FAILURE MECHANISMS OF ULTRA HIGH MOLAR MASS POLYETHYLENE SINGLE FIBERS AT EXTREME TEMPERATURES AND STRAIN-RATES

Abstract

The effects of temperature and strain-rate on the mechanical properties of Ultra High Molar Mass Polyethylene (UHMMPE) single fibers was investigated at eleven temperatures from room temperature (20 °C) to the orthorhombic-hexagonal phase transition (148 °C) and at six strain-rates from quasi-static (10-3 s-1) to dynamic (103 s-1). Dimensional analysis of ballistic limit tests using has shown an underperformance of materials comprised of UHMMPE fibers. A possible explanation is the relatively low melting temperature of UHMMPE fibers (~150 °C) in comparison to other fiber materials, such as poly-aramids (~450 °C). The mechanical properties of UHMMPE single fibers were investigated through a series of 437 tensile tests at 66 temperature-strain-rate combinations. Changes in stress-strain curve shapes were observed with respect to temperature and strain-rate. The transition of stress-curve shape with increasing temperature was observed to be pseudo-brittle, plateauing, necking, and non-failure and transitions between these phases were observed within a strain-rate dependent temperature range. For low and intermediate strain rates, a temperature and strain-rate equivalence is observed: a decadal increase of strain-rate is mechanically equivalent to a ~20 °C decrease in temperature. Strain to failure for dynamic strain rates was invariant over the temperature range of this study. Strength and modulus properties were observed to decrease with increasing temperature and increase with increasing strain-rate. An orthorhombic to hexagonal phase transition occurs between 145 °C and 148 °C and a sudden decrease in strength and moduli was observed. The change in dominant stress-relieving mechanism is proposed. Chain slippage is dominant for the majority of conditions in this study except where scission and straightening are the dominant mechanism. At high temperatures for constrained fibers in the hexagonal phase, chain slippage occurs more frequently due to the trans to gauche conformation. Chain scission is only dominant moments before fiber failure and near the failure surface. Chain straightening is dominant at low strain (0 % to 0.5 %) and at temperatures greater than or equal to the necking temperatures for the quasi-static and intermediate strain-rates and at all temperatures for the dynamic strain-rates.