1. Glass Analysis: Notes, Density Probs, Fracture Qs
Forensic Science
4/8/15

2. Drill Define refraction. What is silica? How is tempered glass made?
HW: Finish (if needed)
Density Problems
Activity 14-1 Glass Fracture Patterns

3. Objectives
IWBAT:
Define the physical and chemical properties of glass.
Define refractive index.
Distinguish between crystalline solids and amorphous solids.
Explain how to determine the order of impacts in a window.
Describe different types of glass breakage.

4. Agenda Drill Pd. 5 & 6 – Finish Notes Density practice problem
Glass is Breaking Up packet
Activity 14-1 Glass Fracture Patterns
Density Problems
Closure

5. Fracture Patterns in Broken Glass
Being an amorphous solid, glass will not break into regular pieces with straight line fractures
Fracture patterns provide clues about the direction, rate, and sequence of the impacts
Forensic Science: Fundamentals & Investigations, Chapter 14

6. Microscopic characteristics can assist in making decision about suspected match
Random stress patterns and breaks are unique

7. Why Radial and Concentric Fractures Form
Impacted glass is compressed on the side it is hit.
It will stretch on the opposite side of the glass, and the tension there will radiate breaks in the glass outward from the point of impact.
Then fractures form in the shape of concentric circles on the same side of the impact.
Forensic Science: Fundamentals & Investigations, Chapter 14

8. Why Radial and Concentric Fractures Form
Glass after an impact shows radial fractures (red) and concentric circle fractures (blue)
Forensic Science: Fundamentals & Investigations, Chapter 14

9. Why Radial and Concentric Fractures Form
Forensic Science: Fundamentals & Investigations, Chapter 14

10. Bullet Fractures
As a bullet passes through glass, it pushes a cone shaped piece of glass out of the glass ahead of it
The exit side of the hole is larger than the entrance side of the hole
Radiating fracture lines from a subsequent shot will stop at the edge of the fracture lines already present in the glass
Forensic Science: Fundamentals & Investigations, Chapter 14

11. Path of a Bullet Passing through Window Glass
perpendicular to the glass
shot from the right
shot from the left
The angles at which bullets enter window glass help locate the position of the shooter
Bits of the glass can fly backward (backscatter), creating trace evidence
Forensic Science: Fundamentals & Investigations, Chapter 14

12. Sequencing Fractures
Radial lines always end in existing radial lines

14. Blunt Object Fractures
Glass fractures caused by a blunt object will show a pattern of fractures but not as regular of a pattern as the fractures from a bullet
The difference is due to the impacting force being dispersed over a greater area.

15. Blunt Object Fractures

16. Heat Fractures Wave shaped
No regular pattern of radial or concentric lines
Show little if any curve patterns along the edges
Expansion of glass occurs first on the side exposed to the heat

17. Heat Fractures

18. Density Practice Problem
A shard of glass is found at a crime scene. It has a mass of g and a volume of mL. What type of glass is it?
Use the table on the Density of Glass Problems WS.

19. What you need to do… With a partner
Glass is Breaking Up packet – turn in when done
On your own sheet – keep these for tomorrow
Activity 14-1 Glass Fracture Patterns
Density of Glass Problems

20. Closure Identify the type of breakage
Wavy breaks that show no curved pattern
Radial and concentric fractures with a small hole
Small, cube-shaped pieces of glass