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«Mechanisms of Dynamic Deformation and Failure in UltraHigh Molecular Weight Polyethylene Fiber-Polymer Matrix Composites A Dissertation Presented to ...»

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Mechanisms of Dynamic Deformation and Failure in UltraHigh Molecular Weight Polyethylene Fiber-Polymer

Matrix Composites

A Dissertation

Presented to

the faculty of the School of Engineering and Applied Science

University of Virginia

in partial fulfillment

of the requirements for the degree of

Doctor of Philosophy

Material Science and Engineering

Mark R. O’Masta

May 2014

APPROVAL SHEET

The dissertation is submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Mark R. O’Masta, Author

This dissertation has been read and approved by the examining committee:

Haydn N.G. Wadley, Advisor James M. Fitz-Gerald, Committee Chair Sean R. Agnew Devin K. Harris Vikram S. Deshpande

Accepted for the School of Engineering and Applied Science:

James H. Aylor, Dean School of Engineering and Applied Science May 2014 i Ultra-high molecular weight polyethylene (UHMWPE) molecules, with molecular weights approaching 107 Da and lengths approaching 10 μm, can be gel spun and drawn into highly crystalline fibers with more than 95% of the molecules oriented in the fiber direction. The very high tensile strength (approaching 4 GPa) and elastic modulus (200 GPa) combined with a very low density (970 kg m-3) result in a fiber with very high specific strength and modulus. While the strength per unit mass of the materials in the fiber direction is ~25 times greater than that of conventional steels, weak (van der Waals) bonds between molecules leads to strengths transverse to the fibers of only a thousandth that in the fiber direction. This weak intermolecular strength also leads to creep deformation under prolonged loading at ambient temperatures, and complete failure of the polymer when the intermolecular bonds “melt” at 155°C. These materials are therefore used in weight sensitive applications, where a high uniaxial stress must be supported for relatively short periods of time. Examples include mooring cables, the sails of racing ships and ballistic impact protection panels. For ballistic applications, the

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to form thin (typically 50 μm thick) unidirectional plies containing ~85% by weight fibers. These plies are then layered to form a cross-ply ([0°/90°]n) structure, and pressed (at 127°C) to create a composite panel. This dissertation investigates the structure, mechanical properties and dynamic deformation and failure mechanisms during the ballistic impact of these UHMWPE reinforced [0°/90°] polymer matrix composites by a model projectile.

Six UHMWPE [0°/90°] polymer composite systems were investigated in the study. The laminates had measured tensile strengths (a fiber dominated property) in the range of 800 – 1100 MPa, which was 500-5,000 times higher than the laminates’ measured interlaminar shear strengths (a matrix dominated property). Digital image correlation techniques have been used to show that the Poisson expansion of a ply under compressive loading was also highly anisotropic, with a Poisson’s ratio of ν23 = 0.5 transverse to fiber direction, and ν13 = 0 in fiber direction. During uniform out of plane (through thickness) compressive loading of [0°/90°] composites, this anisotropic Poisson expansion of adjacent 90° plies has been shown to cause fiber tension in the 0o ply by a shear lag mechanism. Failure of the compressed sample occurs when the tension induced stress in the fibers reaches the plies failure strength (in excess of 1 GPa), and agreed well with experimental data collected on thick laminates with lateral dimensions substantially larger than the shear lag length.

The out of plane compressive strength of the [0°/90°] composites was discovered to be dependent upon the laminate thickness; as the laminate thickness was decreased the strength of the laminates decreased to 60%-70% of the indirect tension strength

