Real-Time Dynamic Impact Strain Deformation 

Measurements of Transparent Poly(urethane urea) 

Materials

by Jian H. Yu and Alex J. Hsieh

ARL-TR-5355 September 2010

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Army Research Laboratory

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ARL-TR-5355 September 2010

Real-Time Dynamic Impact Strain Deformation 

Measurements of Transparent Poly(urethane urea) 

Materials

Jian H. Yu and Alex J. Hsieh

Weapons and Materials Research Directorate, ARL

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Real-Time Dynamic Impact Strain Deformation Measurements of Transparent 

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14. ABSTRACT

Transparent poly(urethane urea) (PUU) materials with tunable microstructures offer an avenue to enable material designs with 

desired dynamic mechanical deformation properties. This report presents recent experimental findings of select model PUU 

materials upon ballistic impact testing. Tailoring the microstructure appears to be a dominant factor in altering the ratedependent mechanical deformation behavior, while promoting phase mixing gives rise to dynamic strain hardening, which is 

validated by the real-time 3D strain evolution measurements via digital photogrammetry. These efforts are a part of an 

ongoing research project focused on investigating the microstructure-property relationships of PUUs, in an attempt to 

elucidate the molecular mechanisms key to simultaneous improvements in both physical and mechanical properties.

15. SUBJECT TERMS

Poly(urethane urea); impact, dynamic strain hardening; photogrammetry

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iii

Contents

List of Figures iv

List of Tables iv

Acknowledgments v

1. Introduction 1

2. Experimental 2

2.1 Materials..........................................................................................................................2

2.2 Small Angle X-ray Scattering (SAXS) ...........................................................................2

2.3 Dynamic Mechanical Analysis (DMA)...........................................................................3

2.4 Ballistic Impact................................................................................................................3

3. Results and Discussion 4

3.1 Extent of Phase Mixing ...................................................................................................4

3.2 Dynamic Impact Deformation.........................................................................................6

4. Conclusion 12

5. References 13

Distribution List 15
iv

List of Figures

Figure 1. Impact experiment setup..................................................................................................3

Figure 2. SAXS data for PUU 2-1-1 2K (black) and PUU 2-1-1 1K (red).....................................5

Figure 3. Plots of tan (loss factor) as a function of temperature obtained via DMA for PUU 

2-1-1 2K (black) and PUU 2-1-1 1K (red).................................................................................6

Figure 4. Mode of deformation of PUU materials after ballistic impact against a 0.22-caliber 

FSP; (a) ductile deformation, and (b) brittle failure. ................................................................7

Figure 5. Plots of von Mises deformation strain history measured by impact of PUU 2-1-1 

2K, PUU 3-2-1 2K, PUU 4-3-1 2K and PUU 3-2-1 1K against a steel sphere at velocity 

of 106 m/s. .................................................................................................................................8

Figure 6. Plots of the maximum von Mises strain as a function of time after impact of PUU 

3-2-1 2K (black), PUU 4-3-1 2K (green) and PUU 3-2-1 1K (red). .........................................9

Figure 7. Plots of von Mises deformation strain history measured by impact of PUU 2-1-1 

2K, PUU 3-2-1 2K and PUU 4-3-1 2K against a steel sphere at velocity of 142 m/s.............10

Figure 8. Plots of the surface shear strain, xy, of PUU 3-2-1 2K and PUU 3-2-1 1K against a 

steel sphere at velocity of 142 m/s...........................................................................................11

Figure 9. Ballistic impact of (a) PUU 2-1-1 650 and (b) PUU 5-4-1 2K against a 0.22-

caliber FSP...............................................................................................................................11

List of Tables

Table 1. Compositions, hard segment contents, and Shore A and D hardness values of select 

PUUs..........................................................................................................................................2
v

Acknowledgments

This research was partially supported by an Institute for Soldier Nanotechnologies (ISN) 6.2 

project funded by the Army through the ISN. Alex J. Hsieh acknowledges the PUU materials 

provided by Dr. Norman Rice of Triton Systems, Inc. through the ISN 6.2 project; he also thanks 

Dr. Frederick L. Beyer of the U.S. Army Research Laboratory (ARL) and Dr. Renaud Rinaldi of 

ISN for providing assistance with Small Angle X-ray Scattering (SAXS)and Dynamic 

Mechanical Analysis (DMA) measurements, respectively. 
vi

INTENTIONALLY LEFT BLANK.
1

1. Introduction

Hyper-elastic materials are currently used in a wide variety of Army and DoD platforms, which

include chem/bio shields or lenses, structure adhesives, foams, composite structures, and films 

for structural retrofit. Transparent poly(urethane urea) (PUU) materials possess good chemical 

barrier properties and have been evaluated to replace polycarbonate in eye-protective and faceshield applications. The chemistries of PUU, like polyurethane, are versatile; PUUs with tunable 

microstructures have shown the potential to enable material designs with simultaneous 

enhancements in both physical and mechanical properties (1).

