Mechanical Properties of Dental Materials

Question 1

Mechanical Properties

Answer 1

the physical science dealing with forces that act on bodies and the resultant motion, deformation, or stresses that those bodies experience

Question 2

Stress

Answer 2

-the force per unit area acting on millions of atoms or molecules in a given plane of material
-its the internal reaction to the external force
-stress = Force per unit area (N/m^2)
-unit of measurement is Megapascal (Mpa)
-Stress is the internal resistance of a material to an external load applied on the material

Question 3

Classification of Stresses

Answer 3

By means of DIRECTION of force, stresses can be classified as: tensile stress, compressive stress, shear stress, flexural stress

Question 4

Tensile Stress (Elongation)

Answer 4

• two forces are directed away from each other in a straight line
• load tends to stretch or elongate a body
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Question 5

Compressive Stress

Answer 5

• Compressive stress occurs when 2 sets of forces are directed towards each other in the same straight line
• also when one end is constrained and the end is subjected to a force towards the constraint
• caused by a load that tends to compress or shorten a body
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Question 6

Shear Stress

Answer 6

• Shear is the results of two sets of forces being directed toward each other
• This type of stress tends to resist the sliding or twisting of one portion of a body over another
• example of this is bracket removal in orthodontics
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Question 7

Flexural Stress

Answer 7

• Force per unit area of a material that is subjected to flexural loading (Bending)

Question 8

Strain

Answer 8

A body undergoes deformation when a force is applied to it
- Strain = Deformation / original length
- strain of the material is reported in percentage %

Question 9

Types of Strain

Answer 9

• Elastic Strain: Its reversible strain, which disappears when force is removed. WHAT WE WANT
• Plastic strain: represents permanent deformation of the material which never recovers when the forces removed

Question 10

Elastic Deformation

Answer 10

• stress is proportional to strain
• NO PERMANENT DEFORMATION
• Proportional limit (elastic limit) = maximum stress that a material will withstand before permanent deformation. Different for posterior and anterior teeth

Question 11

Modulus of Elasticity

Answer 11

• it is the stiffness of a material that is calculated as the ratio of the elastic stress to elastic strain
• the stiff material will have a high modulus of elasticity and the flexible material will have a low modulus of elasticity

Question 12

Plasticity

Answer 12

• When the proportional (Elastic) Limit is exceeded the material exhibits plastic behavior
• Materials that exhibit a LARGE amount of plastic behavior are DUCTILE
• Materials that exhibit a LITTLE or NO plastic behavior are BRITTLE

Question 13

Resilience

Answer 13

• Resilience is associated with springiness of a material but it means precisely the amount of energy absorbed within a unit volume of a structure when it is stressed to its proportional limit

Question 14

Toughness

Answer 14

• toughness = the amount of elastic and plastic deformation energy required to fracture a given material
• Fracture toughness is a measure of the energy required to propagate critical flaws in the structure

Question 15

Brittleness

Answer 15

relative inability of a material to sustain plastic deformation before fracture of a material

Question 16

Ductility

Answer 16

ability of a material to sustain a large permanent deformation under a tensile load up to the point of fracture

Question 17

Malleability

Answer 17

ability of a material to sustain considerable permanent deformation without rupture under compression, as in hammering or rolling in to a sheet

Question 18

Hardness

Answer 18

• the resistance of a material to plastic deformation which is typically produced by an indentation force
• Need hardness so it is not abrated easily