A tensile load is applied to a wire in small increments until it break. If each
stress is plotted on a vertical coordinate and the corresponding strain (change
in length) is plotted on the horizontal coordinate a curve is obtained. This is
known as stress strain curve. It is useful to study some of the mechanical
properties.
Figure (1-8).
strong
stiff flexible
ductile
brittle
weak
R
R: Resilience.
T: Toughness.
T
Figure (1-9).
The stress strain curve is a straight line up to point P after which it curves.
The point P is the proportional limit, i.e. up to point P the stress is
proportional to strain. Beyond P the strain is no longer elastic and so stress is
no longer proportional to strain. Thus proportional stress can be defined as
the greatest stress that may be produced in a material such that the stress is
directly proportional to strain. The proportional limit deals with
proportionality of strain to stress in the structure.
Figure (1-10).
Below the proportional limit (point P) the material is elastic in nature, that is,
if the load is removed the material will return to its original shape. Thus
elastic limit may define as the maximum stress that a material will withstand
without permanent deformation. The elastic limit describes the elastic
behavior of the material.
It is defined as the stress at which a material exhibits a specified limiting
deviation from proportionality of stress to strain.
Yield strength often is a property that represents the stress value at which a
small amount (0.l % or 0.2 %) of plastic strain has occurred. A value of either
0.1 % or 0.2 % of the plastic strain is often selected and is referred to as the
percent offset. The yield strength is the stress required to produce the
particular offset strain (0.1 % or 0.2 %) that has been chosen. As seen in
Figure (1-11); the yield strength for 0.2 % offset is greater than that associated
with an offset of 0.1 %. If yield strength values for two materials tested under
the same conditions are to be compared, identical offset values should be
used. To determine the yield strength for a material at 0.2 % offset, a line is
drawn parallel to the straight-line region (see Figure 1-11), starting at a value
of 0.002, or 0.2 % of the plastic strain, along the strain axis, and is extended
until it intersects the stress-strain curve. The stress corresponding to this point
is the yield strength. Although the term strength implies that the material has
fractured, it actually is intact, but it has sustained a specific amount of plastic
strain (deformation).
Ultimate tensile strength: It is the maximum stress that a material can
withstand before failure in tension.
Ultimate compressive strength: It is the maximum stress that a material can
withstand before failure in compression.
UTS
Figure (1-9): Stress strain plot for stainless steel orthodontic wire that
has been subjected to tension. The proportional limit (PL) is 1020 MPa.
Figure (1-11): Although not shown, the elastic limit is
approximately equal to this value. The yield strength (YS) at a 0.2 %
strain offset from the origin (O) is 1536 MPa and the ultimate
tensile strength (UTS) is 1625 MPa. An elastic modulus value (E) of
192.000 MPa (192 GPa) was calculated from the slope of the
elastic region.
Once the elastic limit of a material is crossed by a specific amount of
stress, the further increase in strain is called permanent deformation, i.e.
the resulting change in dimension is permanent. If the material is
deformed by a stress at a point above the proportional limit before
fracture, the removal of the applied force will reduce the stress to zero,
but the strain does not decrease to zero because plastic deformation has
occurred. Thus, the object does not return to its original dimension when
the force is removed. It remains bent, stretched, compressed, or otherwise
plastically deformed.
As shown in figure (1-11); the stress-strain graph is no longer a straight
line above the proportional limit (PL), but rather it curves until the
structure fractures. The stress strain graph shown in figure (1-11) is more
typical of actual stress-strain curves for ductile materials. Unlike the
linear portion of the graph at stresses below the proportional limit, the
shape of the curve above (P) is not possible to extrapolate because stress
is no longer proportional to strain.
An elastic impression material deforms as it is removed from the mouth.
However, due to its elastic nature it recovers its shape and little
permanent deformation occurs.
It represents the relative stiffness or rigidity of the material within the
elastic range. It is the ratio of stress to strain (up to the proportional limit),
so the stress to strain ratio would be constant.
It therefore follows that the less the strain for a given stress, the greater will
be the stiffness, e.g. if a wire is difficult to bend, considerable stress must
be placed before a notable strain or deformation results. Such a material
would possess a comparatively high modulus of elasticity.
The metal frame of metal-ceramic bridge should have high stiffness. If the
metal flexes, the porcelain veneer on it might crack or separate.
Generally in dental practice, the material used as a restoration should
withstand high stresses and show minimum deformation. However, there
are instances where a large strain is needed with a moderate or slight stress.
