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Reading the clues in fractured glass.

It was 5:30 p.m. on a hot July day in Tempe, Arizona, when Jack went out to his car in the company parking lot and found that the front windshield had been broken. He stormed back to the office and called the police as well as the company security investigator. He was as hot as the day now and wanted to catch the culprits who had vandalized his car.

After investigating the break, the police agreed with Jack that the broken glass was an act of vandals. The company security investigator, however, disagreed. The fracture did not have a radial or concentric pattern, which is characteristic of a fracture cause by impact with an object, explained the investigator. Instead, the fracture was wave shaped, characteristic of fractures caused by excessive exposure to heat.

DETERMINING THE CAUSE OF BROKEN glass is not difficult. The physical properties of glass make it possible to determine the manner in which a piece of glass was damaged. Glass seldom cracks squarely across. It leaves convex or concave edges as a result of bending and stretching before breaking.

Fractures caused by excessive exposure to heat can always be distinguished from those caused by impact, since those that are due to heat do not show a regular pattern of radial and concentric fractures, but are characteristically wave-shaped. If the edges of the glass are examined, no stress lines will be seen. If the stress lines are smooth, or almost so, with no point of impact or penetration present, and no radial or concentric breaks in the glass, then excessive heat caused the damage.

When glass breaks from an impact, it produces both radial (primary) and concentric (secondary) fractures. When glass hit by an object is reconstructed, some of the fractures in the glass will resemble the spokes of a wheel as the fractures radiate outward from the point of impact. These are called radial fractures.

A radial fracture that was caused by an impact appears as a wavy line. Continued movement of the glass will cause extensions to the original fracture, usually in a straight line. Fractures that form a series of broken circles, or arcs, around the point of impact are called concentric fractures. They extend from one radial fracture to another.

After determining that a break is the result of an impact, an investigator may also need to determine the direction of the impact to uncover insurance fraud. The origin of impact can be discovered using the three-r rule: radial fractures only, and the force came from the rear, or opposite side of the glass, at a right angle of the stress line to the side of the glass.

If the glass in question is no longer in its original place such that the investigator can readily see which side was outside, he or she will need to make that determination by analyzing both sides of the glass. If the glass is a window of a house, for example, the inside is usually oily and the outside is usually dirty or dusty.

The investigator should then locate a radial fracture leading from the point of impact and hold the broken edge of the glass toward a light source, taking care at the same time to protect any latent prints. Light should reflect off the radial fracture. When this occurs, lines will be seen that travel across the edge of the broken glass. These lines are called stress lines. Stress lines start out almost parallel to one side, curving toward the other side and ending up almost perpendicular or in a right angle to each other. The direction of force is always from the side opposite or from the rear of the side where the right angle is made.

Safety glass used in automobiles was designed to break and form a web that prevents passengers from going through the glass. Such glass should resist shattering and help contain a passenger who, for example, is inside the vehicle. Safety glass consists of a transparent binding agent, such as sheet vinyl plastic, sandwiched between two sheets of ordinary glass. The binding agent prevents shattering of ordinary glass when it is struck.

Due to the structure of safety glass, cracking is frequently incomplete, neither radial nor concentric fractures penetrate completely from one side to the other. If the concentric cracks appear on only one side and no radial cracks are found, that is the side of impact. If only radial cracks are found on one side, that is the side away from the impact.

If a bullet was fired through a vehicle window, a hole would be seen in the glass where the bullet went through. By examining the hole carefully with one's finger, the side where the bullet entered would feel smooth around the hole, and no sharp edges would be detected unless the finger went inside the hole. The side where the bullet exited would be cone shaped and larger than the entry hole, and it would feel rough.

The shape of the cone differs if the bullet is fired at the window from an angle as compared to straight on. For example, if a bullet is fired at glass from the right, not much chipping is found at the right side of the exit hole, but considerable chipping occurs around the left side of the exit hole. Also, a number of straight and short radial fractures may appear at the right of the entrance hole and one or two long radial fractures occur at the left side of the entrance hole.

This is reversed if the bullet is fired from the left side. The fracture resulting from a bullet should provide a clue about where the subject was standing when the shot was fired.

As can be seen from these examples, the glass at the scene of an incident may be as good as a witness if the investigator knows how to interpret the information at hand. An understanding of fractures--their patterns, their shapes, and the debris of the glass--could help security professionals catch a culprit or save them from pursuing phantom vandals when the damage results from an act of nature.

David R. Stratton, CPP, CFE (Certified Fraud Examiner), is chief of security for the Micro-Rel Division of Medtronic, Inc., in Tempe, Arizona. He is a member of ASIS.
COPYRIGHT 1994 American Society for Industrial Security
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1994 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Crime Scene Analysis; identifying causes of broken glass
Author:Stratton, David R.
Publication:Security Management
Date:Jan 1, 1994
Previous Article:A commitment to cooperation; an interview with 1994 ASIS President Ken Joseph.
Next Article:DIS highlights security procedures.

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