ARL-TR-8022 ● MAY 2017

US Army Research Laboratory

Compression-after-Impact and Bend Fatigue 

Results of Glass/Epoxy Composites with 

Compliant Interlayer and Needling 

Interlaminar Enhancements

by Robert A Haynes, Steven E Boyd, and Bradley D Lawrence

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ARL-TR-8022 ● MAY 2017

US Army Research Laboratory

Compression-after-Impact and Bend Fatigue 

Results of Glass/Epoxy Composites with 

Compliant Interlayer and Needling 

Interlaminar Enhancements

by Robert A Haynes

Vehicle and Technology Directorate, ARL

Steven E Boyd

Weapons and Materials Research Directorate, ARL

Bradley D Lawrence

Bennett Aerospace, Inc., Cary, NC

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4. TITLE AND SUBTITLE

Compression-after-Impact and Bend Fatigue Results of Glass/Epoxy 

Composites with Compliant Interlayer and Needling Interlaminar 

Enhancements

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6. AUTHOR(S)

Robert A Haynes, Steven E Boyd, and Bradley D Lawrence

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US Army Research Laboratory

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Aberdeen Proving Ground, MD 21005-5066

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ARL-TR-8022

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

In this work, we tested 2-D and 3-D woven S2-glass/SC-15 epoxy composite laminates with compliant interlayer and needling 

interlaminar enhancements for single impact performance, compression-after-impact strength, and bend and shear fatigue life. 

The interlaminar enhancements decreased the strength and stiffness of the material but improved the durability. Needling had 

a larger improvement in compression-after-impact strength than the compliant interlayer, while the compliant interlayer had a 

larger improvement in stress-versus-fatigue-life slope than the needling.

15. SUBJECT TERMS

composite materials, interlaminar enhancement, durability, damage tolerance, compliant interlayer

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Robert A Haynes

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iii

Contents

List of Figures iv

List of Tables v

Acknowledgments vi

1. Introduction 1

2. Interlaminar Enhancement Technologies 1

2.1 Compliant Interlayer 2

2.2 Needling 3

3. Test Plan 3

4. Results 6

4.1 Impact Response and Compression after Impact 6

4.2 3-Point Bend 14

4.3 Short-Beam Shear 18

5. Conclusions and Recommendations 19

6. References 20

List of Symbols, Abbreviations, and Acronyms 23

Distribution List 24
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iv

List of Figures

Fig. 1 Representative 3-point bend test setup ................................................. 2

Fig. 2 Low-velocity impact of 2-D woven fabric panels displacement 

response with various interlaminar enhancements ............................... 7

Fig. 3 Low-velocity impact of 2-D woven fabric panels energy response with 

various interlaminar enhancements....................................................... 7

Fig. 4 Low-velocity impact of 2-D woven fabric panels response with 

various interlaminar enhancements....................................................... 8

Fig. 5 Low-velocity impact of 2-D woven fabric panels force response with 

various interlaminar enhancements....................................................... 8

Fig. 6 Low-velocity impact of 2-D woven fabric panels velocity response 

with various interlaminar enhancements .............................................. 9

Fig. 7 CAI residual compressive strength after low-velocity impact of 2-D 

woven fabric panels with various interlaminar enhancements............. 9

Fig. 8 Low-velocity impact displacement response for 3-D TPU interlayer 

composites........................................................................................... 10

Fig. 9 Low-velocity measured impact energy for 3-D TPU interlayer 

composites........................................................................................... 11

Fig. 10 Low-velocity impact response for 3-D TPU interlayer composites ... 11

Fig. 11 Ultrasonic c-scan images of single low-velocity impact damage for the 

3-D T.E.A.M. baseline panels (top row), the 5-mil TPU panels (middle 

row), and the 10-mil TPU panels (bottom row). Individual specimens 

have alphabetic designations A–F. ..................................................... 12

Fig. 12 Ultrasonic c-scan images of single low-velocity impact damage for the 

3-D T.E.A.M. hybrid panels containing 10-mil TPU (top row) and 

20-mil TPU (bottom row). Individual specimens have numeric 

designations; the 10-mil panel only had 5 specimens in sample group.

............................................................................................................. 12

Fig. 13 3-point bend maximum stress vs. fatigue life for panels A–C............ 16

Fig. 14 3-point bend maximum stress vs. fatigue life for panels D–F ............ 16

Fig. 15 3-point bend maximum stress vs. fatigue life for panels G–J............. 17

Fig. 16 3-point bend maximum stress vs. fatigue life for panels K–M........... 17

Fig. 17 Short-beam shear maximum stress vs. fatigue life for panels D–F .... 18
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v

List of Tables

Table 1 Test matrix and sample dimensions (length [cm] × width [cm] × 

thickness [cm]) for evaluation of compliant interlayer (CI) and 

needling interlaminar enhancement techniques in 2-D woven glass 

fabrics.................................................................................................... 4

