Journal of Dental Research, Vol 66, 1569-1575, Copyright © 1987 by International & American Associations for Dental Research Online Journals
Creep of amalgam at low stresses
Y. Papadogiannis, D. B. Boyer and R. S. Lakes
The creep and recovery of dental amalgam were studied with a torsional
creep apparatus. The purpose was to investigate viscoelastic behavior in a
low stress range that might result from normal chewing forces. The creep of
specimens up to six months old was also studied. Four types of alloys for
amalgam were used: two single-composition sphericals (SCS), an
admixture-lathe-cut eutectic (ALE), and a conventional (CON). Constant
torque was applied to dumbbell-shaped specimens for three hr and, after the
stress was released, recovery was followed for 50 hr. All measurements were
made in water at 37 degrees C after storage of the specimens for one week
in 37 degrees C water. Creep was also measured at nine intervals on
specimens aged from three hr to six months. The SCS amalgams exhibited the
least amount of creep, followed by the ALE amalgam. The CON amalgam had the
highest creep. The SCS amalgams approached linear viscoelastic behavior in
the shear stress range of 0.78-4.10 MPa at 37 degrees C. They did not
exhibit steady-state (viscous) creep at stresses less than 4.10 MPa.
Recovery was complete but proceeded more slowly than creep. The ALE and CON
amalgams exhibited viscous creep at 1.23 MPa and higher. The steady-state
creep rate depended on stress, and the stress exponent ranged from 2.3-3.5.
All except the ALE amalgam showed essentially constant creep compliance
when aged up to six months, once the amalgamation reaction was completed
after 24 to 48 hr. The compliance of the ALE amalgam gradually decreased
over the six-month period.
