Mechanical Properties Used to Characterize Dental Materials

Question 1

force

Answer 1

an interaction that, when unopposed, changes the motion of the object
pushing and pulling

Question 2

mechanical properties

Answer 2

used to determine how a material changes or what it can withstand with an applied force
direct consequence of the bonding and structure that develops from the bonding

Question 3

stress

Answer 3

internal force is equal in intensity and opposite in direction the applied external force
inverse relationship of stress with area

Question 4

types of stress

Answer 4

compression
tension
shear
bending
torsion

Question 5

strain

Answer 5

the change in length per unit length of a body when subjected to stress, dimensionless

Question 6

tensions vs compression

Answer 6

tension: causes an elongation of the body
compression: causes shrinkage/shortening of the material

Question 7

elastic strain and response Hooke’s Law

Answer 7

recoverable change in shape, recovery of energy imparted to the material

Question 8

linear elastic response

Answer 8

nominal stress is same as nominal strain

Question 9

stress-strain curves

Answer 9

plotting a stress and resultant strain against each other
slope of line is called Young’s Modulus (E), or Elastic Modulus, strictly a materials property

Question 10

elastic modulus of dental materials

Answer 10

a materials property
directly related to the inter-atomic or intermolecular bonds
gives the relative stiffness of a material
the stronger the basic attraction force, the greater the values of Elastic Modulus and stiffer the material

Question 11

proportional limit: elastic vs plastic

Answer 11

the greatest stress a material can sustain without a deviation from the linear proportionality of stress to strain

irreversible-plastic
reversible-elastic

Question 12

below the proportional limit

Answer 12

no permanent deformation occurs
when stress removed, the structure returns to original dimensions
reversible: the elastic region

Question 13

above the proportional limit

Answer 13

results in a permanent or irreversible strain on the specimen
irreversible: the plastic region

Question 14

plasticity at the atomic level

Answer 14

plastic or permanent strain: strain induced is not recoverable
-for metals, a permanent shifting of planes of atoms
-for polymers, chain sliding as secondary bonds are broken
-upon unloading, any elastic strain is recoverable

Question 15

elastic limit

Answer 15

the stress at which a plastic strain starts

Question 16

yield strength

Answer 16

the stress at which a material exhibits a specified amount of plastic deformation

Question 17

ultimate strength

Answer 17

maximum stress that a material can withstand before failure, specific to type of stress

Question 18

fracture strength

Answer 18

the point at which the material fractures irreparably, not necessarily the highest load

Question 19

ductility

Answer 19

ability of a material to be plastically deformed; indicated by plastic strain
drawn into a wire under tensile force

Question 20

malleability

Answer 20

ability of a material to be hammered into thin sheets without fracturing, compressive force

Question 21

metals tend to be _____, ceramics tend to be ______

Answer 21

ductile, brittle

Question 22

brittleness

Answer 22

relative inability of a material to sustain plastic deformation before fracture of the material occurs
ceramics, composites and amalgams
sustain little (to no) plastic strain before they fracture
fractures at or near its proportional limit

Question 23

resilience

Answer 23

capacity to absorb energy when elastically deformed up to the proportional limit

Question 24

toughness

Answer 24

amount of energy that can be absorbed up to fracture

Question 25

bending tests

Answer 25

measures a flexural strength
3 point bend test: complex stress state, compressive to tensile, neutral axis, shear

Question 26

hardness testing

Answer 26

measuring the resistance to permanent deformation by surface indentation or scratching

Question 27

critical hardness features

Answer 27

indenter material: steel, tungsten carbide, diamond
indenter size and shape: sphere, cone, pyramid, needle
force applied: 1-3000 kg
indentation shape and measurement

Question 28

viscoelasticity

Answer 28

mechanical response of a material is loading or strain rate dependent
elastic solid and viscous fluid
small to negligible dependence for metals and ceramics

Question 29

flaws and mechanical properties

Answer 29

most if not all materials will contain flaws such as cracks and pores typically arising during processing
allows for stresses to become concentrated around these defects

Question 30

fracture toughness (Critical Stress Intensity)

Answer 30

measure of the resistance of a material to catastrophic propagation of flaws under and applied stress

Question 31

stress relaxation

Answer 31

a reduction in stress over time when holding at a fixed strain
rubber bands

Question 32

fatigue strength

Answer 32

when stress is repeated, the strength may be reduced and ultimately cause failure; a progressive fracture under repeated loading is called fatigue

Question 33

fatigue

Answer 33

progressive fracture under repeated (cyclic) loading

Question 34

rotating beam fatigue test

Answer 34

alternating maximum tension and compression at the surface where the diameter is a minimum

Question 35

fatigue properties

Answer 35

endurance limit: stress level below which the material can be cycled to infinity without failure

fatigue strength: maximum stress level a material can withstand without fracture for a specified number of cycles

Question 36

wear

Answer 36

loss of material through the contact of two or more surfaces
undesirable, but when controlled it is beneficial

Question 37

wear results from

Answer 37

inhomogeneity, crystal orientation, phases and inclusions present, microscopic contact, interaction between sliding surfaces, lubrication

Question 38

adhesive wear

Answer 38

formation/disruption of microjunctions

Question 39

corrosive wear

Answer 39

2nd, related to chemical activity

Question 40

fatigue wear

Answer 40

surface weakened by cyclic loading

Question 41

abrasive wear

Answer 41

soft and hard surface in contact