Dental Biomaterials I - Mechanical Properties

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Questions and Answers

What is the SI unit of force?

Pound

Mega Pascal

Newton (correct)

Kilogram

Which of the following correctly describes stress?

The amount of force per unit mass

The total force exerted on a material

The deformation of a material under pressure

The internal reaction to an external force (correct)

What kind of stress occurs when two sets of forces are applied towards each other?

Bending stress

Shear stress

Compressive stress (correct)

Tensile stress

In mechanical properties, how is stress calculated?

Stress = Force/Area Signup and view all the answers

Which of the following is not a characteristic of force?

Duration Signup and view all the answers

What is the primary importance of mechanical properties in dental materials?

Their ability to withstand forces during use Signup and view all the answers

Which unit of stress is most commonly reported?

Newton per millimeter square (N/mm²) Signup and view all the answers

Which term describes materials that show both elastic and viscous behavior?

Viscoelastic materials Signup and view all the answers

What does yield strength indicate in a material?

The stress at which deformation begins Signup and view all the answers

Which of the following statements about elastic modulus is correct?

It is defined as the ratio of stress to strain. Signup and view all the answers

Which material property is considered more important than ultimate strength in dental restorative materials?

Yield strength Signup and view all the answers

What characterizes the relationship between stress and strain according to Hook's law?

Stress is directly proportional to strain up to a limit. Signup and view all the answers

How does the composition of a material affect its elastic modulus?

It can significantly influence stiffness. Signup and view all the answers

What is referred to as fracture strength?

The stress at which a material fractures. Signup and view all the answers

Which of the following materials typically exhibits a low elastic modulus?

Elastomers Signup and view all the answers

What happens to the yield strength of materials when subjected to high stresses in dental function?

It needs to be high to avoid permanent deformation. Signup and view all the answers

What is the primary consequence of using more rigid alloys in dental design?

Thinner sections can be used without risk of bending Signup and view all the answers

What property of impression materials facilitates their removal from the mouth?

Flexibility Signup and view all the answers

What type of stress is characterized by forces acting away from each other in the same straight line?

Tensile stress Signup and view all the answers

Which of the following describes ductility in materials?

Capability to be drawn into wires without fracture Signup and view all the answers

What happens to a body that experiences plastic strain?

It maintains a permanent deformation Signup and view all the answers

Which method is NOT commonly used to measure ductility?

Bend test under tension Signup and view all the answers

How does flexibility contrast with rigidity in materials?

Flexible materials deform under slight stress Signup and view all the answers

How is strain (ε) defined in relation to stress?

Change in length per unit length Signup and view all the answers

What does complex stress refer to?

A combination of all types of stresses developing in a structure Signup and view all the answers

What impact does rigidity of major connectors in partial dentures have?

Controls the stability of the entire denture design Signup and view all the answers

Which of the following factors contributes to the fracture resistance of a dental filling?

Thickness of the base under restoration Signup and view all the answers

Which statement correctly characterizes the engineering stress-strain curve?

It uses a fixed original cross-section area during testing. Signup and view all the answers

What type of stress tends to slide atoms against each other?

Shear stress Signup and view all the answers

In ductility measurement, what does percentage elongation represent?

Increase in length relative to original length Signup and view all the answers

If a wire with an original length of 2 mm stretches to 2.02 mm, what is the strain expressed as a percentage?

1% Signup and view all the answers

What is the primary function of the Universal Testing Machine in relation to stress and strain?

It plots the relationship between load and defamation. Signup and view all the answers

What is the primary advantage of high impact acrylic resin dentures?

They are more resistant to fracture upon dropping. Signup and view all the answers

How does the presence of glass particles in composite resin affect fracture toughness?

It increases fracture toughness by stopping crack propagation. Signup and view all the answers

Which of the following correctly describes fracture toughness?

It indicates a material's resistance to catastrophic flaw propagation under stress. Signup and view all the answers

What does the hardness of a material primarily measure?

