Material Properties: Strength, Ductility, and Toughness

Overview

• Key material properties: Strength, Ductility, Toughness
• Importance in engineering and design

Strength

• Definition: Measure of stress a material can withstand
• Types of Strength:
  • Ultimate Strength: Maximum stress reached during a tensile test
  • Yield Strength: Stress at which material begins to deform plastically
• Stress-Strain Curve:
  • Visual representation from a tensile test
  • Ultimate tensile strength = peak point on the curve
  • Yield strength = point of permanent deformation
• Engineering Design:
  • Structures designed to deform elastically
  • Yield strength is a common failure criterion
• Fatigue Failure:
  • Occurs below yield strength if load varies with time
  • Will be covered in a separate video

Yield Strength Measurement

• Not always clear for all materials
• 0.2% Offset Method:
  • For unclear transition from elastic to plastic
  • Draw line with slope of Young's Modulus and shift 0.2% to the right on curve

Compressive vs. Tensile Strength

• Ductile Materials (e.g., mild steel):
  • Similar yield and ultimate strengths in tension and compression
• Brittle Materials (e.g., concrete, ceramics):
  • Much higher strength in compression than tension
  • Explanation will be provided later

Examples of Strength Values

• Graphene:
  • Strongest tested material
  • Ultimate tensile strength: up to 130 GPa (19,000 ksi)

Ductility

• Definition: Ability of a material to deform plastically before fracture
• Ductile Materials:
  • Large amounts of plastic deformation (e.g., mild steel, gold)
• Brittle Materials:
  • Little to no plastic deformation (e.g., glass, ceramics)
  • Yield strength concept is irrelevant
• Brittle Transition:
  • Materials with strain at fracture less than 5% are considered brittle

Brittle vs. Ductile Strength

• Brittle Strength in Compression:
  • Cracks form under tensile loads but not compressive loads
  • Little or no plastic deformation in brittle materials leads to stress concentrations
• Ductile Materials:
  • Plastic deformation relieves localized stresses

Ductility and Temperature

• Temperatures affect ductility
• Ductile to Brittle Transition Temperature:
  • Important design consideration
  • Example: Titanic’s hull failure due to low temperature

Toughness

• Definition: Ability to absorb energy up to fracture
• Calculation: Area under the stress-strain curve
• Characteristics:
  • High toughness = large area under curve
  • Good balance of ductility and strength
  • Low strength and brittle materials = low toughness

Resilience

• Definition: Ability to absorb energy while deforming elastically
• Corresponds to area under the stress-strain curve in the elastic region
• Materials with high resilience: Suitable for avoiding plastic deformation

Summary

• Yield Strength: Stress at onset of plastic deformation
• Ultimate Tensile Strength: Maximum stress in tensile test
• Ductility: Ability to deform plastically
• Toughness: Energy absorption until fracture
• Resilience: Energy absorption during elastic deformation

Conclusion

• Importance of understanding these properties in material selection and engineering design
• Encouragement to subscribe for further learning