1. FORENSIC SCIENCE Trace Evidence Fiber
11/9/2018
FORENSIC SCIENCE Trace Evidence Fiber

2. Introduction Trace Evidence:
any small pieces of material, man-made or naturally occurring
Most common examples:
Hair
Fiber

3. Test Questions for Trace Evidence
What is it?
Is it man-made or natural?
What is its source?
How common is it?
Can it be identified to a single source?

4. Fiber Evidence
NOTE
Fabric is the type of material and fibers are the “threads” that make up the fabric
The use of fiber evidence in court cases is used many times to connect the suspect to the victim or to the crime scene.
In the case of Wayne Williams, fibers were the entire case.
Williams was convicted in 1982 based on carpet fibers that were found in his home, car and on several murder victims (product rule).

5. Classification Fibers are classified according to their origin:
Natural
Vegetable
Animal or protein
Mineral
Man-Made
Regenerated
Pure Synthetic

6. Vegetable or Plant Fibers
Cotton
Hemp
Linen (Flax)
Ramie
Sisal

7. Protein Fibers
Wool--animal fiber coming most often from sheep but may be goat (mohair,cashmere), rabbit (angora), camel, mink, beaver
Silk--animal fiber that is spun by a silk worm to make its cocoon; fiber reflects light and has insulting properties
Wool Fibers (400X)

8. Mineral Fibers
Asbestos--a natural fiber that was used in fire-resistant substances
Metallics (mylar)--a manufactured mineral fiber
Fiberglass--another manufactured mineral fiber

9. Cellulose Fibers
Rayon--first man-made fiber; soft, lustrous, versatile fiber
Cellulose esters--cellulose is chemically altered to create an entirely new compound not found in nature.
Acetate--less expensive, less polluting than rayon

10. Polymers
Synthetic fibers are made of polymers which are long string of repeating chemical units.
The word polymer means many (poly) units (mer).
The repeating units of a polymer are called monomers.
By varying the chemical structure of the monomers or by varying the way they are weaved together, polymers are created that have different properties.
As a result of these differences, forensically they can be distinguished from one another.

11. Petroleum Plastics (Made from derivatives of petroleum, coal and natural gas)
Nylon--most durable man-made fabric; extremely light weight
Polyester--most widely used man-made fiber
Acrylic--provides warmth from a lightweight, soft and resilient fabric
Spandex--extreme elastic properties

12. Fabric Production
Fabrics are composed of individual threads or yarns, made of fibers, that are knitted, woven, bonded, crocheted, felted, knotted or laminated.
Most are either woven or knitted.
The degree of stretch, absorbency, water repellence, softness and durability are all individual qualities of the different fabrics.

13. Woven Fabric
Woven fabric are made by interlacing warp (lengthwise) and weft (filling) yarns.
Warp run the length of the fabric and parallel to the selvage (the edge of the fabric).
Weft cross over and under the warp threads.
Types include:
Plain
Twill
Satin

14. Woven Fabric PLAIN Simplest and most common weave
Warp and weft pass under each other alternately
Create even patterns of 1/1 and 2/2
Design resembles a checkerboard

15. Woven Fabric
TWILL
Create by passing the warp yarn over one to three weft yarns before going under one
Makes a diagonal weave
Design resembles a stair steps
Denim is the most obvious example

16. Woven Fabric SATIN The yarn interlacing is not uniform
Creates long floats
Interlacing weave passes over four or more yarns
Satin is the most obvious example

17. Knitted Fabric
Knitted fabrics are made by interlocking loops into a specific arrangement.
It may be one continuous thread or a combination. Either way, the yarn is formed into successive rows of loops and then drawn through another series of loops to make the fabric..
Diagram:

18. Identification and Comparison of Fibers
Microscopic examination
Color--compositional differences in the dyes
Diameter
Fibers surface--delustering particles that may be added by manufacturers
Microspectrophotometer--compares fiber colors through spectral patterns
Chromatography--gives a more detailed analysis of the dye composition

19. Identification and Comparison of Fibers (cont.)
Polarizing microscope
Can be used to determine the refractive indices of various fibers.
The fiber is immersed in a fluid that has a comparable refractive index.
The disappearance of the Becke line is observed under the microscope.
In addition, fibers will absorb infrared light in a characteristic pattern.
This can be observed through the use of an infrared microspectrophotometer and a microscope.

20. Collection of Fiber Evidence
Bag clothing items individually in paper bags. Make sure that different items are not placed on the same surface before being bagged.
Make tape lifts of exposed skin areas of bodies and any inanimate objects
Removed fibers should be folded into a small sheet of paper and stored in a paper bag

21. Uniqueness Establishing Individual Characteristics
If there is only one source for the transfer material with a controlled environment where the contact took place
If there is a method available to characterize the material, such as DNA
Otherwise, trace evidence would have only class characteristics.

22. Figure F-1. Cross sections of several natural and synthetic fibers.
Reproduced from Melone, et. al., Lab Manual for Crimminalistics, 6th ed, Prentice Hall.

23. Man, I was nailed when those forensic guys found fibers
from the kid’s math assignment in my teeth.