High-strain-rate tensile mechanical response of a polyurethane elastomeric material

doi:10.1016/j.polymer.2015.03.046

Polymer

Volume 65, 18 May 2015, Pages 72-80

https://doi.org/10.1016/j.polymer.2015.03.046 Get rights and content

Highlights

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The dynamic tensile mechanical response of a soft polymer material is investigated using a split Hopkinson tension bar.
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The critical strain rate for the transition from a rubbery-like behaviour at low strain rates to a glassy-like behaviour at high strain rates at room temperature is determined as 450/s.
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At high strain rate, a ductile deformation mode is processed, however final fracture is revealed to be brittle.
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Craze formation and evolution into cracking is clarified in detail.
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Shielding mechanism is revealed with the crazing and micro cracking in the crack tip zone, contributing to the dynamic tensile toughness.

Abstract

The dynamic tensile mechanical response of a soft polymer material (Clear Flex 75) is investigated using a split Hopkinson tension bar (SHTB). Stress-strain relations are derived to reveal the mechanical properties at moderate and high strain rates. These relations appear to be rate dependent. Under static loading, the polymer exhibits an elastomeric behaviour, while under dynamic loading, the response is elasto-plastic with a hardening branch. The critical strain rate for transition from a rubbery-like behaviour at low strain rates to a glassy-like behaviour at high strain rates at room temperature is determined. The axial and lateral deformation of the specimen in the SHTB test is recorded by a high-speed camera. The final fracture surface is examined by SEM to explore the physical origins of deformation and fracture behaviour: void formation, craze nucleation, craze extension, crack initiation and propagation. Meanwhile, a shielding mechanism is revealed by the observation of crazing and micro cracking in the crack tip zone, which contributes to the dynamic tensile toughness of CF 75 polymer material.

Graphical abstract

Introduction

Polymer materials, with a high specific strength (strength/weight ratio) and high toughness [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], offer an opportunity to meet needs of engineering design. Recently, Clear Flex 75 (CF 75 in short) polymer material was found to be promising in the design concept of transparent armour [11]. It is a urethane rubber produced by mixing two low-viscosity liquid, which is one of the Clear Flex^® series products. However, the time-dependent rheological characteristics are unknown and the mechanical response mechanisms under high-rate loading condition are not well understood. Data and knowledge on the dynamic mechanical response, i.e. the stress-strain relation, are paramount for designing a transparent armour system and for the application in other hybrid protective concepts [12], [13], [14], [15]. Experimental research can provide the basic input data and knowledge to develop a computational model for the analysis of hybrid material systems of CF 75 polymer material.

The SHTB technique has been used to characterize the tensile stress-strain response at high strain rate for a variety of engineering materials [16], [17], [18], [19], [20]. For example, Chen et al. used a SHTB to determine the dynamic tensile stress-strain response and failure behaviour of low-strength and low-mechanical-impedance polymers [16]. Gilat et al. experimentally studied the strain-rate-dependent behaviour of a carbon/epoxy composite at intermediate strain rates conducted with the SHTB technique [17]. However, more challenges are raised for a soft material subjected to a dynamic experiment, because of the low strength, stiffness and wave impedance [20]. Nie et al. successfully characterized a soft rubber using the modified SHTB device to achieve a nearly constant strain rate deformation in the tested specimen [19].

The deformation and failure process of a polymer material is directly coupled to craze nucleation with the formation of voids and fibrils and craze development into a crack [15], [21], [22], [23], [24]. It is expected that similar mechanisms occur in dynamics, so craze and crack formation will determine the deformation and failure process.

In this study, a modified SHTB apparatus built at TU Delft is used to fill the gap of data and knowledge on the dynamic mechanical response of CF 75 polymer material. The rate-dependent behaviour of CF 75 polymer material is revealed by the dynamic tests at different strain rates. A high-speed camera is used to record the axial and lateral dynamic deformation. A scanning electron microscopy (SEM) is used to observe and analyse the corresponding deformation patterns and fracture micrographs, which shows the physical origins of the deformation and fracture mechanism as well as the toughening mechanism.