–  –  –

conjunction with micro-X-ray tomography, two classes of defects have been identified in the [0°/90°] composites. One defect type consisted of tunnel cracks that were parallel to the fibers in a ply and approximately equally spaced in the transverse direction. These are shown to form as a result of anisotropic thermal strains within the laminates during cooling after consolidation processing. The second void-like defect results from missing groups of fibers within each ply. Like the tunnel cracks, this defect extended many centimeters in a ply’s fiber direction. While tunnel cracks were healed during ambient temperature out of plane compression, and therefore had little effect on a laminates out of plane compressive strength, the missing fiber defects significantly degraded the compressive strength of thin laminates. Compression tests using pressure sensitive film and acoustic emission monitoring reveal that regions containing missing fiber defects in thin laminates are shielded from load by defect free regions, which then fail at lower sample pressure during loading. A simple statistical model was developed that successfully predicted the contrast observed in optical and ultrasonic images, and the effect of missing fiber defects upon the out of plane compressive strength.

The dissertation also investigated the mechanisms of projectile penetration during impact of UHMWPE fiber-reinforced composites with a spherical projectile using model targets designed to dynamically load the laminates in different ways. The response of the samples were studied using a combination of synchronized high speed photography with three cameras, and 3D digital image correlation together with post-test characterization via X-ray tomography and optical microscopy. It was found that a rear supported laminate, which was prevented from deflecting, was progressively penetrated by the

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argued that penetration occurred by the indirect tension mechanism. Edge clamped laminates that are allowed to freely deflect have an improved impact resistance, especially if the projectile is fragmented before impacting the laminate, or the laminate is given an out of plane velocity prior to direct impact by the projectile. The results are used to propose a projectile penetration process model that incorporates both the activation of indirect tension and membrane stretching. It predicts that suppression of high compressive stress in the [0o/90o] laminate forces the laminate to respond in a bi-axial membrane stretching mode where the kinetic energy of the projectile is expended in the very significant work needed to stretch the laminate. This hypothesis was tested with a model impact target that spatially distributed the load to the laminate and was found to

–  –  –

Haydn, your guidance and patience have made this journey tenable. You have taught me to be always inquisitive, always a visionary and never deterred. Thank you for your inspiration and enthusiasm and for your enduring encouragement of my (at times unconventional) research endeavors.

I would like to express my gratitude to each member of my committee, Sean Agnew, Vikram Desphande, James Fitz-Gerald and Devin Harris, for your continual advisement and thoughtful review of my work.

I would also like to express my utmost appreciation for all of my collaborators. I thank Vikram Deshpande, Julia Attwood, Kandan Karthikeyan and Ben Russell at the University of Cambridge for your numerous discussions and help in studying this research topic. I sincerely thank Frank Zok, Brett Compton and Nell Gamble at the University of California – Santa Barbara for your help with the ballistic experiments. I am also thankful to Dustin Crayton (UVA) and Jason Cain (Army Research Laboratory)

–  –  –

van der Werff and Ulrich Heisserer at DSM for your knowledgeable insights, fruitful discussions and material support.

I would also like to recognize my laboratory mates and IPM staff: Kumar Dharmasena, Liang Dong, Tommy Eanes, David Glover, Rich Gregory, Ryan Holloman, Toni Kember, Adam Malcom, and Sherri Sullivan. I thank each of you for indispensible help over the years and for providing a richer, fuller and happier graduate experience.

I am forever grateful to my parents for their love, unwavering support and encouragement. Thank you for always pushing me to be a better person.

To my wife, Brenna, you have sacrificed more than anyone in this pursuit. I can never repay all that you have given, your love, affection, advice and support, and all of the sleep, weekends and time apart (both physically and mentally) that we have lost. I give you my deepest and most humble thank you.

