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