Materials, testing and selection
This independent materials study checks whether you can go from a material's internal structure to a measurable property and from there to a choice justified with numbers. The usual content is metals, polymers, ceramics and composites, the tensile test with its stress–strain curve, hardness and impact tests, and selection using specific indices.
Structure and properties
- Many engineering metals are crystalline, with common BCC, FCC or HCP structures. Deformation depends on crystal structure, microstructure and conditions. Grain refinement commonly raises yield strength within conventional grain-size ranges; this is not a universal claim for every scale and material.
- Steel is an Fe–C alloy: in comparable plain-carbon microstructures, increasing carbon commonly raises hardness and reduces ductility; heat treatment and alloying also matter. Around 0.8 % C is the eutectoid (pearlite).
- Polymers are long chains, amorphous or semicrystalline: thermoplastics (remeltable), thermosets (irreversible crosslinks) and elastomers.
- Many ceramics are hard and brittle with ionic or covalent bonding, but electrical and thermal behavior varies; semiconductor and thermally conductive ceramics are important exceptions.
- Composites combine matrix and reinforcement and give anisotropic, high specific stiffness.
Tests
- Tensile test: σ = F/S₀, ε = ΔL/L₀. Only in the linear elastic zone is E = σ/ε. The yield strength marks the end of elastic behaviour; toughness is the whole area under the curve, resilience only the elastic area.
- Hardness: Brinell (ball, HB), Rockwell (different scales use a diamond cone or ball, with HRC using a cone), Vickers (pyramid, HV). It is not a fundamental property but correlates with strength.
- Charpy testing measures absorbed impact energy; testing suitable materials over a temperature range can reveal a ductile-to-brittle transition.
Worked example
A steel specimen 10 mm in diameter and L₀ = 50 mm is loaded to F = 15.7 kN, still in the elastic zone, and stretches ΔL = 0.05 mm. S₀ = π·5² = 78.54 mm², so σ = 15 700/78.54 = 200 MPa. ε = 0.05/50 = 0.001, hence E = 200/0.001 = 200 000 MPa = 200 GPa. If the yield strength of the steel is 250 MPa, the specimen has not yielded and the calculation is valid.
Marks lost
- Mixing up toughness (total area) and resilience (elastic area).
- Using the diameter as if it were the radius.
- Giving E in MPa when GPa is expected, or mixing N and kN.
- Applying E = σ/ε outside the elastic zone.
- Choosing a material by one property only, without the specific index (σ/ρ, E/ρ), cost or corrosion resistance.