This work was funded by the Office of Naval Research (ONR) under grant number N00014-07-1-0764 (Program manager, Dr. D. Shifler) and the Defense Advanced Research Projects Agency (DARPA) under grant number W91CRB-11-1-0005

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Abstract

Acknowledgements

List of figures

List of tables

List of symbols

Chapter 1. Introduction

1.1. Impact resistant polyethylene

1.2. Impact response of polymer matrix composites

1.3. Dissertation goals

1.4. Dissertation outline

1.5. References

Chapter 2. UHMWPE fibers and composites

–  –  –

2.2. Investigated materials

2.3. Laminate consolidation

2.4. References

Chapter 3. Fiber and composite mechanical response

3.1. Fiber tensile response

3.1.1. Methodology

3.1.2. Results

3.2. Laminate tensile response

3.2.1. Methodology

3.2.2. Results

3.3. Inter-laminar shear

3.3.1. Methodology

3.3.2. Results

3.4. Shear strength dependence on compression

3.4.1. Methodology

3.4.2. Results

3.5. Compression of unidirectional laminates

3.5.1. Methodology

3.5.2. Results

–  –  –

3.7. References

Chapter 4. Mechanisms of projectile penetration in HB26 encapsulated aluminum structures

4.1. Materials and sample fabrication

4.1.1. Materials

4.1.2. Sample fabrication

4.2. Impact test protocol

4.3. Impact response of targets

4.3.1. The bare aluminum plate

4.3.2. The front face cutout target

4.3.3. The baseline fully encased target

4.3.4. The rear face cutout target

4.4. Discussion of Dyneema penetration mechanisms

4.4.1. Dyneema plates resting on a strong foundation

4.4.2. Edge supported Dyneema® plates

4.4.3. Residual velocity of the projectile/ejecta

4.5. Concluding remarks

4.6. References

Chapter 5. Defect Dependent Transverse Compressive Strength of Ballistic

–  –  –

5.1. Materials and fabrication

5.1.1. Material Types

5.1.2. Laminate fabrication

5.2. Review of the indirect tension model

5.3. Transverse compressive strength

5.4. Defect characterization

–  –  –

distributions

5.6. Modeling and simulation

5.7. Concluding remarks

5.8. References

Chapter 6. Ballistic impact response of HB26 encased aluminum-alumina hybrid panel target……….

.

6.1. Sample fabrication

6.2. Impact tests and characterization

6.3. Results

6.3.1. Encased hybrid targets – ceramic prism base impacts

6.3.2. Encased hybrid targets – ceramic prism apex impacts

6.3.3. Rear face cutout targets – ceramic prism base impacts.................. 160

–  –  –

6.4. Discussion

6.5. Conclusions

6.6. References

Chapter 7. Discussion

7.1. UHMWPE Composite Characterization

7.2. Impact Response Mechanisms

7.3. Laminate Defect Effects

7.4. Suggestions for future work

7.5. References

Chapter 8. Conclusions

8.1. References

Appendix A. Tensile properties of high-performance fibers

Appendix B. Measurement length changes from teh high speed images......... 203

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Figure 1.1.

(a) Schematic illustration of the cross-section of a UHMWPE fiber (thermoplastic) polymer matrix [0o/90o]2 cross-ply tape used to form the laminate shown in (b) by out of plane compression at a pressure of 20.6 MPa and temperature of 127oC.

Figure 1.2.

The effects of material properties on projectile penetration near the ballistic limit of light targets.

Figure 1.3.

Material property charts comparing the (a) tensile strength and Young’s modulus and (b) specific toughness and extensional wave speed, cL = (E/ρ)1/2, of high performance fibers. Contours of the Cunniff [24] velocity, c*, are also plotted on (b).

Figure 1.4.

(a) Schematic illustration of the deformation mechanisms occurring during

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show (b) the wave propagation along the laminate and (c) the stress state under the projectile.

Figure 1.5.

(a) Schematic illustration of plies within a cross-ply laminate under a uniform compressive stress, σz. Poisson lateral expansion in the fiber direction is much less than that transverse to the fibers. (b) Schematic illustrations of the stress within the composite predicted by the shear lag model.

Figure 2.1.

(a) Packing of polyethylene molecules in an orthorhombic crystal structure.

(b) Crystal texturing in an UHMWPE drawn fiber, where the molecules are oriented in the fiber orientation. This crystal structure can assemble as either (c) a foldedchain lamellar structure or (d) an extended-chain structure

Figure 2.2.



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