Viscoelastic Properties Prepared by Dental Materials Department Yenepoya

Viscoelastic Properties Prepared by: Dental Materials Department Yenepoya Dental College Yenepoya University, Mangalore. 1

Viscoelastic Materials l l Viscosity = The resistance to motion of a liquid Viscoelastic materials l l Properties Elastic solid + Fluid = Materials that have mechanical properties dependent on time (loading rate or strain rate) and temperature. 2

Viscoelastic Model: Spring and Dashpot l Spring: an ideal elastic element l l Will immediately change and return to its original shape upon loading and unloading. Dashpot: a viscous fluid l l Will change its original shape upon loading, depending on time (and temperature). May slowly return to or may not return to its original shape upon unloading. l partial or complete recovery partial recovery no recovery 3

Spring stress input spring 27/06/46 Strain in spring 4

Dashpot stress input dashpot 27/06/46 Strain in dashpot 5

Viscoelastic Models l ***The elastic element reacts instantaneously to a change in load, and the viscous element reacts after a finite time. Combinations of a spring and dashpot stress input Model Strain Response spring dashpot 6

l Maxwell model l l In series Viscous strain remains after load removal. stress input Model Strain Response Maxwell model 7

l Kelvin or Voigt model l In parallel Nonlinear increase in strain with time Strain decreases with time after load removal because of the action of the spring (and dashpot). stress input Model Strain Response Voigt model 8

Examples of Viscoelastic Materials l Mattress, Pillow l Tissue, skin 9

***Important to specify the loading rate with the test results. *** 10

Thixotropic property l l A material that becomes less viscous and more fluid under pressure. A material whose viscosity decreases when subjected to high strain rates. l l Ketchup (http: //www. wcsscience. com/thixotropic/materials. html) Pressure indicator paste (PIP), prophylaxis paste, some impression materials 11

Pressure indicator paste (PIP), Pressure spot Denture base must be adjusted.

Importance of Viscoelasticity l Impression materials l The tear strength of alginate is increased 4 times when the loading rate is increased from 2. 5 to 25 cm/min. l l faster removal stronger impression, less distortion Time is required for the material to recover after impression made, before a plaster model can be poured. 13

l Oral tissue l Tissue must be allowed to recover free of the denture several hours before impression is made. 14

Time-Dependent Properties of Materials l (1) Stress Relaxation l The reduction in stress in a material subjected to constant strain. l e. g. Rubber bands will lose their elasticity when being stretched for a long time. 15

Example l Stress relaxation in orthodontic bands plastic vs. rubber bands 16

l (2) Creep The increase in strain in a material under constant stress. l The flow of a material under a constant load. A deformation is allowed to occur over time. l l e. g. If you put something heavy upon a block of modeling clay, it will be deformed over time. 17

Example l A tungsten lightbulb filament sagging under its own weight. l The defection increases with time due to creep and can lead to touching of adjacent coils, which causes bulb failure. 18

l Amalgam, wax pattern, denture set-up, some impression materials 27/06/46 19

Creep l Is used to rate the resistance of a material to plastic deformation under sustained load. l Creep resistance should be considered for any part or structure that is subjected to sustained load in service. 20

Summary l l l Viscoelastic materials Thixotropic property Time dependent properties l l Stress relaxation Creep 21

End of Viscoelastic Materials