Properties of Materials Principles of EngineeringTM Lesson 6.2 – Properties of Materials Forging new generations of engineers Project Lead The Way, Inc. Copyright 2007

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. e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e-

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 H H

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 Polyethylene (PE) Polypropylene (PP) Cl Cl Polyvinyl Chloride (PVC) Polystyrene

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 W-m Resistivity equation: r=AR/L Conductivity = 1/r 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=-elat/e

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 D – the change in d – total deformation (length and diameter) s – stress, force per unit area (psi) e – 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 s=P/A s=Ee d=PL/EA elat=-vP/EA (from Poisson’s ratio) Hooke’s Law: s/e=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