The resistance to penetration or scratching. Signup and view all the answers

Why should natural teeth not be opposed by porcelain teeth?

Porcelain teeth may cause abrasion to the softer natural teeth. Signup and view all the answers

What is a significant disadvantage of high hardness in co-cr denture-base materials?

They are difficult to finish and polish. Signup and view all the answers

What care should denture-wearing patients take to maintain their dentures?

Avoid soaking dentures in hot water. Signup and view all the answers

How do zirconia particles affect the fracture toughness of porcelain?

They absorb energy needed for crack propagation. Signup and view all the answers

What does the yield strength of a material represent?

The maximum stress a material can withstand without permanent deformation Signup and view all the answers

Which property describes a material's ability to undergo significant plastic deformation before rupture?

Ductility Signup and view all the answers

In a diametral compression test, what type of stress is primarily induced in the brittle material?

Tensile stress Signup and view all the answers

What is the purpose of the three-point bending test?

To determine the flexural strength of materials Signup and view all the answers

Which curve would be expected from a stiff and brittle material during a stress-strain test?

A steep slope with a sharp drop Signup and view all the answers

What characterizes the resilience of a material?

The capacity to absorb energy when deformed and not permanently harmed Signup and view all the answers

Which statement describes malleability in materials?

Ability to be bent or shaped without breaking Signup and view all the answers

Flexible materials that are tough and strong will exhibit what behavior on a stress-strain curve?

They will show considerable plastic deformation before breaking Signup and view all the answers

Study Notes

Dental Biomaterials I - Mechanical Properties

Mechanical properties are a group of properties that describe how materials react to forces.
Restorative materials must withstand forces during fabrication and mastication.
This chapter covers elastic, plastic, and viscoelastic deformation, plus mechanical quantities like force, stress, strain, strength, toughness, hardness, friction, and wear. These are considered in the context of how dental materials perform in the oral environment.

I - Force

Force is the external action that produces or tends to produce motion of a body
Units (kg, lb, N) - SI unit is Newton (N)
A force is defined by magnitude (amount), direction of application, and point of application.
Forces can be static or dynamic.

II - Stress

Stress is the internal reaction to an external force. It's equal in intensity and opposite in direction to the external force.
Stress is calculated as force per unit area (σ = F/A)
Common units include Kg/cm², lb/in², N/mm² (MPa), and N/m² (MN/m²) - MPa being common.

Types of Stress

Compressive stress: Two forces acting towards each other, shortening the material.
Tensile stress: Two forces acting away from each other, stretching or elongating the material.
Shear stress: Two forces acting parallel to each other but not in the same line, causing atoms to slide against each other.

III - Deformation and Strain

Strain is the change in length per unit length of a body when a stress is applied.
Strain is unitless (ε = ΔL/L)
Strain can be elastic (temporary) or plastic (permanent)

IV - Stress-Strain Curve

This curve plots stress against strain, showing how a material responds to increasing load.
It's measured using a Universal Testing Machine.

False and True Curves

False/Engineering Curve: The cross-sectional area of the specimen remains constant.
True Curve: The cross-sectional area of the specimen changes during the test.

Relation Between Stress and Strain (Hook's Law)

Stress is directly proportional to strain up to the proportional limit.
Hook's Law describes this linear relationship.

Terms of the Stress-Strain Curve

Proportional Limit (P.L.): The maximum stress a material can withstand without deviation from proportionality between stress and strain.
Elastic Limit (E.L.): The maximum stress a material can sustain without permanent deformation. Normally very close to the proportional limit.
Elastic, and plastic regions are the portions of the stress strain curve associated with the proportional limit and elastic limit.
Terms such as ultimate tensile strength, yield strength, and fracture strength are relevant to the stress strain curve.

Yield Strength (YS)

Yield strength is the stress level where the material begins to deform.
This property is important in dental restorative materials to minimize permanent deformation under normal function pressures.
Bridge materials should have a yield strength higher than the expected chewing force.

Ultimate Strength

The maximum stress a material can withstand before failure.
Yield strength is usually more critical than the ultimate strength because anything beyond that will cause permanent deformation.

