dh32, ch. 2 Properties of Materials

also important

- color and optical qualities, - maintaining dimensions during procedures

factors affecting dental materials in oral cavity

- constant exposure to saliva, - temperature changes, - chemical changes in oral fluids, - various and changing mechanical forces due to occlusion, - inherent properties/characteristics of the materials themselves

considerations regarding dental materials

- correct storage prior to use, - notation of expiration dates, - following manufacturer directions, - correct environmental factors at time of placement of materials

selection of materials should be influenced by

- effect on oral tissues and - by possible toxic effects if ingested

critical physical properties for various types of dental materials has led to minimum standards or specifications to be developed because

1. materials used to replace portions of teeth are exposed to attack by oral environment and subjected to biting forces, 2. restorative materials are cleansed and polished by various procedures,

26. A dental material gave the following stress-strain curve when tested in tension. Which portions of the curve represent the following properties?

??

14. Which of the following reasons explains why proportional limit and yield strength are important to a dental material? A restoration can be a clinical failure when a significant amount of tarnish occurs. A restoration can be a clinical failure when a significant amount of wettability occurs even though the material does not fracture. A restoration can be a clinical failure when a significant amount of permanent deformation occurs even though the material does not fracture. All of the above

A restoration can be a clinical failure when a significant amount of permanent deformation occurs even though the material does not fracture. Tarnish is a surface reaction of metals. Wettability is a measure of the affinity of a liquid for a solid.

13. The stress at which a material no longer functions as an elastic solid is which of the following? Yield strength Proportional limit Ultimate strength A and B

Both A and B Yield strength and proportional limit are measures of the stress allowed before permanent deformation. Ultimate strength is the stress at which fracture occurs.

4. Materials with high thermal conductivity values are good conductors of heat and cold. Metals have higher conducting values than polymers and ceramic.

Both statements true. Materials with high thermal conductivity values are good conductors of heat and cold. Metals have higher conducting values than polymers and ceramic.

12. The elastic modulus is a measure of the stiffness of a material. Impression materials should have a low elastic modulus value so that they can be readily removed from the mouth.

Both true The elastic modulus is equal to the ratio of the stress to the strain in the linear or elastic portion of the stress-strain curve. The elastic modulus is a measure of the stiffness of a material, and high numbers are not unusual for this property. Because large deflections under stress are not desired, low values are needed for elastic impression materials so that they can be readily removed from the mouth.

5. Which of the following dental materials are considered to have low thermal conductivity properties and would be ineffective as thermal insulators? Cavity varnishes Zinc phosphate cement Zinc oxide-eugenol cement Gold alloys

Cavity varnishes and liners have low thermal conductivity properties and are used in thin layers so thin that they are ineffective as thermal insulators. Cements such as zinc phosphate cement have low thermal conductivities properties but are effective as thermal insulators. Gold alloys exhibit high thermal conductivity properties.

8. Which of the following terms refers to the dissolution of metals in the mouth? Galvanism Corrosion Thermal conductivity Tarnish

Corrosion is the dissolution of metals in the mouth.

3. Which of the following restorative dental materials has an unusual percolation characteristic? Gold alloys Amalgam Composites Ceramics

Dental amalgam is unusual in that percolation decreases with time after insertion. Gold alloys and ceramics do not present percolation issues. Composites, if bonded correctly, could result in stress at the interface due to the difference in the coefficient of expansion, which could lead to failure of the bond over time.

1. Of the following, which describes dimensional change? A measure of how much a material expands The measure of thermal conductivity The percent shrinkage or expansion of a material The generation of electrical currents

Dimensional change is the percent shrinkage or expansion of a material. Thermal conductivity measures how much heat and cold are transmitted. The generation of electrical currents is galvanism.

16. Ductility is the measurement of which of the following? Brittleness Moldability Ability to be hammered into thin sheets Shearing stress

Ductility measures elongation, indicating the amount of plastic strain that can occur before the material fractures, indicating the brittleness of the material. Malleability refers to the ability of a material to be hammered in to thin sheets. Shear stress is the properties of one material in one plane to be forced across another.

