1.2 Engineering Materials

1.2 Engineering Materials

1. Mechanical Properties

1. Strength

   • Capacity to withstand external forces without fracturing or yielding

2. Stiffness

   • Resistance to deformation when subjected to stress

   • Measured by modulus of elasticity

3. Elasticity

   • Ability to return to original shape after deformation when forces are removed

   • Example: Steel has greater elasticity than rubber

4. Plasticity

   • Ability to retain deformation permanently under load

   • Essential for forgings, coin stamping, ornamental work

5. Ductility

   • Capacity to be drawn into wire under tensile force

   • Order of decreasing ductility: Mild steel, copper, aluminum, nickel, zinc, tin, lead

6. Brittleness

   • Tendency to fracture with minimal permanent distortion

   • Example: Cast iron

7. Malleability

   • Ability to be shaped into thin sheets through rolling/hammering

   • Order of decreasing malleability: Lead, soft steel, wrought iron, copper, aluminum

8. Toughness

   • Ability to withstand fracture from high-impact loads (e.g., hammer blows)

   • Decreases with elevated temperatures

9. Resilience

   • Capacity to absorb energy and endure shock/impact loads

   • Quantified by energy absorbed per unit volume within elastic limit

   • Essential for springs

10. Creep

    • Gradual permanent deformation under constant stress at high temperatures over time

    • Critical in design of engines, boilers, turbines

11. Fatigue

    • Failure under repeated stresses below yield point

    • Involves progressive microscopic cracking

    • Critical for shafts, connecting rods, springs, gears

12. Hardness

    • Resistance to wear, scratching, deformation, machinability

    • Ability to cut another metal

2. Material Testing

Purposes of Testing

• Ensure quality

• Test properties

• Prevent failure in use

• Make informed material choices

Forms of Testing

1. Mechanical Tests

   • Material tested to destruction

   • Measures strength, hardness, toughness, etc.

2. Non-Destructive Tests (NDT)

   • Samples/articles tested without damage

Tensile Test

• Evaluates: Strength, Ductility, Elasticity, Stiffness

• Ultimate tensile strength typically reached at Necking

• Slope of linear stress-strain curve = Modulus of Elasticity

• True stress at fracture > Ultimate stress for ductile materials

Hardness Tests

1. Brinell Hardness Test

   • Uses ball-shaped indenter

   • Not for thin materials

   • Formula:

$$\mathrm{HBW} = 0.102\frac{2F}{\pi D(D - \sqrt{D^2 - d^2})}$$

Where:

• $F$ = applied force (N)

• $D$ = diameter of indenter (mm)

• $d$ = diameter of indentation (mm)

1. Vickers Hardness Test

   • Uses square pyramid indenter

   • Accurate; measures diagonal length

   • Used for very hard materials

2. Rockwell Hardness Test

   • Uses diamond cone or steel ball indenter

   • Measures depth of indentation

   • Hardness number format: HRX (e.g., HRC)

Impact Test

• Measures toughness

• Types:

  1. Charpy – Horizontal specimen, U/V-notch

  2. Izod – Vertical specimen, V-notch

• Energy absorbed formula:

$$E(J) = WgR(\cos \beta - \cos \alpha)$$

Bending Test

• Evaluates flexural strength

Creep Test

• Creep strength = max stress causing specified creep in given time at constant temperature

• Three stages:

  1. Primary – decreasing creep rate

  2. Secondary – constant creep rate

  3. Tertiary – rapid creep to failure

Fatigue Test

• Subject specimen to alternating bending stress

• Failure occurs after certain load cycles

• Key terms:

  • $N$ = load cycles

  • $\sigma$ = stress load

  • $K$ = short-term strength

  • $Z$ = fatigue strength

  • $D$ = endurance strength

  • $N_0$ = limit load cycles

3. Fatigue of Metals

• Fatigue Failure: Caused by repetitive/fluctuating stress below tensile strength

• Accounts for ~90% of service failures

• Most influential stress: Tensile stress

• Appearance: Smooth surface with beach marks

• No plastic deformation (no warning)

Stress Cycles Nomenclature

• $\sigma_{\max}$ = Maximum stress

• $\sigma_{\min}$ = Minimum stress

• Stress range:

$$\Delta \sigma = \sigma_{\max} - \sigma_{\min}$$

• Alternating stress:

$$\sigma_{a} = \frac{\Delta\sigma}{2} = \frac{\sigma_{\max} - \sigma_{\min}}{2}$$

• Mean stress:

$$\sigma_{m} = \frac{\sigma_{\max} + \sigma_{\min}}{2}$$

• Stress ratio:

$$R = \frac{\sigma_{\min}}{\sigma_{\max}}$$

• Amplitude ratio:

$$A = \frac{\sigma_{a}}{\sigma_{m}} = \frac{1 - R}{1 + R}$$

S-N Curve

• Plot of stress (S) vs. number of cycles (N)

• High cycle fatigue: $N > 10^5$

• Low cycle fatigue: $N < 10^5$

• Fatigue limit: Stress below which material can endure infinite cycles (~$10^8$ cycles)

• Non-ferrous metals do NOT have fatigue limit

Fatigue Crack Propagation Stages

1. Stage I – Non-propagating crack (~0.25 nm/cycle)

2. Stage II – Stable propagation (widely studied)

3. Stage III – Unstable propagation → failure

4. Creep and Stress Rupture

• Creep: Time-dependent, slow deformation at high temperature under constant stress

• Primary mechanism: Diffusion of atoms

• Creep Test: Measures dimensional changes over hours/days

• Stress Rupture Test: Measures time to failure under high stress and temperature

• Key difference: Creep focuses on deformation; stress rupture focuses on time to failure

5. Corrosion and Control

• Corrosion: Deterioration due to chemical/electrochemical reaction with environment

Types of Corrosion

1. Dry (Chemical) Corrosion

   • Direct reaction with dry gases (O₂, H₂S, SO₂, halogens)

   • Oxidation corrosion (reaction with O₂)

   • Hydrogen embrittlement (H₂S → atomic hydrogen → cracking)

2. Wet (Electrochemical) Corrosion

   • Requires electrolyte and two dissimilar metals

   • Galvanic corrosion – metal with higher negative potential acts as anode

3. Uniform Corrosion

   • Even attack across surface

   • Controlled by coatings, material selection, maintenance

4. Pitting Corrosion

   • Localized holes/pits

   • Caused by chemical imbalances or oxide layer defects

5. Crevice Corrosion

   • In narrow, confined spaces (joints, gaskets)

   • Controlled by design (eliminate crevices), material selection

6. Hydrogen Embrittlement

   • Hydrogen atoms penetrate metal lattice → brittleness

   • Avoid hydrogen sources, use proper materials

7. Intergranular Corrosion

   • Along grain boundaries (due to impurities/sensitization)

   • Use low-carbon stainless steel, heat treatment

8. Stress Corrosion Cracking (SCC)

   • Combined tensile stress + corrosive environment

   • Avoid susceptible materials, reduce stress

9. Microbiologically Influenced Corrosion (MIC)

   • Caused by microorganisms (bacteria)

   • Controlled by monitoring, biocides, coatings

Corrosion Control Methods

• Protective coatings (paint, plating)

• Sacrificial anodes (less noble metal corrodes preferentially)

• Cathodic protection (apply electric current)

• Proper material selection

• Design modifications (eliminate crevices)

• Regular inspection/maintenance

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