Dental Materials Lecture - Mechanical Properties 1k5it
This document was ed by and they confirmed that they have the permission to share it. If you are author or own the copyright of this book, please report to us by using this report form. Report 3i3n4
Overview 26281t
& View Dental Materials Lecture - Mechanical Properties as PDF for free.
More details 6y5l6z
- Words: 1,567
- Pages: 27
3rd Year Dental Materials Science Dr. Graham Cross School of Physics and CRANN SFI Nanoscience Building, Rm 1.5 http://www.tcd.ie/Physics/People/Graham.Cross/
[email protected]
16.11.2007
Dental Materials - Graham Cross
1
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 – 6th Edition 1996 Editors Robert G. Craig, William J. O’Brien, John M. Power, Mosby – Year Book, Inc, St. Louis, USA 16.11.2007
Dental Materials - Graham Cross
2
Mechanical properties of materials
• Stress and strain • Elasticity and viscosity: Solids vs. fluids • Rheology and Plasticity • Viscoelasticity 16.11.2007
Dental Materials - Graham Cross
3
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? 16.11.2007
Dental Materials - Graham Cross
4
Stress • Stress is the force per unit area applied to an object:
Force σ= Area
• Units = N/m2 or Pascals (Pa) • Also: 1 bar = 101.3 kPa • 1 MPa = 106 Pa
• Different ways of applying stress, over a surface: Compressive
16.11.2007
Tensile
Dental Materials - Graham Cross
Shear
5
Adhesion • Adhesion may be defined simply as a force interaction between two materials at an interface where they are in . • Failure occurs at a critical stress level Interface must a solely tensile load:
Mechanical 16.11.2007
Chemical Dental Materials - Graham Cross
6
Area of and stress • Adhesive strength depends on true area limited by roughness:
Chemical Adhesion • Rough surfaces mean small 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 16.11.2007
Dental Materials - Graham Cross
7
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:
dl ε= l Different kinds of strain
16.11.2007
Dental Materials - Graham Cross
8
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 – 8th Edition 1998, John F. McCabe, Angus W. G. Walls, Chapter 2.
16.11.2007
Dental Materials - Graham Cross
9
Elasticity • Reversible stretching, compression, or deforming of a body
Stress σ
• In the linear elastic range, the ratio of stress to strain is called a modulus
σ = Eε
εlimit
εlimit = 0.02 Ceramics/Metals = 0.1 Polymer glasses > 5 Some elastomers! 16.11.2007
Strain ε
• Different modulus are defined for different types of deformation: • Young’s modulus • Shear modulus • Bulk modulus
Dental Materials - Graham Cross
10
Elastic Modulus
Before
After
Polystyrene:
16.11.2007
Bulk B=10 GPa
Young’s E=3 GPa
Dental Materials - Graham Cross
Shear G=1 GPa 11
Shear strain • Shear strain γ is a skew: it changes shape, not volume. • Very important when we consider flow.
dy γ≡ h Shear strain rate: h
16.11.2007
d γ dy h dy dt = = dt dt h dγ vy = dt h
Dental Materials - Graham Cross
12
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! 16.11.2007
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
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! 16.11.2007
A liquid changes shape, not volume, freely
Dental Materials - Graham Cross
14
Solid vs. Liquid Energy
solid
Position
(Low Temperature)
Atoms deep in energy well
16.11.2007
Dental Materials - Graham Cross
vacancy
15
Solid vs. Liquid Energy
solid
Position
(Low Temperature)
Atoms deep in energy well
16.11.2007
Dental Materials - Graham Cross
vacancy
16
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! 16.11.2007
Dental Materials - Graham Cross
17
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 16.11.2007
η Units: Pa s (Poise)
Force
Fluid
Viscosity η Pa s
Air
0.00018
Water
0.0089
Mercury
0.015
Honey
100
Glass
1040 (?)
Dental Materials - Graham Cross
18
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.
16.11.2007
Dental Materials - Graham Cross
19
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 16.11.2007
Dental Materials - Graham Cross
20
Viscosity and setting time of pastes Viscosity η
Time t
• Poor rheological properties - no well defined setting time
16.11.2007
Time t
• Ideal rheological properties - long working time - sudden setting time Dental Materials - Graham Cross
Time t • Good rheological properties - long working time - reasonable setting time 21
Shear rate dependent flow Fluids: Instead of a stress vs. strain curve, we plot a stress vs. strain rate curve a) Dilatant
Newtonian linear fluid
τ =η
dγ dt
b) Pseudoplastic (shear thinning)
Shear Stress τ
Shear Stress τ
b
a
Shear rate 16.11.2007
dγ dt Dental Materials - Graham Cross
Shear rate
dγ dt 22
Plasticity: flow of solids • Ductile behaviour of a solid that occurs above a special shear stress threshold called the “yield stress”: τyield • This occurs for many metals and glassy polymers
Shear Stress τ τyield
• Ceramic materials tend to fracture, not yield
Strain ε
Ductility
Like a liquid, plastic flow of solids involves shape change, not volume change 16.11.2007
Dental Materials - Graham Cross
23
Shearing a solid: Plastic flow Energy
solid
Position
sheared solid Energy
Stress, not temperature, increases the energy level 16.11.2007
Position
One line of atoms changes neighbours Dental Materials - Graham Cross
24
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 16.11.2007
Dental Materials - Graham Cross
25
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. 16.11.2007
Stress
Dental Materials - Graham Cross
Strain σ
Time 26
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 – 8th Edition 1998, John F. McCabe, Angus W. G. Walls, Blackwell, Oxford, UK.