19. Of the following which term is used to measure the energy absorbed at high rates of strain? Static properties Dynamic properties Dynamic resilience Dynamic modulus

Dynamic resilience measures the energy absorbed at high rates of strain, such as from a blow to an athletic mouth protector. Static properties are properties described as having a relatively slow rate of application of the load. Dynamic properties are properties described as having extremely high rates of loading, such as from an impact. Dynamic modulus is a measure of the stiffness of the material at a high rate of strain and is important for mouth protector materials.

6. Of the following terms, which refers to the generation of electrical current resulting from the presence of dissimilar metals in the mouth? Galvanism Corrosion Thermal conductivity Tarnish

Galvanism is the generation of electrical current that results from the presence of dissimilar metals in the mouth. Corrosion is an electrochemical reaction of material, usually metallic, which results in the release of ions from the material, the surface or internal destruction of the material, and the formation of new reaction products. Thermal conductivity is the quantity of heat transferred per second across a unit area (cm2) and unit length (cm) when the temperature difference along the length is 1° C/cm. Tarnish is corrosion of material restricted to its surface, usually resulting in a discoloration of the surface.

18. Which of the following terms refers to the resistance of a material to indentation? Resilience Hardness Toughness Modulus

Hardness is the resistance of a material to indentation. Resilience measures the energy absorbed at high rates of strain such as from a blow to an athletic mouth protector. Toughness measures the energy needed to fracture a material. The modulus is a measure of the stiffness of a material.

2. It is important to maintain dimensions during dental procedures for which of the following reasons? Aesthetics To preserve accuracy of dental restorations To prevent chemical reactions All of the above

Maintaining dimensions during dental procedures is important in the accuracy of dental restorations. Aesthetics are determined by other factors. Maintaining dimensions will not prevent a chemical reaction from occurring.

10. Of the following terms, which one refers to the ratio of the force to the area of distribution? Strain Stress Shear Flexure

Stress is the ratio of the force to the area of distribution. Strain is the change in length per unit length of a material produced by stress. Shear and flexure are two types of stresses.

11. What is the term used to describe the force that results when pulling materials apart? Compressive stress Tensile stress Shear stress Flexure

Tensile stress is the term used to describe the force that results when pulling materials apart. Compressive stress occurs when a material is squeezed together. Shear stress occurs when one portion of the material at one plane is forced to slide by another portion. Flexure occurs when a material is bent.

17. The energy required to deform a material permanently is a criterion of its ___________. The energy necessary to fracture a material is a measure of its ___________. compression; ductility malleability; hardness toughness; resilience resilience; toughness

The energy required to deform a material permanently is a criterion of its resilience. The energy necessary to fracture a material is a measure of its toughness. Malleability refers to the ability of a material to be hammered in to thin sheets. Hardness is the resistance of a material to indention. Toughness measures the energy needed to fracture a material. Resilience measures the energy absorbed at high rates of strain such as from a blow to an athletic mouth protector.

9. Hydrophilic describes a solid that is wetted readily by a liquid. Hydrophilic describes poor wetting of a solid by a liquid.

The first statement is true; the second statement is false. Hydrophilic describes a solid that is wetted readily, not poorly, by a liquid. Hydrophobic describes poor wetting of a solid by a liquid.

15. The point at which a material fractures or ruptures is which of the following? Tensile strength Ultimate strength Shear strength Compressive strength

The stress at which fracture or rupture occurs is called the ultimate strength. Tensile strength is the fracture occurring from tensile stress. Shear strength is the fracture occurring from shear stress. Compressive strength is the fracture occurring from compressive stress.

3. Which of the following statements is true regarding thermal dimensional change of restorative dental materials? A differential expansion of dental material and tooth may result in leakage. Thermal expansion of the restorative material usually does match that of the tooth structure. The thermal coefficient of expansion for a solid generally decreases as the temperature is increased. The thermal coefficient of expansion is uniform throughout the entire temperature range.

