Science of Dental Materials Flashcards

Question

What is stress?

Answer 1

Internal reaction to the external applied force per unit area. It is equal to intensity and opposite in direction to the external applied force per unit area.

Answer 2

Defined as a change in length per unit length.

Answer 3

Tensile stress (Results when the body is subjected to 2 sets of forces directed away from each other in the same straight line) ``` Compressive stress (results when the body is subjected to two sets of forces directed towards each other in the same straight line [normal or axial) ``` Shear stress (results when the body is subjected to two parallel sets of forces directed towards each other not in the same straight line)

Answer 4

2 portions of the stress-strain curve: Elastic portion (obeys hook's law, the strain is directly proportional to the applied stress) Plastic portion (does not obey hook's law, strain is not linearly proportional to the applied stress, when stress is removed the original size and shape is not recovered)

Answer 5

The engineering stress - strain curve is that curve where the applied forces are always divided by the original cross sectional area to calculate the corresponding stress.

Answer 6

Stress related terms: Proportional limit Elastic limit Yield stress Ultimate stress Fracture stress

Answer 7

Hook's law applies directly proportionally between stress and strain until a certain point. This is the proportional limit.

Answer 8

Hook's law applies directly proportionally between stress and strain until a certain point. This is the proportional limit. The greatest stress that a material will sustain without a deviation from the law of proportionality of stress to strain.

Answer 9

It is defined as the maximum stress a material can withstand without permanent deformation. It therefore describes the elastic behavior of the material

Answer 10

For all practical purposes the elastic limit and the proportional limit represents the same stress within the structures and the terms are often used interchangeably

Answer 11

Yield strength

Answer 12

The stress at which the material exhibits a specified limiting deviation from proportionality of stress to strain.(0.2% is usually the limit)

Answer 13

The maximum stress that a material can withstand before failure (fracture or rupture)

Answer 14

Modulus of elasticity (E) It is the constant of proportionality between stress and strain. It represents the slope of the elastic portion of the stress -strain curve This tells us the stiffness of the material.

Answer 15

It is determined from a stress strain curve by calculating the ratio of stress to its corresponding strain within the elastic portion.

Answer 16

Same units as stress since the strain has no units.

Answer 17

The inter - atomic or intermolecular forces of the material are responsible for the property of elasticity. The stronger the basic attraction forces, the greater the values of the elastic modulus and the more rigid the material.

Answer 18

The rigidity of major connectors in partial dentures controls stability of the whole design More rigid alloys have thinner sections that can be used without bending Base under restoration should be rigid in order to increase the fracture resistance of the filling.

Answer 19

Flexible material, when subjected to small stress, | large amount of elastic deformation will occur.

Answer 20

Em = PL/E Em= maximum flexibility PL= proportionality limit E= modulus of elasticity

Answer 21

Maximum flexibility is defined as the strain which occur when a material is stressed to its proportional limit

Answer 22

Flexibility is an important property in elastic impression materials, since it represents the ease by which the impression can be removed from the mouth. Orthodontic wires Endodontic files as well as reamers

Answer 23

The ability of a metal to undergo plastic deformation once the proportional limit has been exceeded. It is known as malleability. When the material is in compression and It is known as ductility when the material is in tension. In other words: the malleability of a metal is its ability to be hammered into thin sheets without fracture, while ductility is its ability to be drawn into wires under force of tension without fracturing

Answer 24

Measured by the % elongation reduction in CSA or the cold bend test

Answer 25

If a material demonstrates no or very little plastic deformation on application of a load it is described as being brittle *Brittle materials are weak in tension for example; dental amalgam has a compressive strength which is nearly six times higher than its tensile strength.

Answer 26

Compressive testing (Diametrical compression test for tension, the indirect tensile test or the brazilian test)

Answer 27

Resilience = Amount of energy absorbed by material when stressed to proportional limit. Toughness: Energy absorbed or required to stress the material to the point of fracture.

Answer 28

2 points bending 3 points bending 4 points bending

Answer 29

Total energy absorbed by material before fracture when struck by sudden load.

Answer 30

2 types available: Charpy and izod

Answer 31

When a material is subject to stresses below the yield stress in a repeated manner for many times (cyclic loading) the strength of the material may be reduced leading to failure.