Table 2 List of material constituents, geometries, densities for the 3-D 

composite materials .............................................................................. 6

Table 3 3-D T.E.A.M. composite panel data from impact and compression 

after impact testing with damage area analysis................................... 13

Table 4 Static bend strength for 3-point bend tests of 2-D woven fabric panels

............................................................................................................. 15

Table 5 Modulus with coefficient of variation (CoV) and slope of the stress vs. 

fatigue life (S-N) curve for 3-point bend tests of 2-D woven fabric 

panels .................................................................................................. 15

Table 6 Strength and modulus from high-speed, static, and 3-point bend tests 

of 2-D woven fabric panels................................................................. 18

Table 7 Static shear strength and modulus with coefficient of variation (CoV) 

and slope of the stress vs. fatigue life (S-N) curve for short-beam shear 

tests of 2-D woven fabric panels......................................................... 19
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vi

Acknowledgments

The authors would like to thank Mr Jim Wolbert, Mr Mike Neblett, Mr Mike 

Thompson, Mr Fred Racine, and Mr Rob Miller for panel processing and sample 

preparation. The authors would also like to thank Mr Jordan Wagner and Mr Art 

Yiournas for conducting the ultrasonic scanning, and Mr Albert Tran for assisting 

with the fatigue testing.
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1

1. Introduction

Composite materials are gaining popularity in structural applications because of 

their high in-plane specific strength and stiffness; however, composites 

traditionally have poor out-of-plane (i.e., interlaminar or through-the-thickness) 

properties.1 Compression-after-impact (CAI) strength is often a driver of composite 

material allowables.2 Further, structural materials subject to repeated loading and 

unloading will degrade (i.e., fatigue); interlaminar damage growth is an important 

driver of fatigue life in composite materials.3

 Improving the interlaminar durability 

and damage tolerance could produce a material with more desirable allowables 

without changing the fiber or matrix. 

Many interlaminar enhancement technologies have been tried and can be 

categorized into 2 main groups: inserting reinforcement through the thickness 

spanning multiple plies, and adding a durable or damage-tolerant layer between 

plies. Braiding, flocking, pultrusion, stitching, tufting, weaving, and z-pinning are 

types of through-the-thickness reinforcement that have been evaluated.4 The 

National Aeronautics and Space Administration has had success with its Pultruded 

Rod Stitched Efficient Unitized Structure (PRSEUS),5 demonstrating that the 

enhancement allowed the structure to carry loads 52% above the requirement, even 

when subject to impact damage. Often thermoplastic sheets are used in interlayertoughened composites.6 Shivakumar and Panduranga7 have reviewed thermoplastic 

interleaving methods and note improvements in the interlaminar fracture toughness 

of nearly 400% in some cases. Many existing interlaminar enhancement 

technologies are expensive to fabricate; the methods presented in this work use 

relatively inexpensive materials and are relatively simple to manufacture.

This work was in support of the Army’s Durable Hybrid Composites and Extremely 

Lightweight, Adaptive, Durable, and Damage Tolerant (XLADD) structures 

programs.

2. Interlaminar Enhancement Technologies

In this work, 2 interlaminar enhancement technologies (compliant interlayer and 

needling) for composite materials are evaluated for their improvement of the CAI 

strength and fatigue life of 2-D and 3-D woven glass fabrics. For the 2-D fabrics, 

the base laminate to which the enhancements are applied is [0]8 composed of a 

24 oz/yd2 woven roving S2-glass/SC-15 epoxy material system.8,9 This is a 

relatively thin laminate as compared to other investigations with these interlaminar 

enhancement technologies. The interlayer is inserted at the midplane, and the 
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needling is performed to each of two [0]4 sublaminates. A 3-D weave S2-glass 

laminate and a hybrid laminate with a Kevlar layer are also investigated. The 3-D 

weave fabric is composed of Owens Corning Shieldstrand S-glass (warp, weft, and 

z-weaver fibers) and woven by T.E.A.M.10,11 The Kevlar layer is a 2-D woven style 

386 from Hexcel.

12 For the 3-D laminates, the thermoplastic polyurethane (TPU)

film interlayer is also inserted between the outer 3-D weave layers at the midplane. 

For the hybrid laminate, 2 TPU film interlayers are used with the woven Kevlar 

placed at the midplane. CAI, quasi-static and fatigue 3-point bend, and quasi-static 

and fatigue short-beam shear testing are performed on select configurations.

2.1 Compliant Interlayer

The compliant interlayer consists of a thin film of thermoplastic ester-based 

polyurethane adhesive polymer (UAF-472)13 located between sublaminates of the 

larger laminate (Fig. 1). Two thicknesses were evaluated for the 2-D materials: 0.13 

and 0.25 mm (5 and 10 mil), located at the midplane of the laminate. For the 3-D 

materials, 2 thicknesses of TPU film were evaluated: 0.25 and 0.51 mm 

(10 and 20 mil). To produce the panels for this work, during manufacturing the film 

is placed between mats prior to a vacuum-assisted resin transfer molding (VARTM) 

process followed by curing.

