Mechanical properties of Dental Materials

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Mechanical properties of dental materials:

Mechanical properties of dental materials By, Dr.S.H.Karthick , II yr PG, Department of Conservative Dentistry & Endodontics

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Mechanical properties Stress Strain Mechanical properties based on elastic deformation Stress-strain diagram Modulus of elasticity Poisson's ratio Flexibility Resilience

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Strength properties Proportional limit Elastic limit Yield strength Diametral tensile strength Flexural strength Fatigue strength Impact strength Toughness Fracture toughness Brittleness Ductility Malleability Hardness Hardness tests Conclusion

MECHANICAL PROPERTIES:

MECHANICAL PROPERTIES Are defined by the laws of mechanics i.e., the physical science that deals with energy and forces and their effects on bodies. It primarily centers around static bodies

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STRESS Stress = Force/ Area SI units: MPa or psi

Types of stresses:

Types of stresses Compressive stress compress or shorten Tensile stress stretch or elongate

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Shear stress A stress that tends to resist the sliding or twisting of one portion of a body over another. Eg: .Orthodontic Bracket removal

Flexural (bending) stress:

Flexural (bending) stress

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Strain Strain = Change in Length / original length Strain (  ) = Deformation / = L – Lo / Lo =  L/ Lo original length

Strain: Elastic or Plastic:

Strain : Elastic or Plastic

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION:

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION STRESS-STRAIN DIAGRAM

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HOOKE’S LAW (1678)

MODULUS OF ELASTICITY :

MODULUS OF ELASTICITY Relative rigidity or stiffness of the material within elastic range Young’s modulus of elasticity Elastic modulus = Stress Strain Units : MPa or GPa ( 1 GPa = 1000 MPa )

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Material with a steep line - higher modulus (more rigid).

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Modulus is a reflection of the strength of the inter-atomic or intermolecular bonds. Stronger the basic attraction forces - greater the elastic modulus & the more rigid/stiff the material. As this property is related to the attraction forces within the material, it is usually same, when the material is in tension or compression. This property is generally independent of any heat treatment or mechanical treatment that a metal or alloy has received but is dependent on the composition of the material.

Poisson’s Ratio:

Poisson’s Ratio For an ideal isotropic material of constant volume the ratio is 0. 5 Most engineering material have values of 0.3 Siméon Denis Poisson (1781-1840)

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Material Poisson’s ratio Amalgam 0.35 Zinc phosphate cement 0.35 Enamel 0.30

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FLEXIBILITY Ability of a material to return to its original form indicates its elasticity, but the strain taking place at elastic limit is known as flexibility. Flexibility is bending capacity . These can be defined as the strain that occurs when the material is stretched to its proportional limit Flexibility with respect to impression material is important Flexibility  Elastic recovery

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Resilience Defined as the amount of energy absorbed within a unit volume of a structure when it is stressed to its proportional limit. The property is often described as "springback potential ." Units : m.MN / m 3 - represents energy per unit volume of material

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A high modulus of resilience is desirable for orthodontic wires - they are capable of storing energy which may then be delivered over an extended period of time.

STRENGTH PROPERTIES:

STRENGTH PROPERTIES Strength is the stress that is necessary to cause fracture or a specified amount of plastic deformation.

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PROPORTIONAL LIMIT The proportional limit is defined as the greatest stress that a material will sustain without a deviation from the linear proportionality of stress to strain.

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ELASTIC LIMIT The maximum stress that a material will withstand without permanent deformation. For all practical purposes, the same as the proportional limit.

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YIELD STRENGTH Defined as the stress at which a material exhibits a specified limiting deviation from proportionality of stress to strain. It is the amount of stress required to produce a predetermined amount of permanent strain usually 0.1% or 0.2% which is called the Percent Offset. Useful property because it is easier to measure.

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For brittle materials such as composites and ceramic - Yield strength can’t be measured

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DIAMETRAL TENSILE STRENGTH Tensile strength is determined usually by subjecting a load, wire etc. to tensile loading ( Uniaxial tension test). But for brittle materials - Diametral Compression Test. Used for materials that exhibit predominantly elastic deformation & no plastic deformation.

