ME1301 Chapter 1: Properties and Testing Methods of Metals

Introduction to Engineering Materials

• Definition of Engineering Materials: These are substances with properties that make them useful in the construction of structures, machines, devices, or products.

• Categories of Materials:

  • Metals
  • Ceramics
  • Semiconductors
  • Polymers (plastics)
  • Glasses
  • Composites
  • Natural substances (e.g., wood)

• Material Selection Process:

  • The selection of materials for components is based on matching the material's properties to the specific service conditions of the component.
  • Fundamental Principle: The properties of materials depend directly upon their internal structure.
  • Selection Steps:
    1. Analyze the application to identify the most important characteristics needed (e.g., strength, stiffness, ductility).
    2. Assess the environment: Will it face repeated force, sudden intense force, high stress with elevated temperatures, or abrasive conditions?
    3. Select appropriate material using handbook data.

• Historical and Modern Context:

  • The Wright Brothers: Used innovative design with common materials of their time: wood, cotton fabric, and steel.
  • Space Travel: Saw over $50$ years of improving use of conventional materials like aluminium for aerodynamic crafts.
  • Modern Aviation: Composite materials have replaced many metallic components in aircraft such as the Boeing 787 and Jumbo A380.

Fundamental Mechanical and Physical Properties

• Mechanical Property: Defined as the characteristic response of a material when a force is applied. Examples include:

  • Strength: The ability to resist larger forces before breaking.
  • Hardness: Resistance to indentation or abrasion.

• Mechanical Testing: The active determination of mechanical properties through methods such as the tensile test.

• Physical Properties: Characteristics determined by methods other than mechanical force application, including:

  • Density
  • Electrical resistivity
  • Thermal conductivity

• Strength Defined: The ability to resist the application of force without rupture.

  • Example: Steel is stronger than aluminium because a steel structure can withstand a higher force before breaking than an identical aluminium structure.

• Types of Applied Forces:

  • Tensile Force: A pulling force acting perpendicular to the surface of the material.
  • Compressive Force: A pressing force acting perpendicular to the surface of the material.
  • Shear Force: A force acting parallel to the surface of the material (e.g., rivets under shear stress).

Concept of Stress

• Definition: The ratio of the applied force to the cross-sectional area of the material on which the force acts.

• Formula for Stress:$\text{Stress} = \frac{\text{Force (N)}}{\text{Cross-sectional Area (mm}^2\text{)}}$

• Types of Stress: Tensile stress, compressive stress, and shear stress.

• Material-Specific Stress Behavior:

  • Steel: Possesses roughly equal compressive and tensile strength.
  • Cast Iron: High compressive strength but low tensile strength.
  • General Rule for Metals: Shear strength is virtually always less than tensile strength.

• Activity 1 Calculation: For a round shaft with diameter $d = 10\,mm$ subjected to a force $F = 30,000\,N$:

  • Area $A = \frac{\pi \times d^2}{4} = \frac{\pi \times 10^2}{4} = 78.54\,mm^2$
  • $\text{Stress} = \frac{30,000}{78.54} = 381.97\,N/mm^2$

The Tensile Test and Standard Specimens

• Purpose: To determine specific mechanical properties by subjecting a test piece of known dimensions to an increasing tensile force until fracture.

• The Test Piece: Materials are machined to standard dimensions to eliminate shape variation effects.

  • Exception: For plain mild steel bars for concrete reinforcement under $40\,mm$ diameter, British Standards specify testing as-rolled bars.
  • Gauge Length ($L_0$): The specific part of the specimen where the test is conducted.
  • Standard Formula (Round Test Piece):$L_0 = 5.65 \times \sqrt{A_0}$$L_0 = 5 \times d$         (where $A_0$ is the original cross-sectional area and $d$ is the original diameter).

• Tensile Properties and the Force-Extension Curve:

  • Elasticity: For forces lower than point A (Elastic Limit), extension is proportional to force. The material returns to its original shape if the force is removed.
  • Elastic Limit (Point A): The point beyond which the material will no longer return to its original dimensions.
  • Yield Point (Point Y): The stress required to start plastic deformation. Removing the force at this stage results in a small permanent extension.
  • Formula for Yield Stress:$\text{Yield Stress} = \frac{\text{Force at yield point (N)}}{\text{Original Cross-sectional area (mm}^2\text{)}}$
  • Ultimate Tensile Strength (U.T.S.) (Point M): The ability to resist tensile force without rupture; the maximum force reached during the test.
  • Formula for Tensile Strength:$\text{Tensile Strength} = \frac{\text{Maximum Force (N)}}{\text{Original Cross-sectional area (mm}^2\text{)}}$
  • Necking: Occurs after point M, leading to fracture.

Proof Stress (Yield Strength for Non-Defining Materials)

• Context: Materials like heat-treated steels and many alloys do not show a well-defined Yield Point. In these cases, Proof Stress is used.

• Definition: The stress required to start a specified amount of permanent extension.

  • 0.1% Proof Stress: The stress producing a permanent extension of $0.1\%$ of the gauge length.
  • Other Standards: Sometimes $0.2\%$ or $0.5\%$ strain is specified.

• Ductility Indicators:

  • Percentage Elongation:$\text{Percentage elongation} = \frac{L_u - L_0}{L_0} \times 100\,\%$         (where $L_u$ is the final gauge length and $L_0$ is the original gauge length).
  • Percentage Reduction in Area:$\text{Percentage reduction in area} = \frac{A_0 - A_u}{A_0} \times 100\,\%$         (where $A_u$ is the smallest cross-sectional area after fracture).

