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a g e

Aya Ayman

Rahaf N awafleh

Shayma Albaloushi

Ala’a Al - Haddad

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“In the first 2 lectures we will learn terms that we need to understand this course,

as well as we will need them to understand which material to use with our

patients”.

Why do we visit the dentist?

It may be for caries treatment, veneers, surgery, tooth implantation,

Dental restoration, Dental fillings, or something related to Orthodontics.

So, it differs from medicine in that we need a lot of materials in order to

do the treatment, more than the materials used in general medicine

Course aims:

➢ To introduce the basics of Material Science.

➢ To understand the characteristics and properties of Materials used

in Dentistry.

➢ To familiarize the students with some Dental Materials used

regularly in dental practice.

Silver amalgam fillings

Composite fillings

Veneers

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Course references:

The Doctor suggests having the book as a reference. Any one of these

will be fine, Applied Dental Materials (it links our study with clinical cases).

*Each chapter has a reference.

For these 2 lectures Craig’s

Restorative Dental Materials is our

reference.

Course outline:

Chemical composition, Physical and Mechanical properties of dental

materials will be discussed during this course.

Dental Material Science

Dental materials consist of the Chemistry, physics

and the biology of each material used in the practice

of dentistry.

principally involved in:

Restorative Dentistry,

Prosthodontics, Pedodontics طب اسنان األطفال, and

Orthodontics علم تقويم األسنان.

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Restorative dentistry

• Filling materials (e.g. Amalgam, Composite)

• Bonding materials & Cements.

Prosthodontics (related to dentures)

• Impression materials (e.g. Alginate, Silicon, Compound)

• Denture materials (e.g. acrylic resin, Cobalt Chromium)

• Casting and Gypsum materials (e.g. dental stone, Plaster of Paris)

• Waxes

We study Dental Materials in order to

be able to transform bad oral hygiene

to that smile with a healthy oral cavity.

Aspects of dental materials:

1. Composition.

2. Properties.

3. Interaction with Human body.

For example, we use Cements in dentistry, but these cements are different than

the construction cement

So, we use biocompatible materials that do not have an interaction with our

body

“Understanding the material behavior → Proper Selection”

Like if you have a boxer patient, you need to use a hard filling material, so it can

handle the high forces that he is exposed to.

We don’t have one choice we have multiple choices, and you have to pick the most

suitable one according to the patient’s situation

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Objectives:

● To differentiate between bulk properties and surface properties.

● To define terms related to mechanical properties of materials.

● To dissect aspects related to stress-strain curves of DMs.

● To give example of applications of mechanical properties in the contemporary

dental practice.

Two types of mechanical properties:

1. Bulk properties: When we apply a force, the whole thickness

of the material will be affected. (Stress and Strain)

2. Surface properties: When we apply a force, only the

surface will be affected.

Bulk properties :

Stress & Strain

a. Most applications of materials in dentistry have a minimum mechanical

property requirement.

“the filling materials should be able to bear more than the pressure applied on it

(biting force of the patient) otherwise it will break or undergo deformation

(because of its elasticity)”.

✓ Ex: the normal biting force is 500N, a patient may have it as 800N, so I

have to use a material that can withstand this force.

b. Certain materials should be sufficiently strong to withstand biting forces

without fracture.

Like in amalgam filling it should have a certain strength, and if it was lower

than the usual it will break. It should also have a certain width that can handle

the biting forces .so, it can be durable in the patient’s mouth

c. Others should be rigid enough to maintain their shape under load.

Meaning that if the filling material that I used was changeable (can change its

shape when exposed to a force then back to its normal shape again after removing

the force) and we don’t want that to happen because then it won’t be functional,

so we need it to be rigid not flexible.

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A) Stress

When an external force is applied to a body or specimen of material under test, an

internal force, equal in magnitude but opposite in direction, is set up in the body.

This internal reaction is called Stress (S or σ).

● It is normally defined in terms of mechanical stress, which is the force

divided by the perpendicular cross-sectional area over which the force is

applied.

● Stress = Force (N)/ Area (m2)

● Units: Pascal (Pa), MPa

✓ Ex: I have 2 rubbers:

the thicker one can hold more load.

1cm

➢ Axial Stress: forces are parallel with the long axis of the material.

1. Compression Stress: forces compress internally toward each other,

perpendicular to the axis of the object.

2. Tensile Stress: forces that are applied on the object in opposite directions,

along the axis of the object. (elongation)

1mm

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➢ Non-Axial Stress: applied forces are not parallel to the long axis of the

material.

1. Shear Stress: forces in opposite directions (different planes) but not along the axis

of the object, as one above and the other below the axis.

