Determination of Unique Fracture Patterns in Glass and Glassy Polymers
Frederic A. Tulleners
John Thornton
Allison C. Baca
Abstract
The study of fractures of glass, glassy type materials, and plastic has long been of interest to the forensic community. The focus of this interest has been the use of glass and polymer fractures to reconstruct past events and to associate items of evidence. One example of this association is the matching of glass fragments from various locations where they can be shown to have come from a common origin. In the materials science community, fractography is the means and methods for characterization of fractured specimens or components in order to study or identify the mechanism of such failures, which is the focus on most of the literature on the subject. The ability to show that each and every fracture is, in fact, unique has not been a matter of consequence or of interest to the engineering or scientific community. In contrast, the basic premise that fractures are not likely to be reproducible is very relevant to the forensic science community. The issue arises when a given fracture pattern is restored or component pieces are physically fitted together and "matched" and the conclusion is drawn that this is unlikely to be possible unless all the components were derived from the same part. Despite the importance of this assumption, very limited research has actually been done to confirm that this is indeed the case. This study documented the very controlled fracture patterns of 60 glass panes, 60 glass bottles, and 60 plastic tail light lens covers. The pane and bottle specimens were fractured with three different types of penetration tips: sharp tip, round tip, and blunt tip. Two basic methods were used to initiate the fractures—dynamic impact from a dropping weight and static pressure from an Instron® 4204 Tensile Tester. The fracture patterns were then documented in great detail in such a manner that allowed the analyst to inter-compare the fracture patterns. This subsequent comparison illustrated the uniqueness of all of the fracture patterns we observed in window glass, bottle glass, and plastic lens materials. Thus, we are substantiating the individuality of glass and polymer fractures under closely controlled conditions.