The thermodynamic incompatibility of the soft segments (SS) and hard segments (HS) leads to a 

phase-separated microstructure in PUUs. Increasing the HS content typically increases the 

stiffness and flow stress levels, as well. On the other hand, by simply altering the molecular 

weight (MW) of the SS—for example, in the case of poly(tetramethylene oxide) (PTMO)—from 

2,000 (2K) to 1,000 (1K) g/mol, PUUs have shown enhanced phase mixing; the glass transition

of the SS shifts towards higher temperatures (1, 2). As a result, PUU materials with PTMO of 1K 

MW display greater rate-dependent mechanical deformation behavior than the corresponding 

PUU materials with 2K MW PTMO (1, 2). Additionally, drastic improvement in permeation 

resistance against chloroethyl ethyl sulfide, which is commonly chosen as a stimulant for the 

chemical warfare blister agent, sulfur mustard gas HD, was observed (1). These include 

significantly longer breakthrough time and slower steady-state flux in the 1K PUU materials 

over the corresponding 2K PUUs having similar HS contents (1). 

We have observed that increasing the Shore A or D hardness values improved the ballistic 

impact performance of polyurethane or PUU materials. In this work, the ballistic impact 

performance of select model PUU materials is evaluated. Further, the composition dependence of 

the mode of deformation upon ballistic impact is investigated to determine whether the extent of 

tensile strain to failure or greater dynamic strain hardening of PUUs is favored to facilitate 

improved impact performance. This study uses photogrammetry, a digital imaging correlation 

technique, to determine the local strain evolution of select PUU materials during the ballistic 

impact testing. This technique permits real-time visualization of a deformation event, as well as 

the determination of geometric properties (e.g., the displacement and strain history) by tracking 

the minute changes on the area of interest. With the advancement in digital imaging technology, 

there has been a strong effort to develop a robust photogrammetric technique for real-time 

mechanical deformation analysis (3–5) in a broad range of materials characterization including 

biomechanics (6, 7), structural engineering (8, 9), and ballistics (10–13). Results from the local 

strain deformation measurements will also be used for correlation with the corresponding 

viscoelastic relaxation data.
2

2. Experimental

2.1 Materials

In this work, a set of model PUU materials with varying molar ratio of PTMO, 4,4’-

dicyclohexylmethane diisocyanate (HMDI) and diethyltoulenediamine (DETA), and molecular 

weight (MW) of PTMO (650, 1000 and 2000 g/mol) are chosen. In the sample nomenclature

(table 1), the numerals ―x-y-z‖ refer to the molar ratio of diisocyanate : diamine chain extender : 

PTMO, and the succeeding ―650,‖ ―1K,‖ or ―2K‖ refers to the MW of PTMO as 650, 1K, or 2K

g/mol, respectively. Table 1 lists the composition, including the HS% and the SS MW of PUUs 

studied in this work. The HS% was calculated using an approach based on the urea content, as 

described by O’Sickey et al. in equation 1 (14):



%HS =

100 (R-1)(Mdi  Mda)

(Mg  R(Mdi)  (R1)(Mda))

, (1)

where R is the molar ratio of the diisocyanate to PTMO, and Mdi, Mda, and Mg are the number 

average MW of the diisocyanate, diamine, and PTMO, respectively. Equation 1 accounts only 

for the portion of diisocyanate that reacts with diamine and, hence, it primarily represents the 

material residing in the hard domains (14). The experimental results described in the following 

sections will be examined in correlation with the HS% values noted here. The Shore hardness 

was measured according to ASTM-D2240 (15).

Table 1. Compositions, hard segment contents, and Shore A and D hardness values of select PUUs.

Sample ID Molar ratio 

HMDI: 

DETA: 

PTMO

MW (gm/mol) 

PTMO

HS 

(%)

Hardness

 Shore A Shore D

PUU 2-1-1 2K 2:1:1

2000

16 81 32

PUU 3-2-1 2K 3:2:1 28 96 54

PUU 4-3-1 2K 4:3:1 37 99 62

PUU 2-1-1 1K 2:1:1

1000

26 96 52

PUU 3-2-1 1K 3:2:1 41 99 70

PUU 4-3-1 1K 4:3:1 51 100 79

PUU 2-1-1 

650

2:1:1 650 33 99 69

PUU 5-4-1 2K 5:4:1 2000 44 99 70

2.2 Small Angle X-ray Scattering (SAXS)

SAXS measurements were carried out with a customized 3 m pinhole collimated SAXS camera. 

