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

2. Properties of Materials

3. 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

4. Overview Characteristics of Metals Characteristics of Nonmetals
Specific Materials Properties
Factors to consider in design

5. Metals and Non-Metals on the Periodic Table

6. Metals – Structure Crystal Lattice molecular structure
Caused by formation of metallic bonds
Easy flow of electrons throughout

7. 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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8. 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

9. 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.

10. Non-Metals – Bonding Covalent Bonds
Share valence electrons to fill valence shells
Simplest example – two hydrogen atoms, one shared pair of electrons H H

11. 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

12. 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

13. 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

14. Polymers Most important non-metals in design
Includes plastics and many other types of synthetic materials
Gigantic molecules formed by carbon chains

15. Some Common Polymers Polyethylene (PE) Polypropylene (PP) Cl Cl
Polyvinyl Chloride (PVC)
Polystyrene

16. Types of Properties Chemical Properties Magnetic Properties
Electrical Properties
Physical Properties
Mechanical Properties

17. Chemical Properties Determined in laboratory
Composition, microstructure, corrosion resistance (metals)
Flammability, chemical resistance (polymers)
Composition, corrosion resistance (composites)

18. Magnetic Properties Most important ferromagnetism
Simply ability of a material to be attracted by magnetic field
Many alloys, oxides, and ceramic compounds display ferromagnetism

19. 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

20. Physical Properties Pertain to interaction with matter and energy
Broad category, includes electrical and magnetic properties

21. 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

22. 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

23. 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

24. 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

25. 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

26. Basic Equations s=P/A s=Ee d=PL/EA elat=-vP/EA (from Poisson’s ratio)
Hooke’s Law: s/e=constant

27. 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

28. 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

29. 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