16.11.2007
Dental Materials - Graham Cross
Time t 27
[email protected]
16.11.2007
Dental Materials - Graham Cross
1
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 – 6th Edition 1996 Editors Robert G. Craig, William J. O’Brien, John M. Power, Mosby – Year Book, Inc, St. Louis, USA 16.11.2007
Dental Materials - Graham Cross
2
Mechanical properties of materials
• Stress and strain • Elasticity and viscosity: Solids vs. fluids • Rheology and Plasticity • Viscoelasticity 16.11.2007
Dental Materials - Graham Cross
3
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? 16.11.2007
Dental Materials - Graham Cross
4
Stress • Stress is the force per unit area applied to an object:
Force σ= Area
• Units = N/m2 or Pascals (Pa) • Also: 1 bar = 101.3 kPa • 1 MPa = 106 Pa
• Different ways of applying stress, over a surface: Compressive
16.11.2007
Tensile
Dental Materials - Graham Cross
Shear
5
Adhesion • Adhesion may be defined simply as a force interaction between two materials at an interface where they are in . • Failure occurs at a critical stress level Interface must a solely tensile load:
Mechanical 16.11.2007
Chemical Dental Materials - Graham Cross
6
Area of and stress • Adhesive strength depends on true area limited by roughness:
Chemical Adhesion • Rough surfaces mean small 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 16.11.2007
Dental Materials - Graham Cross
7
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:
dl ε= l Different kinds of strain
16.11.2007
Dental Materials - Graham Cross
8
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 – 8th Edition 1998, John F. McCabe, Angus W. G. Walls, Chapter 2.
16.11.2007
Dental Materials - Graham Cross
9
Elasticity • Reversible stretching, compression, or deforming of a body
Stress σ
• In the linear elastic range, the ratio of stress to strain is called a modulus
σ = Eε
εlimit
εlimit = 0.02 Ceramics/Metals = 0.1 Polymer glasses > 5 Some elastomers! 16.11.2007
Strain ε
• Different modulus are defined for different types of deformation: • Young’s modulus • Shear modulus • Bulk modulus
Dental Materials - Graham Cross
10
Elastic Modulus
Before
After
Polystyrene:
16.11.2007
Bulk B=10 GPa
Young’s E=3 GPa
Dental Materials - Graham Cross
Shear G=1 GPa 11
Shear strain • Shear strain γ is a skew: it changes shape, not volume. • Very important when we consider flow.
dy γ≡ h Shear strain rate: h
16.11.2007
d γ dy h dy dt = = dt dt h dγ vy = dt h
Dental Materials - Graham Cross
12
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! 16.11.2007
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
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! 16.11.2007
A liquid changes shape, not volume, freely
Dental Materials - Graham Cross
14
Solid vs. Liquid Energy
solid
Position
(Low Temperature)
Atoms deep in energy well
16.11.2007
Dental Materials - Graham Cross
vacancy
15
Solid vs. Liquid Energy
solid
Position
(Low Temperature)
Atoms deep in energy well
16.11.2007
Dental Materials - Graham Cross
vacancy
16
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! 16.11.2007
Dental Materials - Graham Cross
17
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 16.11.2007
η Units: Pa s (Poise)
Force
Fluid
Viscosity η Pa s
Air
0.00018
Water
0.0089
Mercury
0.015
Honey
100
Glass
1040 (?)
Dental Materials - Graham Cross
18
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.
16.11.2007
Dental Materials - Graham Cross
19
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 16.11.2007
Dental Materials - Graham Cross
20
Viscosity and setting time of pastes Viscosity η
Time t
• Poor rheological properties - no well defined setting time
16.11.2007
Time t
• Ideal rheological properties - long working time - sudden setting time Dental Materials - Graham Cross
Time t • Good rheological properties - long working time - reasonable setting time 21
Shear rate dependent flow Fluids: Instead of a stress vs. strain curve, we plot a stress vs. strain rate curve a) Dilatant
Newtonian linear fluid
τ =η
dγ dt
b) Pseudoplastic (shear thinning)
Shear Stress τ
Shear Stress τ
b
a
Shear rate 16.11.2007
dγ dt Dental Materials - Graham Cross
Shear rate
dγ dt 22
Plasticity: flow of solids • Ductile behaviour of a solid that occurs above a special shear stress threshold called the “yield stress”: τyield • This occurs for many metals and glassy polymers
Shear Stress τ τyield
• Ceramic materials tend to fracture, not yield
Strain ε
Ductility
Like a liquid, plastic flow of solids involves shape change, not volume change 16.11.2007
Dental Materials - Graham Cross
23
Shearing a solid: Plastic flow Energy
solid
Position
sheared solid Energy
Stress, not temperature, increases the energy level 16.11.2007
Position
One line of atoms changes neighbours Dental Materials - Graham Cross
24
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 16.11.2007
Dental Materials - Graham Cross
25
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. 16.11.2007
Stress
Dental Materials - Graham Cross
Strain σ
Time 26
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 – 8th Edition 1998, John F. McCabe, Angus W. G. Walls, Blackwell, Oxford, UK.
16.11.2007
Dental Materials - Graham Cross
Time t 27
Related Documents 3h463d
Dental Materials Lecture - Mechanical Properties 1k5it
October 2021 0
Mechanical Properties Of Materials sf4d
January 2021 0
Physical Properties Of Dental Materials 6jzf
April 2022 0
December 2019 46
Mechanical Properties Of Materials (farsi) 2t5c1r
March 2023 0
Mechanical Properties 4x2338
November 2021 0More Documents from "Fatima Siddiqui" 2c10l
Dental Materials Lecture - Mechanical Properties 1k5it
October 2021 0
Edexcel A2 Physics Notes Unit 5, Topic: Thermal Energy. 396w6e
December 2021 0
Ensayo Capitulo 33 Y 34 Guyton sm2p
September 2020 0
Dental Referral Form 2w352
December 2021 0
4p345f
November 2019 189