14,15

Fig. 1 Representative 3-point bend test setup
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The benefit of the compliant interlayer is believed to be its ability to undergo large 

deformation, particularly shear, without fracturing. This leads to a partial decoupling 

of the sublaminates on either side of the compliant interlayer. When a laminate with 

a compliant interlayer experiences significant bending, such as when impacted on its 

face, it has been shown that there will be larger out-of-plane deflection but less 

delamination than if the laminate did not have the compliant interlayer.

16

2.2 Needling

Needling consists of pushing nominally 12.4-mm (0.500-inch) short aramid fibers 

through plies of in-plane fibers.

17 These short fibers are initially randomly oriented 

in a mat, which is laid on top of the laminate; barbed needles designed to catch only 

a few fibers at a time push through the mat and plies, depositing the short fibers as 

z-fiber bundles. The areal density of punctures can be controlled through the 

needling process by modulating the feed rate and stitching pattern geometry. In this 

work aramid fibers are needled through 4 plies of the woven glass fibers with a 

perforation density of 85 perforations/cm2 to create a sublaminate. One laminate 

was needled in error at a higher perforation areal density (greater than 

100 perforations/cm2

); the cured laminate was included in the test regimen. To 

produce panels for this work, 2 needled sublaminates were stacked prior to a 

VARTM process followed by curing.

17 Three control panels having a nominally 

identical layup with the mat of aramid fiber but with 0 perforations/cm2 were 

fabricated and tested as well.

The benefit of needling is believed to be its ability to provide reinforcement 

between individual plies such that when interlaminar tension or shear is 

experienced, the reinforcement will carry the load and prevent the formation and 

spread of delamination. Improvements in Modes I and II fracture toughness have 

been demonstrated.

18

3. Test Plan

For the 2-D materials, 13 panels were fabricated with combinations of the 2

interlaminar enhancement technologies included and subject to CAI, 3-point bend, 

and short-beam shear tests. The tests complemented other durability and damage 

tolerance testing that was performed under the same program17,18; the bend and 

shear tests were chosen specifically to create high shear stresses at the midplane 

where the compliant interlayer was located. Table 1 presents the laminate 

configurations fabricated and tests performed. Panels A–C were manufactured 

earlier than panels D–J, and to ensure consistency, panels D–F duplicate the 

configuration of panels A–C.
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Table 1 Test matrix and sample dimensions (length [cm] × width [cm] × thickness [cm]) for 

evaluation of compliant interlayer (CI) and needling interlaminar enhancement techniques in 

2-D woven glass fabrics

Panel 

label CI Needling CAI(cm)

3PB

(cm)

SBS

(cm)

A None None NA 15.2 × 2.53 × 0.640 NA

B 5 mil None NA 15.2 × 2.53 × 0.663 NA

C 10 mil None NA 15.2 × 2.54 × 0.675 NA

D None None 15.1 × 10.1 × 0.546 15.2 × 2.45 × 0.534 7.60 × 1.14 × 0.530

E 5 mil None 15.1 × 10.1 × 0.559 15.2 × 2.46 × 0.559 7.60 × 1.15 × 0.545

F 10 mil None 15.1 × 10.1 × 0.569 15.2 × 2.47 × 0.562 7.60 × 1.15 × 0.558

G None 85 perfs/cm2 15.1 × 10.1 × 0.632 15.2 × 2.46 × 0.630 NA

H 5 mil 85 perfs/cm2 15.1 × 10.1 × 0.663 15.2 × 2.47 × 0.663 NA

I 10 mil 85 perfs/cm2 15.1 × 10.1 × 0.670 15.2 × 2.45 × 0.665 NA

J None >100 perfs/cm2 15.1 × 10.2 × 0.721 15.2 × 2.47 × 0.723 NA

K None Control NA 15.2 × 2.58 × 0.698 NA

L 5 mil Control NA 15.2 × 2.59 × 0.746 NA

M 10 mil Control NA 15.2 × 2.59 × 0.772 NA

Notes: Tests included compression-after-impact (CAI), 3-point bend (3PB), and short-beam shear (SBS); NA 

indicates testing not performed.