The thermal expansion of the restorative material usually does not match that of the tooth structure; a differential expansion occurs that may result in leakage of oral fluids between the restoration and the tooth. The thermal coefficient of expansion for a solid generally increases as the temperature is increased. The thermal coefficient of expansion is not uniform throughout the entire temperature range.

linear thermal coefficient of expansion of a material

a measure of how much it expands per unit of length if heated 1 degree higher; not uniform throughout entire temp range; usually higher for liquids than for solids

wettability

a measure of the affinity of a liquid for a solid as indicated by spreading of a drop; ex: wetting of tooth enamel by pit and fissure sealants

restoration can be classified as a clinical failure when

a significant amount of permanent deformation takes place even though the material does not fracture

18. Which of the following is/are test(s) for measuring hardness? Knoop, toughness, yield strength, resilence

a. Knoop

8. Which could lead to corrosion in restorative dentistry?

a. a gold alloy contaminated with iron in lab, b. a chemical attack of a metal by components in food or saliva, d. adjacent restorations constructed of dissimilar metals

7. Which of the following is/are examples of galvanism in restorative dentistry?

a. aluminum foil from a baked potato becomes wedged between two teeth and contacts a gold restoration, c. a temporary aluminum crown contacts a gold restoration, d. patient complains of a metallic taste

11. Which of the following statements is/are true? a. avg. biting force on an incisor is about 180 N. b. avg. biting force on a first molar is about 1110 N. c. avg. biting force on a complete denture is about 111 N. d. when a first molar is replaced by a fixed bridge, the biting force on the restored side is about 220 N.

a. avg. biting force on an incisor is about 180 N., c. avg. biting force on a complete denture is about 111 N., d. when a first molar is replaced by a fixed bridge, the biting force on the restored side is about 220 N.

10. Which factor(s) increase(es) the wetting of a solid by a liquid? high surface energy of the solid, low surface energy of the liquid

a. high surface energy of the solid, b. low surface energy of the liquid

19. Which of the following dental materials has/have mechanical properties that are time dependent? human dentin, gold alloy, dental amalgam, alginate, elastomeric impression materials

a. human dentin, c. dental amalgam, d. alginate, e. elastomeric impression materials

9. The contact angle of water on a dental wax is 105 degrees. Which term describes the wettability of the wax?

a. hydrophobic, poor wetting

5. Which describes percolation?

a. percolation usually decreases with time after insertion of dental amalgam, b. percolation is caused by differences in the coefficient of thermal expansion between the tooth and the restorative material when heated or cooled, c. percolation is thought to be undesirable because of possible irritation to the dental pulp and recurrent decay

20. What happens if a load is applied to an elastomeric impression for a long rather than a short time?

a. the permanent strain will be greater

15. Which of the following statements is/are true?

a. the yield strength is always slightly higher than the proportional limit, b. above the stress associated with the yield strength, a material no longer functions as an elastic solid, c. above the stress associated with yield strength, a material will be permanently deformed, even after the applied force is removed

malleability

ability of a material to be rolled into a sheet without fracture

ductility

ability of a material to be stretched without rupturing

dimensional stability

abiltiy of material to maintain its shape and size over time

materials are plastic in their function when

above the stresses of proportional limit of yield strength

thermal conductivity of dental materials, low to high

acrylics, zinc oxide cement, dentin, enamel, composites, ceramic, zinc phosphate cement, dental amalgam, gold alloys

materials with strain-time curve

alginate and impression materials, dental amalgam, human dentin

percent compression

amount of deformation that a material can withstand before rupture when placed under compressive stress

percent elongation

amount of deformation that a material can withstand before rupture when placed under tensile stress

resilience and toughness indicate the energy absorbed up to the proportional limit and the ultimate strength,

and relate to the resistance to deformation and fracture under impact

patients put lower biting forces on bridges and dentures than on normal dentition

average biting force on partial and complete dentures about 111N, or 19% of force with those of normal dentition

maximum biting forces decrease from molar to incisor region

average biting forces on first and second molars are about 580 Newtons

to convert Newtons to pounds, divide Newtons by 4.45

average forces on bicuspids 310 N, cuspids 220 N, incisors 180 N, partial and complete dentures 111 N

13. An alginate impression can withstand a strain of 10% without significant permanent deformation. If the impression must be deformed 0.5 mm to pass over an undercut, how thick should the material be between the tray and the tooth?

