Thermal properties Viscoelasticity Methods for Testing Dental Biomaterials
Thermal properties, Viscoelasticity & Methods for Testing Dental Biomaterials
Session overview • Thermal Properties • Viscoelastic Properties • Test Methods
Thermal Properties
Thermal Expansion The change in unit length of a material per unit temperature change.
Liquid Specimen length Crystal 3 Crystal 2 Crystal 1 Allotropy – the presence of two or more crystal structures at the same chemical composition temperature
Coefficient of thermal expansion L = α T Coefficient of thermal expansion Examples: gold resin composite tooth structure 14. 4 x 10 -6 / o. C 30. 0 x 10 -6 / o. C 11. 4 x 10 -6 / o. C
open margins
fluid in Low T: composite contracts – fluid sucked in.
fluid out Higher T: composite expands – fluid forced out.
fluid in Low T: composite contracts – fluid sucked in.
fluid out Higher T: composite expands – fluid forced out.
Othermal properties: • Melting temperature • Heat of fusion. Energy to convert 1 g of material from solid the liquid (J / g where J is a joule) • Specific Heat. Energy to raise the temperature of 1 g of material by 1 o. C (J / g) • Thermal conductivity. The energy (heat) needed to increase the temperature on the other side 1 m thickness of a material by 1 o. C in 1 s. ( J / s m o. C)
Viscoelastic Properties
Viscoelasticity Deformation that includes both an elastic component and a plastic component. Viscoelasticity is strongly dependent on time and temperature.
stress Viscoelasticity: Strain when a constant stress is applied. 0 ts time creep recovery strain creep 0 ts time
Amalgam Creep Test - ADA Specification No. 1
Static creep: Time-dependent plastic deformation that occurs when a material is placed under a constant load that is less than the its yield strength.
Creep Test - ADA Specification No. 1 • cylinder: 8 mm high by 4 mm in diameter • temperature: 37 o C • stress: 35. 85 MPa
Creep Test - ADA Specification No. 1 (height 4 th hr) - (height 1 st hour) x 100 % creep = initial height
stress relaxation 0 strain time 0 time Viscoelasticity: Stress when a constant strain is applied.
Simulating a purely viscous material strain constant load removed permanent deformation load applied dashpot x t=0 0 tx time t = tx t > tx
Simulating a purely elastic material: constant load applied spring load removed strain No permanent deformation tx 0 time
Spring & dashpot in series – Maxwell Model load removed constant load spring recovers strain load applied dashpot opens 0 dashpot permanent deformation (dashpot) spring opens tx time spring
Spring & dashpot in parallel – Voight Model load removed strain constant load applied 0 spring & dashpot gradually open tx time spring & dashpot gradually close
More like real materials: a 4 element-element viscoelastic model constant load removed s 1 closes instantly load applied d 2 gradually closes strain d 1 and d 2 gradually open s 1 opens instantly 0 s 1 d 1 does not close s 2 tx time at the end of the day there’s a permanent strain. d 2
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