Mechanical Properties

Question 1

Define the “ tensile stress “

Answer 1

• it is one of stress types , occurs when two sets of forces are directed away from each other

Question 2

Define the “ compressive stress “

Answer 2

• it is one of stress types , occurs when two sets of forces are directed towards each other

Question 3

Define the “ shear stress “

Answer 3

• it is one of stress types , it is the result of two sets of forces being directed towards / opposite each other but not in the same direction .

Question 4

Define the “ complex stress “

Answer 4

• it is one of stress types , it is a combination of more than one stress as comprehensive, tensile and shear stress

Question 5

Define the “ elastic “

Answer 5

• it is a type of strain that can be recovered by just removing the force

Question 6

Define the “ stress “

Answer 6

• it is the internal reaction and it is equal in intensity and opposite in direction

Question 7

Define the “ stress “

Answer 7

• it is a deformation / distortion produced as a result of the stress produced within the material .

Question 8

Define the “ plastic “

Answer 8

• it is a strain that can not be recovered by removing the force .

Question 9

Define the “ curve of stress and strain “

Answer 9

• it is when the stress is directly proportional to the strain , until the stress reach the proportional limit

Question 10

Define the “ proportional limit “

Answer 10

• it is the greatest stress that the material can withstand without deviation from the law of proportional between stress and strain

Question 11

Define the “ elastic limit “

Answer 11

• It is the greatest stress that the material can withstand without permanent deformation —> this value describe the elastic behavior of the material

Question 12

Define the “ yield strength “

Answer 12

• it is the stress at which the material exhibit a specific limited deviation from proportionality of stress and strain
• it is the stress at which the material begins to function in a plastic manner

Question 13

Define the “ ultimate strength “

Answer 13

• it is the maximum stress that the material can withstand before fracture

Question 14

Define the “ modulus of elasticity “

Answer 14

• it represent the stiffness of the material within the elastic range
• it depends on the interatomic forces of the material
• ⬆️ strength of attraction forces , ⬆️ greater modulus of elasticity
• also it depends on the heat or mechanical treatment but it is quite dependent on the composition of the material .

Question 15

Define the “ malleability and ductility “

Answer 15

• They are a properties of metals and alloys and indicated workability

Question 16

Define the “ malleability of the material “

Answer 16

• it is the ability of the material to be hammered into thin sheets without fracture .

Question 17

Define the “ ductility of the material “

Answer 17

• It is the ability of the material to be drawn into wires

Question 18

Define the “ elongation “

Answer 18

• it results from application of tensile force

- also indicates workability

Question 19

Define the “ resilience “

Answer 19

• it is the amount of energy needed to deform the material to its proportional limit
• it is an important property of orthodontic wires

Question 20

Define the “ toughness “

Answer 20

• it is the energy required to stress the material to the point of fracture

Question 21

Define the “ fracture toughness “

Answer 21

• it is the amount of energy required to fracture a sample with crack .

Question 22

List the common hardness tests with its usages

Answer 22

• Brinell test —> used for alloys and metals
• knoop test —> used for enamel , dentin , metals and alloys
• vickers test —> used for small areas and very hard materials
• rockwell test —> not suitable for brittle materials , it can be used for materials that exhibit elastic recovery
• shore A test —> used for measuring hardness of rubbers

Question 23

Define the “ wear “

Answer 23

• it is the loss of the material ( the material moves from its site ) and this one happens when a mechanical action ( surfaces slides against each other ) occurs .
• for example —> a normal mastication may cause attrition of tooth structure and restorations by time
• for example —> “ Bruxism “ is a pathological form of “ wear “

Question 24

Define the “ creep “

Answer 24

• it is a slow causing permanent deformation of materials held for long time at stresses below their conventional yield strength —> common in dental amalgam .