Technical terminology from mechanical, civil, electrical, and chemical engineering disciplines.
Annealing is a heat treatment that softens a metal: you heat it, hold it, and cool it slowly. It restores ductility and relieves stress. Hardening does the opposite, using a fast quench to make steel harder and stronger.
Brittleness is a material's tendency to fracture with little or no plastic deformation, absorbing little energy before it breaks. It shows up as low elongation and reduction of area in a tensile test and low absorbed energy in a Charpy impact test.
Ductility is how far a material can be stretched before it breaks. It is measured in a tensile test, as percent elongation or percent reduction of area. Steel, copper and aluminium are ductile; glass and cast iron are not.
Elasticity is a material's ability to return to its original shape after a load is removed. In engineering, it is measured by a modulus such as Young's modulus. In economics, the same word means how strongly demand responds to price.
Hardness is a material's resistance to permanent indentation. It is measured by pressing a hard indenter into the surface under a set load, and reported on a scale such as Rockwell (HRC), Brinell (HB) or Vickers (HV). Each scale has its own number, so a hardness value must always name its scale.
Microstrain (με) is strain expressed in parts per million: 1 με = 10⁻⁶ = 0.0001 %. It is the everyday unit for the small elastic strains measured in metals and structures, mostly with strain gauges.
Quenching is the rapid cooling of a metal from a high temperature by immersion in a cooling medium such as water, oil, or air, to lock in a hardened microstructure.
Shear strain is the change in angle a material undergoes under shear, written γ and measured in radians. For small movements, γ = δ ÷ L. In the elastic range it is tied to stress by the shear modulus G, through τ = G × γ.
Specific weight is the weight of a substance per unit volume, found by multiplying density by gravity (γ = ρ × g). Water is about 9.81 kN/m³, or 62.4 lb/ft³. Engineers also call it unit weight, especially for soil and concrete.
Stress is the internal force per unit area in a loaded material: σ = F ÷ A. Normal stress pulls or pushes across a section, and shear stress slides along it. It is measured in pascals (N/m²), MPa or psi.
Tempering is a heat treatment applied to quenched steel by reheating to a temperature below the critical point and slow cooling, to reduce brittleness and relieve stresses while retaining acceptable hardness.
Tensile strength, or ultimate tensile strength (UTS), is the highest stress a material can carry in tension: the maximum force in a tensile test divided by the original cross-section. Ductile metals reach it before they neck and break.
Tolerance is the permissible range of variation in a dimension or characteristic, defining the acceptable upper and lower limits within which a manufactured part is considered correct.
Torsion is the twisting of a shaft or bar about its long axis by an applied torque. It produces shear stress in the cross-section and an angle of twist along the length.
Torsional stress is the shear stress a torque produces in a shaft. In a round section it rises from zero at the centre to a maximum at the surface: τ = T·r ÷ J, or τ = 16T ÷ πd³ for a solid shaft.
Yield strength is the stress at which a material stops springing back and starts to deform permanently. For metals without a sharp yield point it is defined as the stress that leaves 0.2% permanent strain — the 0.2% proof stress, R_p0.2.