Glass Fractures

Introduction to Glass Fractures

• Glass fractures occur due to applied pressure or force.
• Characteristic of glass: brittle nature, leading to visible fracture marks.
• Fracture analysis can determine the direction and amount of force applied.

Mechanism of Glass Fracture

• Elastic Nature of Glass: Initially acts elastically, bends, then breaks under excessive force.
• Tensile Strength: Glass (and bones) can withstand a certain amount of pressure before breaking.
  • When pressure exceeds tensile strength, fractures occur.

Types of Glass Fractures

1. Radial Cracks

   • Form first, propagate in the opposite direction of applied force.
   • Characteristics:
     • Created on the side opposite to where force is applied.
     • First to appear during a fracture event.

2. Co-centric Cracks

   • Form after radial cracks when pressure continues to be applied.
   • Characteristics:
     • Develop on the same side as the force applied.
     • Appear after radial fractures.

Distinction Between Radial and Co-centric Fractures

• Radial Fractures: Formed on the opposite side of force, appear first.
• Co-centric Fractures: Formed on the same side as force, appear later.

Rib Marks and Stress Marks

• Broken edges of glass show rib or stress marks due to pressure:
  • Rib marks are perpendicular in radial fractures.
  • Indicate direction and point of impact.
• 3R Rule: Radial cracks create rib marks that make right angles on the reverse side of the force.

Characteristics of Different Types of Glass

• Tempered Glass:
  • Fractures into small pieces (dice-like), no rib marks.
  • Breaks easily when tightly held or due to heat.
• Low-Speed Projectiles:
  • Rib marks indicate breaking force application.
  • With higher velocity, the bullet hole size decreases, creating a simpler crack pattern.
  • Entry hole smaller, exit hole larger (loaded vs. unloaded side).

Summary of Key Points

• Glass fractures reveal information about the force applied, including direction and point of impact.
• Understanding the types of fractures (radial vs. co-centric) is crucial for forensic analysis.
• Different glass types behave uniquely under stress, affecting fracture patterns.