2 - Failure Of Materials Flashcards by Sana H

Materials in the mouth
How are restorative materials subjected to a harsh oral environment?

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Mechanical forces
What do occlusal forces range from?
Force of first and second molars?
Force of Incisal ?

Maximum occlusal forces may range from 200 -3500 N

First & second molars~ 400-800N

Incisal forces~ 150-300N

These forces also increase in growing children

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What is the tooth’s structure?

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Describe enamels structure

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Describe dentines structure

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What should the features of Ideal Restorative Materials be?

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What are the desired properties of restorative materials ?

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Failure of materials
Why do materials fail?

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Do dental restorations last forever?

Have limited lifespan

Replacement of failed restorations constitutes a large part of operative work in dental practice

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Disadvantage of dental restorations?

Replacing a restoration may lead to an increase in cavity size

Weaken the tooth

Limited lifespan

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Mechanical Properties of Biomaterials
What are the different types of forces a material experiences in the mouth?

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Mechanical Properties of Biomaterials
What does compression result in

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Why might physical failure of a material occur?

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Mechanical properties of biomaterials
What forces are veneers of porcelain subject to

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What do we use to describe mechanical properties of materials?

Stress strain relationship

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What is stress?

Stress: When an external force is applied on a test specimen an internal force, equal in magnitude but opposite in direction is set up in the body

• STRESS= FORCE/AREA UNITS= Pascal (Pa)

(Textbook says area is cross sectional area?)

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What is strain?

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How do we test dental materials mechanical properties using stress strain relationship

We apply load at a uniform rate and deformation should also occur at a uniform rate

Can use universal testing machine to determine these mechanical properties

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Stress strain curves

1) what do they show?

2) what does the straight line region demonstrate?

3) what does the gradient of the straight line region demonstrate?

4) what does the region under the straight line demonstrate?

5) What is the yield point?

6) what does region under curved part of graph indicate?

7) how does failure point compare in a more brittle material?

8) what is elastic limit ?

9) what is yield stress?

10) what is ultimate stress?

11) what is ultimate strain?

1) demonstrate relationship between stress and strain

2) straight line - linear relationship between stress and strain. As strain increases, stress increases proportionally

3) gradient of the straight line = elastic modulus of material

4) area under straight line = elastic region
Materials in this region will recover from any strain and no change in dimension
(Elastic deformation occurs)

5) yield point = also known as elastic limit . Point beyond which material will experience permanent deformation and change in dimension
On the graph, point the graph is no longer a straight line

6) area under curved - plastic region
Here materials suffer permanent change in dimension. Material will deform and extend until it fails - failure point (End of graph )
Plastic deformation occurs
Has exceeded elastic limit

7) more brittle materials - failure point is closer to yield point

8) elastic limit is the maximum stress a material can withstand without permanent deformation

9) calculate by finding yield point and then going across to y axis and is that value of stress

10) calculate by finding failure point and going across to y axis and is that value of stress

11) calculate by finding failure point and going down to x axis and is that value of strain

Hookes law ?
Young modulus?
Link?

Elastic deformation
Elastic deformation is represented by which part of the stress strain curve?

What is Young’s modulus?

What is proof stress?

Elastic deformation - straight line part

Young’s modulus - gradient of straight line part of graph
It is a measure of stiffness of a material

Proof stress = Point of which a particular degree of permanent stress occurs in the sample, also called offset yield stress
Stress needed to create typically 0.1-0.2% plastic deformation

Elastic deformation

Plastic deformation

What is Youngs Modulus / Elastic modulus?

Measure of stiffness of a material within the elastic range (the linear portion of the curve) - KEY DEFINITON It is also a mechanical property that measures the stiffness of a solid material Defines relationship between stress and strain

What is elastic modulus determined by? What does stiffness depend on? What do we need to know about a material to calculate its stiffness?

Which of these graphs is strongest? Which is most brittle? Which is most ductile? Which is most stiffest?

Why is it important to know the yield point?

Elastic modulus comparisons

Tensile strength What is tensile strength ?

Maximum amount of tensile stress a material/specimen can withstand before failure - either deformation or breaks if brittle Ability of a material to resist a force that tends to pull it apart

How do we test tensile strength ?

Test specimen placed in universal testing machine Uniform load applied to pull apart and tensile strength can be determined

Is it easy to determine tensile strength?

Compressive strength What is compressive strength? How do we determine compressive strength?

Maximum compressive load specimen can withstand without undergoing failure (what failure means was mentioned on prev fc - deformation or breakage) Determined using cylindrical specimens with dimensions in accordance to ISO or ASTM standards

Is tensile test easy to carry out on brittle material?

FROM TEXTBOOK NOT LECTURE For brittle materials tensile test is difficult to carry out and results show high degree of scatter An alternative is compression test - more easily performed on brittle materials

What happens to brittle materials when maximum compressive strength is exceeded? What happens to ductile materials when maximum compressive strength is exceeded?

Compressive strength - Amalgam failure?

What is true stress? Do compressive strengths calculated from equations yield accurate results ? Why / why not? Do ceramics have higher compressive strength or tensile strength? Do composites have higher compressive strength or tensile strength ?

What is diametrical compressive strength also known as Indirect Tensile strength ? How tested? What is it widely used for and why? Why is it widely used?

It is a property for characterising dental composite restorations Cylindrical specimen loaded onto universal testing machine BUT axis is changed , cylinder turned on axis and indirect tensile strength Widely used especially for dental composites as you can prepare small specimens

How do we test flexural strength ? What is flexural strength of a material? What are flexural forces? Is high or low flexural strength desired and why or why not?

BENDING TEST ( or flexure test ) USED TO TEST FLEXURAL STRENGTH Flexural strength - Ability of a material to resist deformation under load For materials that deform significantly but do not break, the load is yield typically measured at 5 percent deformation of outer surface Flexural forces are result of forces generated in clinical situations and dental materials need to withstand repeated flexing, twisting and bending High flexural strength is desirable as once materials are under action of chewing stresses may induce permanent deformation

What is the difference between flexure test and bend test?

Difference between flexure test and bend test is between type of material used and info we have obtained from them Flexure test designed to measure bend strength of brittle material Bend test designed to measure crack resistance of ductile material

How is bending test carried out?

Two types of bending test: Centre loading test / 3 point test OR 4 point bending test Centre loading test - specimen supported by 2 spherical supports. Centre point loading and bending is measured 4 point bending test - 2 supports but instead of loading at the centre using one knife edge, 2 knife edges used to bend / load specimen Surface defects become more apparent with this test

Formula for bending strength - don’t need to memorise but do need to understand

Would test bending in a bridge

What is hardness? Why is hardness important? How can hardness measurement be defined ?

Resistance to permanent surface indentation Sufficient hardness ensures restorations are resistance to surface scratching during mastication or abrasion Important in dentistry as it governs ease of cutting, polishing, resistance to abrasion in service Hardness measurement can be defined as macro, micro or nano- scale according to the forces applied and displacements obtained .

How is hardness measured ?

Vickers hardness of materials?

Vickers indentations near and across the Tooth DEJ