ABSTRACT
Expanding bullets are preferred by law enforcement because of their wounding potential and ability to avoid over-penetration which could result in unintended targets being struck by bullets that perforate their intended targets. Expansion failure for jacketed hollow point (JHP) bullets is commonly attributed to several causes including damage to the bullet's cavity, velocity loss, bullet destabilization and materials from intermediate targets filling the bullet's cavity which can cause expansion failure when the bullet subsequently impacts a soft, fluid-based target such as human tissue or ballistic gelatin. In this study, JHP bullets were fired into ballistic gelatin after passing through selected intermediate targets representing items common to shooting incidents. Velocity loss and bullet destabilization were not factors that contributed to the JHP bullets that experienced expansion failure; however, materials obstructing the bullets' cavities and damage to the bullets' cavities were considered causes for some of the JHP bullet expansion failures. It was determined through this research that most of the target materials caused JHP bullet expansion failure when shored against the ballistic gelatin, but when placed at distances of 7 ft. from the ballistic gelatin, bullets fired through the same target materials did expand. This original and unique study produced findings that are of significant value to shooting incident reconstruction experts and other forensic professionals as shooting incidents can call into question a victim's proximity to a wall or door when a bullet(s) perforated such a target material.
ABSTRACT
In forensic casework, examination of garment damage can provide insight into the mechanism of the specific cause of fiber failure. Different methods of damage impart differing physical characteristics on individual fibers. These alterations are determined by a multitude of factors, among them increased temperature of affected fibers. The process of rapid shear occurs in thermoplastic materials following high-speed impact. It results in distinct features caused by excessive heat generated through the interaction, which is unable to dissipate at a rate that would leave the fibers unchanged. Rapid shear characteristics can be differentiated from other fracture patterns through non-destructive microscopical methods and with a minimal sample size. Fabric samples were shot under heated, chilled, and water-saturated conditions, using ammunition of varying velocities. Analyses performed on the defects were conducted using stereomicroscopy, polarized light microscopy and scanning electron microscopy. Globular-shaped fiber ends, characteristics attributed specifically to rapid shear, were observed in all nylon samples. Through this study, it was determined that the environmental conditions employed did not affect fiber end changes associated with rapid shear.
ABSTRACT
In February 2009, the National Academy of Sciences published a report entitled "Strengthening Forensic Science in the United States: A Path Forward." The report notes research studies must be performed to " understand the reliability and repeatability " of comparison methods commonly used in forensic science. Numerical classification methods have the ability to assign objective quantitative measures to these words. In this study, reproducible sets of ideal striation patterns were made with nine slotted screwdrivers, encoded into high-dimensional feature vectors, and subjected to multiple statistical pattern recognition methods. The specific methods employed were chosen because of their long peer-reviewed track records, widespread successful use for both industry and academic applications, rely on few assumptions on the data's underlying distribution, can be accompanied by standard confidence levels, and are falsifiable. For PLS-DA, correct classification rates of 97% or higher were achieved by retaining only eight dimensions (8D) of data. PCA-SVM required even fewer dimensions, 4D, for the same level of performance. Finally, for the first time in forensic science, it is shown how to use conformal prediction theory to compute identifications of striation patterns at a given level of confidence.
Subject(s)
Discriminant Analysis , Equipment Design , Multivariate Analysis , Principal Component Analysis , Support Vector Machine , Weapons , Forensic Sciences/methods , Humans , Wounds and Injuries/pathologyABSTRACT
Over the last several decades, forensic examiners of impression evidence have come under scrutiny in the courtroom due to analysis methods that rely heavily on subjective morphological comparisons. Currently, there is no universally accepted system that generates numerical data to independently corroborate visual comparisons. Our research attempts to develop such a system for tool mark evidence, proposing a methodology that objectively evaluates the association of striated tool marks with the tools that generated them. In our study, 58 primer shear marks on 9 mm cartridge cases, fired from four Glock model 19 pistols, were collected using high-resolution white light confocal microscopy. The resulting three-dimensional surface topographies were filtered to extract all "waviness surfaces"-the essential "line" information that firearm and tool mark examiners view under a microscope. Extracted waviness profiles were processed with principal component analysis (PCA) for dimension reduction. Support vector machines (SVM) were used to make the profile-gun associations, and conformal prediction theory (CPT) for establishing confidence levels. At the 95% confidence level, CPT coupled with PCA-SVM yielded an empirical error rate of 3.5%. Complementary, bootstrap-based computations for estimated error rates were 0%, indicating that the error rate for the algorithmic procedure is likely to remain low on larger data sets. Finally, suggestions are made for practical courtroom application of CPT for assigning levels of confidence to SVM identifications of tool marks recorded with confocal microscopy.
