Mechanical Properties

MECHANICAL PROPERTIES

Prof. Mohamed Shehata

Most applications of materials in dentistry have a minimum mechanical property requirement .

Certain materials should be sufficiently strong to withstand biting forces without fracture, others should be rigid enough to maintain their shape under load.

Stress: When an external force is applied to a body or specimen of material under test, an internal force, equal in magnitude but opposite in direction, is set up in the body.

Stress = F/A

where F is the applied force A the cross-sectional area A stress resisting a compressive force is referred to as a compressive stress and that resisting a tensile force a tensile stress.

Tensile and compressive stresses, along with shear, are the three simple examples of stress

The unit of stress is the pascal (Pa)

One test method commonly used for dental materials is the three-point bending test or transverse test.

Stress = 3FL 2bd

When a cylinder of a brittle material is compressed across a diameter, a tensile stress is set up in the specimen, the value of the stress being given by

Stress = 2F DTat the axis of cylinder

A diametral compressive tensile test is commonly used when conventional tensile testing is difficult to carry out due to the brittle nature of the test material.

Fracture stress strength:

There is a limit to the value of applied force which a body, or specimen of material, can withstand without fracturing.

In a tensile test, the fracture stress is referred to as the tensile strength of a material whilst a compression test gives a value of compressive strength.

Strain: The application of an external force to a body or test specimen results in a change in dimension of that body.

Strain = Change in length Original length

The strain may be recoverable or the material may remain deformed. A third possibility is that the strain may be partially recoverable.

Stress-strain relationship:

Stress and strain are not independent and unrelated properties, but are closely related.

The application of an external force, producing a stress within a material, results in a change in dimension or strain within the body.

The relationship between stress and strain is often used to characterize the mechanical properties of materials. Such data are generally obtained using a mechanical testing machine.

It can be seen that in this example there is a linear relationship between stress and strain up to the point P.

Further increases in stress cause proportionally greater increases in strain until the material fractures at point T.

The stress corresponding to point T is the fracture stress.

In a tensile test this gives a value of tensile strength, whilst in a compression test value of compressive strength is obtained. The value of stress which corresponds to the limit of proportionality P, is referred to as the proportional limit.

Point E is the elastic limit. This corresponds to the stress beyond which strains are not fully recovered.

The proportional limit is often used to give an approximation to the value of the elastic limit.

The slope of the straight-line portion of the stress-strain graph gives a measure of the modulus of elasticity:

StressModulus of elasticity = Strain

It gives an indication of the rigidity of a material and not its elasticity.

A steep slope giving a high modulus value, indicates a flexible material.

The value of strain recorded between points E and T indicates the degree of permanent deformation which can be imparted to a material up to the point of fracture.

For a tensile test this gives an indication of ductility whilst for a compressive test it indicates malleability.

A ductile material can be bent or stretched by a considerable amount without fracture whereas a malleable material can be hammered into a thin sheet.

A property often used to give an indication of ductility is the elongation at fracture.

The area beneath the curve up to the elastic limit, gives a value of resilience.

Resilience may be defined as the energy absorbed by a material in undergoing elastic deformation up to the elastic limit.

The energy is stored and released when the material springs back to its original shape after removal of the applied stress.

The total area under the stress-strain graph, gives an indication of toughness and may be defined as the total amount of energy which a material can absorb up to the point of fracture.

A material capable of absorbing large quantities of energy is termed a tough material. The opposite of toughness is brittleness.

Notched specimens are generally used to determine the property known as fracture toughness.

Fracture toughness effectively gives a value of the work of creating two new surfaces when cracking occurs.

The equations used to calculate fracture toughness should strictly only be applied to materials which fail by a purely brittle mechanism.

Materials are more likely to behave in a more brittle fashion when stress or strain are increased rapidly.

When the stress is increased very rapidly it may termed an impact test and the important practical property obtained is the impact strength.

The position reached by the pendulum after fracturing the specimen gives a measure of the energy absorbed by the specimen during fracture.

Impact strength is an important property for acrylic denture base materials which have a tendency to fracture if accidentally dropped onto a hard surface.

