ENGR1304: Mechanics of Materials

Mechanics of Materials
.A big part of your design, is a consideration of the material that you are using.
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Mechanics of Materials is the study of the mechanical properties of ceramics, metals, and polymers, and the role of processing and microstructure in controlling these properties.
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Common Material properties to consider:
Ductility, strength, hardness, thermal expansion, thermal conductivity, electrical conductivity, corrosion properties, melting point, density, etc.

Young’s modulus or tensile modulus, or elastic modulus:.

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Shear Modulus (or Modulus of Rigidity)
elasticity for a shearing force.
"the ratio of shear stress to the displacement per unit sample length (shear strain)"
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Shear Modulus for common materials
http://www.engineeringtoolbox.com/modulus-rigidity-d_946.html

Poisson’s ratio:
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Mechanical Properties of Wood:.

http://www.conradlumberco.com/pdfs/ch4-Mechanical-Properties-of-Wood.pdf

Wood: natural, heterogeneous material with knots, splits, non-uniform grain, and variable properties.

Testing
Clear – no knots, uniform straight grain, no splits, homogeneous, defined moisture content - ideal case giving the largest values of forces. (Generally, knots etc. lower strength properties)
Lots of tests, get average values with large standard deviations.

Orthotropic Nature – Strength properties depend on orientation of grain to force.

Because wood is orthotropic, there are 12 different strength measurements:
3 different Young's modulus (one for each direction)
3 Shear modulus (G)
6 Poisson’s ratios

Example tables: http://www.conradlumberco.com/pdfs/ch4-Mechanical-Properties-of-Wood.pdf

Metals:
14 basic bravais-lattices structures of atoms:

Materials properties are controlled by how easily planes of atoms can slide over one another. Cubic lattice structures allow slippage to occur more easily than non-cubic lattices, and so are more ductile.
.Solidification:

Multiple solid crystals (or grains) begin to grow in cooling liquid
Nucleation - when crystals (grains) start to grow

Crystal Packing defects change material properties:

point defects - places where an atom is missing or irregularly placed in the lattice structure. Point defects include lattice vacancies, self-interstitial atoms, substitution impurity atoms, and interstitial impurity atoms.

linear defects - which are groups of atoms in irregular positions. Linear defects are commonly called dislocations.

planar defects - which are interfaces between homogeneous regions of the material. Planar defects include grain boundaries, stacking faults and external surfaces.

Grain boundaries - stop slip planes, and strengthen materials.
The size of the grains depends on how fast you cool it, and how many nucleation sites you get.

Phase diagram of solidification:

Steel
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Steel is an iron alloy, with up to 2.1% Carbon by weight.

Alloy - mixture of a metal + non-metal

Low carbon steel (mild steel) contains less than 0.3% carbon.

Medium carbon steels contain carbon from 0.3 -0.55%.

High carbon steel contain more than 0.5% carbon.

Iron with more than 2% carbon is referred to as Cast Iron.
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Effect of Carbon on steel:.
Increase in carbon in steel:
1) Decreases the ductility of steel.
2) Increases the tensile strength of steel
3) Increases the hardness of steel.
4) Decreases the ease with which steel can be machined.
5) Lowers the melting point of steel.
6) Makes steel easier to harden with heat treatments.
7) Lowers the temperature required to heat treat steel.
8) Increases the difficulty of welding steel.
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Steel with 0.2% Carbon can attain Rockwell C hardness of 49, while an 0.8% carbon steel can be hardened to Rockwell C of 65.
As carbon is added, steel gets harder and becomes difficult to machine.
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Example: steels for springs must have at least 0.45 % carbon to attain required hardness.

Carbon steels, and alloy steels are designated by a four digit number:
- first digit indicates = the main alloying element(s),
- second digit = the secondary alloying element(s),
- last two digits = the amount of carbon,

Example
1060 steel = a plain-carbon steel containing 0.60 wt% C

Mechanical Properties Polymers:

Polymers - an organic material created with a chain of carbon atoms. Includes rubber and synthetic materials such as plastics and elastomers. Can have a wide range of mechanical properties and colors.