Shear and Compression-Shear Response of Polymeric Materials at High Shear Rates
Yungui Hu - Doctoral Dissertation Defense
Advisor: Dr. Ruqiang Feng
Date: Monday, June 9, 2003
Time: 9:00 a.m.
Place: W183 Nebraska Hall
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 polymeric materials under the conditions relevant to those encountered in 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 (TKB) technique is developed. The new TKB rheometer is capable of characterizing the high shear-rate response of polymers at various temperatures above their melting points or in various compression states. With this rheometer, systematic experimental studies are carried out on two widely used polymers, a low-density polyethylene (LDPE) and a linear low-density polyethylene (LLDPE), in various melting states and over a range of shear rates from 370 s-1 to 10,900 s-1, and on three polyborosiloxane-oil compounds (for use as the abrasive carriers in abrasive flow machining) under various combined dynamic compression-shear loadings with pressures up to 3.5 MPa. For the LDPE melt, a rheological modeling study is also conducted.
The experimental results on the LDPE and LLDPE melts indicate that the response of these two materials at high shear rates depends strongly on shear rate, cumulative shear strain and temperature. In particular, the transient behavior exhibits instantaneous rate dependence, which has not been observed in previous low-rate studies including various existing rheological models for LDPE melt. Further modeling study leads to the proposal of a new nonlinear constitutive model for LDPE melt. It is demonstrated that the model calibrated with the experimental data can capture all important features observed in the experimental measurements for the shear rates and temperatures examined.
The compression-shear experiments on the polyborosiloxane-oil compounds and detailed analysis of the experimental results reveal that within the time window of experiment (several hundreds of microseconds), the response of these generally viscoelastic media is predominantly elastic until a strain-controlled shear failure occurs. Although the material elasticity depends weakly on pressure, the failure strain is strongly pressure-dependent.
