Physics and Statistical Models for Physical Match Analysis Utilizing 3D Microscopy of Fracture Surfaces

This fundamental and applied research proposal provides the scientific and quantitative paradigms for forensic comparative analysis of fractured and torn metal and plastic objects, utilizing three dimensional (3D) digital representations of their fractured surfaces and their replicas. An integrated validation study will be conducted on sequentially broken hacksaw blades and their topological replicas. The methodology utilizes 3D spectral analysis of the fracture surface topography, mapped by 3D microscopy (developed under previous NIJ funding). The framework focuses on quantitative statistical measures for the full range of the subclass and individual characteristics of the examined object, and identification of material role (e.g. metal and plastics) on the fracture-feature-characteristic scales. The proposed quantitative forensic comparisons have potential applications across a broad range of fractured materials and/or toolmarks, with diverse textures and mechanical properties. The proposed framework will assist the examiner by providing analytical and statistical support for his/her decision, by estimating with confidence bounds the probability of a true match, helping to arrive at a quantitative match decision. Our methods will use statistical learning tools trained on databases of both matches and non-matches. Three phases of work are proposed:

Yr-1: Develop mathematical and algorithm framework to identify imaging scales for different classes of materials with intrinsic length scales (metal grain size) and without intrinsic length scale (amorphous plastics), identifying key variables, probability distributions, and quantification of different sources of variability.

Yr-2: Perform a systematic study to different fracture scenarios and degrading environmental conditions. We will examine the topology of fracture surfaces exposed to the degrading effects of controlled humidity and pH-level, in order to examine the applicability of the technique to weathered specimens. Testing protocols will be scrutinized by our forensic collaborators for deficiencies and suggest possible improvements.

Yr-3: Perform a validation study on a fully documented consecutively fractured hacksaw sample set (Claytor and Davis, 2010), provided by our forensic scientist collaborator and utilized for the validation study of fracture match using fracture surfaces topology. Identify key issues of 3D microscopies of surface replicas.

Successful advancement of the proposed technique has the potential to provide a new investigative machine-based analysis with quantified error probabilities that can be applied in performing physical matches for a variety of materials. This research will be conducted in response to the NIJ’s expressed need for expanding knowledge underlying forensic science, and in collaboration with forensic scientists working in forensic laboratories.

This project contains a research and/or development component, as defined in applicable law, and complies with Part 200 Uniform Requirements - 2 CFR 200.210(a)(14).

CA/NCF