Sample dimensions are provided in Table 1. As expected, inclusion of the 

compliant interlayer and the needling-induced Z-fiber architecture produce thicker 

laminates. Low-velocity impact performance was determined through testing per 

ASTM D 7136,

19 and CAI strength was measured per ASTM D 7137.20 

Three-point bend testing was conducted using ASTM D 726421 as a guide. The 

supports and loading nose diameters were 1.27 cm (0.500 inch), and the span of the 

supports was nominally 12.7 cm (5.00 inches); the span-to-thickness ratio was 

approximately 20 inches this work; see Fig. 1 for a representative test setup. For all 

panels, 6–8 specimens were tested in quasi-static monotonic loading to determine 

the strength. Testing was conducted at 4.06 mm/min (0.160 inch/min). Four 

additional specimens each from Panels A–C were tested at higher rates: 2 at 

406 mm/min (16 inches/min) and 2 at 8130 mm/min (320 inches/min); this was to 

investigate rate sensitivity of the compliant interlayer. Four samples from each 

panel with 6–8 specimens in each sample were subject to fatigue at 2 Hz and an 

R-ratio of 0.1, with each sample experiencing a maximum load of 90%, 80%, 70%, 

and 60% of the ultimate strength, respectively. 

Short-beam shear testing was conducted on panels D–F using ASTM D 234422 as a

guide. The supports and loading nose diameters were 3.18 mm (0.125 inch), and the 

span of the supports was nominally 2.54 cm (1.00 inch); the span-to-thickness ratio 

was approximately 4 inches in this work. Six specimens were tested in quasi-static 

monotonic loading to determine the strength. Testing was conducted at 4.06 mm/min 

(0.160 inch/min). Four samples from each panel with 6 specimens in each sample 
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were subject to fatigue at 2 Hz and an R-ratio of 0.1, with each sample experiencing 

a maximum load of 90%, 80%, 70%, and 60% of the ultimate strength, respectively.

Because of improvements in impact performance and durability observed in thicksection composites,

14,15 the testing methodology presented for the 2-D materials 

was also extended to 3-D weave fabric materials. Three composite panels with the 

T.E.A.M. S-glass weave were manufactured; a baseline panel containing no TPU 

film interlayers and a 5- and 10-mil TPU panel. The panels were specifically 

manufactured to evaluate the effect of the TPU film interlayer and its thickness on 

impact and postimpact properties. Though no 3-D fabrics were needled, 2 hybrid 

panels were manufactured to include a 2-D woven layer of Hexcel Kevlar 49 with 

2 interlayers of 10- and 20-mil TPU. The Kevlar layer was included in an attempt 

to combine the tear and punching shear resistance of Kevlar with the decoupling 

effect of the TPU interlayers with the overall aim of improving the impact durability 

of the composite. There is no corresponding 3-D T.E.A.M. hybrid panel with no 

TPU because of processing issues encountered with dry spots and voids during 

infusion.

All 3-D panels were tested for single low-velocity impact performance per ASTM 

D 713619 and for CAI strength per ASTM D 7137.

20 The lay-ups were specifically 

designed to place the interlaminar enhancement (either TPU film or combination 

of TPU film and Kevlar) at the midplane and to manufacture panels with an 

approximate areal density of 10 kg/m2 (2 psf). The impact and CAI panels were cut 

using a waterjet to nominal dimensions of 101.6 × 152.4 mm (4.00 ×

6.00 inches) according to ASTM D 7136; the loading surfaces were then machined 

flat using a diamond-coated machine tool for maximum uniformity and to minimize 

damage to the sample. Table 2 gives the composite material panel dimensions, layup, and areal densities. After the panels were impacted and before CAI testing, the 

panels were ultrasonically scanned to produce a c-scan image of the damage area 

from the single impact. The damage area was analyzed digitally with ImageJ23 and

reported. No 3-point bending or short-beam shear testing was performed on the 

3-D composite materials.
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Table 2 List of material constituents, geometries, densities for the 3-D composite materials

Composite (SC-15 Resin) Lay-up

Thickness

(mm)

Areal density

kg/m2 (lb/ft2

)

3-D T.E.A.M. (no TPU) 6.2 10.5 (2.14)

3-D T.E.A.M. + 5-mil TPU 6.6 11.0 (2.25)

3-D T.E.A.M. + 10-mil TPU 6.8 11.2 (2.28)

3-D T.E.A.M. + 10-mil TPU + Kevlar 7.9 ...

3-D T.E.A.M. + 20-mil TPU + Kevlar 8.4 13.4 (2.7)

4. Results

The results are categorized by test: CAI, 3-point bend, and short-beam shear.

4.1 Impact Response and Compression after Impact

The results for the impact testing of the panels with 2-D woven glass fabrics are 

provided in Figs. 2–6 for displacement versus time, energy versus time, force versus 

displacement, force versus time, and velocity versus time, respectively. Of 

particular note are the larger forces and smaller displacements experienced by the 

overneedled specimens. A comparison of CAI strength for all panels is provided in 

Fig. 7. Inclusion of a compliant interlayer alone improved the strength by up to 13% 

as compared to the baseline. Inclusion of needling alone improved the strength by 

21%. Inclusion of both technologies improved the strength by up to 43%. 

