12.7: Striated evidence
- Page ID
- 53160
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Striated evidence occurs when a hard object comes in contact with a softer material. This is most commonly found in residential, agricultural, and commercial burglaries, but can also be found at homicide or sexual assault scenes where the suspect gained entry to an area by forcibly breeching a door or window. Although ballistic evidence is, in fact, striated evidence, for the moment we will primarily focus on tool mark evidence. A tool mark is any impression, cut, gouge or abrasion left on an object that is softer than the object, tool that caused the marks.
Tool marks impressions are divided into three categories:
- Compression tool marks
- Cutting tool marks
- Sliding tool marks
- Saw and drill marks
Just as in footwear and tire tread impression evidence, all tools are manufactured through a variety of means, all of which create class characteristics. Once the tools are employed, however, their individual history results in unique individualistic characteristics through accidental variations on the edges and surfaces of the tool from use.
Compression tool marks
Compression tool marks are the most common marks encountered in the crime scene and are typically the type of evidence that will be found at a burglary scene. When a tool is pressed into a softer material, it will leave an impression that usually contains a clearly identifiable outline of the tool, such as a screwdriver, pry-bar, hammer, etc.
Cutting tool marks
Cutting tool marks are usually found on chains, hasps, locks, or chain link fences and are created by tools such as a pair of bolt cutters, hacksaws, etc.
As the tool cuts the softer metal, it transfers marks and striations from the harder cutting surface to the softer surface of the item being cut. Those marks and striations can be macroscopically examined and documented, which can then be compared to the tool when recovered.
Sliding tool marks
Sliding tool marks are created when the surface of a tool slides along the surface of a softer material leaving patterns of parallel striations.
Photograph the impression, cast it, and if possible, collect the item. Trace evidence may be located on the tool, if it can be found, and may provide a link to the impression area (i.e. paint chips, metal fragments, etc.). Never fit the tool back into the found tool marks! After the suspect tool has been checked for trace evidence, the tool mark examiner will make several comparison impressions that will be microscopically compared with the tool and the impression evidence submitted from the crime scene.
Drill and Saw Marks
When a drill or saw are used to bore or cut through a softer material, the resulting marks can identify the tool that was used by the distinctive impression that is left behind. Saw marks can reveal the pattern or number of teeth per unit length of the blade and may also provide identifying features of worn or missing teeth on the blade. The drill can provide information regarding the spiral “rifling” within the bore of the hole and may also exhibit the characteristics of the tip of the drill bit. Metal shavings from the saw or drill can also provide information as to their metallic construction, which can be matched to the blade or bit that created the cut or hole.
Fracture matching
Fracture matching or mechanical fit is a unique form of evidence. It is also a method of individualization. An item can be associated with a specific source based on the unique way the pieces fit together. The match of unique damage of an object makes it identifiable to the exclusion of all others.
Fracture matching, or mechanical fit, is also a useful tool for the forensic technician in cases of automotive accidents where the vehicle has left the scene. A close examination of the accident scene will generally result in the discovery of an ejected piece of the vehicle. That piece can later be fracture matched to its original source.
