Mechanical Properties Used to Characterize Dental Materials Flashcards
force
an interaction that, when unopposed, changes the motion of the object
pushing and pulling
mechanical properties
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
stress
internal force is equal in intensity and opposite in direction the applied external force
inverse relationship of stress with area
types of stress
compression
tension
shear
bending
torsion
strain
the change in length per unit length of a body when subjected to stress, dimensionless
tensions vs compression
tension: causes an elongation of the body
compression: causes shrinkage/shortening of the material
elastic strain and response Hooke’s Law
recoverable change in shape, recovery of energy imparted to the material
linear elastic response
nominal stress is same as nominal strain
stress-strain curves
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
elastic modulus of dental materials
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
proportional limit: elastic vs plastic
the greatest stress a material can sustain without a deviation from the linear proportionality of stress to strain
irreversible-plastic
reversible-elastic
below the proportional limit
no permanent deformation occurs
when stress removed, the structure returns to original dimensions
reversible: the elastic region
above the proportional limit
results in a permanent or irreversible strain on the specimen
irreversible: the plastic region
plasticity at the atomic level
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
elastic limit
the stress at which a plastic strain starts
yield strength
the stress at which a material exhibits a specified amount of plastic deformation
ultimate strength
maximum stress that a material can withstand before failure, specific to type of stress
fracture strength
the point at which the material fractures irreparably, not necessarily the highest load
ductility
ability of a material to be plastically deformed; indicated by plastic strain
drawn into a wire under tensile force
malleability
ability of a material to be hammered into thin sheets without fracturing, compressive force
metals tend to be _____, ceramics tend to be ______
ductile, brittle
brittleness
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
resilience
capacity to absorb energy when elastically deformed up to the proportional limit
toughness
amount of energy that can be absorbed up to fracture
bending tests
measures a flexural strength
3 point bend test: complex stress state, compressive to tensile, neutral axis, shear
hardness testing
measuring the resistance to permanent deformation by surface indentation or scratching
critical hardness features
indenter material: steel, tungsten carbide, diamond
indenter size and shape: sphere, cone, pyramid, needle
force applied: 1-3000 kg
indentation shape and measurement
viscoelasticity
mechanical response of a material is loading or strain rate dependent
elastic solid and viscous fluid
small to negligible dependence for metals and ceramics
flaws and mechanical properties
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
fracture toughness (Critical Stress Intensity)
measure of the resistance of a material to catastrophic propagation of flaws under and applied stress
stress relaxation
a reduction in stress over time when holding at a fixed strain
rubber bands
fatigue strength
when stress is repeated, the strength may be reduced and ultimately cause failure; a progressive fracture under repeated loading is called fatigue
fatigue
progressive fracture under repeated (cyclic) loading
rotating beam fatigue test
alternating maximum tension and compression at the surface where the diameter is a minimum
fatigue properties
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
wear
loss of material through the contact of two or more surfaces
undesirable, but when controlled it is beneficial
wear results from
inhomogeneity, crystal orientation, phases and inclusions present, microscopic contact, interaction between sliding surfaces, lubrication
adhesive wear
formation/disruption of microjunctions
corrosive wear
2nd, related to chemical activity
fatigue wear
surface weakened by cyclic loading
abrasive wear
soft and hard surface in contact