b. 5 mm strain = deformation/length .10 = 0.5 mm/5

6. Which restorative materials has/have values of thermal conductivity similar to human enamel and dentin? dental amalgam, composites, zinc phosphate cements, gold alloys

b. composites, c. zinc phosphate cements

16. Rank the following dental materials in order of increasing tensile strength: dental amalgam, gold alloy, human dentin, human enamel

b. human enamel, dental amalgam, human dentin, gold alloy

1. Which of the following describes the purpose of the American National Standards Institute and the ADA specifications?

b. specification measure critical physical and mechanical properties of materials to establish minimum standards, d. knowledge of materials that meet minimum requirements ensures quality control and is helpful in the selection of materials

thermal dimensional change

because of changes in temperature in mouth

materials are elastic in their function when

below the proportional limit or yield strength

12. An amalgam has a force of 111 N applied over a contact area of 0.645 mm2. Which of the following is the stress applied to the amalgam. Would you expect the amalgam to fracture?

c. 172 MPa, no

4. Rank the following dental materials in order in increasing values of their coefficient of thermal expansion: dental amalgam, human teeth, ceramic and unfilled acrylics

c. ceramic, human teeth, dental amalgam, unfilled acrylic

17. Rank the following in order of increasing compressive strength: unfilled acrylic, dental amalgam, human dentin, human enamel

c. unfilled acrylic, human dentin, human enamel, dental amalgam; - or acrylic, dentin, amalgam, enamel?

strain

change in length per unit length of a material produced by stress; units of strain are dimensionless

percolation

clinical effect of difference of coefficients of thermal expansion; when restoration is cooled and shrinks, fluid leaks in and then when temp returns to normal, fluid is forced out of space; possible irritation to dental pulp and recurrent decay

effects of aging cannot be easily replicated in the mouth

clinician must use info from manufacturer plus experimental data gained to make an informed estimate of minimal requirements for certain mechanical and physical properties and its intended uses

gold alloys have about same stiffness as human enamel and

composites and zinc phosphate cement are in same range as human dentin

14. Which of the following dental materials has/have an elastic modulus value that is similar to human enamel? zinc phosphate cement, human dentin, dental amalgam, gold alloy

d. gold alloy

physical properties: properties other than mechanical that depend on the physics of the material itself. - physical = physics

density, thermal conductivity, electrical conductivity, thermal expansion

to measure linear thermal expansion of materials

determine difference in length of a specimen at two tempertures; expressed as a coefficient of thermal expansion

percent of elongation measures

ductility

resilience

energy required to deform a material permanently

toughness

energy required to fracture a material

volumetric dimensional change

equal to three times the linear dimensional change for a specific material

strain-time curves useful when

for materials in which the strain is dependent on the the time the load is maintained

elastomeric impression materials exhibit considerable strain when a stress is applied

gold alloys and human enamel show lower strain under stress

Knoop hardness

hardness of dental materials

degree of wetting depends on relative surface energies of solids and liquids and on their inter-molecular attraction

high-energy solids and low-energy liquids encourage good wetting

poor thermal conductors

human enamel and dentin

22. If the contact angle of a water droplet on the surface of a dental material is greater than 90 degrees, the material is classified as _______.

hydrophobic; poor wetting

compressive strength

if fracture occurs from compression; when pressed together; forces applied opposite but toward each other

shear strength

if fracture occurs from shear; one portion is forced to slide over another portion

tensile strength

if fracture occurs from tensile stress; forces pulled opposite and away from each other

ductility and malleability indicate amount of plastic strain, or deformation, that can occur before the material fractures

indicate brittleness of the material

21. The property that measures the expansion of a material per unit length for every degree of temperature change is called the _________.