Overneedling improved the strength by 73%.
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Fig. 2 Low-velocity impact of 2-D woven fabric panels displacement response with various 

interlaminar enhancements

Fig. 3 Low-velocity impact of 2-D woven fabric panels energy response with various

interlaminar enhancements
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Fig. 4 Low-velocity impact of 2-D woven fabric panels response with various interlaminar 

enhancements

Fig. 5 Low-velocity impact of 2-D woven fabric panels force response with various 

interlaminar enhancements
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Fig. 6 Low-velocity impact of 2-D woven fabric panels velocity response with various 

interlaminar enhancements

Fig. 7 CAI residual compressive strength after low-velocity impact of 2-D woven fabric 

panels with various interlaminar enhancements
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The single low-velocity impacts on the 3-D panels are represented in Figs. 8–12. 

Panel displacement (deflection calculated from Eqs. 9 and 10 in ASTM D 713619) 

versus time is plotted in Fig. 8. The impact energy (measured energy given in Eq. 

5 in ASTM D 713619) versus time is plotted in Fig. 9. Finally, the measured impact 

force profile (measured from the impact tup) is plotted versus the displacement 

given in Fig. 8 to yield the force-deflection impact curve of Fig. 10. Relevant data 

values for impact, damage assessment, and CAI strength are stated in Table 3. 

Ultrasonic c-scans of the 3-D panels are presented in Figs. 11 and 12 for the 

nonhybrid 3-D T.E.A.M. panels and the hybrid panels, respectively.

Fig. 8 Low-velocity impact displacement response for 3-D TPU interlayer composites
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Fig. 9 Low-velocity measured impact energy for 3-D TPU interlayer composites

Fig. 10 Low-velocity impact response for 3-D TPU interlayer composites
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Fig. 11 Ultrasonic c-scan images of single low-velocity impact damage for the 3-D T.E.A.M.

baseline panels (top row), the 5-mil TPU panels (middle row), and the 10-mil TPU panels 

(bottom row). Individual specimens have alphabetic designations A–F.

Fig. 12 Ultrasonic c-scan images of single low-velocity impact damage for the 3-D T.E.A.M.

hybrid panels containing 10-mil TPU (top row) and 20-mil TPU (bottom row). Individual 

specimens have numeric designations; the 10-mil panel only had 5 specimens in sample group.

A B C D E F

No TPU

A B C D E F

5 mil

A B C D E F

10 mil

1 2 3 4 5

10 mil

20 mil

1 2 3 4 5 6
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Table 3 3-D T.E.A.M. composite panel data from impact and compression after impact 

testing with damage area analysis

Composite 

(SC-15 Resin)

Single impact

ASTM 7136 Damage

area

(% of aperture)

CAI strength

ASTM 7137

Peak (MPa) load

(kN)

Peak 

displacement

(mm)

3-D T.E.A.M.

(no TPU) 11.7 ± 0.11 6.65 ± 0.05 9.6 ± 1.1 154.7 ± 6.8

3-D T.E.A.M. + 

5-mil TPU 11.7 ± 0.74 6.28 ± 0.29 11.6 ± 2.7 157.4 ± 7.0

3-D T.E.A.M. + 

10-mil TPU 12.1 ± 0.13 6.64 ± 0.12 11.1 ± 1.0 158.9 ± 5.8

3-D T.E.A.M. + 

10-mil TPU + 

Kevlar

14.0 ± 0.16 6.35 ± 0.15 5.9 ± 0.6 149.4 ± 9.0

3-D T.E.A.M. + 

20-mil TPU + 

Kevlar

14.1 ± 0.25 6.61 ± 0.20 6.6 ± 0.6 137.1 ± 1.5

In Figs. 8–10 and Table 3, there is little distinction between the baseline, 5-mil, and 

10-mil panels in the impact data. All samples have similar peak loads and 

deflections, CAI strengths, and damage areas. One notable difference is the damage 

areas revealed on the c-scans of Fig. 11 and Table 3. The 3-D T.E.A.M. baseline 

panel has 2% less damage area per aperture (the cut-out area of the impact support 

in ASTM D 713619) than both TPU interlayer panels. The 3-D T.E.A.M. panels 

with 5- and 10-mil TPU seem to have a “halo” of damage surrounding the 

immediate dark blue damage under the impactor. This effect was observed for 

thick-section composites and is known to be an area of matrix cracking due to the 

compliant TPU interlayer allowing the composite sublayers to decouple and allow 

more flexure under the impact, as determined through sectioning and microscopy. 

The damage area of the baseline panel is less because the damage is more localized 

matrix cracking, fiber crushing, delamination, and, perhaps, fiber punching under 

the impactor. From Fig. 9, the impact energy of the 3-D T.E.A.M. baseline, 5-mil, 

and 10-mil TPU panels was approximately 44 J, which is consistent with ASTM D

7136.

19 Increasing the impact energy may provide a stronger delineation between 

impact response of the baseline, 5-mil, and 10-mil panels.

The 3-D T.E.A.M. hybrid panels with 10- and 20-mil TPU were impacted at 50 J 

according to ASTM D 7136.

