MATERIAL PROPERTIES - MECHANICAL, ELECTRIACAL AND CHEMICAL


PROPERTIES OF MATERIAL


MECHANICAL PROPERTIES


Elasticity/Stiffness

             This is a measure of elastic deformation of a body under stress which is recovered when the stress is released. The ratio of stress to strain in the elastic region is known as stiffness or modulus of elasticity (Young’s Modulus). When the stress goes beyond the elastic limit the material will no longer return completely to its original dimension.

Yield (or Proof Strength)

          Stress needed to produce a specified amount of plastic or permanent deformation. (Usually a 0.2 % change in length)

Ultimate Tensile Strength (UTS)

         The maximum stress a material can withstand before fracture.

Ductility

         The amount of plastic deformation that a material can withstand without fracture.

Hardness

          The resistance to abrasion, deformation, scratching or to indentation by another hard body. This property is important for wear resistant applications.

Toughness

           This is commonly associated with impact loading. It is defined as the energy required to fracture a unit volume of material. Generally, the combination of a high UTS and a high ductility results in a higher toughness.

Fatigue Strength and Endurance Limit

           Fatigue failure results from a repeated cyclic application of stress which may be below the yield strength of the material. This is known to be the most common form of mechanical failure of all engineering components. The number of stress cycles needed to cause fatigue failure depends on the magnitude of the stress. Below a certain stress level material does not fail regardless to the number of cycles. This is known as endurance limit and is an important parameter in many design applications.

Creep Resistance

           The plastic deformation of a material which occurs as a function of time when the material is subjected to constant stress below its yield strength. For metals this is associated with high temperature applications but polymers may exhibit creep at low temperatures.
                                         

Brittleness

          Ability of a material to break or shatter without significant deformation when under stress; opposite of plasticity

Bulk modulus

          Ratio of pressure to volumetric compression (GPa)

Compressive strength

          Maximum stress a material can withstand before compressive failure (MPa)

Creep

          The slow and gradual deformation of an object with respect to time

Viscosity

          A fluid's resistance to gradual deformation by tensile or shear stress; thickness

Yield strength

         The stress at which a material starts to yield plastically (MPa)

Young's modulus

          Ratio of linear stress to linear strain (MPa)

STRESS AND STRAIN DIAGRAM





ELECTRICAL PROPERTIES


Electrical conduction

         Materials are classified based on their electrical propertiesas conductors, semiconductors and insulators. New to thisgroup is super conductors.
        Electrical conductivity of a material is defined in terms ofease of charge flow through it.Charge that flows comprised of either electrons, ions,charged holes, and their combinations.
        Ohm’s law relates the current and applied voltage:
                     V = IR
                     where V – applied voltage (volts)
                     I – current (amperes)
                     R – resistance (ohms)

         Material’s electric resistance is NOT an intrinsic-property i.e. it depends on object geometry.
         Electrical resistivity, defined as follows, is an intrinsicproperty, inverse of which called conductivity.

Semiconductivity

          Electrical properties of semiconductors are unique, in thesense that their electrical properties are extremely sensitiveto even minute concentrations of impurities.
          Two kinds of semiconductors – intrinsic and extrinsic.
          For intrinsic semiconductors, their electrical behavior isbased on inherent electronic structure of the pure material.
          On the other hand, if the electrical properties are dominatedby impurities, they are called extrinsic semiconductors.
           In semiconductors, the valence and conduction bands do notoverlap as in metals, but they possess enough electrons inthe valence band those can be promoted to the conduction
band at a certain temperature.

Ferro-electricity

           Ferro-electricity is defined as the spontaneous alignment ofelectric dipoles by their mutual interaction in the absence ofan applied electric field.
           It arises from the fact that the local field increases inproportion to the polarization. Thus, ferro-electric materialsmust possess permanent dipoles.
           Ex.: BaTiO3, Rochelle salt (NaKC4H4O6.4H2O), potassiumdihydrogen phosphate (KH2PO4), potassium niobate(KNbO3).
           These materials have extremely high dielectric constants atrelatively low applied field frequencies. Thus, capacitorsmade from ferro-electric materials are smaller thancapacitors made of other dielectric materials.

Piezo-electricity

           Piezo-electricity, or pressure electricity, is defined aspolarization induced by the application of external force.
           Thus by reversing the direction of external force, direction of the field can be reversed i.e. the application of an external electric field alters the net dipole length and causes a dimensional change.
            Hence piezo-electric materials are useful as transducers – devices that convert mechanical stress into electrical energy and vice versa.
            Application for these materials includes microphones, ultrasonic generators, sonar detectors, and mechanical strain gauges.
            Ex.: Barium titanate, lead titanate, lead zirconate (PbZrO3), ammoinium dihydrogen phosphate (NH4H2PO4), and quartz.