Section snippets

Experimental procedure

Clear Flex^® is a commercially available ‘smooth-on’ product obtained as a two-component elastomeric polyurethane with Part A (a polyol) and Part B (an isocyanite). These two parts are water white clear urethane liquid pre-polymers. By mixing Part A and Part B with a weight ratio of A:B = 1:1.75, Clear Flex 75 (CF 75 in short) is prepared, which is a polyurethane elastomeric material. It has a branched structure of molecular chains with a random distribution. CF 75 is a transparent, flexible and

Mechanical properties at various strain rates

Dynamic tensile tests were carried out on a CF 75 polymer material. The striker bar impact velocity is changed to vary the strain rate. The characterization of the dynamic mechanical response of CF 75 polymer material is processed at different loading speeds/strain rates. Yielding stress and maximum stress at various strain rates are collected to investigate the effect of strain rate on the dynamic tensile mechanical properties, which reveals the strain rate sensitivity of CF 75 polymer

Summary

This paper treats the mechanical response of CF 75 polymer material under dynamic tensile loading. Experimental data are presented at different strain rates. The inherent physical origins for deformation and fracture behaviour and the corresponding toughening mechanisms are explored by means of SEM recordings.

The engineering stress-strain curve of CF 75 polymer material under dynamic tension loading shows the following characteristics: initial linear elasticity, non-linear transition to global

Acknowledgements

Authors acknowledge Mr. G.W. Nagtegaal and Mr. K. van Beek of TU-Delft, Mr. E.P. Carton and Mr. E.C.M. van Daelen of TNO, for their contributions to the experiments and the financial support from the Dutch Technology Foundation, STW with the project number of 10615 and TNO.

References (52)

Z. Bartczak et al.
Toughness mechanism in semi-crystalline polymer blends: I. High-density polyethylene toughened with rubbers
Polymer
(1999)
F.H. Gojny et al.
Influence of different carbon nanotubes on the mechanical properties of epoxy matrix composites - a comparative study
Compos Sci Technol
(2005)
M. Costas et al.
Static and dynamic axial crushing analysis of car frontal impact hybrid absorbers
Int J Impact Eng
(2013)
J. Huang et al.
Comparison of fracture behavior of nylon 6 versus an amorphous polyamide toughened with maleated poly(ethylene-1-octene) elastomers
Polymer
(2006)
M. Dimchev et al.
Effect of carbon nanofibers on tensile and compressive characteristics of hollow particle filled composites
Mater Des
(2010)
O. Okada et al.
Fracture toughness of nylon 6 blends with maleated ethylene/propylene rubbers
Polymer
(2000)
P. Musto et al.
Toughness enhancement of polyimides by in situ generation of silica particles
Polymer
(2004)
C.R. Siviour et al.
The high strain rate compressive behaviour of polycarbonate and polyvinylidene difluoride
Polymer
(2005)
J.H. Yang et al.
Brittle-ductile transition of PP/POE blends in both impact and high speed tensile tests
Polymer
(2003)
R. Gensler et al.
Influence of the loading rate on the fracture resistance of isotactic polypropylene and impact modified isotactic polypropylene
Polymer
(2000)

W. Chen et al.

Tension and compression tests of two polymers under quasistatic and dynamic loading

Polym Test

(2002)

A. Gilat et al.

Experimental study of strain-rate-dependent behavior of carbon/epoxy composite

Compos Sci Technol

(2002)

S.Q. Fu et al.

Tension testing of polycarbonate at high strain rates

Polym Test

(2009)

C.M. Landis et al.

Crack velocity dependent toughness in rate dependent materials

Mech Mater

(2000)

S. Hong et al.

Cohesive-zone laws for void growth - I. experimental field projection of crack-tip crazing in glassy polymers

J Mech Phys Solids

(2009)

J.E. Field et al.

Review of experimental techniques for high rate deformation and shock studies

Int J Impact Eng

(2004)

H. Zhao et al.

On the use of a viscoelastic split Hopkinson pressure bar

Int J Impact Eng

(1997)

R. Smerd et al.

High strain rate tensile testing of automotive aluminum alloy sheet

Int J Impact Eng

(2005)

J. Yi et al.

Large deformation rate-dependent stress-strain behavior of polyurea and polyurethanes

Polymer

(2006)

A. Mulliken et al.

Mechanics of the rate-dependent elastic-plastic deformation of glassy polymers from low to high strain rates

Int J Solids Struct

(2006)

M.F. Omar et al.

Effect of molecular structures on dynamic compression properties of polyethylene

Mater Sci Eng A

(2012)

Q.W. Zhang et al.

Effect of strain rate on the tension-compression asymmetric responses of Ti-6.6Al-3.3Mo-1.8Zr-0.29Si

Mater Des

(2014)

D. Sherman et al.

Dynamic instabilities in {111} silicon

J Mech Phys Solids

(2008)

E.J. Kramer et al.

Theory of slow, steady state crack growth in polymer glasses

Polymer

(1984)

Y.L. Liang et al.

The toughening mechanism in hybrid epoxy-silica-rubber nanocomposites (HESRNs)

Polymer

(2010)

M.N. James et al.

Fatigue crack growth and craze-induced crack tip shielding in polycarbonate

Polymer

(2012)

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