Flashcards
intro - glass
Cards in this deck (41)
- 1glass definition“an inorganic product of
fusion that has cooled to a
rigid condition without
crystallization” - 2glass composition what substance accounts for the majority of manufactured glass globally?Soda-lime glass
- 3glass compositionSand (main
component AKA
primary former)
Soda ash (lowers the
melting point)
Limestone (makes
glass insoluble in
water, making it
chemically durable)
Cullet (scraps of waste
glass) - 4glass composition Sand (main component AKA primary former)SiO2 - 63 to 74%
- 5glass composition Soda ash (lowers the melting point)Na2CO3 - 12 to 16%
- 6glass composition Limestone (makes glass insoluble in water, making it chemically durable)CaO - 7 to 14%
- 7glass compositionThe impurities in these
main products, can be
used by the forensic
scientist to discriminate
glass of the same type. - 8Flat Glass – Float ProcessMost common glass
Manufactured in flat sheets
Can come as float glass, rolled glass or sheet glass
commonly used in doors, windows
Sand & other mixtures needed to be made into a liquid which
forms the solid sheet of glass
the thickness is determined by spreading out the glass using
liquid tin. It then hardens. The tin remains a liquid because its
hardening point is lower than that of glass.
The glass is then cut by scoring, so that to weaken the glass
along a particular line
This may be done using machinery, however can be done
manually for made to order products - 9Laminated GlassA composite typically of a sheet of plastic between two sheets of
flat glass.
• Flat or shaped laminates may be produced and are dominant in
modern windscreens.
• The two sheets may be taken from one pallet, in which case they
are typically very similar, or from two pallets, in which case they
may differ.
• Some applications deliberately call for colour differences in the
two sheets.
• Laminates are noted for their ability to resist penetration, acting
as security glass or to restrain passengers within a vehicle during
collisions - 10Toughened Glasscalled Tempered Glass
The end product is sometimes referred to as safety
glass
When broken, it breaks into “diced” pieces
that do not have sharp or pointed edges
it absorbs impact
used in phones and vehicles
is achieved by rapid cooling of the heated glass
using air jets. This compresses the surfaces and
places the centre in tension. - 11Types of fractures 3 typesLOW VELOCITY IMPACT
HIGH VELOCITY IMPACT
THERMAL - 12Low Velocity Impact Fracturesproduce fractures which radiate outward from the point of impact (radial cracks).
If the glass is in a firm position, concentric cracks can form around the point of impact.
sequence of impacts can be deduced when the cracks caused by a subsequent impact terminate at previously formed cracks
• By observing the Wallner lines (ridges) on the radial cracks, direction of breaking force can be determined.
impact site is not preserved, the glass must be reconstructed.
Wallner lines on the radial cracks nearest the point of impact are at right angles to the side opposite, or to the rear,
of the impact.
This phenomenon is referred to as the 3R rule, (Radial cracks are at Right angle to the Rear.) The 4R rule is
unreliable for laminated glass, tempered glass, and small windows tightly held in a frame (Koons et al. 2002). - 13High Velocity Impact FracturesA cone or crater is produced by a projectile
If the projectile passes through the glass, the opening on the exit side
is larger than the opening on the entry side (if the projectile goes through at an angle there will be an elongated hole)
size of the hole and the diameter of the crater cannot be used to
reliably predict the size of said projectile
Radial cracks may also occur. The sequence of multiple impacts can be
deduced in the same way as low velocity, using the 3R rule. - 14Thermal FracturesIf an area of a glass pane gets hotter than the adjacent area thermal
stress occurs. The greater the stress the more chance of a fracture.
The hot glass expands and the cooler area doesn’t. If the cool area isn’t
strong enough thermal stress fracture will occur.
Toughened glass is unlikely to be fractured this way
Rarely occurs due to glass fault, but rather what conditions it’s.
exposed to. - 15failure upon tension1. Glass doesnt break upon compression, it breaks upon tension
2. the glass bends elastically with the near surface under compression and the far surface under tension, when force is applied.
