DIRECTIONAL PROPERTIES DIRECTIONAL PROPERTIES OF MATERIALS OF MATERIALS

andand

APPLICATION OF APPLICATION OF CONTROLLED ANISTROPYCONTROLLED ANISTROPY

By: Rub Nawaz Ansari

DIRECTIONAL PROPERTIES DIRECTIONAL PROPERTIES OF MATERIALSOF MATERIALSSubstances in which measured properties

are independent of the direction of measurement are isotropic

The physical properties of single crystals of some substances depend on the crystallographic direction in which measurements are taken; this directionality of properties is termed as anisotropy

Common examples of anisotropic materials are wood and composites.

an amorphous solid is isotropic.

In contrast, crystalline materials are generally anisotropic, so the magnitude of many physical properties depends on direction in the crystal.

Crystalline Crystalline Amorphous Amorphous

The dependence of the physical properties (mechanical, thermal, electrical, magnetic, and optical) of a substance on direction (as opposed to isotropy, the independence of properties from direction).

However, not all the properties of crystals are Directional. The density and specific heat capacity of all crystals are independent of direction.

Daily Life Examples of Directional Daily Life Examples of Directional PropertiesPropertiesExamples of anisotropy are: mica plates split easily into thin leaves

only along a certain plane (parallel to this plane the cohesive forces between the mica particles are very small)

meat is more easily cut along the fiberscotton material is torn easily along a

thread (the strength of the material is the lowest in these directions).

When a sphere made of an isotropic substance is heated, it expands uniformly in all directions, that is, it remains a sphere. A crystalline sphere changes its shape when heated, turning into an ellipsoid, (Figure a)

It may happen that a sphere upon heating will expand in change in shape of a crystalline sphere (dashed circles) upon heating one direction and contract in another (perpendicular to the former Figure b).

FigureFigure

Mechanical anisotropyMechanical anisotropyMechanical anisotropy is the variation of

mechanical properties—strength, solidity, viscosity, and elasticity—in various directions.

Polycrystalline materials (metals and alloys), consisting of an aggregation of randomly oriented crystal grains (crystallites), are in general isotropic or nearly isotropic.

The anisotropic properties of polycrystalline materials become manifest if as a result of processing (annealing, rolling, and so on) a preferred orientation of the individual crystals is developed in some direction (structure).

Thus, during the rolling of steel plate the grains of the metal are oriented in the direction of the rolling, and anisotropy occurs as a result (chiefly in the mechanical properties)

for example, in rolled steels the yield limit, and the ultimate elongation along and across the direction of rolling differ by 15 to 20 percent (up to 65 percent).

TEXTURETEXTURESometimes the grains in polycrystalline materials have a

preferential crystallographic orientation, in which case the material is said to have a “texture.”

The magnitude of a measured property represents some average of the directional values. In material science, texture is the distribution of crystallographic orientations of a polycrystalline sample.

A sample in which these orientations are fully random is said to have no texture. If the crystallographic orientations are not random, but have some preferred orientation, then the sample has a weak, moderate or strong texture.

Anisotropy in polycrystalline materials can also be due to certain texture patterns often produced during manufacturing of the material.

In the case of rolling, "stringers" of texture are produced in the direction of rolling, which can lead to vastly different properties in the rolling and transverse directions.

Some materials, such as wood and fiber-reinforced composites are very anisotropic, being much stronger along the grain/fiber than across it.

Directionally dependent physical properties of anisotropic materials are significant due to the affects it has on how the material behaves.

For example, in the case of fracture mechanics, the way the microstructure of the material is oriented will affect the strength and stiffness of the material in various directions therefore affecting direction of crack growth.

Mechanical analogy of Mechanical analogy of anisotropic responseanisotropic response

This can be demonstrated with a simple mechanical model, consisting of a mass supported by two springs.

This can be demonstrated with a simple mechanical model, consisting of a mass supported by two springs.

The following photographs show the response of the model under the application of various forces.

Model with no force applied Model with horizontal force

producing horizontal displacement

(parallel response) 90o

Model with vertical force producing Model with 45º displacement fromvertical displacement non-45º force (non-parallel (parallel response) 0o anisotropic response) 45º

Note that the displacement of the mass is only parallel to the force when the force acts parallel or perpendicular to the springs. These are the directions of the principal axes.

When heating the section cut perpendicularly to the c-axis, the observed shape is a circle, showing that the thermal conductivity is the same in all directions in this plane.

However, when using the section cut parallel to the c-axis, the shape seen is an ellipse, which shows that the thermal conductivity in this plane is direction-dependent.

Images of heat flow patterns

Applications of Controlled Applications of Controlled AnistropyAnistropyAnisotropy may be induced in a material

during the manufacturing through processes like rolling or forging or Aneealing.

This induced anisotropy gives rise to the concept of orientation-dependent material properties such as yield strength, ductility, strain hardening, fracture strength, or fatigue resistance. Inclusion of the effects of anisotropy is essential in correctly predicting the deformation behavior of a material.

ALUMINIUMALUMINIUMAnnealing may change the anisotropy of

tensile properties. In the as-rolled condition Aluminium, the

transverse direction is the strongest and 45 degrees to the rolling direction is the weakest.

On annealing at 280 and 300 °C the anisotropy is unchanged but annealing at 340 °C affects anisotropy.

Aluminum alloy castings can display the tensile properties of most forgings, extrusions and rolled plate.

Because wrought products are normally characterized by finely recrystallized grain structures with specific anisotropy and highly textured microstructural features, ductility in longitudinal directions is typically greater than in castings that contain coarser grain structures.

Anisotropic behavior in a rolled aluminum-lithium sheet material used Anisotropic behavior in a rolled aluminum-lithium sheet material used in aerospace applications. The sketch relates the position of tensile in aerospace applications. The sketch relates the position of tensile bars to the mechanical properties that are obtainedbars to the mechanical properties that are obtained

A number of fan blades for cooling automotive and truck engines, manufactured through stamping blades from cold-rolled steel sheet.

All produced at the same time, have failed by the initiation and propagation of a fatigue crack transverse to the axis of the blade All other fan blades perform satisfactorily.

Cast IronCast Iron

Cast iron, is composed of anisotropic graphite inclusions in a metal matrix. One application of cast iron can be found in truck engine cylinder heads.

A problem present in cylinder heads is thermo-mechanical fatigue (TMF). TMF is related to the thermal stresses developed due to the start up-shut down thermal cycling, which leads to valve bridge cracking and ultimately to TMF failure.

TMF cracks at valve bridges

Types of Graphite FlakesTypes of Graphite Flakes

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