Properties of Engineering Materials Notes

Properties of Engineering Materials

Importance of Engineering Materials Properties

  • Understanding engineering materials relies heavily on knowledge of their specific properties.

  • Knowledge is acquired through:

    • Scientifically-designed tests.

    • Previous experiences of engineers, designers, and architects.

  • Study perspectives:

    • Qualitative or descriptive viewpoint.

    • Quantitative viewpoint rooted in scientific testing.

Classification of Properties

  • The range of material properties is diverse. Key properties are categorized as follows:

1. Physical Properties

  • Dimensions & Shape: Physical measurements and configurations.

  • Porosity: Ratio of voids to volume expressed as a percentage.

  • Density or Specific Gravity: Mass per unit volume; density assessed via displacement techniques.

  • Moisture Content: Water content affecting strength and properties.

  • Macrostructure: Visible structure features.

  • Microstructure: Structural features at a microscopic level.

2. Chemical Properties

  • Oxide or Compound Composition: Specific chemical makeup.

  • Acidity or Alkalinity: pH level impact.

  • Resistance to Corrosion or Weathering: Durability against environmental hazards.

3. Physico-Chemical Properties

  • Water-Repellent Action: Ability to resist moisture absorption.

  • Shrinkage and Swell Due to Moisture Changes: Volume changes with humidity variations.

4. Mechanical Properties

  • Strength: Categories include:

    • Tension

    • Compression

    • Shear

    • Flexure (under various conditions: static, impact,
      fatigue, etc.)

  • Stiffness: Resistance to deformation under load.

  • Toughness: Energy absorption before failure.

  • Elasticity: Material's ability to return to original shape.

  • Plasticity: Permanent deformation behavior beyond elastic limit.

  • Ductility: Material's ability to be drawn into wires.

  • Brittleness: Tendency to fracture without significant deformation.

  • Hardness: Resistance against scratching, abrasion, and penetration.

  • Wear Resistance: Durability against wear and tear.

5. Thermal Properties

  • Specific Heat: Heat needed to raise the temperature of unit mass by one degree.

  • Thermal Conductivity: Ability to conduct heat; rated comparatively.

6. Electrical and Magnetic Properties

  • Electrical Conductivity: Movement of electrons and ions; contrast between conductors and insulators.

  • Magnetic Permeability: Relation to electron behavior; categorization includes diamagnetic, paramagnetic and ferromagnetic materials.

7. Acoustical Properties

  • Sound Transmission: Ability for sound waves to pass through.

  • Sound Reflection: Surface behavior regarding sound waves.

8. Optical Properties

  • Light Transmission: Levels of transparency and reflection.

  • Color & Lustre: Identification characteristics.

Important Physical, Thermal, and Electrical Properties

Density and Specific Gravity

  • Density: Defined as mass per unit volume, influenced by:

    • Atomic weights of components.

    • Number of atoms per volume.

  • Measuring methods involve displacement of liquid.

  • Specific Gravity: Ratio of material's density to that of water, with water's gravity being 1.

Porosity

  • Measured as either apparent or true:

    • Apparent: Based on absorbed water.

    • True: Based on true density, involves volume of voids.

  • Porosity impacts properties like strength and heat conductivity.

Moisture Content

  • Affects strength properties; wood is a primary example:

    • Green wood contains 2-3x water compared to dry.

    • Seasoning reduces moisture down to 7-16% and significantly impacts strength.

  • Alters the volume of materials, such as wood swelling or shrinking with moisture changes.

Specific Heat and Thermal Conductivity

  • Specific Heat: Affects heating and cooling rates; important in materials like water due to high specific heat.

  • Thermal Conductivity: Rating materials against silver (100 rating):

    • Metals show high conductivity (e.g., Copper: 90, Aluminium: 48).

    • Non-metals and insulation materials show much lower ratings.

Thermal Expansion

  • General properties of materials include:

    • Expansion and contraction with temperature change; particle energies influence this.

    • Coefficient of thermal expansion correlates with melting point; lower melting points generally exhibit higher expansion rates.

Electrical Properties

Electrical Conductivity

  • Conductors vs. Insulators: Conductivity dictated by atomic structure:

    • Metals are good conductors due to valence electrons.

    • Insulators (like rubber, glass) have high resistivity.

  • Defined resistivities:

    • Conductors: 1.6imes10−81.6 imes 10^{-8} to 1.4imes10−6extextohmmeter1.4 imes 10^{-6} ext{ } ext{ohm meter}.

    • Insulators: 101010^{10} to 1022extextohmmeter10^{22} ext{ } ext{ohm meter}.

Heat Resistance & Refractory Materials

  • Heat resistance determined by:

    • Melting point, chemical stability, oxidation resistance.

  • Refractory materials maintain properties under high temperatures, though may have low thermal/shock resistance.

Magnetic Behavior

Magnetic Properties

  • Arise from electron behavior; unpaired electrons lead to magnetism.

  • Classification of materials based on magnetism:

    • Diamagnetic: All electrons paired.

    • Paramagnetic: Unpaired electrons in metals (e.g., sodium).

    • Ferromagnetic: Unpaired electrons in iron, nickel; potential for permanent magnetism.

  • Curie Point: Temperature above which ferromagnetic materials lose magnetic properties (e.g., pure iron at 768°C).

Optical Properties

  • Optical characteristics determine material identification:

    • Transparency Levels: Transparent, translucent, opaque properties.

    • Lustre Appearance: Reflective properties of clean metal surfaces.

  • Luminescence: Emission of light when cooled, different from lustre.

Mechanical Properties

Significance in Engineering

  • Primary concern for engineers focusing on machine and structure integrity.

Key Mechanical Properties

  • Strength: Resistance to failure.

  • Hardness: Resistance to surface deformation (assessed using Mohs' scale).

  • Elasticity: Ability to return to original form after load application.

  • Stiffness (Young's Modulus): Resistance to load-induced deformation.

  • Plasticity: Allows permanent shape change under stress.

  • Workability (Formability): Ability to deform without failure.

  • Fatigue: Behavior under cyclic loading.

  • Toughness: Energy absorption before fracture; tested through comparative failure energy.

Key Definitions and Tests

  • Hardness Tests and Mohs' Scale:

    • Mohs' Scale: Ranks minerals by scratch hardness (e.g., Talc at 1 to Diamond at 10).

  • Specific material examples:

    • Mild steel is tougher than glass, absorbing more energy before failing.

Summary of Strength Properties Influences

  1. Type of chemical bonding.

  2. Crystalline lattice structure.

  3. Imperfections in the crystal structure.

  4. Macrostructure influencing stress points.