Glass Analysis Notes Density Probs Fracture Qs Forensic

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

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

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.

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

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

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

Why Radial and Concentric Fractures Form n Impacted glass is compressed on the side it is hit. n 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. n Then fractures form in the shape of concentric circles on the same side of the impact. Forensic Science: Fundamentals & Investigations, Chapter 7

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

Why Radial and Concentric Fractures Form Forensic Science: Fundamentals & 9

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

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

Sequencing Fractures • Radial lines always end in existing radial lines

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.

Blunt Object Fractures

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

Heat Fractures

Density Practice Problem • A shard of glass is found at a crime scene. It has a mass of 1. 239 g and a volume of 0. 4956 m. L. What type of glass is it? • Use the table on the Density of Glass Problems WS.

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

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