Thermal Properties

MEE1004: Mechanics of Materials - Lecture 8

Thermal properties

Quantify the response of materials to heat.

Two important reference termperature

Melting temperature T_m

Glass transition temperature T_g

Crystalline and non crystalline thermal properties

1. Crystalline materials: defined T at which they melt (T_m)

2. Non-crystalline solids: transition T from a true solid to a very viscous liquid (T_g)

Stiffness of Polymers

Thermal Conductivity (λ)

Heat conduction rate through a solid at steady-state (temperature profile does not change with time)

Unit: W/m.K

Thermal Diffusivity (a)

Conduction rate of a temperature pulse (transient, non-constant flow of energy)

Unit: m²/s

Thermal diffusivity equation

λ: thermal conductivity

C_p: specific heat capacity

ρC_p: volumetric heat capacity

ρ: density

Solids thermal properties

λ/a is a constant

Foams thermal properties

Low λ, do not transmit much heat

Change T relatively quickly

Good for insulating applications

Temperature dependence and material properties

At low temperatures most metals and all polymers become brittle.

Exceptions: FCC metals such as copper, stainless steel.

Creep

Slow and continuous deformation of a material that occurs as a stress less than σ generally at elevated temperatures.

Creep factors

Time, load, (temperature)

Creep curves

Creep v diffusion

Creep requires relative motion of atoms

Diffusion is the spontaneous intermixing of atoms over time

Creep in polycrystalline materials by diffusion

Grain boundaries act as sources and sinks for vacancies

If a vacancy joins a boundary, an atom must leave it (crystal eaten away)

If a vacancy leaves a boundary, an atom must joint it (face grows)

Creep by dislocation climb

The diffusion can unlock dislocation from obstacles

Half plane of atom diffused away - dislocation climb

Creep fracture

Fracture occurs due to creep (tertiary creep stage) by creating voids that nucleate on grain boundaries

Thin walled pressure vessel hoop stress equation

σ: hoop stress or circumferential stress

p: internal pressure

R: radius

t: wall thickness

Thermal conductivity

Rate at which heat is conducted through a solid a steady state (temperature profile does not change with time).

Thermal diffusivity

How quickly a temperature pulse can transverse a material of known thickness when a heat source is applied briefly on one side. Governs transient heat flow.

How strengthening processes affect thermal conductivity

Strengthening mechanisms such as solid solution and precipitation increase the yield strength of metals but introduce scattering sites for phonons which can reduce thermal conductivity.