linear thermal expansion, expressed as coefficient of thermal expansion

liquids generally wet higher energy solids well; ex: water on metals

liquids bead up on lower-energy solids like wax; add a wetting agent like a detergent to the water to lower surface tension or energy

wear

loss of material occurring as a result of function

percent of compression measures

malleability

proportional limit is point that

material begins to deform plastically when stress is applied to it

hardness is the resistance of a material to indentation

material considered hard if it strongly resists indentation by a hard metal such as a diamond; - no direct relationship between hardness and yield strength or wear resistance, except in materials of the same type

thermal conductivity

measure of heat transferred and related to rate of heat flow

dynamic modulus

measure of stiffness of a material a high rate of strain; important for mouth protector materials

elastic modulus

measure of stiffness; higher values = more stiff; - ex: enamel and gold more stiff; want lower values for impression materials so they can be removed easier from mouth

dynamic resilience

measures energy absorbed at high rates of strain, like from a blow to an athletic mouth protector

proportional limit and yield strength

measures of the stress allowed before permanent deformation; indicate the stress at which the material no longer functions as an elastic solid; strain recovers below these values if stress is removed, and permanent deformation of material occurs above these values

materials with high thermal conductivity values are good conductors of heat and cold

metals have higher values than polymers and ceramics;

high contact angle wetting, - contact angle more than 90 degrees

poor wetting of solid; - hydrophobic

elastic modulus equal to

ratio of the stress to the strain in the linear or elastic portion of the stress-strain curve; elastic modulus = stress/strain

higher tensile strengths result at more rapid rates of applying the load

recommended that alginate impressions be removed from mouth in a rapid motion

fatigue

result of continual stress on materials which will eventually lead to fracture of the material

low contact angle wetting - contact angle less than 90 degrees; - good wetting

solid is wetted readily by liquid; - hydrophilic

23. When the deformation of a material is divided by the length of the material, the quotient is called the ________.

strain

stress-strain curves important when

strain is independent of length of time that a load is applied

24. When the force applied to a material at fracture is divided by the area over which the force was applied, the quotient is called the ________.

stress

ultimate strength

stress at which fracture occurs

flexion

stress from bending

tensile

stress from pulling

compressive

stress from pushing

shear

stress from sliding

torsion

stress from twisting

proportional limit

stress on the stress-strain curve when it ceases to be linear or when ratio of the stress to the stain is no longer is no longer proportional

7. The surface reaction of metals in the mouth from components in saliva or foods is which of the following terms? Galvanism Corrosion Thermal conductivity Tarnish

tarnish

mechanical properties measure a materials reaction to applied forces; - all mechanical properties will relate back to stress and strain - mechanical = measurement

tensile strength, elongation, modules of elasticity, fracture toughness, fatigue strength

bond between two material usually measured in

tension or in shear; expressed as the stress necessary to cause rupture of the bond

limitations of lab testing...impossible to replicate oral conditions of mouth in the lab

tests only address individual properties without necessarily looking at interactions of various properties

adsorption

the concentration of molecules at the surface of a solid or liquid; ex: components of saliva at the surface of tooth structure, or of a detergent adsorbed on the surface of a wax pattern

corrosion

the dissolution of metals in the mouth; - as a result of galvanism, material goes into solution and roughness and pitting occur; - may also result from chemical attack of metals by components in food or saliva

stress, measured in MPa, or megapascals; 1 MPa = 145 lbs/inch squared

the force per unit area; distributed over an area; smaller the area, the larger the value of the stress

galvanism

the generation of electrical currents that the patient can feel; - resulting from presence of dissimilar metals in the mouth

clinically, the shorter the time and the less force applied to the impression material initially,

the lower the permanent strain and the more accurate the impression

dimensional change

the percentage of shrinkage or expansion of a material

sorption = adsorption + absorption

the solubility of materials in the mouth and the sorption of oral fluids by material are important criteria in their selection

yield strength

the stress at some arbitrarily selected value of permanent strain, and thus always slightly higher than the proportional limit

absorption

the uptake of liquid by a bulk solid; ex: absorption of water by acrylic polymers

25. An alginate impression material gave two different stress-strain curves when tested rapidly and slowly. As a result, the properties (plastic and elastic) are better evaluated using a strain-time test than a stress-strain test. t/f?

true

types of stresses are considered to evaluate properties of various materials

types of forces applied include compressive, tensile, shear, twisting moment, bending moment/flexure

yield strength of dental materials, low to high; all indicate yield strength in compression, except for gold which is yield strength in tension; - brittle materials have more compressive strength than tensile strength

unfilled acrylics, composites, dentin, gold, enamel

tarnish

when polished amalgams become dull and discolored with time