Date of Graduation
Eberly College of Arts and Sciences
Forensic and Investigative Science
Keith B. Morris
Finite element analysis (FEA) is a technique which has not been widely used in Forensic Science in general and firearms examination in particular. The general utility of FEA was assessed by the simulation of scenarios commonly encountered during the investigation of firearm related crimes. As a test, simulations included the perforation of a windshield and sheet metal with a 9 mm projectile, the development of pseudo subclass characteristics during the manufacturing of a breech face, and the impact of a projectile with another projectile in air and with a solid body. Results from the simulations were compared to projectiles and substrates used in empirical testing which mimicked the simulated scenarios. Observation of the deformation caused in both methods of testing demonstrated visual similarity. The main aim of this project was to expand the knowledge-base regarding sources of variability in breech face impressions on cartridge cases fired by a single firearm. It was proposed that the gas flow within the primer cup during discharge is non-uniform. This is due to interaction between the generated gas and the anvil which is contained inside the primer cup. As gas flows around the arms of the anvil, it is directed away from the rear surface of the arms which results in localized areas of low pressure. Concurrently, the areas unobscured by the anvil are subjected to higher pressures. This translates into localized areas of high and low force in the interaction between the primer surface and breech face. The resulting areas of high force will result in greater impression development while areas of low force will result in less development. It was hypothesized that cartridges fired from a single firearm with similar anvil orientations will have greater similarity in impression development while cartridges fired with dissimilar anvil orientations will result in less similarity. This study investigated the effect of anvil orientation on breech face impression development in two ways, theoretically and experimentally. Computer aided design (CAD) and FEA were used to model the gas flow in the primer cup and the subsequent impression development during discharge. A 9 mm Luger cartridge was modeled according to the Sporting Arms and Ammunition Manufacturers Institute (SAAMI) specifications. The 3D model was then imported into Ansys Workbench for FEA simulation. Results of fluid simulations modeling the gas flow at the point of maximum pressure during discharge showed localized areas of high and low pressure across the inner base of the primer cup. Structural simulation to model the development of breech face impressions suffered from multiple limitations resulting in a reduced model being used. While simulation of breech face impressions for the entire breech face was unattainable, the reduced model returned impression generation demonstrating the feasibility of FEA for modeling breech face impressions once the discussed limitations are remedied. Cartridges were reloaded for experimental data collection. Alliant Unique powder was used and five charge weights ranging from the suggested starting charge weight to the suggested maximum charge weight were chosen with equal increments between each charge weight. The anvil orientation inside the primer was marked before each primer was seated into the primer pocket of the cartridge case. Ten anvil orientations were chosen ranging between 0 degrees and 162 degrees in 18 degree increments. Ten repetitions for each charge weight-angle combination were performed. The loaded cartridges were discharged in an H-S Precision Universal Receiver and the fired cartridge cases were scanned using a Sensofar s Neox confocal microscope. The resulting surface profiles were compared using the Congruent Matching Cells (CMC) algorithm provided by the National Institute of Standards and Technology (NIST). The results from CMC analysis were analyzed to determine the presence or absence of significant differences in CMC results between anvil orientations and charge weights. Non-parametric statistical tests (Kruskal-Wallis and Dunns) were conducted to determine significant differences between delta angle and delta charge weight groups. The Cohen d test was used to assess practical significance of these results. Significant differences were found between groups for both angle orientation and charge weight. Overall, the highest CMC percent values are returned when the anvil orientation and charge weight are the same between the two fired cartridges.
Hamilton, James A., "Finite Element Modeling and Analysis of Breech Face Impressions" (2023). Graduate Theses, Dissertations, and Problem Reports. 12093.
Available for download on Wednesday, July 24, 2024