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Method : A compressive load is placed by a flat plate against the side of a short cylindrical specimen or disk. The vertical force produces a tensile stress along the side of the disc that is perpendicular to the vertical plane that passes through the centre of the disk. Fracture occurs along the vertical plane. Here the tensile stress is directly proportional to the compressive load applied.

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FLEXURAL STRENGTH (TRANSVERSE STRENGTH OR MODULUS OF RUPTURE ) Ability to bend before it breaks. It is a measure of how a material behaves when under multiple stresses. It is measured by subjecting a beam of the material to three- or four-point loading which results in the development of compressive stresses on the top of the beam, tensile stresses on the bottom, and shear stresses on the sides. Compressive stresses convert to tensile ones through the neutral axis along the center of the beam .

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To evaluate flexural strength of a dental material, it is generally used bar-shaped specimens with dimension of 25 mm in length X 2 mm in width X 2 mm in height Specimens are placed on two supports and a load is applied at the center. This test is known as three-point bending test .

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FATIGUE STRENGTH When a stress is repeated a great number of times , the strength of the material may be drastically reduced and ultimately cause failure. Fatigue - defined as a progressive fracture under repeated loading. Fatigue strength - the stress at which a material fails under repeated loading.

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Failure under repeated / cyclic loading is therefore dependent on, The magnitude of the load and The number of loading repetition. For some materials stress can be loaded infinite number of times without failure - Endurance Limit .

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The imperfections lead first to the development of micro-cracks, which coalesce and ultimately lead to a macroscopic crack and failure.

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A rough brittle material would fail in fewer cycles of stress.

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IMPACT STRENGTH

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Charpy -Type impact tester A pendulum is released that swings down to fracture the centre of a bar specimen that is supported at both ends. The energy lost by the pendulum during the fracture of the specimen can be determined by the comparison of the length of the swing after the impact with that of its free swing when no impact occurs. Units : joules, foot-pounds, inch–pounds

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IZOD IMPACT TESTER The specimen is clamped vertically at one end. The blow is delivered at a certain distance above the clamped end instead of at the center of the specimen supported at both ends as described for the Charpy - type impact test. With appropriate values for velocities and masses involved, a blow by fist to jaw can be considered an impact situation.

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A material with low elastic modulus and high tensile strength is more resistant to impact forces. But if both the values are low, impact resistance is also low. Rank of Resilience Material Elastic modulus ( Gpa ) Tensile strength ( Mpa ) 1 Composite Resin 17 30 – 90 2 Dental porcelain 40 50 – 100 3 Polymethyl methacrylate 3.5 60 4 Amalgam 21 27 – 55 5 Alumina ceramic 350 - 418 120

MASTICATION FORCES AND STRESSES:

MASTICATION FORCES AND STRESSES Because of their dynamic nature, biting stresses during mastication are difficult to measure. The average maximum sustainable biting force is approximately 756N. However it is different for different regions in the mouth and also from person to person. Biting force - generally higher for males than females. 39 Region Biting force Molar 400 to 890N Premolar 222 to 445N Incisor region 89 to 111N

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TOUGHNESS Defined as the amount of elastic and plastic deformation energy required to fracture a material. Toughness depends on strength and ductility. A tough metal may be strong, but a strong metal may not be tough.

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FRACTURE TOUGHNESS A measure of the resistance of a material to failure from crack propagation in tension. Given in units of stress times the square root of crack length. i.e. MPa . m ½ MN .m -3/2 Because fracture toughness relates to crack propagation as opposed to crack initiation, surface condition is of little importance. Single-edge notch, three-point loading test

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BRITTLENESS Relative inability of a material to sustain plastic deformation before fracture of a material occurs. Eg : Amalgam, Ceramics & Composites are brittle at oral temperature 5 -55°C. Materials that are brittle usually have a very ordered atomic structure which does not permit the easy movement of dislocations. Hence they are brittle.

