- Physical Properties Definition: Physical properties of materials are those characteristics that can be observed without changing the material itself, such as color, density, and melting point.
- Density Explained: Density is a fundamental property defined as mass per unit volume, important for identifying material suitability in different applications.
- Specific Heat: Specific heat determines how much heat a material can absorb before it changes temperature, crucial for engineering thermal properties.
- Thermal Expansion: Materials expand when heated, and the coefficient of thermal expansion quantifies this change, vital for engineering applications.
- Physical Properties of Materials: Understanding these properties is essential for selecting materials that will perform well under specific conditions in engineering tasks.
Engineers compare material properties before choosing a material for a product, process or operating environment. The physical properties of materials can be measured without changing chemical identity. This page also includes several mechanical and manufacturing properties, whose reported values depend on the test method, temperature, direction, loading rate and material condition.
- Density
- Specific gravity
- Phase-change temperatures
- Coefficients of thermal expansion
- Specific heat capacity
- Specific latent heat
- Fluidity
- Weldability
- Elasticity
- Plasticity
- Porosity
- Thermal conductivity
- Electrical Conductivity
Density of Materials
Density of a material is its mass divided by its volume. The symbol is ρ. In the SI system, mass density is measured in kg/m3. Density normally changes with temperature and pressure, so precise values should state the measurement conditions.
If m is mass in kg and V is volume in m3,
the material density is
Specific Gravity of Materials
Specific gravity, also called relative density, is the ratio of a material’s density to a stated reference density at specified conditions. Water is the usual reference for solids and liquids. Because it is a ratio of two densities in the same units, specific gravity has no unit.
State Change Temperatures
A substance can occur as a solid, liquid or gas. A phase-change temperature marks equilibrium between phases, but its value depends on pressure and composition. Glass transitions and solid-state transformations are additional changes that do not fit this simple three-state list.
The common phase-change temperatures are:
Melting point-For a pure substance at a stated pressure, this is the equilibrium temperature at which solid and liquid phases coexist. It may be stated in oC or K.
Boiling point-This is the temperature at which a liquid’s vapour pressure equals the external pressure. It changes when the external pressure or composition changes and may be stated in oC or K.
Freezing point-For a pure substance at equilibrium and a stated pressure, the freezing point equals the melting point. In practice, a liquid can supercool before crystals nucleate, so the observed start of freezing may be lower than the equilibrium temperature stated in oC or K.
Coefficient of Thermal Expansion
Most materials change dimensions when temperature changes. A coefficient of thermal expansion relates fractional dimensional change to temperature change over a stated range. The value can depend on temperature and direction; materials with anisotropic structure need direction-specific coefficients. Three coefficients commonly used for isotropic geometry are:
Coefficient of Linear Thermal Expansion
The coefficient of linear thermal expansion relates change in length to original length and temperature change. It is denoted by αL.
Here, l is initial length, Δl is change in length and Δt is temperature change. The unit of αL is K⁻¹ or per oC.
Coefficient of Area Thermal Expansion
The coefficient of area thermal expansion relates change in area to original area and temperature change. It is denoted by αA.
Here, A is initial area, ΔA is change in area and Δt is temperature change. The unit of αA is K⁻¹ or per oC.
Coefficient of Volume Thermal Expansion
The coefficient of volume thermal expansion relates change in volume to original volume and temperature change. It is denoted by αV.
Here, V is initial volume, ΔV is change in volume and Δt is temperature change. The unit of αV is K⁻¹ or per oC.
Specific Heat of Materials
Specific heat capacity is the heat required per unit mass for a temperature rise of 1oC under stated conditions. This page denotes it by S; the symbol c is also common.
Here, m is mass in kg, Q is heat transferred in joules and Δt is the temperature change. The coherent SI unit is J/(kg·K); a temperature interval of 1oC equals 1 K.
Latent Heat of Materials
Specific latent heat, denoted here by L, measures the energy absorbed or released per unit mass during a phase change at stated conditions. During an equilibrium phase change at constant pressure, energy can transfer while temperature remains at the transition value.
Here, Q is energy transferred in joules and m is mass in kg. The coherent SI unit of specific latent heat is J/kg.
Fluidity of Materials
In rheology, fluidity is the reciprocal of dynamic viscosity for a Newtonian fluid. In metal casting, fluidity instead means how far molten metal can flow and fill a mould before solidifying. Casting fluidity also depends on temperature, solidification range, surface tension, mould conditions and geometry, so it is not simply the inverse of viscosity.
Weld Ability of Materials
Weldability describes whether a material and joint can be welded by a specified process to meet the required performance. It depends on composition, thickness, joint design, heat input, restraint, preheat, filler material and post-weld treatment rather than on the base material alone.
Elasticity of Materials
Elasticity is the ability to recover deformation after a load is removed. Recovery is expected only while the material remains within its elastic range and may be time-dependent in polymers and other viscoelastic materials.
Plasticity of Materials
Plasticity is the ability to undergo permanent deformation without fracture. When stress exceeds the elastic limit or yield condition, removing the load leaves a permanent strain; the amount depends on material state, temperature, loading rate and stress condition.
Porosity of Materials
Porosity is the fraction of a material’s bulk volume occupied by voids. Pores may be open or closed and can arise from trapped gas, solidification shrinkage, particle packing, sintering, degradation or deliberate processing. Porosity can affect density, strength, permeability, thermal conductivity and electrical insulation.
Thermal Conductivity of Materials
Thermal conductivity relates conductive heat flux to temperature gradient in Fourier’s law. A higher value means more heat flows for the same geometry and temperature difference.
For one-dimensional steady conduction, it can be described as the heat-transfer rate through unit area divided by the temperature gradient normal to that area. The property can depend on temperature and direction.
Its coherent SI unit is W/(m·K).
Electrical Conductivity of Materials
Electrical conductivity, denoted by σ, relates current density to electric field under stated conditions. For an isotropic material in the linear regime, conductivity is the reciprocal of resistivity. The coherent SI unit is siemens per metre, S/m; mho per metre is an older name.
