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Questions and Answers
What is the SI unit of force?
What is the SI unit of force?
Which of the following correctly describes stress?
Which of the following correctly describes stress?
What kind of stress occurs when two sets of forces are applied towards each other?
What kind of stress occurs when two sets of forces are applied towards each other?
In mechanical properties, how is stress calculated?
In mechanical properties, how is stress calculated?
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Which of the following is not a characteristic of force?
Which of the following is not a characteristic of force?
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What is the primary importance of mechanical properties in dental materials?
What is the primary importance of mechanical properties in dental materials?
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Which unit of stress is most commonly reported?
Which unit of stress is most commonly reported?
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Which term describes materials that show both elastic and viscous behavior?
Which term describes materials that show both elastic and viscous behavior?
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What does yield strength indicate in a material?
What does yield strength indicate in a material?
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Which of the following statements about elastic modulus is correct?
Which of the following statements about elastic modulus is correct?
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Which material property is considered more important than ultimate strength in dental restorative materials?
Which material property is considered more important than ultimate strength in dental restorative materials?
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What characterizes the relationship between stress and strain according to Hook's law?
What characterizes the relationship between stress and strain according to Hook's law?
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How does the composition of a material affect its elastic modulus?
How does the composition of a material affect its elastic modulus?
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What is referred to as fracture strength?
What is referred to as fracture strength?
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Which of the following materials typically exhibits a low elastic modulus?
Which of the following materials typically exhibits a low elastic modulus?
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What happens to the yield strength of materials when subjected to high stresses in dental function?
What happens to the yield strength of materials when subjected to high stresses in dental function?
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What is the primary consequence of using more rigid alloys in dental design?
What is the primary consequence of using more rigid alloys in dental design?
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What property of impression materials facilitates their removal from the mouth?
What property of impression materials facilitates their removal from the mouth?
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What type of stress is characterized by forces acting away from each other in the same straight line?
What type of stress is characterized by forces acting away from each other in the same straight line?
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Which of the following describes ductility in materials?
Which of the following describes ductility in materials?
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What happens to a body that experiences plastic strain?
What happens to a body that experiences plastic strain?
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Which method is NOT commonly used to measure ductility?
Which method is NOT commonly used to measure ductility?
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How does flexibility contrast with rigidity in materials?
How does flexibility contrast with rigidity in materials?
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How is strain (ε) defined in relation to stress?
How is strain (ε) defined in relation to stress?
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What does complex stress refer to?
What does complex stress refer to?
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What impact does rigidity of major connectors in partial dentures have?
What impact does rigidity of major connectors in partial dentures have?
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Which of the following factors contributes to the fracture resistance of a dental filling?
Which of the following factors contributes to the fracture resistance of a dental filling?
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Which statement correctly characterizes the engineering stress-strain curve?
Which statement correctly characterizes the engineering stress-strain curve?
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What type of stress tends to slide atoms against each other?
What type of stress tends to slide atoms against each other?
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In ductility measurement, what does percentage elongation represent?
In ductility measurement, what does percentage elongation represent?
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If a wire with an original length of 2 mm stretches to 2.02 mm, what is the strain expressed as a percentage?
If a wire with an original length of 2 mm stretches to 2.02 mm, what is the strain expressed as a percentage?
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What is the primary function of the Universal Testing Machine in relation to stress and strain?
What is the primary function of the Universal Testing Machine in relation to stress and strain?
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What is the primary advantage of high impact acrylic resin dentures?
What is the primary advantage of high impact acrylic resin dentures?
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How does the presence of glass particles in composite resin affect fracture toughness?
How does the presence of glass particles in composite resin affect fracture toughness?
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Which of the following correctly describes fracture toughness?
Which of the following correctly describes fracture toughness?
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What does the hardness of a material primarily measure?
What does the hardness of a material primarily measure?
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Why should natural teeth not be opposed by porcelain teeth?
Why should natural teeth not be opposed by porcelain teeth?
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What is a significant disadvantage of high hardness in co-cr denture-base materials?
What is a significant disadvantage of high hardness in co-cr denture-base materials?
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What care should denture-wearing patients take to maintain their dentures?
What care should denture-wearing patients take to maintain their dentures?
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How do zirconia particles affect the fracture toughness of porcelain?
How do zirconia particles affect the fracture toughness of porcelain?
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What does the yield strength of a material represent?
What does the yield strength of a material represent?
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Which property describes a material's ability to undergo significant plastic deformation before rupture?
Which property describes a material's ability to undergo significant plastic deformation before rupture?
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In a diametral compression test, what type of stress is primarily induced in the brittle material?
In a diametral compression test, what type of stress is primarily induced in the brittle material?
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What is the purpose of the three-point bending test?
What is the purpose of the three-point bending test?
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Which curve would be expected from a stiff and brittle material during a stress-strain test?
Which curve would be expected from a stiff and brittle material during a stress-strain test?
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What characterizes the resilience of a material?
What characterizes the resilience of a material?
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Which statement describes malleability in materials?
Which statement describes malleability in materials?
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Flexible materials that are tough and strong will exhibit what behavior on a stress-strain curve?
Flexible materials that are tough and strong will exhibit what behavior on a stress-strain curve?
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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.