 Dielectric behavior concept

             Dielectric is a material separating two charged bodies. For a material to be a good dielectric, it must be an electrical insulator. Dielectric materials are used in capacitors, devices used to store the electric energy. Electrical conduction in ionic ceramics
             Charge can also be conducted via ions - called ionic conduction. This may occur either in conjunction with or separately from electronic conduction.
             Several types of compounds show exceptionally high ionic conductivity.
             Such phases fall into three broad categories: halide and chalcogenides of silver and copper; oxides with ฮฒ-alumina structure; and oxides of fluorite structure.
             Ex.: La2CuO4(Tc = 30 K), YBC compounds – yttrium doped perovskite structure, YBa2Cu3O7 (Tc = 92 K).
             By properly engineering the point defects, it is possible to convert ceramics into semiconductors. Ex.: Indium tin oxide 

Electrical conduction in polymers

              Polymers are, in general, insulators. They can be made conductors in two ways: (1) introducing an additive to the polymer to improve conductivity, and (2) creating polymers
with inherent conductivity.
             (1) Adding ionic compound or Introducing conductive fillers such as carbon black.
             (2) Inherent conductivity by doping.
              Ex.: polyparaphynylene, polypyrole, polyaniline, acetalpolymers.
            Some other polymers such as polyphthaocyanine can be cross-linked by special curing processes to raise its conductivity.

Chemical properties


Phase Transformation Temperatures

             When temperature rises and pressure is held constant, a typical substance changes from solid to liquid and then to vapor. Transitions from solid to liquid, from liquid to vapor, from vapor to solid and visa versa are called phase transformations or transitions. Since some substances have several crystal forms, technically there can also be solid to another solid form phase transformation.
Phase transitions from solid to liquid, and from liquid to vapor absorb heat. 
              The phase transition temperature where a solid changes to a liquid is called the melting point. The temperature at which the vapor pressure of a liquid equals 1 atm (101.3 kPa) is called the boiling point. Some materials, such as many polymers, do not go simply from a solid to a liquid with increasing temperature. Instead, at some temperature below the melting point, they start to lose their crystalline structure but the molecules remain linked in chains, which results in a soft and pliable material.
             The temperature at which a solid, glassy material begins to soften and flow is called the glass transition temperature.


Density

             Mass can be thinly distributed as in a pillow, or tightly packed as in a block of lead. The space the mass occupies is its volume, and the mass per unit of volume is its density.

            Mass (m) is a fundamental measure of the amount of matter. Weight (w) is a measure of the force exerted by a mass and this force is force is produced by the acceleration of gravity. Therefore, on the surface of the earth, the mass of an object is determined by dividing the weight of an object by 9.8 m/s2 (the acceleration of gravity on the surface of the earth). Since we are typically comparing things on the surface of the earth, the weight of an object is commonly used rather than calculating its mass.



Specific gravity

              Specific gravity is the ratio of density of a substance compared to the density of fresh water at 4°C (39° F). At this temperature the density of water is at its greatest value and equal 1 g/mL. Since specific gravity is a ratio, so it has no units. An object will float in water if its density is less than the density of water and sink if its density is greater that that of water. Similarly, an object with specific gravity less than 1 will float and those with a specific gravity greater than one will sink. Specific gravity values for a few common substances are: Au, 19.3; mercury, 13.6; alcohol, 0.7893; benzene, 0.8786. Note that since water has a density of 1 g/cm3, the specific gravity is the same as the density of the material measured in g/cm3.

Thermal conductivity


              Thermal conductivity (ฮป) is the intrinsic property of a material which relates its ability to conduct heat. Heat transfer by conduction involves transfer of energy within a material without any motion of the material as a whole. Conduction takes place when a temperature gradient exists in a solid (or stationary fluid) medium. Conductive heat flow occurs in the direction of decreasing temperature because higher temperature equates to higher molecular energy or more molecular movement. Energy is transferred from the more energetic to the less energetic molecules when neighboring molecules collide.


Thermal expansion

               When heat is added to most materials, the average amplitude of the atoms' vibrating within the material increases. This, in turn, increases the separation between the atoms causing the material to expand. If the material does not go through a phase change, the expansion can be easily related to the temperature change. The linear coefficient of thermal expansion ( a) describes the relative change in length of a material per degree temperature change. As shown in the following equation, a is the ratio of change in length ( Dl) to the total starting length (li) and change in temperature ( DT).



Magnetic Permeability

                 Magnetic permeability or simply permeability is the ease with which a material can be magnetized. It is a constant of proportionality that exists between magnetic induction and magnetic field intensity. This constant is equal to approximately 1.257 x 10-6 Henry per meter (H/m) in free space (a vacuum). In other materials it can be much different, often substantially greater than the free-space value, which is symbolized ยต0.

                 Materials that cause the lines of flux to move farther apart, resulting in a decrease in magnetic flux density compared with a vacuum, are called diamagnetic. Materials that concentrate magnetic flux by a factor of more than one but less than or equal to ten are called paramagnetic; materials that concentrate the flux by a factor of more than ten are called ferromagnetic. The permeability factors of some substances change with rising or falling temperature, or with the intensity of the applied magnetic field.


Electrical cunductivity

                Ellectrical conductivity is a measure of how well a material accommodates the movement of an electric charge. It is the ratio of the current density to the electric field strength. Its SI derived unit is the Siemens per meter, but conductivity values are often reported as percent IACS. IACS is an acronym for International Annealed Copper Standard, which was established by the 1913 International Electrochemical Commission.

Corrosion

               Corrosion involves the deterioration of a material as it reacts with its environment. Corrosion is the primary means by which metals deteriorate. Corrosion literally consumes the material reducing load carrying capability and causing stress concentrations. Corrosion is often a major part of maintenance cost and corrosion prevention is vital in many designs. Corrosion is not expressed in terms of a design property value like other properties but rather in more qualitative terms such as a material is immune, resistant, susceptible or very susceptible to corrosion.

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