3. fracture occurs on the far surface at a point defect which then proceeds towards the near surface
4. if more tension needs to be relieved then more cracks will occur between the radial cracks on the near surface. which forms the concentric fractures. - 16Exceptions to the 3R rule• Toughened glass
• Thermal fractures (no
point of impact)
• Very small panes that
are held tightly so
that they cannot bend - 17wallner lines diagram
- 18Wallner LinesStress marks
Rib shaped marks
Indicate propagation direction
Help determine course of events - 19wallner lines Primary Linessurface/internal flaw was already present
- 20wallner lines Secondary Linesoccur when fracture reaches terminal velocity
- 21wallner lines Tertiary Linesa result of shock and vibration
- 22crack propagation direction
- 23Hackle marks• Hackle marks are coarse parallel marks lying
parallel to the direction of fracture
• They are often used to match two pieces of glass.
• The easiest marks to see on the glass edge are the curved shell-like or conchoidal marks.
• Can get lines or “mist hackles” - 24• Concentric cracksare fractures forming in an approximately circular
pattern around the point of impact. They are usually in straight
segments that terminate in an existing radial crack. - 25Cone or crateris a funnel-shaped area of damage caused by a high velocity impact.
- 26Hackleis a line on the crack surface running parallel to the local
direction of crack spreading. - 27• Radial cracksare fractures extending outward from the point of
impact. - 28Wallner lines (ridges)are rib-shaped marks with a wave-like pattern.
Wallner lines are called rib marks or ridges to describe their shape
and are almost always concave in the direction from which the crack
was propagating. - 29Fractures caused by projectiles• Produce cone/crater fractures
• Cause high velocity fractures
• Can tell you the angle of a shot
• Produce sequence penetrations
• Shows you point of impact - 30If the bullet hits at an acute angle from the rightthe fragments
will spray to the left
and the exit hole
will be larger than
the entrance hole &
vise versa - 31Sequence of shotsRadial fractures associated with the 2nd shot will run out when they meet a fracture from the first shot.
- 32trace evidence - glassFragments recovered from a suspects clothing or hair will be in the range of 0.25- 1mm. Number and distribution of fragments is vital.
Endurance of evidence is dependant on the suspects subsequent activities & their textiles.
Wool clothing will hold more fragments than leather. Glass from clothing
>significant than a piece embedded in a shoe.
Tiny fragments cannot be identified using visual characteristics and therefore will need further analytical density processing (i.e. refractive index).
Glass needs to be classified. (where is it from?) - 33Analytical techniquesRefractive Index
Elemental Analysis
Chemical impurities - 34Refractive IndexLight has wave properties
• A beam of light travels from a gas (air) into a solid (glass)
and thus velocity decreases causing the beam to bend
downwards
• The refractive index measures how much light has been
bent (refracted) as it enters the glass.
• Gives composition and thermal history of glass. - 35Refractive Index – Oil Immersion• Questioned glass is placed in silicone oil (resistant to decomposition)
• As the oils temperature increases the refractive index decreases
• This is placed on a hot microscope stage
• Initially a bright halo occurs (Becke Line)
• This disappears where there is a match point (the refractive index of the glass
and oil are the same)
• If the physical evidence and the reference sample has the same match point,
the glass is the same
• The GRIM system can do this automatically using a database. - 36RI – Toughened vs Flat GlassToughened glass can be distinguished by
heating the fragments to a temperature
higher than 600 degrees Celsius – AKA
annealing - 37RI – Toughened vs Flat Glass what is annealingAnnealing alters the optical properties in
glass. - 38RI – Toughened vs Flat Glass The RI for toughened glass ismuch higher
than that in flat glass. - 39Elemental AnalysisScanning Electron Microscopy
(SEM)
X-Ray Fluorescence
(XRF)
Optical Emission
Spectrometer
(OES) - 40Elemental Analysis
- 41failure upon tension
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- - Forensics Flashcards on intro - glass, created by megan trent on 13/11/2019.
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