Mechanical properties of dental material

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Mechanical properties of dental material
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• Strain
• When the external force or load is applied to a
  material the phenomenon of strain occurs this
  is a change in dimension of the material ( the
  change in length, or deformation per unit length
  )
• Deformation of length
• Strain
• Length

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Strain
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Types of strain

• 1- temporary of elastic strain
• Which disappears on removal of the external
  force. The material will return to its original
  shape.
• 2- Permanent or plastic strain
• Which will not disappear on removal of the
  external force. The material will not return to
  its original shape.

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• Stress
• Associated with strain is the phenomenon of
  stress this is an internal force/unit area in a
  material, equal and opposite to the applied load
  or force/unit area.
• Force
• Stress
• Area

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Stress
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Types of stress

• 1) Tensile stress
• Tension results in a body when it is subjected to
  two sets of forces directed away from each other
  in the same straight line.
• 
  

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• 2) Compressive stress
• Compression results when the body is subjected to
  two sets of forces directed towards each other in
  the same straight line

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• 3) Shear stress
• Shear is the result of two sets of forces
  directed towards each other but not in the same
  straight line.

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• 4) Complex stresses
• A single type of stress is extremely difficult to
  induce in a structure so in practice the stresses
  within a material are complex. (complex stresses
  are produced by 3 point loading)
• 
  compression
  
  
  
  shear
  
  
  
  
  
  
  
  
  
  
  
  
  
  
• 
  
  tension

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Stress strain curve

• Stress
• MPa
• ultimate strength
• Yield strength
• Proportional limit ,
  elastic limit
  
  
  
  
  
  
  
  
  
  
  Strain

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• Stress strain curve
• A convenient means of comparing the mechanical
  properties of materials is to apply various
  forces to a material and to determine the
  corresponding values of stress and strain . A
  plot of the corresponding values of stress and
  strain is referred to as a stress- strain curve.
  Such a curve may be obtained in compression,
  tension, or shear.

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From the stress strain curve, the following
properties can be drawn

• 1) Proportional limit (P.L)
• It is defined as the maximum stress that a
  material will withstand without deviation from
  the low of proportionality of stress to strain
  (it describes the relation between stress and
  strain)

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• 2) Elastic limit (E.L)
• It is defined as the maximum stress that a
  material will withstand without permanent
  deformation resulting. (it describes the elastic
  behavior of the material)
• 3) Yield strength (Y.S.)
• It is the stress at which the material begins to
  function in a plastic manner. (defined as the
  stress at which a material exhibits a specified
  limiting deviation from proportionality of stress
  to strain.

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• 4) Ultimate strength (U.S.)
• If higher and higher forces are applied to a
  material, a stress will be reached at wich the
  material will fracture. If the fracture occurs
  from tensile stress, the property is called the
  tensile strength, and, if in compression, the
  compressive strength.
• The ultimate tensile strength is therefore
  defined as the maximum stress that a material can
  withstand before failure (fracture or rupture) in
  tension, whereas the ultimate compressive
  strength is the maximum stress a material can
  withstand in compression. It is calculated by
  dividing the load by the original cross-sectional
  area.

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• 5) Modulus of elasticity or (Youngs Modulus)
  (E)
• It is the constant of proportionality between
  stress and strain. It represents the slope of the
  elastic portion of the stress strain curve. It
  is a measure of rigidity or stiffness
• Materials with higher Youngs modulus value are
  said to be stiffer or more rigid than those of
  low Youngs modulus values because they require
  much more stresses to produce the same amount of
  strain.

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Modulus of elasticity or (Youngs Modulus) (E)

• 
  2
• stress Kg/cm
  2
• Elastic modulus Kg/cm
• strain Cm/cm

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Modulus of elasticity or (Youngs Modulus) (E)
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• 6) Flexibility
• Maximum flexibility is the strain resulting in
  the material when the stress reaches the elastic
  limit.
• This is very important for impression materials,
  which often must be severely deformed to be
  removed from undercuts, but must have the ability
  to spring back without suffering any permanent
  change in shape.

