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Shear and compression-shear response of polymeric materials at high shear rates
Polymeric materials may exhibit complex rheological behavior that depends on time history of strain, temperature, and pressure. Experimental and theoretical characterizations of the rheological response of these materials under the conditions relevant to their production or application processes are of great importance for computerized process modeling and optimization. ^ In this work, a novel polymer rheometer based on the torsional Kolsky bar technique is developed. The rheometer is capable of measuring the high shear-rate response of polymers at various temperatures or in various compression states. With this rheometer, two systematic experimental investigations are carried out. One is on two widely used polymers, low-density polyethylene (LDPE) and linear low-density polyethylene, for various melting states and over a shear rate range of 370–10,900 s−1. The other is on three polyborosiloxane-oil compounds, which are abrasive carriers for abrasive flow machining, under various dynamic compression-shear loadings with pressures up to 3.5 MPa. For the LDPE melt studied, phenomenological modeling of its rheological response is also performed. ^ The experimental results on the polymer melts indicate that the high shear-rate response of the materials depends strongly on shear rate, cumulative shear strain and temperature. The transient behavior exhibits instantaneous rate dependence, which has not been observed in previous low-rate studies. Calibrated with the experimental data, a new constitutive model developed for the LDPE melt is shown to be able to capture all of the important features observed in the experiments including the material's temperature dependence and relaxation behavior during unloading. ^ The compression-shear experiments on the polymer-oil compounds and detailed data analysis reveal that within the time window of experiment (∼500 μs), the response of these generally viscoelastic materials is predominantly elastic and relatively insensitive to pressure until a microfracture-induced shear failure occurs. However, the shear stress at failure depends on both pressure and shear rate. ^
Hu, Yungui, "Shear and compression-shear response of polymeric materials at high shear rates" (2003). ETD collection for University of Nebraska - Lincoln. AAI3152612.