Fracture Strength

The stress level where the material fractures.

Elastic Modulus (Young's Modulus)

Measures a material's stiffness or rigidity within its elastic range.
The ratio of stress to strain.
Common units are MPa or GPa

Elastic Modulus (Young's Modulus) - Material Values

Several Types of dental materials have different elastic moduli and elastic strength values.
Examples include: Enamel, Dentin, Gold Alloys, etc.
Elastomers and polymers have low elastic modulus values.

Elastic Modulus (Young's Modulus) - Significance

The stronger the interatomic or intermolecular forces, the higher the elastic modulus and the stiffer the material.
The property generally depends on material composition, not heat treatment or mechanical processing.
The rigidity of connectors in dentures affect the stability of the whole design. Stiffer alloys allow for thinner components.

II - Strain Terms

Flexibility: The ease in which a material or structure bends, with slight stress and large elastic strain.
Opposite to stiff or rigid materials.
Important in impression materials due to ease of removal from the mouth.

Ductility and Malleability

Ductility is a material's ability to deform permanently under tensile force without breaking.
Malleability is a material's ability to deform permanently under compressive force without breaking (hammered or rolled into thin sheets).

Brittleness

Materials that fracture at or near their proportional limit without significant deformation.
Many dental materials (porcelain, cements, amalgam) are brittle.
Dental amalgam has a high compressive strength compared to its tensile strength

III - Energy terms (Resilience and Toughness)

Resilience: The energy needed to permanently deform a material to the proportional limit.
Toughness: The total energy absorbed by the material before fracturing.
Acrylic resin often is more resilient than porcelain, absorbing more energy and transmitting less to the jaw bone.

Resilient and Tough Materials

Resilient Material: High ability to absorb energy without permanent deformation.
Tough Material: High ability to absorb energy without fracturing.

Summary of Stress-Strain Curve Terms

Stiff/rigid materials have high strength/low ductility, and high toughness.
Flexible materials have low strength/high ductility, and low toughness

Different Stress-Strain Graphs

Various types of stress-strain graphs illustrate different material behaviors.

Evaluation Sheets

Evaluation sheets address specific definitions and applications of mechanical properties.

Diametral Compression Test

Used to measure the tensile strength of brittle dental materials (cements, amalgam, investments).

Transverse Bending Test

Measures the flexural strength of materials loaded in a three-point bending configuration.
The top of the beam experiences compressive stress, the bottom tensile stress, and shear stress at the supports.

Fatigue

Failure of a structure subjected to repeated loading below its proportional limit. It causes alternating/cyclic stresses.
The S-N curve plots stress against number of cycles to fracture.
Fatigue limit/endurance limit is the stress that can be repeatedly applied—without fracture. It is determined from the S-N curve.

Importance of Mechanical Properties in Dental Materials

Understanding and applying mechanical properties help in choosing the right materials for specific dental applications.
Designing functional and durable dental restorations.

Impact Strength

Measures a material's resistance to fracture caused by sudden impact.
Important for complete dentures to prevent catastrophic failure (falling).

Fracture Toughness

Measure of a material's resistance to catastrophic crack propagation.
Presence of filler particles, such as glass or zirconia, typically increases fracture toughness

Surface Mechanical Properties (Hardness, Wear, and Friction)

Hardness: Resistance to penetration, indentation, or scratching—measured using different methods.
Wear: Material loss due to mechanical action. This can be undesirable except finishing and polishing processes, where it's desirable.
Friction: Resistance to motion of one material over another—caused by molecular bonding at surfaces. Useful for dental implants, to reduce motion with surrounding tissues.
Denture-wearing patients must be cautioned on proper brushing techniques. Hard brushes can lower surface hardness and cause wear.

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Description

Explore the mechanical properties that influence dental materials in the oral environment. This quiz covers essential concepts such as force, stress, and the behavior of materials under various conditions. Test your understanding of how restorative materials perform during fabrication and mastication.