Material Properties: Physical Characteristics

Properties and Characteristics of Materials

• Materials Science and Engineering, First Semester, A.Y. 2024-2025

Physical Properties

• Physical properties are characteristics observed without altering a substance's chemical composition and can change with variables like heat.
• Density implies the weight of a material; higher density rates indicate heavier materials
  • Density can be determined using the formula: ρ = m/V
  • ρ = density
  • m = mass
  • V = volume
• Melting point represents the minimum temperature required for a solid material to transition into a liquid state.
• Boiling point represents the minimum temperature needed for a liquid to transform into a gaseous state
  • Water's boiling point under standard conditions is 100°C or 212°F.
• Color refers to the reflective property of a material.
• Size and shape, which are dimensions of any metal, reflect shape and size of material, length, width, height, and depth
  • Size and shape determines specific rectangular, circular, spherical, and other sections.
• Specific Gravity is defined as the ratio of a material's density to the density of a reference material, often water
  • It's also called relative density and it does not have any unit.
• Porosity is related to the quantity of voids within solid materials and occurs when dissolved gases evaporate
  • When materials in a melting condition solidfy, these gases get evaporated and leave behind voids.

Mechanical Properties

• Mechanical behavior reflects a material's response/deformation to an applied load/force.
• Stress and strain can be ascertained by a simple stress-strain test
  • These tests are conducted for metals at room temperature, using the Universal Testing Machine.
• Engineering stress (σ) is the instantaneous load divided by the original specimen cross-sectional area
  • Stress calculation: σ = F/A, F = Force applied, A = Area
• Tensile stress (σt) calculation: στ = Force Applied / Area perpendicular, during tests of gradually increasing tensile load applied uniaxially
• Compression tests are similar to tensile tests, but the force is compressive, causing specimen contraction.
  • Compressive stress formula: σε = Force Applied / Area perpendicular
• The shear stress (τ) using a pure shear force is computed according to: τ = shear force / Area parallel
• The torsional stress (τ) a variation of pure shear in which a structural member is twisted, is computed by: τ= Tc/ J, where:
  • T= twisting moment
  • C=distance from the center
  • J= polar moment of inertia
• Engineering strain (∈) using tensile/compression testing is expressed as change in length divided by the original length.
  • Strain calculation: ∈ = ΔL/L, Where: ΔL= final length - original length, L= original length
• Shear strain (γ) in pure shear is the tangent of the strain angle θ
  • In torsion, this strain is related to the angle of twist (Ø)
• Stress-Strain Behavior, the degree to which a structure deforms depends on the magnitude of the imposed stress
  • For most metals stressed in tension and relatively low levels, stress and strain are proportional, exhibited by: σ= Ε∈
  • σ =stress
  • E= modulus of elasticity
  • ∈= strain
  • This equation is based on Hooke's Law, where the stress is directly proportional to strain.
• Elastic deformation is deformation in which stress and strain are proportional
  • Elastic deformation is nonpermanent, when the applied load is released, the piece returns to its original shape
• Poisson's ratio (v) is the ratio of lateral to axial strains and always positive
  • Calculation: v = - εx/εz = - εy/εz
  • Poisson's ratio values range between 0.25 and 0.35 for many metals and alloys.
• Shear and elastic moduli are related to Poisson's ratio, according to: E = 2G (1 + v), where G = shear modulus of elasticity or modulus of rigidity.
• Plastic deformation is permanent and occurs when stress is removed, and the material doesn't return to its previous dimension.
• Tensile properties are determined using tensile testing and can be explained using the stress-strain diagram.
  • Proportional Limit: It is the region in the strain curve which obeys hookes law and the stress is directly proportional to the strain produced in the material,
    • The ratio of stress with strain gives proportionality constant known as young's modulus.
  • Elastic Limit: Represents the point up to which the material returns to its original position when load is removed
    • Beyond this limit, it cannot return to its original position, initiating plastic deformation.
  • Yield Point: Material starts to deform plastically
    • After the yield point is passed there is permanent deformation develops in the material and which is not reversible
    • Point B is the upper yield stress point and point C is the lower yield stress point.
  • Ultimate Stress/Tensile Strength: It is the maximum stress a material can handle before failure
    • Point D is the ultimate stress point.
  • Fracture or Breaking Point: Represents the point in the stress-strain curve at which the failure of the material takes place.
    • Point E is the breaking point.
• Ductility indicates how easily a material gets deformed under tensile stress and is often categorized by the ability of material to get stretched into a wire by pulling or drawing.
  • It is the ability of the material to deform under tensile stress
  • Ductility is the opposite of brittleness, it is temperature dependent and increases with rise of temperature.
• Ductility can be given as percent maximum elongation
  • % Elongation = (lf - lo)/lo x 100%
    • lf: is the fracture length
    • lo: is the original gauge length
• Ductility of the material can also be determined using percent reduction of area.
  • % Reduction Area = (Ao - Af)/Ao x 100%
    • Ao: is the original area
    • Af: is the area at the point of fracture
• Resilience is the ability of a material to absorb energy when deformed elastically by stress and release energy when stress is removed.
  • Proof resilience is defined as the maximum energy that can be absorbed without permanent deformation
  • The modulus of resilience is defined as per unit volume without permanent deformation, its unit is joule/m3.
• Modulus of resilience calculation: Ur = ∫σdε from 0 to εy, and assuming a linear elastic region, Ur = 1/2 σyεy
  • εy= strain at yielding
  • σy= yield strength of the material
• Toughness is the ability of material to absorb the energy and gets plastically deformed without fracturing, with its value determined by energy per unit volume (Joule/ m³).
  • Material should have good strength and ductility for good toughness.
• Hardness is the ability of material to resist permanent shape change due to external stress. There are various measure of hardness:
  • Scratch Hardness is the ability to oppose scratches to the outer surface layer due to external force
  • Indentation Hardness is the ability to oppose the dent due to punch of an external hard and sharp object
  • Rebound Hardness, is also called dynamic hardness that determined with a diamond tipped hammer dropped from a fixed height on the material
• Brittleness indicates how easily a material gets fractured when subjected to a force/load and is converse to ductility
  • It is temperature dependent: ductile metals become brittle at low temperatures.
• Malleability indicates how easily a material gets deformed under compressive stress and is the ability of material to be formed into a thin sheet by hammering or rolling
  • It is temperature dependent and increases with rise of temperture.
• Creep is the tendency of material to move slowly and deform permanently under external mechanical stress within the limit of yielding
  • Creep is more severe in materials subjected to heat for a long time.
  • Slip in material is a plane with high density of atoms.
• Fatigue is the weakening of material caused by the repeated loading of material
  • Microscopic cracks form at grain boundaries and interfaces under cyclic loading, leading to fracture
  • Structure shape significantly affects fatigue.

