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Properties of Materials

Background

• Two types: Metals and Nonmetals• All materials display certain properties and

characteristics• Based on sciences of physics and chemistry• Depending on properties different materials

suited for different uses• Necessary to take properties into account when

choosing materials to use in design

Overview

• Characteristics of Metals

• Characteristics of Nonmetals

• Specific Materials Properties

• Factors to consider in design

Metals and Non-Metals on the Periodic Table

Metals – Structure

• Crystal Lattice molecular structure

• Caused by formation of metallic bonds

• Easy flow of electrons throughout

Metals – Bonding• Low number of valence

electrons• Shells overlap to form a

“sea” of electrons• Electrons are free moving

between valence shells• Movement of electrons

holds molecules together• Attaction in metallic bonds

is between the positive metal ions in the lattice and the “sea” of electrons.

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Metallic Properties Explained by Bonding

• Dense – atoms tightly packed in lattice structure• High M.P. and B.P. – high energy level required

to break strong force of attraction• Conduct electricity – free electrons allow easy

flow of electrons• Lustrous – free electrons reflect light• Conduct heat – vibrations transmitted through

electrons• Ductility – the amount that any material yields

under shear stress

Malleability

• Malleability is a physical property of metals and metalloids, or generally of any kind of matter. A malleable metal can easily be deformed, especially by hammering or rolling, without cracking.

• Malleability occurs as a result of the metallic bonding found in most metals; the sea of free electrons formed during the loss of electrons from the outer-most electron shells of the metal atoms allow layers of the metal to slide over one another. This makes metals malleable.

Non-Metals – Bonding

• Covalent Bonds• Share valence

electrons to fill valence shells

• Simplest example – two hydrogen atoms, one shared pair of electrons

HH

Non-Metallic Properties Explained by Bonding

• Do not conduct electricity – no free electrons

• Low M.P. and B.P. – weak attraction between atoms in the molecules

Structure of Covalent Networks

• Atoms bond to form network solids

• Display different properties than single covalent bonds

• Not separate molecules but continuous networks

• Example: diamond (carbon network)

• Note: Each carbon should have four bonds; a few have only three

C

C

C

C

C

Properties of Covalent Networks

• Poor conductors – no free electrons

• High M.P. – strong covalent bonds hold atoms in place, large amounts of energy required to break bonds

• Hard, brittle – lattice form makes solids hard, yet bonds break under stress, making them brittle

Polymers

• Most important non-metals in design• Includes plastics and many other types of

synthetic materials• Gigantic molecules formed by carbon chains

Some Common Polymers

Cl

Polyethylene (PE)

Polyvinyl Chloride (PVC)

Polypropylene (PP)

Polystyrene

Cl

Types of Properties

• Chemical Properties

• Magnetic Properties

• Electrical Properties

• Physical Properties

• Mechanical Properties

Chemical Properties

• Determined in laboratory

• Composition, microstructure, corrosion resistance (metals)

• Flammability, chemical resistance (polymers)

• Composition, corrosion resistance (composites)

Magnetic Properties

• Most important ferromagnetism

• Simply ability of a material to be attracted by magnetic field

• Many alloys, oxides, and ceramic compounds display ferromagnetism

Electrical Properties

• Resistivity and conductivity• Resistivity rate of current flow based on cross-

sectional area, resistance, and length• SI unit -m• Resistivity equation: =AR/L• Conductivity = 1/• Metals (conductors) have low resistivities,

ceramics and polymers (insulators) have high resistivities

Physical Properties

• Pertain to interaction with matter and energy

• Broad category, includes electrical and magnetic properties

Important Physical Properties

• Melting Point – Temperature at which a material changes between solid and liquid states

• Density – Mass per unit volume (m/V)• Specific Gravity – Ratio of mass to mass of an equal

volume of water• Curie Point – Temperature where magnetization of

ferromagnetic materials by outside forces is no longer possible

• Refractive Index – Ratio of velocity of light to velocity of light in a vacuum

Important Physical Properties

• Thermal Conductivity – Rate of heat flow (K), English units ºF-h-ft2/Btu-in.

• Thermal Resistivity – R=1/K• Thermal Expansion – Rate of elongation when

heated for a given temperature range (m/ºC)• Heat Distortion Temperature – Temperature at

which a specified amount of deflection is shown in a polymer under a specified load

Important Physical Properties

• Water Absorption – Percent weight gain in a polymer when immersed in water for a given length of time

• Dielectric Strength – Highest withstandable potential difference of an insulating material without electrical breakdown (given time and thickness)

• Specific Heat – Ratio of amount of heat required to raise a mass of a substance 1 degree to the amount required to raise the same mass of water 1 degree

• Poisson’s Ratio – Negative ratio of lateral strain to axial strain of a bar when subjected to axial forces– v=-lat/

Mechanical Properties

• Describe material when a force is applied to it

• Determined through testing, usually involving destruction of material

• Extremely important to consider in design

Symbols Used in Mechanical Properties

– the change in – total deformation (length and diameter) – stress, force per unit area (psi) – strain (inches per inch)• E – modulus of elasticity, Young’s modulus (ratio

of stress to strain for a given material)• P – axial forces

Basic Equations

=P/A=E=PL/EAlat=-vP/EA (from Poisson’s ratio)

• Hooke’s Law: /=constant

Important Mechanical Properties

• Tensile Strength – Ratio of maximum load to original cross-sectional area

• Yield Strength – Stress at which a material deviates a specified amount from Hooke’s Law

• Compressive Strength – Maximum withstandable compressive stress

• Flexural Strength – Outer fiber stress when a beam is loaded and deflected to a certain strain value

• Shear Strength – Stress required to fracture

Important Mechanical Properties

• Percent Elongation – Increase in gage length after fracture

• Percent Reduction in Area – Difference between original cross-sectional area and minimum cross-sectional area after fracture

• Hardness – Resistance to plastic deformation• Impact Strength – Energy required to fracture a

given volume• Endurance Limit – Maximum stress below which

a material maintains elasticity

Important Mechanical Properties

• Creep Strength – Constant stress that causes a set quantity of creep in a given time (temperature constant)– Creep – Permanent strain

• Stress Rupture Strength – Nominal stress in a tension test at fracture