Glass 2

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  • Created by: s013693b
  • Created on: 02-05-14 23:21
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  • 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.
      • 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
    • 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

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