Forging new generations of engineers

1. Forging new generations of engineers

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. 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-

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