A Rigaku Ultrax18 rotating anode with a copper target was operated at 40 kV/60 mA to generate 

X-rays with  = 1.5418 Å. Two-dimensional (2D) data sets were collected using a Molecular 
3

Metrology 2D multi-wire area detector placed ~65 cm from the sample. SAXS data of PUU 2-1-

1 2K and PUU 2-1-1 1K were highlighted to show the influence of SS MW on microstructure. 

2.3 Dynamic Mechanical Analysis (DMA)

DMA testing was carried out using a TA Instruments Q800 DMA in an oscillatory tension mode. 

Film samples of 2 mm in width and 1 mm thick were used, and the grip-to-grip distance was 

about 10 mm. Measurements were conducted at a frequency of 1 Hz for a temperature range of 

–100 to 150 C at a heating rate of 1 C/min. 

2.4 Ballistic Impact

Ballistic impacts measurements were carried out with either a 17-grain (1.1-g), 0.22-caliber 

fragment simulating projectile (FSP) or a 5.56 mm diameter stainless steel sphere (Type 302, 

0.69 g). The gas gun was pressurized at different pressures with helium to propel the projectile to 

reach select impact velocities. The speed of the projectile was tracked with Doppler radar (BR3502, Infinition Inc.). The PUU target (20 cm × 15 cm × 0.3 cm) was sandwiched in a target 

frame with a circular opening that was 7.62 cm in diameter. The targets were spray-painted with 

a random pattern of white and black dots (figure 1). The dot size that appeared on the digital 

image was about 8 pixels in diameter. 

Figure 1. Impact experiment setup.

Two high-speed cameras (Photron SA1, Photron USA, Inc.) were used to generate stereo image 

pairs of the impact area. The two cameras were placed behind the target fixture. Details of the 
4

camera set-up and images collection can be found in (13). The stereo images were analyzed 

using a commercially available photogrammetric software program, ARAMIS (GOM GmbH, 

Germany, distributed by Trilion Quality Systems in the USA). The cameras were calibrated with 

a series of standard dot images (13, 17). ARAMIS has a built-in algorithm for calculating the 

displacement. Details of the data analysis, including the determination of the displacement 

measurement sensitivity and the strain calculation sensitivity, can be found in (12, 17). The 

ARAMIS software allows for calculation of displacement history in the transverse direction 

(yy), as well as the corresponding shear component (xy). In this work, the values of von Mises 

strain calculated based upon the assumption of a plane-stress loading condition (18) were also 

used in the strain history plots for the purpose of comparing the overall local strain deformation.

3. Results and Discussion

3.1 Extent of Phase Mixing

As previously mentioned, altering the SS MW from 2K to 1K g/mol resulted in greater phase 

mixing in the 1K PUU materials than in the 2K PUU materials. The extent of phase mixing is 

characterized by comparing the scattering intensity profiles obtained from SAXS and the 

viscoelastic relaxation from DMA. 

Figure 2 is a highlight comparing the scattering intensities for PUU 2-1-1 2K and PUU 2-1-1 1K. 

PUU 2-1-1 2K exhibits a well-defined scattering peak, suggesting the presence of isolated HS 

domains, as is typically observed in microphase-separated PUUs. The scattering characteristics

differ when the SS MW is varied. As expected, longer SS (2K MW), while keeping the same 

molar ratio, contributes to a larger inter-domain spacing (correspondingly, a lower q value). PUU 

2-1-1 1K nevertheless exhibits a significant decrease in scattering intensity and a much broader 

scattering peak, suggesting greater phase mixing. 
5

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

q (1/angstroms)

Intensity (a.u.)

Figure 2. SAXS data for PUU 2-1-1 2K (black) and PUU 2-1-1 1K (red).

Variation in the SS MW also has a large effect on the SS glass transition temperature (Tg). A 

shift of the SS Tg ~18 C towards a higher temperature in PUU 2-1-1 1K than in PUU 2-1-1 2K 

indicates that PUU 2-1-1 1K exhibits greater phase mixing. This is consistent with the SAXS 

data. Also, the nature of the tan peaks is vastly different between these two PUU materials. 