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• 7) poissons ratio
• The increase in length of a material under
  tension is associated with a decrease in
  cross-sectional area. The increase in length is
  known as axial strain and decrease in cross
  sectional area is know as lateral strain.
• lateral
  strain
• Poissons ratio
• axial
  strain

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• 8) Ductility and malleability
• Ductility is the ability of a material to
  withstand plastic deformation under tensile
  stress without fracture. Malleability, is the
  ability of a material to withstand plastic
  deformation compressive stress without fracture.
• In other words, the malleability of a metal is
  its ability to be hammered in to thin sheets
  without fracturing, while, ductility is its
  ability to be drawn into wire without fracturing
  .ductility is measured by the percentage of
  elongation.

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• A material which has good ductility shows high
  elongation before fracturing. The percentage
  elongation represents the maximum amount of
  permanent deformation.
• increase in
  length
• Percentage elongation
  X 100
• 
  original length

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• 9) Brittleness
• If a material showed no or very little plastic
  deformation on application of load it is
  described as being brittle, in other words, a
  brittle material fractures at or near its
  proportional limit. More over ,brittle materials
  are weak in tension for example, dental amalgam
  has compressive strength which is nearly six
  times higher than its tensile strength.

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• Ductile material Brittle material
• 1) Is the ability of a 1) brittle material
• Material to withstand fractures at or near
• Plastic deformation its proportional
  limit.
• Under tensile stress
• Without fracture.
• Fracture occur far Fracture occur at or
  
• Away from P.L near P.L

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• Necking takes place No necking, but
• Before fracture crack propagation
• takes
  place till
• fracture
• Example are gold Examples are
• Alloys and nickel amalgams, porcelain,
  
• Chromium alloy. And composites.

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• 10) Resilience
• The modulus of resilience is the maximum amount
  of energy a material can absorb without
  undergoing permanent deformation. It is
  represented by the area under the elastic portion
  of the stress strain curve.
• Acrylic resin denture teeth are more resilient
  than porcelain teeth and consequently absorb most
  masticatory forces and transmitted less to the
  underlying bone, preserving it.

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Resilience
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• 11)Toughness
• It is the energy required to stress the material
  to the point of fracture. It is represented by
  the area under the elastic and plastic portion of
  the stress-strain curve. Therefore toughness of a
  material is the ability to absorb energy. The
  toughest materials are those which high
  proportional limits and good ductility. However
  two highly different materials can have the same
  toughness.

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Toughness
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• 12) Fracture toughness
• It is the ability of the material to resist
  fracture through its resistance to crack
  propagation. In general, high fracture toughness
  indicates good resistance to crack propagation

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Cantilever bending

• The pending properties of many materials are
  equally or more important than their tensile or
  compressive properties. The bending properties of
  wires, endodontic files and reamers are specially
  important.
• Bending properties are usually measured by
  clamping a sample at one end and applying a force
  at a fixed distance from the face of the clamping.

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• As the force is increased and the sample is bent,
  corresponding values for the bending (angular
  deflection) and the bending movement (force x
  distance) are recorded.
• All instrument will be permanently bent if the
  bending angle exceeds the value at the end of he
  portion of the curve.
• Bending moment force x distance

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Mechanical test

• 1) Diameter compression test (indirect tensile
  test)
• 2) Transverse strength test
• 3) Hardness test

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Diameter compression test (indirect tensile test)

• The diametral compression test or indirect
  tensile test used to measure the tensile strength
  of brittle materials. These brittle materials
  include dental amalgam, cements, ceramics and
  gypsum products. These materials are much weaker
  in tension than in compression thus this
  contributes to their failure in service.

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• In this test a disk of the brittle material is
  compressed diametrically in a testing machine
  until fracture occurs. The compressive stress
  applied to the specimen introduces tensile stress
  in the material. 2P
• tensile stress --------
• DT ?
  P load, D diameter , T
  thickness

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Transverse strength test

• In practice, the stresses within the material are
  complex. Thus if a beam is in tension, and the
  top is in compression. Shear stresses are also
  present. The transverse strength of a material is
  obtained by loading a bar which is supported at
  each end with the load applied in the middling.
  It is often described as the modulus of rupture
  or flexure strength.

