelocity impact; composite overwrapped pressure vessel

* Corresponding author. Tel.: +1 915 706 8341; fax: +1 281 483 3908 E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 14th Hypervelocity Impact Symposium 2017.

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 14th Hypervelocity Impact Symposium 2017. 10.1016/j.proeng.2017.09.744

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1. Introduction Composite overwrap pressure vessels (COPV) are commonly used in spacecraft propulsion systems as they provide a reduced mass compared to all metallic vessels [1]. The effect is achieved by replacing the metallic vessel with a much thinner liner and reinforcing it with a composite overwrap such as a carbon-fiber reinforced plastic (CFRP) [2]. This design allows a high fraction of the stress in the pressurized vessel walls to be carried by the highly-directional, composite overwrap strength [3]. While the directionality of the composite overwrap allows the efficient use of strength to contain the pressurized contents, the composite overwrap’s brittle nature near failure makes determining the effects from flaws difficult to predict [4]. This potential brittle failure mode at high stress states can have a large effect on COPV performance and jeopardize mission success; therefore, designers must diligently balance the advantage gained in strength to weight ratio of COPV’s with the inherent risk of failure associated with implementing it into spacecraft architecture [5]. While the space environment is host to a variety of hazards, a principal threat to COPV integrity is impact of micrometeoroid and orbital debris (MMOD) [6]. Studies by NASA and other agencies suggest the MMOD population will grow in the next 100 years, even if space operations discontinue [7]. With no feasible method to reduce the ever increasing MMOD population, the most effective means of mitigation to this threat is to shield critical hardware and track known orbital debris [8]. As related to composite structures, studies of the effects of hypervelocity impact to unshielded reinforced composites have demonstrated that severe fiber damage and extensive delamination of laminae in layered composites is probable [9-12]. This has motivated loosely-constrained, initial assumptions regarding catastrophic failure in survivability analyses ranging from any damage to a vessel up to full penetration of the composite overwrap. This difficulty in identifying a critical damage is due largely to limited research in hypervelocity impacts of an energized COPV and the broad array of COPV designs. Despite limited knowledge of COPV failure threshold, two critical COPV failure modes have been established, the “leak-before-burst” (LBB) and catastrophic rupture [13]. Standards for spaceflight pressure vessel designs require validation that a vessel, at or below maximum expected operating pressure, fails within the LBB failure mode threshold [14]. Because this standard does not incorporate hypervelocity impact damage within its scope, initial assumptions regarding the COPV response to hypervelocity impact can only be inferred. If a projectile exceeds the ballistic limit of the COPV, the lowest severity failure expected is rapid, uncontrolled venting and depressurization of the COPV, i.e. the “leak-before-burst” mode. Furthermore, expectations are that beyond a certain threshold above the ballistic limit, catastrophic rupture will occur. While this has been demonstrated with smaller scale consumer grade COPV, consumer products are not designed to the same vigilance aerospace hardware is and thus comparison between the two cannot be made [15,16]. Given that the threshold for catastrophic failure from hypervelocity impact is not known, and such a failure leads to the uncontrolled release of stored energy, there is significant uncertainty that a spacecraft can recover from catastrophic failure of a COPV. To lessen the uncertainty associated with COPV failure threshold, NASA has invested resources for better understanding this MMOD risk. The hypervelocity impact testing reported herein has been conducted on a series of non-flight, full scale COPV provided by NASA’s International Space Station program. To maximally illustrate the effects of MMOD impact damage to pressurized COPV, the test articles have been subjected to hypervelocity impact in the absence of shielding. In this paper, the details of a NASA Engineering and Safety Center (NESC) sponsored experimental study into failure threshold of COPV is presented. To further the scope of this effort, a numerical model has been developed which is comparative to the experimental testing. The preliminary data obtained from the numerical model is presented as well. 2. Experiment Hypervelocity impact testing of COPV has been conducted under the guidance of NASA’s Johnson Space Center (JSC) Hypervelocity Impact Technology (HVIT) group in Houston, Texas. A series of thirty impact tests occurred on non-flight surplus assets provided by NASA’s International Space Station program and testing took place at NASA’s

