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PENETRATION MECHANICS OF UHMWPE SOFT LAMINATES

K. Ayotte (BME), B. Gama, R. Adkinson (ARL), J.W. Gillespie Jr.

University of Delaware . Center for Composite Materials . Department of Mechanical Engineering

INTRODUCTION

Ultra High Molecular Weight Polyethylene (UHMWPE) fibers possess extraordinary physical and mechanical properties: low specific weight, high modulus, high strength, high impact resistance, and high cut and abrasion tolerance to name a few [1]. Commercial examples of UHMWPE include: DSM Dyneema® and Honeywell Spectra®. Current commercial uses include: High strength ropes, marine ropes and lines, gloves, body armor (vests and helmets), storm protection panels, etc.

MATERIALS PROCESSING

UHMWPE soft laminate composites are made using compression molding with [0/90/0/90]N (N=1,2,4,6,...80) sub-laminate architecture and N being the number of layers in the finished composite.

609-mm x 609-mm (24-in x 24-in) soft laminates are received from the Army Research Laboratory for 20 layers and greater (N=20,30,40,60,80). All soft laminates with less than 20 layers are fabricated in house at UD-CCM using compression molding on a 150 ton hot press.

300 275 4000

QS-PST METHODOLOGY

Proven capable of quantifying ballistic damage mechanisms and energy dissipation in thick-section composites.

Specimens are tested at varying SPR (support span to punch diameter ratio, DS / DP) and STR (support span to specimen thickness ratio, DS / HC) to mimic various ballistic energy dissipation mechanisms. By varying the test SPR, one can test specimens for both shear and bending dominated loading. The resulting load-displacement data is used to calculate the specific energy absorption at each SPR and then is compared with ballistic results.

QS-PST fixture schematic

257°F

OBJECTIVES

Use Quasi-static Punch Shear Test (QS-PST), Dynamic Impact Punch Shear Test (DI-PST), and Ballistic test methodology to characterize failure mechanisms and energy dissipation of UHMWPE Soft Laminates. Earlier studies on thick section composites have shown that QSPST mimics ballistic energy dissipation mechanisms. Can this be proven with UHMWPE Soft Laminates?

[1] Xu,T., Farris, R.J. Comparative Studies of Ultra High Molecular Weight Polyethylene Fiber Reinforced Composites. Polymer Engineering and Science, 2007, Vol. 47 ,pp.1545-1553

Temperature, F

Load Temperature

3500

250 225 200 175 150 125 100 75 500 50 25 0 60 1500 3000

3000psi

2500

140°F

2000

1000

0

5

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No. of Layers Thickness (mm) Density(g/cm3) AD (kg/m2) 1 0.246 0.97 0.238 2 0.532 0.97 0.516 3 0.737 0.97 0.715 4 1.070 0.97 1.038 6 1.586 0.97 1.539 8 2.080 0.97 2.018 12 3.162 0.97 3.067 16 4.232 0.97 4.105 20 5.243 0.97 5.086 30 7.871 0.97 7.635 40 10.414 0.97 10.102 60 15.730 0.97 15.258 80 21.080 0.97 20.448

QS-PST set up

Time, Minutes

Load, psi

Manufacturing cycle for DSM

Dyneema®

Thickness, Density, and Areal Density Corresponding to No. of Layers of UHMWPE Soft Laminate

QS-PST

A new fixture has been fabricated for large SPRs due to the large displacement of the punch needed to completely penetrate the specimens. A longer punch was made, but restraints were needed around the punch in order to prevent it from bending under high loads.

DYNAMIC IMPACT PUNCH SHEAR TEST (DI-PST)

BALLISTIC TESTING

Ballistic experiments are conducted at Chesapeake Testing, MD, using a custom built ballistic test fixture. Fragment Simulating Projectiles (FSP) and Right Circular Cylinder (RCC) projectiles are used. SPR for ballistic experiments is SPR = 13.33. Impact velocity (VI) and residual velocity (VR) of the projectiles are measured using infrared velocity screens and a V50 is statistically recorded. V50 is The baseline ballistic limit, which refers to the impact velocity at which a projectile completely penetrates an armor component 50% of the time.

VI VR DS HP DP HC

BALLISTIC DAMAGE MECHANISMS

On impact, a longitudinal stress wave will travel along the fibers, while a transverse stress wave will travel radially outwards from the point of impact. The material is strained at a uniform rate. However, the interaction of the longitudinal and transverse stress waves results in a the increase of the local strain rate. Failure will only occur when the ultimate strain of the fiber has been exceeded [2]. Therefore, fibers, like UHMWPE, that have high tensile strengths and large failure strains can absorb larger amounts of Fiber energy absorption energy.

(AMPTIAC Quarterly Vol. 9) [2] Taylor, S.A., Carr, D.J. Post Failure Analysis of [0/90] Ultra High Molecular Weight Polyethylene Composite after Ballistic Testing. Journal of Microscopy, 1999, Vol. 196, Pt. 2, pp. 249-256

Simple Schematic of DI-PST set up

DI-PST is used to dynamically test soft laminates in a controlled environment. A high pressure gas gun shoots a steel projectile in to 30cal punch which impacts the specimen. Behind the projectile is a Piezotronic force sensor that outputs a certain voltage that is read by a picoscope upon impact. The voltages are calibrated to a specific force and then recorded.

