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3. TerminologyConcentric 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. 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 GuidelineThis 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 UseFracture 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 HandlingExtreme 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. Analysis
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7.2.1. Low-velocity impact fractures | |||
7.2.1.1.
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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. | ||
7.2.1.2.
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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. | ||
7.2.1.3.
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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 |
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7.2.2.1.
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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. | ||
7.2.2.2.
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Radial cracks may also develop from high-velocity impact (see Section 7.2.1.3 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
(Frechette 1990). |
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.
Frechette, V. D. Advances in Ceramics, Failure Analysis of Brittle
Materials, American Chemical Society, Westerville, Ohio, Volume
28, 1990.
Koons, R. D., Buscaglia, J., Bottrell, M., and Miller, E. T. Forensic
glass comparisons. In: Forensic Science Handbook. 2nd
ed. R. Saferstein, ed. Prentice-Hall, Upper Saddle River, New Jersey,
2002, Volume I, pp. 161-213.