For example in orthodontic appliance, a spring is often bent a considerable
distance under the influence of a small stress. In such a case, the structure
is said to be flexible and it possesses the property of flexibility. The
maximum flexibility is defined as the strain that occurs when the material
is stressed to its proportional limit.
It is useful to know the flexibility of elastic impression materials to
determine how easily they may be withdrawn over undercuts in the mouth.
It is the amount of energy absorbed by a structure when it is stressed not to
exceed its proportional limit.
Resilience can be measured by calculating the area under the elastic portion
(straight line portion) of the stress strain curve calculating (the area of the
triangle=1/2 bh).
Resilience has particular importance in the evaluation of orthodontic wires.
An example: The amount of work expected from a spring to move a tooth.
It is the energy required to fracture a material. It is also measured as the
total area under the stress strain curve (elastic and plastic portions of
stress strain curve). Toughness is not as easy to calculate as resilience.
Figure (1-12): The area under stress strain graph may be used to calculate
either (a) resilience or (b) toughness.
It is the relative inability of a material to sustain plastic deformation
before fracture of a material occurs.
Brittleness is generally considered as the opposite of toughness, glass is
brittle at room temperature. It will not bend appreciably without breaking.
It should not be wrongly understood that a brittle material is lacking in
strength, from the above example of glass we see that its shear strength is
low, but its tensile strength is very high, if glass is drawn into a fiber, its
tensile strength may be as high as 2800 MPa.
Ductile
Brittle
Nylon
Acrylic
Many dental materials are brittle, e.g. porcelain,
acrylic, cements, gypsum products.
It is the ability of a material to withstand a permanent deformation under
a tensile load without rupture. A metal that can be drawn readily into a
wire is said to be ductile. It is dependent on tensile strength. Ductility
decrease as the temperature increased.
Figure (1-13): Schematic of
different type of deformation in
brittle (glass, steel file) and
ductile (copper) materials of
the same diameter and having
a notch of the same dimension.
Figure (1-14): Stress strain plots of materials that exhibit different mechanical properties.
(UTS) ultimate tensile stress, (PL) proportional limit.
It is the ability of a material to withstand considerable permanent
deformation without rupture under compression as in hammering or rolling
into a sheet. It is not dependent on strength as is ductility. It increases with
raise in temperature.
Gold is the most ductile and malleable metal. This enables
manufacturer to beat it into thin foils. Silver is the second.
It is the reaction of a stationary object to a collision with a moving object.
Impact strength: it is the energy required to fracture a material under an
impact force.
Dentures should have high impact strength to prevent it from breaking if
accidentally dropped by patient.
Pendulum
A structure subjected to repeated or cyclic stress below its proportional limit
can produce abrupt failure of the structure. This type of failure is called
fatigue.
Restorations (filling, crown, denture) in the mouth are subjected to cyclic
forces of mastication, so these restorations should be able to resist fatigue.
The hardness is the resistance to permanent surface indentation or
penetration.
The value of hardness, often referred to as the hardness number, depends
on the method used for its evaluation. Generally, low values of hardness
number indicate a soft material and vice versa.
Used for measuring hardness of metal and plastic materials.
Figure (1-15): Shapes of hardness indenter points (upper row
and the indentation depressions left in material surfaces (lower row).
The measured dimension M that is shown for each test is used to
calculate hardness. The following tests are shown:
Brinell test: A steel ball is used, and the diameter of the indentation is
measured after removal of the indenter.
Rockwell test: A conical indenter is impressed into the surface. Under a
minor load (dashed line) anti a major load (solid line), and M is the
difference between the two penetration depths.
Vickers test: A pyramidal point is used, and the diagonal length of the
indentation is measured.
Knoop test: A rhombohedral pyramid diamond tip is used, and the long
axis of the indentation is measured.
Figure (1-17): Vickers indentation.
Figure (1-18): Vicat penetrometer used to
determine initial setting time of gypsum
products.
After a substance has been permanently deformed (plastic deformation),
there are trapped internal stresses; such situations are unstable. The
displaced atoms are not in equilibrium positions through a solid-state
diffusion process driven by thermal energy, the atoms can move back
slowly to their equilibrium positions, the result is a change in the shape or
contour of the solid as the atoms or molecules change positions. The
material warps or distorts.
This stress relaxation leads to distortion of elastomeric impressions.
Waxes and other thermoplastic materials like compound undergo
relaxation after they are manipulated.