Engineering Materials & Mechanics Basics Notes

Last reviewed 16 Sept 2026 · 9 min read

Classification of engineering materials

Group	Examples	Characteristics
Ferrous metals	Cast iron, wrought iron, carbon steels, alloy steels, stainless steel	Iron-based; strong, economical; most (except stainless) prone to corrosion
Non-ferrous metals	Aluminium, copper, zinc, lead, tin, titanium, nickel and their alloys	Corrosion resistance, conductivity, lightness (Al, Ti)
Polymers (plastics)	Thermoplastics (PVC, polyethylene), thermosets (epoxy, Bakelite), elastomers	Light, corrosion resistant, low strength and stiffness, temperature sensitive
Ceramics and glasses	Bricks, tiles, cement, glass, alumina	Hard, brittle, heat resistant, strong in compression
Composites	Reinforced concrete, FRP, plywood, fibre-reinforced cement	Combine properties of constituents

Ferrous materials

Material	Carbon content (approx.)	Properties / uses
Wrought iron	Very low (below about 0.1%) with slag	Ductile, malleable, corrosion resistant; largely obsolete
Low carbon (mild) steel	Up to about 0.25–0.3%	Ductile, weldable, tough — structural sections, reinforcement bars, sheets
Medium carbon steel	About 0.3–0.6%	Stronger — shafts, rails, gears
High carbon steel	About 0.6–1.5%	Hard — tools, springs, wires
Cast iron	About 2–4% (above the steel range of roughly 2%)	Brittle, strong in compression, good castability, damping — machine beds, pipes, manhole covers
Alloy steels	With Ni, Cr, Mo, V, Mn etc.	Improved strength, toughness, hardenability
Stainless steel	At least about 10.5% chromium (forms passive oxide film)	Corrosion resistant — kitchen equipment, cladding, chemical plants

Iron–carbon diagram: steel up to about 2.1% C; cast iron beyond (practical cast irons 2–4% C).
Types of cast iron: grey (graphite flakes, machinable), white (hard, brittle), malleable, ductile (spheroidal graphite/nodular) — ductile iron pipes for water supply.

Non-ferrous alloys

Alloy	Composition	Uses
Brass	Copper + zinc	Fittings, valves, taps
Bronze	Copper + tin	Bearings, bushes, statues
Duralumin	Aluminium + copper + magnesium + manganese	Aircraft structures
Gunmetal	Copper + tin + zinc	Valves, pump parts
Solder	Tin + lead (or lead-free alloys)	Joining

Mechanical properties

Property	Meaning
Strength	Ability to resist load without failure (tensile, compressive, shear)
Stiffness	Resistance to deformation — measured by modulus of elasticity
Elasticity	Ability to regain original shape after load removal
Plasticity	Ability to undergo permanent deformation without fracture
Ductility	Ability to be drawn into wires under tension (large elongation) — mild steel, copper, aluminium
Malleability	Ability to be hammered/rolled into sheets under compression — gold, lead, aluminium
Hardness	Resistance to indentation, scratching, wear
Toughness	Ability to absorb energy before fracture (area under stress–strain curve)
Resilience	Energy absorbed within elastic limit (modulus of resilience $σ_{y}^{2} /2 E$ )
Brittleness	Fracture with little deformation — cast iron, glass, concrete
Fatigue	Failure under repeated/fluctuating stresses below static strength; endurance limit
Creep	Slow time-dependent deformation under constant load, especially at high temperature
Machinability, weldability, castability	Manufacturing properties

Stress–strain diagram of mild steel (tension test)

Key points:

Proportional limit — end of linear (Hooke's law) region.
Elastic limit — maximum stress without permanent set.
Upper and lower yield points — sudden yielding (plastic flow) at nearly constant stress.
Strain hardening region — stress rises with strain.
Ultimate tensile strength — maximum stress; necking begins.
Fracture (breaking) point — engineering stress appears lower due to reduced area.

Percentage elongation and reduction in area measure ductility.
Materials without a clear yield point (aluminium, high-strength steel) use 0.2% proof stress.
Brittle materials (cast iron, concrete) fail with little plastic deformation.

Hooke's law and elastic constants

FormulaElastic relations

Stress $σ = P / A$ ; strain $ε = δ L / L$ Hooke's law: $σ = E ε$ → elongation $δ L = \frac{P L}{A E}$ Poisson's ratio $ν = - \frac{lateral strain}{longitudinal strain}$ (steel ≈ 0.3; concrete ≈ 0.15–0.2) Shear: $τ = G γ$ Relations: $E = 2 G (1 + ν) = 3 K (1 - 2 ν)$ , and $E = \frac{9 K G}{3 K + G}$

Typical E: steel ≈ 200 GPa, aluminium ≈ 70 GPa, copper ≈ 110–120 GPa, concrete (M25) ≈ 25 GPa.

Hardness and impact tests

Test	Method	Notes
Brinell (HB)	Hardened steel/tungsten carbide ball (commonly 10 mm) pressed with load (e.g. 3000 kgf); indentation diameter measured	$H B = \frac{2 P}{π D ( D - D ^{2} - d ^{2} )}$ ; suits softer/medium metals, castings
Rockwell (HRB, HRC …)	Depth of penetration by ball or diamond cone under minor and major loads	Quick, direct reading; widely used
Vickers (HV)	Square-based diamond pyramid (136°)	All materials, thin sections, microhardness
Mohs scale	Scratch hardness 1–10	Minerals
Izod and Charpy impact tests	Pendulum strikes a notched specimen; energy absorbed measured	Toughness, ductile–brittle transition; Izod — cantilever specimen; Charpy — simply supported

Heat treatment of steels

Process	Procedure	Purpose
Annealing	Heat above critical temperature, slow furnace cooling	Soften, improve ductility and machinability, relieve stresses
Normalising	Heat above critical temperature, cool in still air	Refine grain structure, uniform properties (stronger than annealed)
Hardening	Heat above critical temperature, rapid quenching (water/oil)	Increase hardness (martensite) — but brittle
Tempering	Reheat hardened steel to a lower temperature, then cool	Reduce brittleness, relieve stresses, improve toughness
Case hardening (carburising, nitriding, cyaniding, induction/flame hardening)	Harden surface only	Hard wear-resistant surface with tough core — gears, cams

Thermo-mechanically treated (TMT) bars used as reinforcement are produced by quenching the hot-rolled bar surface and self-tempering — giving a hard outer layer and ductile core.

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