Fatigue properties:

Many materials which are used as restoratives or dental prostheses are subjected to intermittent stresses over a long period of time.

The stresses encountered may be far too small to cause fracture of a material.

Failure may occur by a fatigue process.

This involves the formation of a microcrack, this crack slowly propagates until fracture occurs.

Final fracture often occurs at quite a low level of stress.

As the applied cyclic stress increases, the number of cycles to failure decreases.

Fatigue properties may be studied in oneof two ways:It is possible to apply cyclic stress at a given magnitude and frequency and to observe the number of cycles required for failure. The result is often referred to as the fatigue life of a material.

Selection of given number of the cyclic stress which is required to cause fracture within this number of cycles. The result in this case is referred to as the fatigue limit.

One of the most important factors involved in such tests is the quality of the specimen used in the test.

Stress concentrations within materials can occur to an extent where cracks can propagate to cause failure within the normal lifetime of the material.

Abrasion resistance:

Wear can occur by one or more of a number of mechanisms.

Wear caused by indenting and scratching of the surface by abrasive wear .

Wear due to intermittent stresses is termed fatigue wear.

Wear of certain materials can often be attributed to chemical degradation. Such processes are often referred to as erosion processes.

Hardness: The hardness of a material gives an indication of the resistance to penetrationwhen indented by a hard asperity.

The value of hardness, often referred to as the hardness number.

Generally, low values of hardness number indicate a soft material and vice versa.

Common methods used for hardness evaluation include Vickers, Knoop, Brinell and Rockwell.

Vickers and Knoop both involve the use of diamond pyramid indentors.

In the case of Vickers hardness, the diamond pyramid has a square base, whilst for Knoop hardness, one axis of the diamond pyramid is much larger than the other.

The brinell hardness test involves the use of a steel ball indentor producing an indentation of circular cross-section.

The hardness is a function of the diameter of the circle for Brinell hardness and the distance across the diagonal axes from Vickers and Knoop hardness.

Measurements are normally made using a microscope.

The case of Rockwell hardness, a direct measurement of the depth of penetration of a conical diamond indentor is made.

Hardness is often used to give an indication of the ability to resist scratching.

Hardness is also used to give an indication of the abrasion resistance of a material.

Elasticity and viscoelasticity: The elastic limit is the value of stress beyond which the material becomes permanently distorted.

Although elastic limit is an important property it does not fully characterize the elastic properties of a material.

Elastic properties are often defined in terms of the ability of a material to undergo elastic recovery.

When a material undergoes full elastic recovery immediately after removal of an applied load it is elastic.

If the recovery takes place slowly, or if a degree of permanent deformation remains, the material is said to be viscoelastic.

Models involving the use of springs and dashpots can be used to explain the elastic and viscoelastic behavior of materials.

This type of behavior has important practical significance for many dental materials.

Elastic materials become distorted when being removed over undercuts.

The permanent deformation depends on the applied load and the time for which that force is applied.

Creep and stress relaxation are two other phenomena which can be explained using the viscoelasticity models.

Creep involves a gradual increase in strain under the influence of a constant applied load similar to that which takes place in the Maxwell model.

Stress relaxation involves the application of a constant strain.

Stress relaxation is a measure of decreasing stress at constant strain.

Under such conditions the stress decreases as a function of time for Maxwell-type viscoelastic materials.

Creep tests have more practical significance for dental materials.

A constant load is applied to a test specimen in either compression or tension.

The strain or creep is measured as a function of time.

Dynamic creep tests are also carried out.

Rheological propertiesRheology is the study of the flow or deformation of materials.

A study of the rheological properties of liquids and pastes normally involves the measurement of viscosity.

Viscosity () is given by the equation: = Shear stress () Shear rate (E)

Further characterization of the rheological properties of materials is obtained by reference to the equation:

Shear stress = K (Shear rate)

Viscosity values of materials are temperature-dependent.

Time-dependence of viscosity (working times and setting times):

Manipulation becomes impossible when viscosity has increased beyond a certain point.

The time taken to reach that point is the working time of the material.

The setting time is to the time taken for the material to reach its final set state or to develop properties which are considered adequate for that application.