Elsevier

Materials & Design

Volume 50, September 2013, Pages 124-129
Materials & Design

Experimental investigation on the tensile behavior of polyurea at high strain rates

https://doi.org/10.1016/j.matdes.2013.02.063Get rights and content

Abstract

Elastomeric polymers, like polyurea, are finding relevance in strengthening applications and as energy absorbing materials for structures and systems subjected to impulsive loadings. Understanding the dynamic behavior of these materials is essential for their application as an effective protective and retrofitting material. This paper presents the findings of a series of uniaxial tensile tests that were conducted on polyurea over the strain rate region from 0.006 to 388 s−1. The stress–strain behavior of the polyurea sample was established at different strain rates, and key mechanical properties of the material were determined. The results indicate that the stress–strain behavior of the material at high strain rates were considerably non-linear and exhibited significant rate dependency. The characteristics of the stress–strain curves at quasi-static and at higher strain rates can be described as rate-dependent linear elastic/piecewise linear strain hardening. Based on the findings of the experiments, the influence of strain rate effects on the modulus and yield stress of the polymer were analyzed and empirical correlations to describe the dynamic increase factor (DIF) of the modulus of elasticity and the yield stress at high strain rates were proposed.

Highlights

► Experimental program to analyze the tensile behavior of polyurea at high strain rates. ► Stress–strain behavior of polyurea was considerably non-linear and exhibited significant rate dependency. ► The modulus of elasticity of polyurea was enhanced with the amplification of strain rates. ► Empirical correlations to describe the DIF of modulus of elasticity and yield stress at higher strain rates were proposed.

Introduction

Substantial efforts have been undertaken in recent years to identify novel techniques to enhance the survivability of structures and structural systems under severe impulsive loadings. Elastomeric polymers, like polyurea, have attracted the attention of researchers, due to their feasibility in strengthening structures, and as energy absorbing materials in structural and ballistic systems, subjected to such blast, ballistic and impact loadings [1], [2], [3], [4], [5], [6], [7], [8]. Initial attempts to utilize polyurea for structural strengthening applications were undertaken to evaluate its viability in enhancing the resistance of masonry structures to blast effects [1], [2]. The findings from these investigations indicated that besides enhancing the resistance of the structures to blast effects, the application of polyurea coatings was also effective in minimising the fragmentation and collapse potential of the unreinforced masonry walls [1], [2]. Subsequent efforts by various researchers further assessed the feasibility and characteristics of this material in enhancing the capacity of metallic structures [3], [4], and composite structural systems [5], [6], to resist severe impulsive loadings. On studying the application of polyurea coatings on steel plates, Amini et al. [3] found that the applied coating would benefit in enhancing the resistance of steel plates to blast loadings, in terms of failure mitigation and energy absorption, only when it is applied on the unloaded face of the plate. On the other hand, the application of the coating on the blast-receiving face of the plate would in turn elevate the damage of the steel plate [3].

Polyurea has also been investigated as a potential material for suspension-pad for combat helmets, as a mitigating technique against blast-induced traumatic brain injury. The researchers found that the application of polyurea as a suspension-pad material could result in significant reduction in the peak loading experienced by the brain in a high blast peak pressure situation, thus deeming it as a preferred choice for such application [7]. In a more recent study, Grujicic et al. [8] investigated polyurea as an adhesive layer material in analyzing the ballistic and structural capacity of a ceramic/polymer composite armor system.

Polyurea is derived from the reaction of an isocyanate component and a polyamine, which is odorless, exhibits low shrinkage resistance to moisture, and bonds well with steel, concrete and plastic [6]. Considering that impulsive loadings (as applied in all the examples discussed above) are usually associated with high loading rates, it is therefore essential to synthesize the characteristics and behavior of the material under high strain rates prior to developing and optimizing their functionality as a feasible protective and strengthening material.

Over the decades, various instrumentations and testing procedures have been introduced and developed to synthesize the dynamic characteristics of polymeric materials. Yi et al. [9] investigated the compressive stress–strain behavior of one polyurea and three polyurethane samples using the split Hopkinson pressure bar (SHPB) system. The polymeric samples demonstrated highly non-linear stress–strain relationships, indicating strong hysteresis and rate dependency. Subsequently, Sarva et al. [10] analyzed the same polyurea sample and one of the polyurethane samples over a wide range of strain rates, from 10−3 to 104 s−1. Sarva et al. [10] used the Zwick screw drive mechanical tester for low to moderate (10−3–10−1 s−1) strain rate compression tests, an enhanced servo-hydraulic axial testing machine (MTS 810) for moderate to intermediate (100–102 s−1) strain rate tests, and two SHPB configurations for higher strain rate tests. Besides being similar to the findings of Yi et al. [9], Sarva et al. [10] also deduced that the polyurea’s stress–strain curves indicated a transition from rubber-like behavior at low strain rates to leather-like behavior at high strain rates.

