Glass 2
- Created by: s013693b
- Created on: 02-05-14 23:21
View mindmap
- Glass 2
- Advantages
- Large source of potential glass evidence
- Large amounts may be retrieved, eg a whole windscreen
- Used as an investigative aid eg origins, dates
- Depending on circumstances, evidence can refute the allegation that a person was involved in a crime
- Provides links - associate evidence
- Fracture analysis, used for reconstruction
- Glass is stable, doesn't degrade over time
- Variation in manufacture allows discrimination between different types of glass
- Disadvantges
- Easily contaminated
- Expensive to analyse
- Large fragments are easy to analyse but don't persist on clothing for long
- Difficult to discriminate between same glass types
- Commercial glass is very similar in composition
- Difficult to discriminate between same glass types
- Usual methods of analysis are of little help
- Fracture Pattens
- Strong in compression
- Weak under tension and bending
- Griffith Theory
- Flaws act as stress concentrators
- Poncelet Theory
- Fractures only requires prolonged period of stress
- Radial- direction of force applied
- Concentric- oppostite direction to force applied
- Order of Fracturing
- 1. Force applied, glass bends elastically
- Near surface under compression, outside surface under tension
- 2. Fracture will initiate at defective point and radiate outwards from the point of orgin
- 3. Fracture may finish but if still under stress, tension will occur between radial fractures on near surface.
- 4. Fragments between radial fractures break; fractures begin on near side of glass producing concentric circle fractures around the point of origin.
- 1. Force applied, glass bends elastically
- Types of fracture
- Mirror
- Slow fractures - low stress eg. thermal fractures
- Mist
- Cracks form to release stress
- Hackle
- Stiations
- Marks appear parallel to each other
- Coarse marks
- Perpendicular to conchoidal marks
- Wallner Lines
- Ripples
- Tempered Glass
- Distinctive fracture pattern
- Wont show direction of impact
- Hertzian
- High impact- firearms
- Flake of glass ejected from opposite side
- Crater - opening away from direction of force
- Mirror
- Analysis
- Fractrography
- No databases, just an expert
- Computerised methods which analyse geometric patterns
- Impact or thermal stress
- Type of force
- Amount of energy
- Point of impact
- Order of multiple impacts
- Problems of fracture analysis
- Distortion of fracture may cause uncertainty in matches
- Post fracture damage may occur
- Segments missing making physical fit impossible
- Physical Fit
- Fractrography
- No databases, just an expert
- Computerised methods which analyse geometric patterns
- Fractrography
- Physical Fit
- Fractrography
- Advantages
Comments
No comments have yet been made