Chemical Properties

• pH measures a solution's acidity or basicity
  • pH less than 7 is acidic
  • pH greater than 7 is basic or alkaline
• In solution, pH is the molar concentration of hydrogen ions (H+); low pH indicates high hydrogen ion concentration, and vice versa.
• Hygroscopy is the ability of a substance to attract and hold water molecules through absorption or adsorption
  • Hygroscopic substances include sugar, honey, glycerol, ethanol, methanol, diesel fuel, sulfuric acid, methamphetamine, and many salts.
  • Engineering polymers like nylon, ABS, polycarbonate, and cellulose are hygroscopic.
• Surface tension is a property of liquid surfaces that allows resistance to external forces due to cohesion of like molecules.
• Specific surface area is used to determine the type and properties of a material and is a material property of solids, measures the total surface area per unit of mass, solid, bulk volume, or cross-sectional area.
• Reactivity is the rate at which a chemical substance undergoes a chemical reaction; it depends on physical properties and contaminants (impure compounds)
  • In crystalline compounds, the crystalline form also affects reactivity.
• Corrosion resistance is observed with materials for which corrosion is thermodynamically unfavorable.
  • Some metals have naturally slow reaction kinetics; those include zinc, magnesium, and cadmium

Thermal Properties

• Heat capacity indicates a material's ability to absorb heat from surroundings/external and is the energy amount required to produce a unit temperature rise in joules per kelvin.
• Heat capacity calculation: C = dQ/dt, where dQ is the energy required to produce a dT temperature change.
• Specific heat capacity is the heat capacity per unit mass of a material in J/kg-K, cal/gK, or BTU/lbm
• Molar heat capacity is the heat capacity per mole of a pure substance (J/mol-K).
• Thermal expansion is the extent to which a material expands upon heating; expressed as reciprocal temperature
  • AL/L = α₁ΔΤ: Where AL= change in length , L= initial length, αl = coefficient of linear expansion, ΔΤ= change in temperature
  • AV/V = ανΔΤ: Where AV= change in volume , V = initial volume, aV= coefficient of volume expansion, ΔΤ-change in
• Thermal conductivity measures a material's ability to transfer heat measured in watts per kelvin per metre (W/Km) or BTU/ft °F
  • Thermal conductivity formula: q = -k*(dT/dx)
  • q denotes the heat flux
  • k is the thermal conductivity
  • dT/dx is the temperature gradient through the conducting medium.
• Thermal stresses are induced in a body as a result of temperature changes, which can lead to fracture and may be calculated using σ = Εα₁(Το - Ττ) = Εα₁ΔΤ
  • E = modulus of elasticity
  • a1= linear coefficient of thermal expansion
    • Heating where Tf > To leads to compressive stress
    • Cooling where Tf < To leads to tensile stress

