🔹 Introduction

Choosing a suitable material for machine elements depends on its properties, cost, availability, and other factors.

🔹 Classification of Engineering Materials

Engineering materials are broadly classified into Metals and Non-Metals.

• Ferrous Metals: Cast iron, Wrought iron, Steel
• Non-Ferrous Metals: Aluminium alloys, Copper alloys (Brass, Bronze), Nickel, Silver, Zinc, etc.

🔹 Ferrous Materials

Cast Iron

Alloy of iron, carbon, and silicon. Hard and brittle. Carbon content: 1.7%–3%.

• Grey Cast Iron – Graphite form, inexpensive, high compressive strength.
• White Cast Iron – Carbon as Fe₃C, hard, abrasion resistant.
• Malleable Cast Iron – Annealed white cast iron, tough, machinable.
• Spheroidal/Nodular Cast Iron – Graphite in nodules, high tensile strength.
• Austenitic Cast Iron – Alloyed with Ni, Cr, Mo, Cu, Mn; used in automobile parts.
• Abrasion Resistant Cast Iron – Alloyed for wear resistance.

Wrought Iron

99.5% pure iron, tough, malleable, ductile, easily forged/welded. Used in chains, crane hooks, railway couplings.

Steel

Most important engineering material. Alloy of iron and carbon (<1.7%).

• Plain Carbon Steel – Properties depend on carbon percentage.
• Alloy Steel – Contains Ni, Cr, W, V, Mn, Si, Co, Mo for special properties.
• Stainless Steel – Corrosion resistant, e.g., 18/8 steel (18% Cr, 8% Ni).

🔹 Non-Ferrous Metals

Aluminium

Lightweight, corrosion resistant, strengthened with Cu, Mn, Si, Mg.

Duralumin

Al alloy with 4% Cu, 0.5% Mn, 0.5% Mg. Used in automobiles and aircraft.

Y-Alloy

Al alloy with Cu, Mn, Ni, Si. High strength at elevated temperatures.

Magnalium

Al alloy with 2–10% Mg. Lightweight, strong, used in aircraft and automobiles.

Copper Alloys

• Brass (Cu-Zn): Ductility varies with Zn content. Highly corrosion resistant.
• Bronze (Cu-Sn): Hard, brittle at high Sn content. Gunmetal (Cu-Sn-Zn) used in boiler fittings.

🔹 Non-Metals

• Timber: Low cost, elastic, used in foundry patterns.
• Leather: Flexible, wear resistant, used in belt drives.
• Rubber: High bulk modulus, used for sealing and vibration isolation.
• Plastics: Thermosetting (Bakelite, Epoxy) and Thermoplastics (PVC, Polythene).

🔹 Mechanical Properties of Materials

• Elasticity: Regain shape after deformation.
• Plasticity: Permanent deformation beyond yield point.
• Hardness: Resistance to indentation (Brinell, Rockwell, Vickers tests).
• Ductility: Ability to elongate before rupture.
• Malleability: Ability to be rolled into sheets.
• Brittleness: Sudden fracture without warning.
• Resilience: Resistance to shock, energy absorption.
• Toughness: Ability to absorb energy before fracture.
• Creep: Slow deformation under constant load over time.

🔹 Equations & Figures

Plasticity Criterion (Mises-Henky):

\[(\sigma_1 - \sigma_2)^2 + (\sigma_2 - \sigma_3)^2 + (\sigma_3 - \sigma_1)^2 = 2\sigma_y^2\]

Plastic Flow Condition:

If \(P > P_m\), plastic flow occurs.

Resilience Equation:

Strain energy/unit volume = \(\frac{\sigma^2}{2E}\)