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JSC White Sands Test Facility (WSTF) Remote Hypervelocity Test Laboratory (RHTL) in Las Cruces, New Mexico [17]. This assessment included subjecting pressurized and unpressurized COPV to hypervelocity impact. In this regard, an impact on a pressurized COPV which generates a catastrophic rupture, and thus the destruction of the test article, can be compared against the impact damage generated on its unpressurized counterpart. For pressurized tests, the internal pressure varied between 27.7 and 29.4 MPa; the vessel is rated for a 29.7 MPa maximum designed operating pressure. Impact of unpressurized COPV occurred for tanks in an ambient condition with respect to the gas gun target chamber. For both conditions, the target chamber was evacuated for the duration of the test; nominal was < 1 torr. To assess the dependence COPV failure threshold has on impact location, impact tests occurred in the cylindrical and shoulder regions. Also, impact obliquities of 0°, 45° and 60° have been assessed to determine the effects obliquity has on failure threshold for this test article. This enabled the identification of the impact conditions most likely to produce a catastrophic rupture of a pressurized COPV. 2.1. COPV target description This vessel is constructed of two materials, an interior metallic liner and external reinforced-composite overwrap, which share the stress loads acting on the vessel during operation. However, it is the composite overwrap which bears much of the COPV internal stress. The metallic liner serves primarily as an impermeable barrier and as the anchor for which the composite overwrap is installed. The interior metallic liner of this vessel is a Samtech liner (SK-1343-3) made from the aluminum alloy Al 6061-T6. The basis of the composite overwrap is a tow type CFRP of Toray T1000 carbon-fiber. The CFRP tows are wrapped over the liner in layers comprised of circumferential and high-angle helical windings. Seen in Fig. 1, a representative cross-sectional view of a test article is presented, including thicknesses of the liner and overwrap regions, and has been obtained from computed tomography (CT) scanning. The liner features, in general, a minimum thickness of approximately 1.97 mm in its cylinder region and gradually increases in wall thickness through the dome and boss regions of the vessel. The composite overwrap thickness in the cylindrical region of the vessel is approximately 2.5 mm, however, fluctuation in the wrap thickness is apparent and can attributed to manufacturing variation. Furthermore, the overwrap thickness in the dome region of the vessel increases from 1.5 mm near the cylinder/dome transition of the vessel to an approximate maximum of 5 mm at the boss location, and again, manufacturing anomalies resulting in thickness variation are apparent.

Fig. 1. Computed Tomography (CT) scan of a test article in the assessment series.

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3. Test Results Summary Post-test photography, Fig. 2-3, illustrates the range of damage which has been observed from hypervelocity impact testing in this assessment. The test parameters and results are presented in Table 1. The damage results are abbreviated into three qualitative categories respective of the damage incurred for each test. Tests denoted Pass indicate that catastrophic failure did not occur during a pressurized test. The denotations Vent and Rupture refer to and describe the failure mode observed after an impact produced a perforation in the vessel wall. Finally, Perforation and Crater indicate the extent of damage incurred in an impact test of an unpressurized tank. The presence of an asterisk (*) indicates that a projectile fully penetrated the composite overwrap but did not perforate the aluminum liner.

Fig. 2. Post-test CT scan imagery. (a) Pass, HITF16212; (b) Full perforation of COPV wall, HITF16211.

Fig. 3. Ruptured COPV, Test No. HITF16162.

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Table 1. Test results summary Test Id

Target Location

COPV Pressure (MPa)

COPV temperature (C)

Projectile Material

Projectile Diameter (mm)

Projectile Mass (g)

Impact Angle (deg)

Velocity (km/s)