Electric Properties

• Electrical properties are important in material selection and processing.
• Ohm's Law relates the current / time rate of charge passage to applied voltage: V = IR, with R being a resistance of material.
• Electrical resistivity (ρ) is independent of geometry but related to R via the expression ρ = RA/l
  • l is the distance between points where voltage is measured
  • A is the cross-sectional area perpendicular to the current direction.
• Formula for Ohm's law and relation of resistivity: ρ = VA/Il
• Electrical conductivity (σ), the ease with which a material conducts current, is the inverse of electrical resistivity (ρ) measured as: σ = 1/ρ.
  • It has SI units of siemens per metre (Ω·m)-¹ and CGSE units of inverse second (s¯¹).
• Ohm's law can be expressed as: J = σΕ, with J representing current density (I /A) and E representing electric field intensity (V / I).
• Electrical resistivity is the reciprocal of conductivity, with resistivity mechanisms acting independently and represented by: Ptotal = Pt + Pi + Pa
  • P₁ = individual thermal
  • p = impurity
  • P = deformation resistivity
• Capacitance is related to charge stored on a plate: C = Q/V Q = Charge V = voltage applied across the capacitor
  • Units are coulombs per volt, or farads (F)
• Permittivity measures a medium's resistance in forming an electric field measured in farads per meter (F/m)
• Relative permittivity, or dielectric constant, is equal to the ratio of K = Er = Em/E0

Magnetic Properties

• Materials are classified as diamagnetic, paramagnetic, or ferromagnetic based on response to external magnetic fields.
• Magnetism is the phenomenon of materials exerting attractive or repulsive forces.
• Magnetic dipoles are found in magnetic materials
  • Dipoles are thought of as small bar magnets composed of north and south poles.
• Magnetic field strength (H) is the externally applied magnetic field
  • Measurement: H = NI / l.
    • N= number of turns
    • l = length
    • I = magnitude
• Magnetic flux density/magnetic induction represents the magnitude of the internal field strength within a substance and relates to field strength as B = μΗ
  • μ= permeability
• Permeability is a property of the specific medium through which the H field passes and in which B is measured; dimensions in webers per ampere-meter (Wb/A-m)
• Diamagnetism is a very weak, induced, non-permanent magnetism that persists only while an external field is applied induced by changes in electron orbital motion.
• Paramagnetism: Some materials have permanent atomic dipole moments due to incomplete cancellation of electron spin.
  • The orientation of these atomic magnetic moments are random
• Both diamagnetic and paramagnetic materials are considered nonmagnetic in the absence of an external field.
• Ferromagnetism: Certain metallic materials possess permanent magnetic moment in the absence of an external field and manifest large magnetizations
  • Includes transition metals like iron, cobalt, nickel, and gadolinium.
• Antiferromagnetism: Magnetic moment coupling between adjacent atoms/ions results in an antiparallel alignment
  • In these materials neighboring atoms have spin moments in opposite directions
• Ferrimagnetism: Ceramics exhibit permanent magnetization
  • Macroscopic characteristics are similar to ferromagnets, but the distinction lies in the source of net magnetic moments.

Optical Properties

• Optical properties are a material's response to electromagnetic radiation, particularly visible light.
• Electromagnetic radiation is wavelike and consists of electric and magnetic components, including light, heat, radar, radio waves, and x-rays.
• All electromagnetic radiation traverses a vacuum at light velocity which is related to vacuum's electric permittivity and magnetic permeability as c = 1/√Εο μο
• Frequency (v) and wavelength (λ) are functions of velocity: c = λν
• Radiation is composed of photons, whose energy is quantified as: E = hv = hc/λ
  • h= Planck's constant
• The intensity Io of a beam incident on the surface must equal the sum of intensities of transmitted, absorbed, and reflected beams.
• Radiation intensity indicates watts per square meter corresponding to energy transmitted per unit of time across a unit area perpendicular to the direction of propagation
• Transparent materials transmit light with little absorption/reflection
• Translucent materials transmit light diffusely
• Opaque materials do not transmit visible light.
• Refraction is the bending of light transmitted into a transparent material because of a decrease in velocity/ and happens at the interface
  • Refraction is described by index of refraction, n: c = n/v
• Index of refraction, n, is the ratio of light velocity in a vacuum to its velocity in the medium
• Reflection occurs as light passes from one medium to another
  • Degree of reflectance depends on the indices of refraction and angle of incidence.
  • Reflectivity can be calculated at normal incidence using R = (n₂ - n₁/n₂ + n₁)^2
• Absorption: Photon absorption happens via electron excitation from the valence band to higher states within the conduction band.
  • Intensity of absorbed radiation depends on character/path length of the medium, with transmitted radiation decreasing over distance
    • Intensity formula: I= Io‘e^βx
      • I=incident coefficient
      • B = the absorption coefficient