Engineering vs. True Stress/Strain

• Engineering Stress: $\frac{\text{Actual Force}}{\text{Original Area}}$
• True Stress: $\frac{\text{Actual Force}}{\text{Actual Area}}$
  • Property: True stress is always larger than engineering stress because the actual area decreases during the tensile test.
• Strain:$\text{Strain} = \frac{\text{Extension (mm)}}{\text{Gauge Length (mm)}}$
• Engineering Strain: $\frac{\text{Actual Extension}}{\text{Original Gauge Length}}$

Young's Modulus of Elasticity ($E$)

• Definition: The ratio of Stress to Strain within the elastic region (between point O and A on the curve).
• Physical Meaning: Indicates the stiffness of a material, which determines elastic deflection under load.
• Stiffness Examples:
  • A stiff tennis racket offers better ball control than a less stiff one.
  • Steel beams deflect less than aluminium beams because steel has a higher $E$ value.
• Calculation: Represented by the slope (gradient) of the linear portion of the stress-strain curve.     $E = \frac{\text{Stress}}{\text{Strain}}$
• Comparison Values:
  • Steel: $E \approx 206\,kN/mm^2$
  • Aluminium: $E \approx 69\,kN/mm^2$

Specific Material Properties

• Ductility: The ability to undergo permanent plastic deformation under tensile stress before fracture.
  • Example: Copper is highly ductile (wire drawing).
  • High ductility is indicated by high percentage elongation ($\approx 70\%$) and reduction of area.
• Malleability: The ability to undergo permanent plastic deformation under compressive stress without fracture.
  • Key for: Hammering, forging, riveting, and cold heading.
  • Examples: Lead, gold, silver, copper.
• Brittleness: The tendency to fracture without visible permanent plastic deformation.
  • Examples: Chalk, fully hardened steel, cast iron.
  • Note: Excessive cold working causes brittleness.
• Elasticity: Ability to undergo temporary deformation and return to original size.
  • Example: Springs and elastomers.
• Plasticity: Ability to undergo permanent deformation.
  • Example: Hot forging crankshafts or horseshoes.
• Comparison at Fracture:
  • Brittle metals: Very small elongation/area reduction; appearance is crystalline.
  • Ductile metals: Higher elongation/area reduction; appearance is fibrous or "cup and cone."

Hardness Testing Methods

• General Principles: Hardness is usually directly proportional to tensile strength and inversely proportional to machinability. Increased hardness typically results in decreased ductility.

• Brinell Hardness ($HB$):

  • Procedure: A hardened steel ball of diameter $D$ is forced into the surface with a standard load $F$ for $15$ seconds.
  • Formula:$HB = \frac{2F}{\pi D(D - \sqrt{D^2 - d^2})}$
  • Minimum Thickness: Must be $\\ge 15$ times the depth of impression for soft materials, and $\\ge 7$ times for hard materials.
  • Advantages: Close relation to U.T.S.; uses a single linear scale.
  • Disadvantages: Requires different loads for different materials; extremely hard metals can deform the indentor ball.

• Vickers Pyramid Hardness ($HV$):

  • Indentor: Square-based diamond pyramid with a $136^\circ$ apex angle.
  • Formula:$HV = 1.854 \times \frac{F}{d^2}$         (where $d$ is the arithmetic mean of the two diagonal lengths $d_1$ and $d_2$).
  • Advantages: Geometrically similar impressions regardless of depth; no need for $F/D^2$ ratios; accurate for very hard materials; suitable for thin sheets.
  • Disadvantages: Requires extensive surface finish preparation; expensive indentor.

• Rockwell Hardness ($HRA, HRB, HRC$):

  • Mechanism: Measures the depth of penetration under two loads (minor and major).
  • Minor Load ($10\,kgf$): Used to take up "slackness" and eliminate surface irregularity errors.
  • Scales:
    • Scale B: $100\,kgf$ major load, $1/16$ inch steel ball. Used for copper alloys and aluminium.
    • Scale C: $150\,kgf$ major load, $120^\circ$ diamond cone (Brale). Used for hardened steel harder than B-100.
    • Scale A: $60\,kgf$ major load, Brale indentor. Used for cemented carbide and thin steel.
  • Advantages: Rapid; direct digital/dial readout.

Impact, Fatigue, and Creep

• Toughness: The ability to withstand shock or impact loading. It is the energy required to fracture a material.

  • Ductile metals are generally tougher than brittle ones.

• Impact Tests:

  • Purpose: To indicate toughness, brittleness, monitoring heat treatment, and quality control of welds.
  • Izod Test: Test piece is held at one end (cantilever fashion).
  • Charpy Test: Test piece is supported at both ends; striker hits the centre.
  • Notching: A notch is machined into the piece to create a stress concentration point to encourage fracture.
  • Hounsfield Balanced Impact Test: Uses two pendulums moving in opposite directions to strike an $8\,mm$ diameter specimen.

• Fatigue: Failure at stresses far below yield strength due to repeated stress cycles (reversed, alternating, or fluctuating).

  • Fatigue Resistance: Critical for shafts, spindles, and springs.

• Creep: Slow, permanent deformation under a constant load below the tensile strength, usually occurring at high temperatures.

  • Significance: Crucial for gas turbines, steam turbines, and boilers.

Data Reference: Material Properties Table

Discussion and Critical Thinking

• Ceramic Coffee Cups: Can support the weight of a fire truck if loaded in pure compression but break easily when dropped because they are brittle and cannot withstand the sudden impact/shock load.
• Material Failure Cases: Material-related factors played significant roles in historical disasters such as the sinking of the Titanic and the collapse of the World Trade Centre on September 11, 2001.