Damage Result

HITF16159

Cylinder

29.2

50.5

Al 2017-T4

1.00

0.00146

45

7.41

Pass*

HITF16160

Cylinder

28.8

35

Al 2017-T4

1.52

0.00510

45

7.06

Vent

HITF16274

Cylinder

0

…

Al 2017-T4

1.51

0.00505

45

7.00

Perforation

HITF16161

Cylinder

27.7

41.7

Al 2017-T4

2.38

0.01975

45

7.08

Rupture

HITF16178

Cylinder

0

…

Al 2017-T4

2.38

0.01974

45

7.09

Perforation

HITF16162

Cylinder

29.1

29.4

Al 2017-T4

2.01

0.01191

45

7.01

Rupture

HITF16211

Cylinder

0

…

Al 2017-T4

2.01

0.01196

45

7.00

Perforation

HITF16163

Shoulder

29.0

38.9

Al 2017-T4

1.52

0.00514

45

7.04

Pass*

HITF16212

Shoulder

0

…

Al 2017-T4

1.52

0.00509

45

6.86

Crater*

HITF16164

Shoulder

29.1

50.5

Al 2017-T4

2.01

0.01190

45

7.19

Vent

HITF16327

Shoulder

0

…

Al 2017-T4

2.01

0.01191

45

7.05

Perforation

HITF16165

Cylinder

29.1

35.5

Al 2017-T4

1.72

0.00741

45

7.23

Vent

HITF16275

Cylinder

0

…

Al 2017-T4

1.71

0.00736

45

6.53

Perforation

HITF16331

Cylinder

0

…

Al 2017-T4

1.72

0.00746

45

7.10

Perforation

HITF16166

Cylinder

28.1

29.4

Al 2017-T4

1.51

0.00505

0

7.24

Vent

HITF16328

Cylinder

0

…

Al 2017-T4

1.51

0.00505

0

6.65

Perforation

HITF16332

Cylinder

0

…

Al 2017-T4

1.51

0.00506

0

7.32

Perforation

HITF16167

Cylinder

29.1

44.4

Al 2017-T4

1.72

0.00741

0

7.01

Rupture

HITF16329

Cylinder

0

…

Al 2017-T4

1.72

0.00740

0

7.11

Perforation

HITF16168

Cylinder

29.1

42.7

Al 2017-T4

2.31

0.01176

60

7.03

Rupture

HITF16393

Cylinder

0

…

Al 2017-T4

2.31

0.01175

60

7.20

Perforation

HITF16169

Cylinder

29.1

42.7

Al 2017-T4

2.01

0.01191

60

7.08

Vent

HITF16394

Cylinder

0

…

Al 2017-T4

2.01

0.01196

60

7.01

Perforation

HITF16170

Cylinder

29.1

40.5

Al 2017-T4

2.50

0.02290

45

4.07

Rupture

HITF16395

Cylinder

0

…

Al 2017-T4

2.50

0.02289

45

4.11

Perforation

HITF16171

Cylinder

29.4

44.4

Al 2017-T4

2.01

0.01194

45

4.08

Pass*

HITF16504

Cylinder

28.8

20

440C SS

1.09

0.00525

45

7.10

Vent

HITF16513

Cylinder

0

…

440C SS

1.09

0.00528

45

7.03

Perforation

HITF16505

Cylinder

28.7

10.5

440C SS

1.29

0.00861

45

7.10

Rupture

HITF16514

Cylinder

0

…

440C SS

1.29

0.00861

45

7.01

Perforation

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4. Simulation While the experimental trials provide valuable insight into failure characteristics of unshielded pressurized COPV, the impact testing is constrained by the upper limit of projectile acceleration. As evident in Table 1, testing primarily took place in the 7 km/s regime. Beyond this threshold, testing was not feasible and thus numerical analysis is a valuable method for expanding the dataset. This is important because solid particles in the MMOD environment carry a much wider velocity domain than what testing can reproduce. To this end, a numerical model has been designed to broaden the conclusions made from experimental testing beyond the original dataset. Numerical simulations have initially been compared to unpressurized COPV tests. Because the numerical model reported herein has not been fully validated, the scope of this section will be to briefly discuss the design of the model and report on preliminary results obtained from it. 4.1. Model Development A hydrodynamic stress analysis code, CTH, has been used for simulating hypervelocity impact in this study. Unstressed COPV impact simulations have been compared to the unpressurized impact tests. Each simulation used a three-dimensional rectilinear (3DR) calculation with a mesh volume defined by 150 m mesh cell widths, distributed equally in three dimensions in the calculation space. Mie-Gruneisen equation of states (EOS) were assigned to the liner and CFRP overwrap, and a native CTH sesame tabulated EOS for aluminum was assigned to the projectile. Elastic perfectly plastic stress models were assigned to the liner and CFRP, and a Johnson-Cook strength model was assigned to the projectile. Details regarding user defined parameters of the EOS and elastic-plastic models can be reviewed in Tables 2 and 3, respectively. Table 2. User defined Mie-Gruneisen EOS values. EOS

Parameter

Value

Units

Composite Overwrap

Density

1.54

g/cm3

Sound speed

2.34e05

cm/s

Hugoniot linear coeff.

1.5

-

Gruneisen parameter

2.0

-

Specific heat

1.98e-2

J/(kg-k)

Table 3. Elastic-plastic user defined values. Elastic-plastic model

Parameter

Value

Units

Aluminum 6061-T6

Yield strength

2.55e09

MPa

Poisson’s ratio

0.35

-

Yield strength

2.21e09

MPa

Poisson’s ratio

0.36

Composite Overwrap (CFRP)

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4.2. Simulation Results The results of unpressurized COPV impact tests and simulations are shown as a plot of impact velocity and projectile diameter, shown in Fig. 4. In general, the results of simulation agree with the outcome of the hypervelocity impact testing, i.e. the simulation predicted a failure for the same test parameters which triggered a failure in the experimental test series. The extent of damage in the unpressurized test no. HITF16212 is shown in Fig. 5a as a CT cross section of the impact crater. A simulated impact of the COPV shoulder wall which mirrored the test parameters of HITF16212 gives insight into how the model performed. In both cases, experiment and simulation, the impact parameters resulted in crater and initiation of spall in the rear surface of the vessel liner. However, the material model governing the metallic liner over-performed compared to experimental testing. This is evident in that the simulation yielded approximately a 2.0 mm dimple of undetached spall on the inside of the liner, Fig. 5b, whereas the experimental test yielded approximately a 2.5 mm fracture feature. To expand on the data set, additional impacts were computed at 4 km/s and 12 km/s. The model predicts a projectile and impact velocity combinations consistent with the general trend established by both testing and computation. 3.00