PUU 2-1-1 2K exhibits a very distinct and intense peak at –64 oC, along with a weaker shoulder 

at 21 oC, as shown in Figure 3. The more intense and dominant peak relates to the relaxation of 

the SS rich phase, and the weaker shoulder presumably relates to SS in the inter-phase regions 

(1, 2, 16). PUU 2-1-1 1K, on the other hand, displays a single broad tan peak, which appears to 

be a result of the merging of the two SS relaxation peaks that is observed in PUU 2-1-1 2K (1,

2). 
6

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

-150 -100 -50 0 50 100

Temperature ( C ) Loss factor

Figure 3. Plots of tan (loss factor) as a function of temperature obtained via DMA for PUU 2-1-1 

2K (black) and PUU 2-1-1 1K (red).

3.2 Dynamic Impact Deformation

Earlier results showed that increasing the hard segment content in the 2K PUUs conversely 

increased hardness, resulting in improved ballistic impact performance in PUU 4-3-1 2K 

(37 HS%) over the PUU 2-1-1 2K (16 HS%). Figure 4a is a photograph that shows a typical 

ductile mode of failure observed in PUUs after impact against a 0.22-cal FSP. It is noted that the 

size of the penetration-induced opening was much smaller than the diameter of the projectile, 

which makes it plausible that the hyper-elastic tensile strength and elongation characteristics may 

play an important role during the impact deformation. This is significantly different from a 

typical plug failure seen in polycarbonate.

In this work, a new set of model PUUs consisting of a broad range of compositions is evaluated

to determine the influence of SS MW on the impact performance of PUUs. It is noted that PUUs 

with 1K MW SS generally have higher HS% than the corresponding 2K PUUs having the same 

stoichiometric molar ratio of diisocyanate : diamine chain extender : PTMO. For example, the 

HS% of PUU 2-1-1 1K and PUU 2-1-1 2K are 26% and 16%, respectively. In the 1K series, the 

stiffness and harness can further increase drastically by increasing the HS%; PUU 4-3-1 1K is 

significantly stiffer than PUU 2-1-1 1K. While PUU 2-1-1 1K exhibits a ductile mode of 

deformation, PUU 4-3-1 1K displays a change in the failure mode from a ductile to a brittle 

failure, as shown in figure 4b. The composition dependence of the impact response appears to 

correlate well with the corresponding quasi-static tensile deformation; PUU 4-3-1 1K exhibits 

the highest modulus and flow stress values but the lowest strain to failure among the select 

model PUUs. Results from an earlier work indicated that the strain to failure under tensile testing 
7

decreased as the HS content increased, suggesting an increasing continuity, percolation, or 

networked hard domains structure as the HS content is increased (1). 

 

Figure 4. Mode of deformation of PUU materials after 

ballistic impact against a 0.22-caliber FSP; 

(a) ductile deformation, and (b) brittle failure.

The question is whether a greater strain-to-failure or dynamic strain hardening is more desired in 

terms of the overall impact performance for these hyper-elastic PUU materials. In this work, a 

digital imaging correlation technique, which allows for real-time visualization of an impact 

deformation event, is utilized to determine the local strain evolution of select PUU materials. 

Figure 5 shows the time histories of the deformed surfaces of the targets, including PUU2-1-1 

2K, PUU 3-2-1 2K, PUU 4-3-1 2K, and PUU 3-2-1 1K, which were impacted by a steel sphere at 

about 106 m/s. The von Mises strain contours are overlaid over the deformed surface. The fullfield displacement profiles reveal that PUU exhibits global bending deformation, and 

additionally the composition dependence of the extent of dynamic deformation strain among 

these model PUUs. 
8

x

y

z

Figure 5. Plots of von Mises deformation strain history measured by impact of PUU 2-1-1 2K, PUU 3-2-1 2K, 

PUU 4-3-1 2K and PUU 3-2-1 1K against a steel sphere at velocity of 106 m/s.

PUU 2-1-1 2K is the softest elastomer among the select PUU materials. Increasing the hard 

segment content in the 2K PUU series renders the PUU 4-3-1 2K stiffer; it, therefore, displays 

more resistance to strain deformation as compared to PUU 3-2-1 2K and PUU 2-1-1 2K. On the 

other hand, the dynamic strain deformation profile is distinctly different in PUU 3-2-1 1K than 

the other PUU 2K materials. The PUU 3-2-1 1K is apparently much more resistant to the 

perforation, and it also shows the most effectiveness in dynamic strain hardening among these

four model PUUs. Figure 6 compares the maximum von Mises strain data as a function of time 

after impact at 106 m/s. The maximum of the von Mises strain refers to the location encountering 
9

the maximum deformation near the vicinity of impact. The PUU 3-2-1 1K reaches the max strain 

value at the shortest time—about 0.13 ms—compared to 0.18 ms and 0.19 ms observed in PUU 

4-3-1 2K and PUU 3-2-1 2K, respectively. Additionally, the max strain value reached in PUU 3-

2-1 1K is about 70% of that of PUU 4-3-1 2K and less than 50% of that seen in PUU 3-2-1 2K. 

