Non-metallic Materials: Polymers, Ceramics, Composites and Concrete
Non-metallic Materials treats polymers, ceramics, composites and concrete as engineering materials in their own right. The examiner looks for the correct failure criterion - Weibull for ceramics, viscoelastic limits for polymers, fibre-direction dependence for composites - rather than metal-derived intuition applied to the wrong material.
Polymers
Behaviour is set by molecular weight, chain architecture, crystallinity and glass transition temperature. Below Tg a thermoplastic is glassy and stiff; above Tg it creeps, so the design allowable becomes an isochronous stress-strain curve at the service temperature and time, not a single modulus. Thermosets such as epoxy, polyester and phenolic are cross-linked: they do not melt, but they are brittle and degrade by UV photo-oxidation and, for polyester, by hydrolysis. Environmental stress cracking is slow crack growth in polyethylene under detergent contact well below yield, and is a classic forensic finding in tanks and pipework.
Ceramics
Strength is flaw-controlled and described by a Weibull modulus, typically 5-15 for engineering alumina, so the weakest of a thousand components is far weaker than the tested sample. Design must use a lower-bound strength for the actual stressed volume, apply proof testing, and avoid tensile stress concentrations. Thermal shock resistance scales with sigma_f(1-nu)/(E alpha); transformation-toughened zirconia and fibre reinforcement give rising R-curve behaviour.
Composites
Rule of mixtures gives axial stiffness E1 = Vf Ef + Vm Em; transverse stiffness and shear are far lower and matrix-dominated. Failure modes are fibre fracture, matrix cracking, debonding, delamination and pull-out. Carbon fibre bolted to aluminium forms a strong galvanic cell, so joints need insulating isolation.
Worked example
60 vol% E-glass (72 GPa, 2.54 g/cm3) in epoxy (3.5 GPa, 1.20 g/cm3): E1 = 0.6x72 + 0.4x3.5 = 44.6 GPa; density = 0.6x2.54 + 0.4x1.20 = 2.00 g/cm3; specific stiffness 22.3, below steel at 26.4. The same lay-up in carbon (230 GPa, 1.80) gives 139 GPa and 89 - a genuine step change. The arithmetic, not the label composite, decides the application.
Traps
Applying rule of mixtures to transverse or shear loading; treating Weibull strength as deterministic and ignoring the size effect; checking polymer creep only at 20 C; assuming a laminate is isotropic; ignoring moisture uptake, UV and thermal expansion mismatch over the service life.