NESC - Fail CTH Simulation - Fail

NESC - Pass CTH Simulation - Pass

2.50

d, mm

2.00 1.50 1.00 0.50 0.00 2.00

4.00

6.00

8.00

V, km/s

10.00

12.00

14.00

Fig. 4. Simulation and Experiment results

Fig. 5. Comparison of test number HITF16212; (a) CT scan image, (b) CTH simulation result

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5. Conclusions This impact testing has demonstrated that pressurized COPV have a limited capacity to incur damage from high energy impacts. What’s more interesting, the COPV’s which passed testing did so while incurring full perforation of the CFRP overwrap. In the tests where perforation of the vessel wall did occur, it appears that impact energy plays a role in the failure mode of the vessel. The initial findings from this experimental work, in general, indicate that assumptions about COPV robustness to MMOD impact could be conservative. While the internal stress state of a COPV, post impact, has not been determined, the results generated by this series of testing and computational simulation certainly motivate further analysis. The preliminary simulation results show that refinement of the COPV composite overwrap EOS model is necessary. However, the qualitative findings of the preliminary simulations agreed with the experimental outcomes. References [1] Kezirian, M.T., K. L. Johnson, and S. L. Phoenix. “Composite Overwrapped Pressure Vessels (COPV): Flight Rationale for the Space Shuttle Program.” AIAA SPACE 2011 Conference & Exposition (2011). [2] Vasiliev, V.V., and Robert Jones. “Composite Pressure Vessels: Analysis, Design, and Manufacturing.”, (2009). [3] Thesken, J.C., et al. “A Theoretical Investigation of Composite Overwrapped Pressure Vessel (COPV) Mechanics Applied to NASA Full Scale Tests.” (2009). [4] Raju, I. S., D. S. Dawicke, and R. W. Hampton. "Some Observations on Damage Tolerance Analyses in Pressure Vessels." 58th AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference (2017) [5] Murthy, P.L.N., et al. “Stress Rupture Life Reliability Measures for Composite Overwrapped Pressure Vessels.” 48th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference (2007). [6] Kessler, D.J. “Orbital Debris Issues.” Advances in Space Research 5.2 (1985): 3-10. [7] Percy, T. K., and B. D. Landrom. "Investigation of National Policy Shifts to Impact Orbital Debris Environments." Space Policy 30.1 (2014):23-33. [8] Christiansen, E.L.; “Handbook for Designing MMOD Protection." NASA, JSC-64399, Version A, JSC-17763 (2009) [9] Yew, C. H., and R. B. Kendrick. "A Study of Damage in Composite Panels Produced by Hypervelocity Impact." International Journal of Impact Engineering 5.1-4 (1987): 729-38. [10] Tennyson, R.C., and C. Lamontange. “Hypervelocity Impact Damage to Composites”. Composites Part A: Applied Science and Manufacturing 31.8 (2000): 785-94. [11] Alexander, C. S., C. T. Key, and S. C. Schumacher. "Dynamic Response and Modeling of a Carbon fiber- epoxy Composite Subject to Shock Loading." Journal of Applied Physics 114.22 (2013) [12] Cherniaev, A., Tellichev, I. "Numerical Simulation of Impact Damage Induced by Orbital Debris on Shielded Wall of Composite Overwrapped Pressure Vessel." Applied Composite Materials 2.6 (2014): 861-84. [13] McLaughlan, P. B., S. C. Forth, and L. R. Grimes-Ledesma. "Composite Overwrapped Pressure Vessels: A Primer." NASA/SP-2011-573 (2011). [14] Chang, J.B., and R.W. Seibold. “Operational Guidelines for Spaceflight Pressure Vessels.” DOT-VNTSC-FAA-05-01 Vol. The Aerospace Corporation, 2005. [15] Mespoulet, J. et al. “Numerical Investigations of Hypervelocity Impacts on Pressurized Aluminum-Coposite Vessels.” Procedia Engineering 103. The 13th Hypervelocity Impact Soymposium (2015): 373-80. [16] Héreil, P., Mespoulet, J. and Plassard, F. “Hypervelocity Impact of Aluminum Projectiles Against Pressurized Aluminum-Composite Vessel.” Procedia Engineering 103.The 13th Hypervelocity Impact Symposium (2015): 181-8. [17] Remote Hypervelocity Test Laboratory. (2015, March 23). Retrieved April 05, 2017, from https://www.nasa.gov/centers/wstf/site_tour/remote_hypervelocity_test_laboratory/index.html