Crime Scene Management: Chapter 8.7 - Glass Fragmentation

when glass is hit by a small fast-moving object if fractures to produce a circular hole with cracks both encircling and radiating outwards

cracks encircling hole → concentric fractures

cracks radiating from hole → radial fractures

the hole is narrowest on impact side and so its relatively easy to determine the direction in which the object was travelling

slow or large moving objects will be irregular or the pane may be completely broken

info may be obtained from examination of fractures

as glass is hit by an object, it initially bends away from object until it reaches a point where it shatters

during this bending, stress marks, known as Walner Lines are formed within the glass

if glass still housed in window frame are examined edge on, using a low-power microscope or a magnifying glass, it is possible to see these stress marks

if the facture is radial then the perpendicular section of the stress mark is on the reverse side of the impact direction

for concentric factures the perpendicular section section is on the side of impact

toughened glass does not form radial and concentric fractures and impact direction information cannot be gathered

if a sheet of glass has received several high-speed impacts but remains intact then its possible to determine the sequence of impacts

this is because a new fracture always terminates at an existing fracture

its important to gather as many samples as possible from a crime scene so that either a physical fit or a compositional analysis can be made to any fragments found on a suspect

the glass from a crime scene must be handled and stored very carefully to avoid further fragmentation

as no two panes of glass fragment in the same way, any physical fit has very strong evidential value