Creep, recovery and high strain rate response of soft roll cover materials

Abstract

Polymer covered rolls are used in paper machines to extend the contact area between two mating rolls and also to damp vibrations excited in the roll systems. On the other hand, viscoelastic covers can also be a source of vibration problems because of incomplete recovery of time dependent deformation between two consecutive compression cycles. In this paper, the viscoelastic behavior of two relatively hard particle-reinforced epoxy based calender roll covers as well as a soft polyurethane based coater roll cover material was studied at different loading rates. Because the high loading rates common in modern paper machine operation cannot normally be achieved using servohydraulic testing machines, a compressive split Hopkinson pressure bar (SHPB) technique was applied. In a roll cover, the stress-state is multi-axial, including also shear components, and therefore compressive SHPB tests do not fully simulate the nip loading but provide, nevertheless, a good basis for comparison of different materials. Also long duration creep tests at different temperatures were made using servohydraulic testing machines to construct master curves, which extend the time scale of creep data up to hundreds of hours.