(displacement)

2. Torsion Stress: forces in opposite directions at the edges to twist the object.

we use it a lot with “Withdrawal of nerve using files, each file has a certain

torsion stress value if the force that

you are applying on the tooth is higher than it (the file’s

torsion stress), it may cause the file to break inside the

tooth”

3. Bending or Flexural: Two opposite forces applied

perpendicularly to the longitudinal axis at the same side.

“If we have a patient who lost a tooth then we will make a

bridge between the two adjacent teeth, the stress which is in the middle of the bridge

(above the empty area, which is not a supported area) is the flexural stress (on the

fulcrums) “. (يعني انا بضغط من االطراف بنفس االتجاه ف بنحني الجسم من النص)

B) Strain مطاوعة

Each type of stress is capable of producing a corresponding deformation in a

material’s bulk (deformation or change in the geometry). Described as:

● A change in length per unit length when stress is applied

● Strain = Change in length (Deformation)/Original length

● Strain (ε)= ΔL/L0, unitless.

Ex1: if the length was 10 cm then it becomes 12 cm, so the strain = 2/10

Ex2: the changing in shape with tension:

Width: 1

Length: 10

Width: 0.5

Length: 20

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Stress – Strain Curve (S-S curve):

● Used to characterize DMs.

● Independent of the Geometry of the tested object,

For example, Fracture toughness of Amalgam = 500 (only one value according

to the s-s curve, even if we change the thickness).

because in stress we divide by surface area and in strain we divide by original

length, so in each point of the curve we divide by the same dimensions)

volume), as if we’re excluding Geometry. ( واحنا بنستخدم الطول لحساب 2أطول من الجسم 1,,, مثال عندي الجسم ةبس باحجام مختلف املادةيعني هال ملا نجيب نفس

شان هيك بتضل ع) stressلحساب ال املساحة( واحنا بنستخدم 2اكثر من الجسم 1)بالتالي اكيد مساحة سطح الجسم strainال

. ( ةيعني الحجم ما بأثر على القيم ةبينهم ثابتالنسبة

● Stress-Strain Vs Force Deformation curve

✓ Stress-Strain curve: we will always get the same curve for same-type

material, despite its different geometry.

للمادة الوحدة ""يعني شو ما غيرت بالحجم بتضل العالقة بينهم نفسها

✓ Force Deformation curve: we can’t compare different DMs, because we’ll

get more than one curve for the same-type material depending on their

geometry.

● Engineering Stress-Strain Vs True Stress-Strain curve

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Resilience: 1.The resistance of a material to permanent deformation.

2. Indicates the amount of energy necessary to deform

the material to the proportional limit.

➢ A → Proportional Limit

➢ Elastic region

➢ Plastic region

Proportional limit:

(The maximum point when the strain and stress

relationship is linear, always in elastic behavior)

The greatest stress that a material will sustain without a deviation from the

proportionality of stress to strain, below which no permanent deformation happens.

Ex: when we apply 10 N a change of 1 mm occurs, 20 N → 2 mm, 30 N → 3 mm

but if we apply 60 N → 5.25 mm, when it’s not linear anymore.

➢ Elastic region: the material is elastic and can get back to its original shape, the

region below the curve (up to point A)

➢ Plastic region: permanent deformation occurs, the region below the nonlinear

curve (after point A)

(Elastic # Plastic)

EX: a piece of rubber, it’s Proportional limit = 15 N, its length = 10 mm, if the load

is less than 15 N it will go back to its original shape (elastic region), but if the load is more

than the limit the length will increase (plastic region)

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Elastic Limit:

before the elastic limit it gets back to the original shape (elastic behavior)

but after elastic limit it doesn’t (plastic deformation).

1. The maximum stress that a material will withstand without

permanent deformation.

2. Super-elastic Materials: Nonlinear elastic behavior.

So, is there any difference between proportional limit and elastic limit?

- Proportional limit indicates the end of linearity. (Point A)

- Elastic limit indicates the end of elasticity. (Point B)

- they may be the same point or different points

- From the point of proportional limit until the elastic limit the material shows

elasticity but not linearity.

Elastic behavior: forces in the same plane, the molecules are still together

(cohesion) so it goes back to its original shape after removing the force, but

with plastic behavior the molecules separate from each other so the shape

changes.

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Yield Strength (YS) OR Yield Stress: “important”

1. The stress at which a material:

➢ begins to function in plastic manner. (plastic deformation)

➢ exhibits a defined amount of permanent strain.

2. YS indicates a degree of permanent deformation (Percent offset)

which is usually 0.2%. (the certain value of the YS = 0.2% of

the maximum deformation)

3. YS indicates a functional failure (initiation of failure)!!!

Ex: YS of Amalgam = 800N, the patient’s biting force may reach

800N so the Amalgam will start to crack and change in shape then

it will become nonfunctional anymore.

