This document describes the guidelines for assessing fracture features
as they relate to forensic glass analysis.
2. Reference Documents
|Scientific Working Group for Materials Analysis
Trace evidence recovery guidelines
Quality assurance guidelines
|American Society for Testing and Materials Standard
||C1256 Standard Practice for Interpreting Glass
Concentric cracks are 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.
Cone or crater (Hertzian cone) is a funnel-shaped area of
damage caused by a high-velocity impact.
Hackle is a line on the crack surface running parallel to
the local direction of crack spreading.
Radial cracks are fractures extending outward from the point
Ream is an imperfection; nonhomogeneous layers of flat glass.
Wallner 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.
4. Summary of Guideline
This guideline discusses the characterization and assessment of
the significance of fracture features in glass as observed through
microscopic and macroscopic examination. This guideline also describes
the reconstruction of broken glass objects.
5. Significance and Use
Fracture patterns are unique. Fracture features in a piece of glass
reflect the nature of the glass and the direction of travel and
velocity of the breaking object. Glass fracture examinations can
provide information as to the direction of the breaking force and
the sequence of multiple impacts. A physical match of two pieces
of glass establishes that they came from the same source to the
exclusion of all other sources.
6. Sample Handling
Extreme care should be taken to prevent further breakage of the
glass during any phases of packing, shipping, or unpacking of glass
shards. Proper sample preparation and technique are prerequisites
for obtaining reliable results. See
the Scientific Working Group for Materials Analysis Collection,
Handling, and Identification of Glass.
7.1. Physical Reconstruction
Ensure that all pieces of glass could have originated from the
same object (see
the Scientific Working Group for Materials Analysis Initial
Examinations of Glass). Coatings, the float surface, and
other features may be used to aid in the orientation of glass
pieces prior to reconstruction. Align the edges of two pieces
of glass that appear to match physically. Two pieces of glass
will not slip past one another with gentle pressure when there
is a physical match. Examine the broken edges using low-power
light microscopy to observe corresponding Wallner lines (ridges)
and/or hackle marks on the matching pieces of glass. Features,
such as surface scratches or ream, may also match across a fracture.
7.2. Types of Fractures
Low-velocity impact, high-velocity impact, and thermal fractures
may be observed in glass and can be differentiated.
||7.2.1. Low-velocity impact fractures
|Low-velocity projectiles produce cracks in the
glass, which radiate outward from the point of impact (radial
cracks). If a pane is firmly held on all sides, concentric cracks
can form around the point of impact. The sequence of multiple
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, the direction of breaking force can often be
determined. Observe only the Wallner lines on the radial cracks
nearest the point of impact. If the impact site is not preserved,
the glass must be reconstructed. The original orientation of
the glass must be known to complete the determination.
|The ridges (Wallner lines) on 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 4R rule, (Ridges on 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).
7.2.2. High-velocity impact fractures
|A high-speed projectile striking a piece of glass
will produce a cone or crater. If the projectile
passes through the glass, the opening on the exit side will
be larger than the opening on the entry side. If the impact
site is not preserved, the glass must be reconstructed to observe
any coning effects. However, because of the small size of the
shattered fragments at the impact site, the reconstruction of
a sufficient portion of the object to display coning effects
may not be possible. The size of the hole and the diameter of
the crater cannot be used to reliably predict the size of the
projectile. Projectiles that pass through the glass at an angle
to the surface produce an elongated hole.
|Radial cracks may also develop from high-velocity
impact (see Section 188.8.131.52 for the 4R rule). The sequence of
multiple impacts can be deduced when the cracks caused by a
subsequent impact terminate at previously formed cracks.
7.2.3. Thermal fractures
In nontempered glass a typical heat crack is curved, has
a smooth edge, and has no indication of the point of origin
of the crack. Localized heating of thick pieces of glass can
cause cracks with a feathered appearance. The side to which
the heat was applied cannot be determined from fracture edges
Glass fragments that are reconstructed with coinciding edges came
from the same object. If the direction of impact and/or the sequence
of impact have been determined, they should be reported.
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