3 rd Year Dental Materials Science

Size: px

Start display at page:

Download "3 rd Year Dental Materials Science"

Andra Flowers
7 years ago
Views:

1 3 rd Year Dental Materials Science Dr. Graham Cross School of Physics and CRANN SFI Nanoscience Building, Rm Dental Materials - Graham Cross 1

2 Topics Oct. 26: Basic metallurgy and alloys Nov. 2: Properties of materials, thermals Nov. 16: Mechanics of solids and fluids Textbooks Further Reading Applied Dental Materials 8 th Edition 1998, John F. McCabe, Angus W. G. Walls, Blackwell, Oxford, UK. Restorative Dental Materials 10th Edition 1997 Editor Robert G. Craig, Mosby Year Book, Inc, St. Louis, USA Notes on Dental Materials 6th Edition 1992 Editor E.C. Combe, Churchill Livingstone, Edinburgh, UK Phillip s Science of Dental Materials 10th Edition 1996, Editor Kenneth J. Arusavice, W.B. Saunders Company Philadelphia, USA Dental Materials, Properties and Manipulation 6 th Edition 1996 Editors Robert G. Craig, William J. O Brien, John M. Power, Mosby Year Book, Inc, St. Louis, USA Dental Materials - Graham Cross 2

3 Mechanical properties of materials Stress and strain Elasticity and viscosity: Solids vs. fluids Rheology and Plasticity Viscoelasticity Dental Materials - Graham Cross 3

4 Solids vs. liquids We all understand generally what the difference between solid and a liquid is, but in practice this difference can be blurred.. A very general distinction is this: Elastic behaviour When you apply and then remove a force, fast or slow, the object returns to its original shape! Inelastic behaviour (flow) When you apply and remove a force, the shape of the object is permanently changed. How can we understand the reaction of materials to forces independently of the geometry of the tested object? Dental Materials - Graham Cross 4

5 Stress Stress is the force per unit area applied to an object: σ = Force Area Units = N/m 2 or Pascals (Pa) Also: 1 bar = kpa 1 MPa = 10 6 Pa Different ways of applying stress, over a surface: Compressive Tensile Shear Dental Materials - Graham Cross 5

6 Adhesion Adhesion may be defined simply as a force interaction between two materials at an interface where they are in contact. Failure occurs at a critical stress level Interface must support a solely tensile load: Mechanical Chemical Dental Materials - Graham Cross 6

7 Area of contact and stress Adhesive strength depends on true contact area limited by roughness: Chemical Adhesion Rough surfaces mean small contact area, so a small force makes a large stress at local points on surface, causing failure Polishing a surface to make it smooth increases area and reduces stress Dental Materials - Graham Cross 7

8 Strain Strain ε is a measure of the change in dimension of an object that occurs by the application of stress. It is defined as a relative displacement: ε = dl l Different kinds of strain Dental Materials - Graham Cross 8

9 Stress vs. strain curve A principle way to characterize mechanical properties of solid materials. Many properties can be determined from it: Elastic modulus Tensile strength Yield strength Ductility Resilience Fracture toughness Stress σ Strain ε This is an intrinsic signature of a material Why would a force vs. displacement curve not be? See: Applied Dental Materials 8 th Edition 1998, John F. McCabe, Angus W. G. Walls, Chapter Dental Materials - Graham Cross 9

10 Elasticity Reversible stretching, compression, or deforming of a body In the linear elastic range, the ratio of stress to strain is called a modulus Stress σ σ = Eε ε limit Strain ε ε limit = 0.02 Ceramics/Metals = 0.1 Polymer glasses > 5 Some elastomers! Different modulus are defined for different types of deformation: Young s modulus Shear modulus Bulk modulus Dental Materials - Graham Cross 10

11 Elastic Modulus Before After Polystyrene: Bulk B=10 GPa Young s E=3 GPa Shear G=1 GPa Dental Materials - Graham Cross 11

12 Shear strain Shear strain γ is a skew: it changes shape, not volume. Very important when we consider flow. γ dy h h Shear strain rate: dγ dyh dy dt = = dt dt h dγ = dt Dental Materials - Graham Cross 12 v y h

13 Simple fluid flow Consider fluid between to large plates of area A: What shear stress τ must be applied between the two plates to get vy? Shear Force τ= x h y Force Area vy Newton s law of fluid flow: Shear stress τ is proportional to the flow velocity gradient normal to flow: v dγ τ y h = dt Stress is proportional to shear strain rate! velocity in y direction NB: Fluid velocity at walls is zero with respect to wall (Fluid sticks to the walls) Dental Materials - Graham Cross 13