19 Figure 8 and Table 3 show that the 20-mil TPU panel 

deflects to 6.6 mm, 4% more under impact than the 10-mil panel, although both 

have similar peak loads. From the force-deflection response of Fig. 10, the impact 
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stiffness (kN/cm) of the hybrid, the 20-mil sample is noticeably less than the 

10-mil panel, with a decrease of 14% at 1-cm deflection. The damage area for the 

hybrid 20-mil panel is approximately 1% greater as determined by digital image 

analysis from Figs. 11 and 12. These data are consistent with the fact that adding 

twice the thickness of compliant TPU film at the midplane of the composite will 

result in more compliance, greater deflection, and increased impact damage due to 

that deflection. The CAI strength is also diminished by 8% as stated in Table 3. 

The 2-D Kevlar layer has a distinct effect on the impact response of the 3-D 

T.E.A.M. composites, as demonstrated in Figs. 8–10 and Table 3. The key 

difference can be seen in the damage areas and the CAI strengths. The hybrid 

composites sustained a 20% larger impact energy, yet both the hybrid 10 and 

20-mil TPU panels had smaller damage areas as compared to the group. The hybrid 

10-mil TPU panel had a 47% drop in damage area over the 3-D T.E.A.M. 10-mil 

TPU panel. The CAI strengths were noticeably lower overall with the 2-D Kevlar 

layer. The hybrid 10-mil TPU composite had a 6% drop in CAI strength. The 

addition of the 2-D Kevlar layer improves composite durability under impact but 

compromises in-plane mechanical properties.

4.2 3-Point Bend

The average static flexural strengths and moduli of the specimens from each panel 

are provided in Tables 4 and 5, respectively, along with the percent change relative 

to Panel D, which was the baseline with no interlaminar enhancement techniques 

applied. Panels A–C had an approximately 10% lower strength than Panels D–F, 

possibly due to manufacturing variations. With just needling, the strength and 

modulus dropped by 22% and 14%, respectively. The compliant interlayer alone

performed slightly better than the needling alone. In general, the 5-mil interlayer 

performed slightly better than the 10-mil interlayer, but having both needling and 

an interlayer performed worse than one interlaminar enhancement technique alone. 

The predominant failure mode was compression under the loading nose.
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Table 4 Static bend strength for 3-point bend tests of 2-D woven fabric panels

Panel 

label CI Needling Strength 

(MPa)

CoV 

(%)

Change 

from 

baseline

A None None 404 4.5 ...

B 5 mil None 355 9.9 ...

C 10 mil None 365 5.7 ...

D None None 461 4.2 Baseline

E 5 mil None 400 6.1 –13.2%

F 10 mil None 389 3.5 –15.6%

G None Standard 359 19 –22.1%

H 5 mil Standard 362 7.3 –21.5%

I 10 mil Standard 332 5.4 –28.0%

J None Over 396 2.9 –14.1%

K None Control 400 2.4 –13.2%

L 5 mil Control 326 2.4 –29.3%

M 10 mil Control 414 5.0 –10.2%

Note: CI = compliant interlayer, and CoV = coefficient of variation.

Table 5 Modulus with coefficient of variation (CoV) and slope of the stress vs. fatigue life 

(S-N) curve for 3-point bend tests of 2-D woven fabric panels

Panel

label CI Needling Modulus

(GPa)

CoV

(%)

Change 

from 

baseline 

Slope of the 

S-N curve

(MPa/logecycles)

A None None 17.9 3.0 ... –18.0

B 5 mil None 16.3 2.2 ... –15.0

C 10 mil None 16.6 5.2 ... –17.3

D None None 21.5 3.1 Baseline –19.1

E 5 mil None 19.5 4.4 –9.30 –17.3

F 10 mil None 16.8 5.4 –21.9 –16.0

G None Standard 18.5 2.3 –14.0 –19.7

H 5 mil Standard 14.7 8.6 –31.6 –14.2

I 10 mil Standard 10.5 16 –51.2 –17.1

J None Over 15.2 1.5 –29.3 –14.9

K None Control 9.25 1.7 –57.0 –20.4

L 5 mil Control 11.7 2.7 –45.6 –37.4

M 10 mil Control 13.0 2.8 –39.5 –15.4

Stress-fatigue life curves are provided in Figs. 13–17. The slope of the stressfatigue life curve can be considered to be a measure of durability, where the slope 

is found from a linear regression between the maximum stress and the natural log 

of the number of cycles to failure; the slopes are provided in Table 5. With the 

exception of the control samples, all samples incorporating a compliant interlayer 

exhibited a higher durability by 10%–20% than samples without a compliant 

interlayer; on average 5-mil samples performed 6.8% better than 10-mil samples. 