4. Example of Dental applications:

➢ Crown fabricating materials (YS> biting force)

➢ Bending orthodontic wires (Bending force> YS)

What is the difference between Yield Strength and Elastic Limit?

Elastic Limit → at its end the deformation starts (end of elastic, beginning of plastic)

Yield Strength → plastic deformation already happened (with a value = 0.2 %)

How to determine Yield Strength?

1. Along the strain axis (x axis),

the percent offset (0.2%) is

given.

2. You calculate the whole strain.

3. you draw a line from the 0.2%

of the strain to the curve (this

line should be parallel to the

proportional limit slope).

4. The parallel line meets with the

curve at a point that indicates

the YS (point 3 on the curve).

1. Proportional limit

2. Elastic Limit

3. Yield St and

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How could I know the YS point?

➢ It’s in the plastic behavior

➢ It’s with the offset

Elastic Modulus (E): “important”

Young’s modulus OR modulus of elasticity.

➢ Is a measure of elasticity of the material (How stiff the material is in the

elastic range)

✓ Ex: if you expand a piece of rubber it will go back to its original

shape because of its elasticity, but a piece of wood won’t, so they

have different modulus of elasticity.

➢ When the modulus of elasticity the stiffness

High E, less elasticity, more stiffness

✓ Ex: If the elastic modulus of material A =10, material L=20

material A which has a lower modulus of elasticity, is more

elastic so it is less stiff, while material L which has a higher

modulus of elasticity is less elastic so it’s stiffer.

How to determine Elastic modulus? The Slope of stress-stain up

to the proportional limit linear portion (slope increases) indicates an

increase of the modulus of elasticity.

➢ Elastic modulus =Stress/Strain

➢ The slope of the curve

➢ Unit:

Pascal

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➢ Amalgam is a good choice if we are looking to replace Enamel but not so if

we want to replace Dentine, why?

Because when you try to replace something, you choose a material close to

the original material properties and characteristics.

➢ We can use cobalt chromium for making dentures because it’s very stiff

and can reserve its shape

Material A has a higher

modulus of elasticity

(slope A > slope B)

thus material A is less

elastic so it’s stiffer and

rigid, while Material B

is more elastic.

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Poisson’s ratio

The ratio of lateral to axial strain within the elastic

range.

n = - e trans / e longitudinal

Range: -1 - 0.5 (the range for all materials with elasticity)

For Enamel, dentine, amalgam, composite ~ 0.3) the acceptable value of the

dentistry materials)

Under tensile loading, there is a reduction in cross section (necking). Green

arrows

Under compressive loading, there is an increase in cross section. Black arrows

(He studied the relationship between the deformation of length and deformation of

width, when we expand a piece of rubber and apply certain stress “longitudinal

train” on it, so the length increases, cross section decreases, the result was:

transverse relationship, we put a )–( sign because there is an increasing value and a

decreasing value). (ي بقللال)و التغير في العرض (بزيد)اللي يعني بقيس النسبة بين التغير في الطول

Ductility and Malleability

Ductility: the ability of a material to be plastically deformed and

shaped into wire by means of tension.

Meaning that I have a material that I applied tensional stress on it to do a

plastic deformation to be able to change its shape like a wire

If the material is not ductile (brittle), then it will have lower

deformation so it will break easily.

due to tension strength.

Rubber →if I applied a tension stress it will elongate as long as it’s in the elastic range, once

it becomes in the plastic range and I keep pull it may break (elastic doesn’t mean ductile)

Cobber → not elastic but ductile

المادةيعني انا عندي جسم بضل اشد فيه وطوله بصير اكثر لحتى يتحول لشي زي السلك من الوسط وهاي (

) اعلىيعني بكون عندها القدرة انها ما تنكسر وتتحمل قيمة شد ductileبنسميها

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Malleability: the ability of a material to deform under pressure (compressive stress)

and hammered or rolled into thin sheets without fracturing.

Example of Ductile and Malleable dental

materials:

● Gold (the highest Ductility) and silver

● Metal alloys.

(Which were mainly used for fillings, crowns and bridges).

Ductile Vs Brittle Materials (like a piece of biscuits):

C is the most ductile (plastic deformation)

A is the most brittle (breakable and no plastic deformation)

Notice that the Brittle-material curve has NO PLASTIC REGION, up to

elastic limit and then it’ll break, whereas the Ductile-material curve

shows plastic deformation, up to the elastic limit then it will Continue.

" ❤..قل للذين فوضوا أمرهم الى لله أنه الله لن يخذلهم أبدا "