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Brittle materials are sensitive to internal flaws/cracks/voids and do not respond well to tensile or bending forces because these forces tend to propagate the flaws/cracks/voids. Brittle materials do well under compressive forces, however, because they tend to close cracks. A brittle material may not necessarily be weak. Example : a cobalt chromium partial denture alloy has 1.5 % elongation but UTS of 870 MPa .

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DUCTILITY Ability of materials to sustain a large permanent deformation under a tensile load before it fractures. Gold is the most ductile metal, second is silver and Platinum comes third in ductility.

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MEASUREMENT OF DUCTILITY There are 3 common methods for measurement of ductility: 1) Reduction in area of tensile test specimens. 2) Maximum number of bends performed in a Cold bend test. 3) Percent elongation after fracture.

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An increase in temperature decreases ductility because a material's strength generally decreases with an increase in temperature. Ductility is unrelated to proportional limit. One way that ductility is used in dentistry is as a measure of Burnishability . Burnishability Index is defined as the percentage elongation divided by the yield strength. Therefore, the greater the ductility and the lower the yield strength, the greater the burnishability

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MALLEABILITY Ability of a material to sustain permanent deformation without rupture under compression as in hammering or rolling into a sheet Gold is the most malleable, second is silver & aluminium third An increase in temperature generally results in a increase in malleability because malleability is dependent upon dislocation movement, and dislocations generally move more easily at a higher temperature.

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Ductility Malleability Gold Gold Silver Silver Platinum Aluminium Iron Copper Nickel Tin Copper Platinum Aluminium Lead Zinc Zinc Tin Iron Lead Nickel

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HARDNESS In mineralogy the relative hardness of a substance is based on its ability to resist scratching. In metallurgy & in most other disciplines the concept of hardness that is most generally, accepted is its resistance to indentation . The indentation produced on the surface of a material from an applied force of a sharp point or an abrasive particle results from the interaction of numerous properties.

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SURFACE HARDNESS TESTS

Brinell hardness test:

Brinell hardness test Small, hardened steel ball Indentation – round Diameter of the dent

Rockwell hardness test:

Rockwell hardness test Ball or metal cone indenter Depth of the indentation is measured

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Advantages : Disadvantages :

Vickers Hardness Test:

Vickers Hardness Test Pyramid shaped diamond with a square base Diagonals of the square-shaped indentation

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Significant advantage: Testing very small specimens Used on materials that are soft as well as hard. Ceramic

Knoop hardness test:

Knoop hardness test Rhombohedral pyramid diamond Indentation - Rhombic Length of the largest diagonal is measured.

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Ideal for measurement of elastic materials Advantages : same as the Vickers test. Disadvantages :

Barcol hardness test:

Barcol hardness test Depth of cure of resin composites. Spring loaded needle with a diameter of 1 mm 10% decrease in Barcol hardness of a resin composite results in a 20% decrease in the flexural strength.

Shore hardness test:

Shore hardness test Measure the hardness of rubbers & soft plastics Values  0 to 100

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Material Compressive Strength ( MPa ) Tensile strength ( MPa ) Modulus of Elasticity ( GPa ) Hardness Cast metal 1140 177 – 202 335 DFG 46 – 50 55 – 69 Amalgam 396 – 423 48 – 64 52 – 58 120 GIC 150 6.6 9 48 Composite 280 – 385 40 – 54 16 – 21 43 – 70 Ceramics 150 24 – 37 70 - 107 591 Enamel 384 10.3 84 - 130 355 – 431 Dentin 297 105.5 14.7 339 – 418

Conclusion:

Conclusion Single property  Combined properties

References:

References Phillip’s science of dental materials 11 th edition. Restorative dental materials, Craig 12 th edition Dental Materials & their selection by William J. O’Brein 4 th edition Fundamentals of colour by Stephan.J.Chu Esthetics in dentistry, by Goldstein Esthetic dentistry:An artists science, Sharat reddy and Ratnadep Patil . 64

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Thank you