ENSC1004 - Lecture 7

|← Back to Home

ENSC1004 - Lecture 6
Strength and Ductility 1 - Slides

Universal Mechanical Testing Machine

• The universal mechanical testing machine graphs force over $\Delta l$ of a material while applying pressure to it
• The machine also can use special grips to conduct specialised testing
  e.g. shear, fatigue, compression

Structure:

• The frame: sustains the maximum load the machine can apply
• The drive: electrical motor or hydraulic power to move the crosshead
• Load cell: sensor to measure the force
• Extensometer (usually mounted on the sample) measures the displacement of the sample
• The control panel and display
• Interface with computer and data acquisition.

This is how we find stress-strain diagrams for different materials (as stress can be obtained from force and strain obtained from $\Delta l$)

Understanding Deformation Behaviour

• Elastic deformation $-$ initial deformation

  • Deformation fully recoverable
  • For most metals and ceramics, initial elastic region is linear
    • Described by the Hooke’s Law: $\sigma =Eϵ$
  • Some metals show non-linear elastic behaviour (e.g. cast irons)
    • When load is removed, deformation still fully recovered

• Plastic deformation $-$ when applied load is above critical value (yield strength, ${\sigma }_y$)

  • Non-linear deformation
  • Permanent residual strain (see slides)
    • Also known as plastic strain

Stress-Strain Graph Key Features:

1. Start with pure elastic deformation up to the point of yielding at P
2. Elastic and plastic deformation after yielding, reaching a maximum stress at M, where necking occurs
3. Localised elastic and plastic deformation at the neck until final rupture at F

---

ENSC1004 - Lecture 8