(a) (b) (c) (d) (a) Five "half moon disks are stacked around the punch. (b) The collar is slid up, (c) one disk is removed, and (d) the collar is slid back down as the punch head descends.

© 2010, University of Delaware, all rights reserved

PENETRATION MECHANICS OF UHMWPE SOFT LAMINATES

(Continued)

QS-PST ­ LOAD UNLOAD RELOAD

Load Unload Reload (LUR) Testing is done to observe the unloading and reloading stiffness at various displacement during the test. Cycle: Punch is displaced 1.27-mm (0.05-in), then unloaded back to 0.635-mm (0.025-in), then reloaded to 1.905-mm (0.075-in), then unloaded back to 1.27-mm, etc.

24 22 20 18 16

QS-PST RESULTS

No. of Layers = 20 HC = 5.243-mm No. of Layers = 40 HC = 10.414-mm

BALLISTIC TESTING RESULTS

20 Layers, 30cal FSP 40 Layers, 16Grain RCC 40 Layers, 30cal FSP

SPR = 1.5

VI = 1421 m/s, VR = 0 m/s VI = 1128 m/s, VR = 0 m/s VI = 831 m/s, VR = 0 m/s

SPR = 2.0

VI = 1622 m/s, VR = 401 m/s VI = 1200 m/s, VR = 344 m/s VI = 859 m/s, VR = 94 m/s

Load, P, kN

14 12 10 8 6 4 2 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13

SPR = 2.5

VI = 1264 m/s, VR = 522 m/s VI = 1032 m/s, VR = 642 m/s

VI = 2419 m/s, VR = 1099 m/s

SPR = 3.0

Displacement, , mm.

(a) (b) (a) LUR results for 6L, SPR = 1.5, HC = 1.586-mm and (b) a zoomed in section

SPR = 4.17 As SPR increases, less shear deformation and more fiber pull is obser- SPR = 6.67 ved

VI = 3424 m/s, VR = 1837 m/s

VI = 1489 m/s, VR =768 m/s

VI = 1313 m/s, VR = 1007 m/s

The red line is the unloading stiffness, the pink line is the reloading stiffness and the x-int. is considered o With these results, we can gather data on all the unloading and reloading stiffnesses and corresponding displacements and try and understand the non-linear loading of soft laminates.

High speed photo below V50 (Picture credit to Ref. [2])

QS-PST DAMAGE MECHANISMS

1 ­ The "knee" before the first local peak 2 ­ After first failure 3 ­ After second failure

30

Load, P, kN

QS-PST RESULTS

120

BALLISTIC COMPARISON

SPR = 1.5, ET = 453 J SPR = 2.0, ET = 561 J SPR = 2.5, ET = 653 J SPR = 3.0, ET = 720 J SPR = 4.17, ET = 771 J SPR = 6.67, ET = 997 J 40L, HC = 10.414-mm

CONCLUSIONS

There are 3 main damage modes observed for QS-PST: inter-laminar matrix damage, shear cutting of fibers, and combined tension-shear cutting/failure of fibers. Stiffness reduces with increasing SPR, however, the peak load obtained in different SPRs are comparable to each other There are 4 main damage modes observed for Ballistic: sling shot failure, shear cutting of fibers, physical inter-laminar delamination, and wrinkles in the rear face of the soft laminate. Below V50, more back face deformation is observed in ballistic testing and as VI increases, more shear cutting is seen.

70 60 50 40 30 20

SPR = 1.5, ET = 173 J SPR = 2.0, ET = 275 J SPR = 2.5, ET = 294 J SPR = 3.0, ET = 364 J 20L, HC = 5.243-mm

1200

100

25

60

Total Dissipated Energy, ET, Joule

80

Load, P, kN.

1000

800 20L Ballistic E50 QS-PST 20L: 5.243mm ... 40L: Ballistic E50 QS-PST 40L: 10.414mm

20

40

Load, P, kN

1

15 10

10 0

20

600

1 2 1

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Displacement, , mm

Displacement, , mm.

20L, HC = 5.243, Varying SPR

50

40L, HC = 10.414, Varying SPR

16L, HC = 4.232-mm 12L, HC = 3.162-mm 8L, HC = 2.080-mm 6L, HC = 1.5863-mm 4L, HC = 1.0620-mm 2L, HC = 0.5317-mm 1L, HC = 0.2456-mm

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Displacement, , mm

40

0

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Before the first failure, the fibers have only been drawn and back face deformation is occurring. As displacement increases, more shear cutting is seen.

0

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Load, P, kN

30

SPR = DS/DP

20

1 2 E50 mPV50 2

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E50 20L: 307J 40L: 823J

Displacement, , mm

SPR = 2.5, DS = 19.05-mm, Varying Thickness

The ballistic limit energy for soft laminates of: 20L mimics QS-PST at SPR = 2.55 40L mimics QS-PST at SPR = 4.75

ACKNOWLEDGEMENTS

Funding for this work is provided by ARL-CMR MIPR (Soft Laminate). Special thanks to ARL-PI, Rob Adkinson.

© 2010, University of Delaware, all rights reserved

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