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Transverse strength
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Clinical significance

• 1) Denture base materials in which a stress of
  this type is applied to the denture during
  mastication.
• 2) Long bridge spans in which the biting stress
  may be severe.

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Hardness and hardness test

• Hardness is the resistance of the material to
  scratching, indentation or penetration.
• It is a surface property not related directly to
  any other mechanical property i.e. strong or
  stiff materials are not necessary hard.
• Hardness cant be seen or calculated from
  stress-strain curved but only by using one of the
  following Brinel,Knoop,Vickers,Rockwell,and
  shore A hardness test.

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Brinell hardness test

• A steel ball is pressed into the surface of the
  material under a specified load. The load is
  divided by the area of the surface of the
  indentation. Thus, the smaller the indentation
  the larger the hardness number becomes, and the
  harder the material is. This test is used to
  determine the hardness of the metallic materials.
  it is expressed in B.H.N

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Brinell hardness test
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Brinell hardness test
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Disadvantages

• 1) It is difficult to measure the indentation
  area.
• 2) Not suitable for measuring hardness of brittle
  materials because the steel ball will fracture
  it.
• 3) Not suitable for measuring hardness of elastic
  materials because the indentation is recovered on
  removal of the steel ball.

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Rockwell hardness test

• Rockwell hardness test is similar to Brinell test
  in that steel ball or cone is used. Instead of
  measuring the diameter of the indentation, the
  depth is measured directly by a dial gauge on the
  instrument.
• Advantage it is a rapid and easy method for
  measuring hardness.
• Disadvantage as for the Brinell test, Rockwell
  test is not suitable for brittle and elastic
  materials.

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Vicker hardness test

• Vicker hardness test a diamond square based
  pyramid (cone) is used. The Vickers hardness
  number is determined by dividing the load by the
  area of indentation which is square and not round
  as in the Brinell test. This test is easy and
  suitable for brittle materials but not for
  elastic materials. It is expressed in V.H.N.

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Knoop hardness test

• Knoop hardness test uses a diamond cone designed
  to give an indentation having a long and a short
  diagonal(7 1). The load may be varied over a
  wide range, from one gm to more than a Kg, so
  that values for both hard and soft materials con
  be obtained. It is expressed in K.H.N.

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Advantages

• 1)Easy measuring of indentation depth.
• 2) Can test hardness of brittle materials without
  fracture.
• 3) Can test hardness of elastic materials because
  when the indentation is made. The stresses are
  distributed in such a manner that only the
  dimensions of the short axis are subject to
  change by relaxation while the dimensions of the
  long axis remain unchanged.
• 4) Hardness for both soft and hard materials can
  be measured.

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Shore A hardness test

• The hardness tests described previously cannot be
  used to determine the hardness of the rubbers,
  since the indentation disappears after the
  removal of the load. An instrument called a Shore
  A is used in the rubber industry to determine its
  hardness. The indenator is attached to a scale
  that is graduated form 0 to 100. if the indentor
  completely penetrates the sample, a reading of 0
  is obtained, and if no penetration occurs, a
  reading of 100 results.

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Clinical significance

• 1) Denture wearing patients must take care not
  to be aggressive during the cleaning of their
  dentures by using brushes with hard bristles.
• 2)Hardness is an important property to consider
  for model and die materials on which crown and
  bridge wax patterns are made, because a soft
  surface may become scratched, affecting the
  accuracy of the final restoration.

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• Impact strength
• It is describe to know the effects of the
  application of a sudden force to a material
  because under these condition materials are often
  more brittle.
• Fatigue strength
• The repeated application of small stress (below
  the P.L) to an object causes tiny (very small)
  cracks to be generated within its structure.
  These tiny cracks do not cause failure
  immediately. With each application of stress, the
  cracks grow until the material breaks. Metal,
  ceramics can all fail by fatigue. Fatigue is
  the fracture of a material when subjected to
  repeated (cyclic) small stresses below the P.L.

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Fatigue strength
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• Creep
• Creep is defined as the time dependant plastic
  deformation that occurs in an object subjected to
  a small load below its E.L(P.L).

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