Meanwhile, Shim and Mohr [11] used an Instron 8800 hydraulic universal testing machine, two conventional SHPB configurations, and a modified SHPB system with hydraulic actuators to characterize the high strain rate behavior of polyurea in compression. They established that the influence of strain rate is more pronounced at larger strains [11]. More recently, Shim and Mohr [12] reported the findings of relaxation experiments, continuous and multi-step compression experiments using the hydraulic universal testing machine for strain rates from 10−3 to 101 s−1 and compressive strains up to 1.0. A rate-dependent non-linear viscoelastic finite strain constitutive model was developed based on the findings [12]. On the other hand, Omar et al. [13] also used the SHPB system to analyze the high strain rate (up to 1100 s−1) compressive behavior of three polymers (polyethylene, polypropylene and polycarbonate).

It can be observed from the discussions that most of the researches on the high strain rate characteristics of polymers, particularly polyurea, have focused on their compressive properties. Very limited research has been undertaken to investigate the tensile behavior of polyurea at high strain rates. Roland et al. [14] investigated the high strain rate mechanical behavior of polyurea in tension at strain rates of 0.06–573 s−1 using a drop weight test instrument. The design of the instrument was inspired by the modified pendulum impact equipment design, as discussed in Bekar et al. [15], and Hoo Fatt and Bekar [16]. Investigations have also been undertaken by Pathak et al. [17] up to a strain rate of 830 s−1. Meanwhile, Qiao and Wu [18] performed low strain rate uniaxial tensile tests on polyurea in the strain rate region from 5.3 × 10−4 to 5.1 × 10−2 s−1. A computer-controlled single-axial Instron load frame was used in the tests, where the rate-dependent tensile behavior and the effect of stoichiometry on the mechanical behavior of polyurea were analyzed [18].

This paper presents the findings of an experimental program undertaken to analyze the tensile behavior of a polyurea sample at increasing strain rates from 0.006 to 388 s−1. Two testing systems, a MTS servo-hydraulic machine and an Instron high rate testing system were used in the study. These findings are part of a comprehensive research project initiated by the authors to characterize various polyurea samples at low, intermediate and high strain rates. The findings presented herein are of one polyurea sample. Ultimately, in order to develop a more accurate material model for polyurea, the tensile behavior of the material at varying strain rates needs to be fully understood and discretized. The findings from this study can be used to derive and calibrate the necessary parameters for materials models, such as the C01 and C10 empirical material constants for the Mooney–Rivlin material model used in finite element codes such as ANSYS® AUTODYN® [19].

The stress–strain characteristics of the polyurea sample at quasi-static and high strain rates were analyzed. Based on the findings of the experiments, empirical correlations to describe the dynamic increase factor (DIF) of the polymer’s modulus of elasticity and yield stress at high strain rates were developed. The findings of this study are envisaged to enhance the present limited knowledge on the tensile characteristics of polyurea at high strain rates.

Section snippets

Materials and specimen preparation

The polyurea specimens were prepared (cut) from polyurea sheets. The polyurea sheets were prepared by spray-on procedure and the cast sheets were allowed to cure at room temperature. After annealing the cast sheets at room temperature for about 1 month, the test specimens were die-cut from the cast sheets to the dimensions shown in Fig. 1 (with an allowance of ±0.5 mm). The specimens were then ground to smoothen their surfaces and to a thickness of 6 mm.

Dynamic tensile test systems

Uniaxial tensile tests were performed at

Results and discussions

The findings of the experimental studies and their discussions are provided in the following sections.

Conclusions

The dynamic tensile properties of a polyurea sample were investigated in this study and the stress–strain characteristics of the material at various enhancing strain rates were presented. It was observed that the stress–strain behavior of the polyurea at high strain rates was considerably non-linear and exhibited significant rate dependency. The behavior of the polyurea sample can be described as rate-dependent linear elastic/piecewise linear strain hardening.

One of the key findings of this

Acknowledgments

The authors would like to extend their sincere appreciation to Dr. Cuong Nguyen, Dr. Jianhu Shen, Mr. Shanqing Xu and Dr. Tracy Ruan for their assistance throughout the high-rate tensile test program. This research was performed during the PhD candidature of the first author at The University of Melbourne. Deep gratitude is extended to the Ministry of Higher Education of Malaysia, and to Universiti Kebangsaan Malaysia for providing the funding for his Doctoral studies. The assistance provided

References (27)

  • C.M. Roland et al.

    High strain rate mechanical behavior of polyurea

    Polymer

    (2007)
  • C. Li et al.

    A hyper-viscoelastic constitutive model for polyurea

    Mater Lett

    (2009)
  • J.S. Davidson et al.

    Explosive testing of polymer retrofit masonry walls

    J Perform Constr Facil

    (2004)
  • Cited by (0)

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