3D Characterization and Comparison of Fracture Surfaces

As New Investigators; we propose to investigate the underlying scientific basis for forensic analysis of fractured and torn surfaces, by employing the fundamentals from the field of fracture mechanics and the nature of the material behavior. This quantitative approach has the potential to enhance the ability of forensic scientists to capture, visualize and analyze fracture patterns, and possibly provide new methodologies for trace evidence. The project will employ spectral analysis of 3D fracture surface topography-measurements to associate or to differentiate fracture surfaces in the performance of physical comparisons. We will utilize an understanding of material failure mechanisms (developed in the field of fracture mechanics), with digital image analysis, to construct protocols for the association (or exclusion) of pairs of surfaces. A material’s fracture surface consists of 3-D features, with associated spatial frequency signatures, that are dictated by the material’s intrinsic microstructure and external loading history. The topography of a fracture surface is dependent on the ratio of the local material resistance to fracture vs. the local stress state (i.e. load severity), and this relationship can be used to forensically compare fracture surfaces. The quantitative expression of complex microstructural details, combined with the quantitative characteristics of applied load, have the potential to provide quantitative signatures for fracture surfaces, expressed as distributions of the spatial sizes and orientations of the features of a fracture surface. These, then, can be used to support the discriminant analysis of fracture match, yielding a statistical expression of fracture match. A 3D spectral analysis of fracture surface, based on the use of white light non-contact surface profilometers, will be evaluated to provide fracture surface measurements. The proposed analysis will be self-calibrated for fracture-feature-characteristics identification. This self-calibration should strengthen the methodology, and should expand its potential application across a broad range of fractured materials, with diverse textures and mechanical properties. Moreover, it would provide ease of use for forensic examiners, especially when a user-friendly interface with the analysis tools is developed. The analytical protocol will be examined to access fracture-match threshold(s), reliability and uncertainty(ies) of measurement. Successful preliminary work supported by the USDOE Ames Laboratory-Midwest Forensic Resource Center suggests a two-year development. First phase, Year1, a detailed validation study will be conducted on controlled laboratory samples from prey tool steel, with focus on analysis tool assessment and improvements. Efforts will focus on morphological measurement practices, development of a mathematical framework for describing non-continuous fracture events, and establishment of measurement uncertainties. Second phase, Year-2, a broader range of materials class such as metals, glass and plastic fragments will serve as the subjects for further protocol applicability and evaluation. We will explore the role of environmental degradation effects (moisture, heat/cold and corrosives) on the topology of fracture surfaces, to ascertain the applicability of the technique to weathered specimens. Our forensic collaborator will examine the testing protocol to identify its shortfalls and possible improvements. Assuming developmental success, the proposed technique may be utilized in; (i) Evaluating the 3D surface characterization for representative metal, glass and plastic fragments, (ii) analysis of fracture fragments or torn sections where a visual jig-saw match between fragments cannot be established, (iii) understanding the role of environmental deterioration of fracture surfaces, and (iv) possible expansion to address fibrous materials and torn taps. A detailed report with the scientific findings and implications will be generated, for forensic analysts’ use of spectral analysis of 3D fracture surface topography-measurements to associate, or differentiate, metal, ceramic and plastic fracture surfaces. This research will be conducted in response to the NIJ’s expressed need for knowledge underlying forensic science disciplines, and in collaboration with forensic scientists working in a forensic laboratory.