With the exception of the control and overneedled samples, the needled samples 

were within 3% of the nonneedled panels. Again, with the exception of the control 

samples, the overneedled sample exhibited a 21% higher durability than the other 
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16

samples without a compliant interlayer. The control samples exhibited a 43% worse 

durability on average than the noncontrol samples. The predominant failure mode 

was compression under the loading nose.

Fig. 13 3-point bend maximum stress vs. fatigue life for panels A–C

Fig. 14 3-point bend maximum stress vs. fatigue life for panels D–F
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Fig. 15 3-point bend maximum stress vs. fatigue life for panels G–J

Fig. 16 3-point bend maximum stress vs. fatigue life for panels K–M
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Fig. 17 Short-beam shear maximum stress vs. fatigue life for panels D–F

The high-speed static test results are provided in Table 6. A 14% increase in the 

strength was observed in the 16 inches/min samples over the 0.16 inches/min 

samples, suggesting a rate dependency, but there was a negligible difference in 

strength between the 16 and 320 inches/min samples. A negligible difference in 

modulus was seen between the 0.16 and 16 inches/min samples, whereas a 6.9% 

increase in modulus was seen between the 0.16 and 320 inches/min samples.

Table 6 Strength and modulus from high-speed, static, and 3-point bend tests of 2-D woven 

fabric panels

Panel 

label CI Needling

16 inches/min 320 inches/min

Strength 

(MPa)

Modulus 

(GPa)

Strength 

(MPa)

Modulus 

(GPa)

A None None 422 18.7 459 19.0

B 5 mil None 426 16.9 415 17.8

C 10 mil None 424 16.3 420 17.5

4.3 Short-Beam Shear

The average static shear strengths and moduli of each panel are provided in 

Table 7. The 5- and 10-mil compliant interlayer samples exhibited a 7.7% decrease 

and an 18% increase in strength, respectively, as compared to the sample without a 

compliant interlayer. Figure 17 shows the stress versus fatigue life plots for the 3
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19

panels tested in short-beam shear; the slopes of the curve fits are provided in 

Table 7. The 5- and 10-mil compliant interlayer samples exhibited a 1.7% increase 

and a 23% decrease in durability, respectively, as compared to the sample without 

a compliant interlayer.

Table 7 Static shear strength and modulus with coefficient of variation (CoV) and slope of 

the stress vs. fatigue life (S-N) curve for short-beam shear tests of 2-D woven fabric panels

Panel 

label CI Needling Strength (MPa)

CoV 

(%)

Modulus

(GPa)

CoV 

(%)

Slope of the 

S-N curve

(MPa/logecycles)

D None None 72.8 4.0 4.07 9.3 –70.5

E 5 mil None 67.2 4.7 3.72 4.4 –69.3

F 10 mil None 85.8 3.6 5.87 4.3 –86.7

5. Conclusions and Recommendations

All interlaminar enhancement technologies improved the CAI strength and 

decreased the 3-point bend strength and modulus for the 2-D composites. The 

overneedled sample appeared to have the best overall properties; it had the best CAI 

strength, one of the highest retained 3-point bend strengths, and one of the lowest 

stress-versus-fatigue-life slopes. It is recommended that this technology be 

investigated further. Adding a compliant interlayer generally resulted in higher 

durability, although the results were mixed between the 2 thicknesses. Needling 

had a larger improvement in CAI strength than the compliant interlayer, while the 

compliant interlayer had a larger improvement in stress-versus-fatigue-life slope 

than the needling. Slight increases in strength and stiffness were observed when 

testing at higher rates, indicating the presence of some form of rate dependency for 

these composite architectures.

Adding TPU films to the 3-D T.E.A.M. composites was inconclusive, possibly 

because of the use of a relatively low impact energy level (44 J) pursuant to ASTM 

D 7136.

19 Doubling the energy for these single impact tests may better elucidate 

the benefits of adding TPU films on impact durability through interlaminar 

toughening. An improvement in the impact response and durability was 

demonstrated by adding a 2-D woven Kevlar layer, but with a corresponding 

reduction in in-plane CAI strength. The results indicate that for all composites 

manufactured with TPU film interlayers, adding thicker TPU films results in a more 

compliant and thicker composite panel.
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6. References

1. O’Brien TK. Composite interlaminar shear fracture toughness, GIIC: shear 

measurements or sheer myth? Bucinell RB, editor. West Conshohocken (PA): 

American Society for Testing and Materials; 1998. p. 3–18. (Composite 

materials: fatigue and fracture; vol. 7, ASTM STP 1300).

2. Hodge AJ, Nettles AT, Jackson JR. Comparison of open-hole compression 

strength and compression after impact strength on carbon fiber/epoxy laminates 

for the Ares I composite interstage. Huntsville (AL): National Aeronautics and 

Space Administration; 2011 Mar. Report No.: NASA/TP-2011-216460.

3. Talreja R. Fatigue of composite materials. Lancaster (PA): Technomic Pub. 

Co.; 1987.