The HS content in PUU 3-2-1 1K is nevertheless slightly higher than that of PUU 4-3-1 2K (41%

versus 37%). These results indicate that dynamic strain hardening is most significant via altering 

the SS MW, as seen in PUU 3-2-1 1k, among these PUUs.

0

5

10

15

20

25

30

35

40

45

0.0000 0.0001 0.0002 0.0003 0.0004 0.0005

time (s)

Max. von Mises strain (%)

Figure 6. Plots of the maximum von Mises strain as a function of time after impact of PUU 

3-2-1 2K (black), PUU 4-3-1 2K (green) and PUU 3-2-1 1K (red).

The effect of impact velocity was also evaluated. Figure 7 is a plot of the von Mises strain 

evolution of the PUU 2K materials impacted at 142 m/s, which displays similar composition 

influence on the dynamic strain hardening shown in figure 5.
10

Figure 7. Plots of von Mises deformation strain history measured by impact of PUU 2-1-1 2K, PUU 3-2-1 2K 

and PUU 4-3-1 2K against a steel sphere at velocity of 142 m/s.

The ARAMIS software also allows for calculation of the shear strain deformation. Figure 8

displays the surface shear strain, xy, at times after impact. Based on the evolution of the shear 

strain propagation data, it clearly shows that PUU 3-2-1 1K exhibits a faster rate in transverse 

strain wave propagation than PUU 3-2-1 2K, as indicated by faster expanding shear strain

contours seen at time 59.3 s and 133 s. This suggests that better energy dissipation is 

associated with the dynamic stiffening in PUU 3-2-1 1K. These measurements also correlate 

very well with the viscoelastic relaxation data, confirming that the extent of phase mixing, which 

is composition-dependent, has a significant influence on the high strain-rate deformation in these 

hyper-elastic PUU materials.
11

Figure 8. Plots of the surface shear strain, xy, of PUU 3-2-1 2K and PUU 3-2-1 1K against a steel sphere at 

velocity of 142 m/s.

Based on these dynamic impact deformation findings, two additional PUU targets, PUU 2-1-1 

650 and PUU 5-4-1 2K, are also included for evaluation. Both these PUU materials have Shore 

A and Shore D hardness values similar to those of PUU 3-2-1 1K (see Table 1). PUU 2-1-1 650 

exhibits a brittle mode of failure (figure 9a), despite the fact that HS% of PUU 2-1-1 650 is 33% 

much lower than 41% for PUU 3-2-1 1K. This indicates that greater phase mixing promotes 

dynamic stiffening, yet the choice of SS MW needs to be optimized. PUU 5-4-1 2K has HS 

content of 44%, and it displays a ductile deformation like the other 2K PUU materials (figure 

9b). Further materials characterization of these PUU materials is in progress.

 

(a) (b)

Figure 9. Ballistic impact of (a) PUU 2-1-1 650 and (b) PUU 5-4-1 2K against 

a 0.22-caliber FSP. 
12

4. Conclusion

A better understanding of molecular influence is invaluable to the design of PUUs for articles 

that manifest improved protective abilities for Warfighters against the ballistic impact and blast 

loading hazards. In this work, ballistic impact deformation measurements of select model 

transparent PUU materials were carried out. The high-speed photogrammetry has demonstrated 

the capability to reveal and differentiate the composition dependence of the 3D strain 

deformation histories, which cannot be easily observed quantitatively by other methods. Results 

indicated that dynamic stiffening by altering the extent of phase mixing appeared to be most 

important to facilitating the impact stress wave propagation. These high strain-rate mechanical 

deformation measurements correlated very well with the viscolelastic relaxation data via DMA. 

Thus, for bulk PUU usage, the choice of SS MW, though critical to phase mixing, requires 

optimization in addition to maintaining HS% below a threshold value, to ensure a ductile mode 

of deformation.
13

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3 US ARMY RSRCH LAB

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MAIL & RECORDS MGMT

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ADELPHI MD 20783-1197

1 US ARMY RSRCH LAB

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BLDG 4600

ABERDEEN PROVING GROUND MD 

21005-5066

7 HCS US ARMY RSRCH LAB

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 M. VANLANDINGHAM

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A. RAWLETT

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