Materials: properties, structure, testing and selection
This block of the PAU Tecnología e Ingeniería II 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
- Metals are crystalline (BCC, FCC, HCP). FCC metals (Al, Cu, austenite) are ductile; grain boundaries strengthen, so a finer grain size raises the yield strength.
- Steel is an Fe–C alloy: as %C rises, strength and hardness rise and ductility falls. Around 0.8 % C is the eutectoid (pearlite).
- Polymers are long chains, amorphous or semicrystalline: thermoplastics (remeltable), thermosets (irreversible crosslinks) and elastomers.
- Ceramics have ionic/covalent bonds: hard, brittle, electrically and thermally insulating.
- 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 (cone, HRC), Vickers (pyramid, HV). It is not a fundamental property but correlates with strength.
- Charpy impact reveals the ductile–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.