4. Ranatunga V, Clay S. Study of delamination onset under mode-II loading in 

translaminar reinforced composites using acoustic emission techniques. 

Dayton (OH): American Society for Composites; 2014.

5. Lovejoy A, Rouse M, Linton K, Li V. Pressure testing of a minimum gauge 

PRSEUS panel. 52nd AIAA/ASME/ASCE/AHS/ASC Structures, Structural 

Dynamics and Materials Conference, Structures, Structural Dynamics, and 

Materials and Co-located conferences; 2011 Apr 4–7; Denver, CO.

6. Hayes B, Gammon L. Optical microscopy of fiber-reinforced composites.

Novelty (OH): ASM International; 2010.

7. Shivakumar K, Panduranga R. Interleaved polymer matrix composites - a 

review. 54th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, 

and Materials Conference. 2013 Apr 8–11; Boston, MA.

8. S2-glass. Aiken (SC): AGY; c2017 [accessed 2017 May 2]. 

http://www.agy.com/products/s-glass/.

9. SC-15 resin. Benicia (CA): Applied Poleramic Inc.; c2017 [accessed 2017 May 

2]. http://www.appliedpoleramic.com/.

10. ShieldStrand woven fabrics. Toledo (OH): Owens Corning Composite 

Materials, LLC; 2010 Jan [accessed 2017 May 2]. 

http://www.ocvreinforcements.com/pdf/products/ShieldStrand_S_Fabrics.pdf.

11. Style 0015-02. Woonsocket (RI): Textile Engineering and Manufacturing 

(T.E.A.M.); nd [accessed 2017 May 2]. http://www.teamtextiles.com/.
Approved for public release; distribution is unlimited.

21

12. Kevlar 49. Seguin (TX): Hexcel; 2014 June [accessed 2017 May 3]. 

http://www.hexcel.com/user_area/content_media/raw/HRH_49_us.pdf.

13. Polyurethane adhesive films. Pine Brook (NJ): Adhesives Films, Inc.; c2013 

[accessed 2017 May 2].

http://www.adhesivefilms.com/Polyurethane__UAF_Series.html.

14. Boyd SE, Wolbert JP. Multi-impact durability and processing of thick-section 

carbon-glass/epoxy hybrid composites toughened with thermoplastic 

polyurethane inter-layer films. Aberdeen Proving Ground (MD): Army 

Research Laboratory (US); 2012 Sep. Report No.: ARL-TN-0501.

15. Boyd SE, Emerson RP, Bogetti TA. Low-velocity, multi-impact durability 

performance of thick-section 3WEAVE S2-glass/SC-15 composites toughened 

with thermoplastic polyurethane inter-layer films. Aberdeen Proving Ground 

(MD): Army Research Laboratory (US); 2013 Aug. Report No.: ARL-TR6547.

16. Alfredsson KS, Bogetti TA, Carlsson LA, Gillespie JW Jr, Yiournas A. Flexure 

of beams with an interlayer: symmetric beams with orthotropic adherends. J

Mechanics of Materials and Structures. 2008;3(1):45–62.

17. Emerson RP, Cain J, Simeoni M, Lawrence B. Processing and evaluation of 

3D-reinforced needled composite laminate. Aberdeen Proving Ground (MD): 

Army Research Laboratory (US); Sep 2012. Report No.: ARL-TR-6107.

18. Emerson RP, Lawrence B, Montgomery A, Safriet S. Improvements to the 

processing and characterization of needled composite laminates. Aberdeen 

Proving Ground (MD): Army Research Laboratory (US); 2014 Jan. Report No.: 

ARL-RP-472.

19. ASTM D 7136. Standard test method for measuring the damage resistance of a 

fiber-reinforced polymer matrix composite to a drop-weight impact event. West 

Conshohocken (PA): ASTM International; 2015.

20. ASTM D 7137. Test method for compressive residual strength properties of 

damaged polymer matrix composite plates. West Conshohocken (PA): ASTM 

International; 2012.

21. ASTM D 7264. Standard test method for flexural properties of polymer matrix 

composite materials. West Conshohocken (PA): ASTM International; 2015.

22. ASTM D 2344. Standard test method for short-beam strength of polymer 

matrix composite materials and their laminates. West Conshohocken (PA): 

ASTM International; 2016.
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23. ImageJ: Image processing and analysis in Java. nd [accessed 2017 May 2].

https://imagej.nih.gov/ij/.
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List of Symbols, Abbreviations, and Acronyms

2-D 2-dimensional

3-D 3-dimensional

3PB 3-point bend

CAI compression after impact

CI compliant interlayer

CoV coefficient of variation

PRSEUS Pultruded Rod Stitched Efficient Unitized Structure

SBS short-beam shear

TPU thermoplastic polyurethane

VARTM vacuum-assisted resin transfer molding

XLADD Extremely Lightweight, Adaptive, Durable, and Damage Tolerant
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