Constitutive Modeling of Polyamide Split Hopkinson Pressure Bars for the Design of a Pre-stretched Apparatus
This paper aims to model the constitutive behavior of polyamide material used in the Split Hopkinson Pressure bars (SHPB). The Hopkinson bars apparatus is employed for the mechanical characterization of many materials under high strain rates at large strains. Nevertheless, testing soft materials is a challenging task regarding their low impedance properties and the difficulty to achieve a dynamic equilibrium. To address that issue, polyamide (nylon) SHPB are employed. However, the application of the pre-stretched technique to tensile apparatus using polyamide bars may provide a flexible mechanical characterization device reaching moderate to high strain rates at large strains. It requires bars of several meters where wave attenuation and dispersion are dominant. Moreover, the design of such apparatus is extremely complex with respect to the sample shape and rigidity as well as connectors. While analytical techniques are proposed in the literature, they are not sufficient to provide guidance in the design and the optimization of a pre-stretched apparatus.
Therefore, the aim of the present study is to develop a finite element model of polyamide SHPB. Various experimental tests are conducted using compressive polyamide SHPB. These tests are computationally modeled using the Radioss explicit FE code through an axisymmetric analysis. The generalized Maxwell model is chosen to consider the viscoelastic material properties. An optimization procedure by inverse method is applied using both experimental and numerical strain signals to identify the material coefficients.
Experimental tests are repeatable for each test configuration. The viscoelastic model parameters of the bars are identified through one configuration and validated against three others. This model gives very satisfactory results and presents interesting predictive abilities.
KeywordsHopkinson bars Viscoelastic bars Experimental testing Constitutive modeling Inverse technique
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