Abstract
Knowledge of the properties of soft, viscoelastic materials at high strain rates are important in furthering our understanding of their role during blast or impact events. Testing these low impedance materials using a metallic split Hopkinson pressure bar setup results in poor signal to noise ratios due to impedance mismatching. These difficulties are overcome by using polymeric Hopkinson bars. Conventional Hopkinson bar analysis cannot be used on the polymeric bars due to the viscoelastic nature of the bar material. Implementing polymeric Hopkinson bars requires characterization of the viscoelastic properties of the material used. In this paper, 30 mm diameter Polymethyl Methacrylate bars are used as Hopkinson pressure bars. This testing technique is applied to polymeric foam called Divinycell H80 and H200. Although there is a large body of of literature containing compressive data, this rarely deals with strain rates above 250s−1 which becomes increasingly important when looking at the design of composite structures where energy absorption during impact events is high on the list of priorities. Testing of polymeric foams at high strain rates allows for the development of better constitutive models.
Highlights
Material characterisation at high strain rates is concerned with measuring the change in mechanical properties, such as yield strength, work hardening, and ductility, at higher strain rates, which can deviate from quasistatic results [1, 2]
The main technique in the range of 500 s−1–5000 s−1 is the Split-Hopkinson Pressure Bar (SHPB), which is capable of achieving high uniform uni-axial stress loading of a specimen in compression at nominally constant strain rates of the order of 103 s−1
The shifting of the signals occours in the frequency domain utilising the propagation coefficient and an Inverse Fast Fourier Transform (IFFT) is used to tranfer the signal back to the time domain
Summary
Material characterisation at high strain rates is concerned with measuring the change in mechanical properties, such as yield strength, work hardening, and ductility, at higher strain rates, which can deviate from quasistatic results [1, 2]. The changes in these properties at high strain rates has become increasingly important as improvements have been made to the understanding of high speed machining operations, ballistic, and impact events where the changes in these properties can have adverse effects. Specimen stress is inferred using elastic elements in series with the specimen
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