14 Why shear is important for flow Force between atoms Bonding energy Repulsive Distance of separation Difficult! Attractive Compressive/tensile stress: - Changing the distance of separation of atoms is difficult (volume change) Shear stress: - Changing neighbours between atoms is much easier (shape change) Easy! A liquid changes shape, not volume, freely Dental Materials - Graham Cross 14

15 Solid vs. Liquid Energy solid Position (Low Temperature) Atoms deep in energy well Dental Materials - Graham Cross vacancy 15

16 Solid vs. Liquid Energy solid Position (Low Temperature) Atoms deep in energy well Dental Materials - Graham Cross vacancy 16

17 Solid vs. Liquid Energy solid Position (Low Temperature) Atoms deep in energy well vacancy Energy liquid Position (High Temperature) Atoms can hop over energy barrier! Dental Materials - Graham Cross 17

18 Viscosity Viscosity η describes the way momentum is transferred by a fluid during flow For simple fluids it is a constant of proportionality between shear stress and shear rate (Newton): dγ τ dt dγ τ =η dt η Units: Pa s (Poise) Force Fluid Viscosity η Pa s Air Water Mercury Honey 100 Glass 1040 (?) Dental Materials - Graham Cross 18

19 Rheology Study of the flow of all materials, including solids and complex liquids such as polymer melts, colloids, suspensions, slurries, pastes, etc. Consider a complex fluid, a polymer melt: What happens when you shear this material? Molecules both flow and they change their shape they relax Gives rise to both shear rate (dγ/dt) and time dependent behaviour Dental Materials - Graham Cross 19

20 Time dependent material response Due to mechanical reasons such as relaxation time of constitutive particles in transient (ie. non-steady) flows Or due to chemical reasons such as setting times Usually viscosity will be used to measure this: Initial low viscosity for dispensing and moulding Followed by large increase in viscosity during setting Working time time the material can be easily manipulated Setting time time at which viscosity goes very high Dental Materials - Graham Cross 20

21 Viscosity and setting time of pastes Viscosity η Time t Time t Time t Poor rheological properties - no well defined setting time Ideal rheological properties - long working time - sudden setting time Good rheological properties - long working time - reasonable setting time Dental Materials - Graham Cross 21

22 Shear rate dependent flow Fluids: Instead of a stress vs. strain curve, we plot a stress vs. strain rate curve Newtonian linear fluid dγ τ = η dt a) Dilatant b) Pseudoplastic (shear thinning) Shear Stress τ Shear Stress τ b a Shear rate dγ dt Shear rate Dental Materials - Graham Cross 22 dγ dt

23 Plasticity: flow of solids Ductile behaviour of a solid that occurs above a special shear stress threshold called the yield stress : τ yield This occurs for many metals and glassy polymers τ yield Shear Stress τ Ceramic materials tend to fracture, not yield Strain ε Ductility Like a liquid, plastic flow of solids involves shape change, not volume change Dental Materials - Graham Cross 23

24 Shearing a solid: Plastic flow Energy solid Position sheared solid Energy Stress, not temperature, increases the energy level Position One line of atoms changes neighbours Dental Materials - Graham Cross 24

25 Viscoelasticity Response of materials with both elastic and viscous character: time dependent Eg. Elastomers Two important forms: Creep Stress relaxation Visualized by combining mechanical components of Springs (elastic): instant response to stress Dash-pots (viscous): slow response Dental Materials - Graham Cross 25

26 Creep Time dependent dimensional change of materials under constant stress. Eg. Weight of a gold filling, effect on elastomer padding layer σ 0 Important for dental amalgams: - Melting temperature is close to room temperature - Teeth clenching - Creep may be precursor to fracture at filling edge Stress Dental Materials - Graham Cross Strain σ Time 26

27 Stress relaxation When a viscoelastic material is under constant strain a gradual reduction in stress can occur Stress σ Eg. Dental waxes, resins, and gels Manipulate into shape, then stress drops over time This can, in turn, lead to dimensional changes on other surrounding loaded structures. For more examples, see: Applied Dental Materials 8 th Edition 1998, John F. McCabe, Angus W. G. Walls, Blackwell, Oxford, UK. Time t Dental Materials - Graham Cross 27

Notes on Polymer Rheology Outline

Notes on Polymer Rheology Outline 1 Why is rheology important? Examples of its importance Summary of important variables Description of the flow equations Flow regimes - laminar vs. turbulent - Reynolds number - definition of viscosity