Answer 32

Fatigue is a progressive fracture under repeated loading or it is the stress at which a material fails under repeated loading.

Answer 33

Fatigue tests are performed by subjecting the specimen to alternating stress application below the yield stress until fracture occurs

Answer 34

Magnitude of the load Number of cycles

Answer 35

Lots of alternating forces in the oral cavity due to mastication, grinding, as well as presence of prosthetic materials.

Answer 36

A surface property. Numerous factors influence hardness making the term difficult to define. In mineralogy it is the ability to resist scratching In metallurgy it is the resistance to indentation or penetration

Answer 37

Many methods are used to measure hardness. All of the tests depend on the penetration of some small symmetrically shaped indentor into the surface of the material being tested. Force application to the indentor produces a symmetrically shaped indentation which can be measured under a microscope for depth, area or width of the indentation produced. By applying certain equation relating the force to the area or depth of indentation, a hardness number is obtained. The smaller the indentation the higher is the number, the harder is the material and vice versa

Answer 38

The indentor may be made of steel, tungsten carbide or diamond, and shaped as a sphere, cone or pyramid. Each tool will give rise to an indentation area of definite shape and dimensions. Different loads can be applied to the tool depending on the material (range from 1-3000 Kg)

Answer 39

A hardened steel ball is pressed into a polished surface of a material under a specified load. Load is divided by the area of the surface of the indentation.

Answer 40

Advantages: The Brinell test is used to determine the hardness of the dental metallic materials Disadvantages: The Brinell test is not suitable for determining the hardness of brittle material; the steel ball tends to fracture brittle materials thus the indentation is not well defined. Not suitable for those that exhibit elastic recovery; resilient materials tend to recover upon removal of the steel ball, and the indentation is inaccurate.

Answer 41

The Rockwell hardness test is similar to Brinell test in that a steel ball or cone is used. The depth is measured directly by a dial gauge on the instrument.

Answer 42

Advantages: It is a rapid method for hardness determination. It is good for testing viscoelastic materials. Disadvantages: A preload is needed Greater time is required The indentation may disappear immediately on removal of load.

Answer 43

Diamond shape of a square based pyramid is used Vickers hardness number is computed by dividing the load by the area of indentation.

Answer 44

Advantages: Test is suitable for brittle materials It is especially useful in measuring the hardness of small areas and very hard materials. Disadvantages: Not suitable for materials with elastic recovery.

Answer 45

``` The Knoop hardness test employs a diamond indenting tool ground to a pyramid shape thus giving a diamond or rhomboid indentation having a long and short diagonal of an approximate ratio of 7 : 1 ```

Answer 46

Advantages: Can be employed to examine material that vary in hardness. Hardness of tooth enamel can be compared to that of gold, porcelain, resin, and other tooth restorative materials by this test It is suitable for materials with elastic recovery because when indentation is made the stresses are distributed in such a way that only the minor axis gets smaller.

Answer 47

The science which study the flow and deformation of matter.

Answer 48

In fluid pastes which subsequently solidify Mixed pastes that are adapted to required shapes Setting of such materials involves a change in viscosity with time, and then the development of an elastic modulus as solification proceeds.

Answer 49

Period during which the material can be manipulated ideally with no thickening taking place.

Answer 50

Time when the material has reached a given degree of rigidity.

Answer 51

Static creep (time dependent plastic strain on a material in response to a constant or static load that produces stresses below proportional limit) ``` Dynamic creep (is the creep of a material when the load (below proportional limit) is applied in a cyclic manner. ) ```

Answer 52

Creep usually occurs at temperatures near the softening point of the material. • Since the softening temperatures of most metals and ceramics are far above mouth or room temperatures they do not creep in dental application. • However, many polymers such as waxes, rubber and plastics are near their softening point at room or mouth temperatures and can creep considerably. The term creep has also become useful in describing the flow of set amalgam samples under an applied load.

Answer 53

Strain-time curves are sometimes used when strain depends on the time the load is maintained (e.g. alginate, rubber impression material)

Science of Dental Materials Flashcards

(77 cards)