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 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

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

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

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

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

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

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

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 mL. 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