trace evidence: fiber · fibers on clothing/body of one person transfer to clothing/body of second...

Trace Evidence: Fiber

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What are fibers?

• A fiber is the smallest unit of a textile material that has a length many times greater than its diameter.

• Used in forensic science to create a link between a crime

and a suspect.

• Fibers exchanged between 2 individuals, individual and

object, or 2 objects

• Considered to be CLASS EVIDENCE because they are

mass produced.

• Sensitive evidence – 95% of all fibers may be lost within

24 hours.

• Only the fibers you would not expect to find are

investigated. – Example: pink fibers found all over the victim’s body found on a

pink carpet vs. pink fibers found on a suspect’s pants

What are Fibers Used for In Forensics?

• Fibers can originate from many sources:

– Carpet

– Clothing

– Linen

– Furniture

– Insulation

– Rope/ligature

– Tape

Where do Fibers Come From?

How are Fibers Transferred?

Primary Transfer

• When fiber is transferred

from a fabric directly onto

person or person’s clothing

• Fabric suspect

• Fabric victim

Secondary Transfer

• When already transferred

fibers on clothing/body of

one person transfer to

clothing/body of second

person

• Suspect victim

• Victim suspect

How are Fibers Collected?

• Special vacuums

• Sticky tape

• Forceps

How Do Forensic Scientists Use

Fibers?

• Evidence must be evaluated

• Especially important for fibers because

there are SO many of them

• Value of fiber evidence in crime

investigation depends on its potential

uniqueness

• How is this determined? . . .

Fiber Uniqueness

• Type of fiber

• Fiber color

• Number of fibers found

• Where the fiber was found

Fiber Uniqueness Cont

• Textile the fiber originated from

• Multiple Fiber Transfers

• Type of Crime Committed

• Time between crime and discovery of fibers

Two Types of Fibers

• Natural

– derived from plants or animal hair

• Synthetic

– manmade

Natural Fibers: Animal Fibers

• Sheep (wool)

•most common; the end use is often determined by

coarseness

• fine wool fibers are used for clothing, while coarse

wool fibers are used in carpeting

• Goat (mohair, cashmere)

• Camel (wool)

• Llama

• Alpaca (wool)

• Fur fibers from mink, rabbit, beaver, etc.

• Silk (fiber from the cocoon of the silkworm)

Natural Fibers: Plant Fibers

• Cotton (ribbon-like shape with irregular twists; most common)

• Flax (Linen)

• Ramie

• Sisal (often used in linen rugs)

• Jute (used in ropes)

• Hemp (the common name for cannabis for industrial or non-

drug use; typically used for rope or sack)

• Kapok (fiber from kapok tree seed pods; used in pillows and

mattress stuffing)

• Coir (coconut husks; used in carpet, rugs)

Natural Fibers

Synthetic Fibers

More than half of all fibers used in the production of textile

materials are manmade.

• Polyester (most common; wrinkle resistant; often spun with

cotton)

• Nylon (very common; elastic and strong; lustrous and silk-like

when stretched)

• Acrylics (wool-like, soft and warm; quick drying and resistant

to moths)

• Rayon (cellulose-derived, regenerated, thin fiber)

• Acetates (cellulosed-based, wrinkle-resistant fiber)

Special Fibers

• Aramid fiber is a light, but strong, synthetic fiber.

• Heat-resistant aramid fiber is typically used for bullet-

proof vests, military applications, and racing tires

• Fire-resistant aramid fiber is used for firemen or disaster

response teams.

Fiber Identification

• https://www.youtube.com/watch?v=cn6K1

m7yH0I

Polymers 1. Synthetic fibers are made of polymers, which are long

chains of repeating chemical units.

2. The word polymer means many (poly) units (mer).

3. The repeating units of a polymer are called monomers.

4. By varying the chemical structure of the monomers or by varying the way they are joined together, polymers are created that have different properties.

5. As a result of these differences, they can be distinguished from one another forensically.

Fiber Comparisons

• Microscopic Comparisons

• Color

• Diameter

• Surface markings

• Delustering agents

• Cross-sectional shape

Filament Cross Sections

1. Synthetic fibers are forced out of a nozzle when

they are hot, and then they are woven.

2. The holes of the nozzle are not necessarily

round; therefore, the fiber filament may have a

unique shape in cross section.

Round 4-lobed

Octalobal

Irregular

Multi-lobed or Serrate

Trilobal

Dogbone or Dumbbell

• Polarized Light Microscopy – determines birefringence

(difference between two refractive indices) using polarized light

Fiber Comparisons (continued)

• Fourier Transform Infrared Spectroscopy (FTIR) –

analysis of a fiber’s chemical composition based on its ability

to absorb light at different wavelengths

• UV-Visible Microspectrophotometry – distinguishes

slight/subtle color differences based on absorption of light at

different wavelengths

Fabric Production

• Forensic Significance – if fabric sample

found at crime scene, it can be traced to a

particular type of clothing or to a particular

weave pattern that could help identify the

type of fabric

• Fiber – a fine, slender piece of thread or filament

• Yarn – a twisted aggregate of fibers

• Textiles – woven fibers; fabrics

Types of Fibers

Fabric Production

1. Fabrics are composed of individual threads or yarns that are made of fibers and are knitted, woven, bonded, crocheted, felted, knotted, or laminated.

2. Most are either woven or knitted.

3. The degree of stretch, absorbency, water repellence, softness, and durability are all individual qualities of the different fabrics.

Weave Terminology

1. Yarn—a continuous strand of

fibers or filaments that may

be twisted together

2. Warp—lengthwise yarn

3. Weft—crosswise yarn

4. Blend—a fabric made up of

two or more different types of

fibers

Lengthwise threads (the warp) are woven by crosswise threads

(the weft) in a pattern.

Common Weave Patterns

• Plain (firm, tends to wrinkle)

• Basket (not very durable, shrinks when washed)

• Satin (not durable, shiny surface)

• Twill (very strong, dense and compact, soft)

• Leno (open weave, easily distorted)

Weaving – Woven Fabrics

Two Possible Conclusions in Fiber Comparisons

• The Questioned fiber could have originated from the Known

sample.

• The Questioned fiber did not originate from the Known

source.

• The number of fibers is directly proportional to the likelihood of

actual contact (i.e., the greater the number of fibers, the more

likely that contact actually occurred.)

Significance of Fiber Evidence

Greater number of fiber types

More unique the material

Greater significance

Significance of Fiber Evidence (continued)

Resources

• Saferstein, Richard. Forensic Science: An Introduction. New Jersey: Pearson Prentice Hall, 2008.

• Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 8th ed. Upper Saddle River, NJ: Pearson Prentice Hall, 2004.

• Saferstein, Richard. Forensic Science Handbook. Englewood Cliffs, NJ: Prentice-Hall, 1982.

• Bertino, Anthony J. Forensic Science: Fundamentals and Investigations. Mason, OH: South-Western Cengage Learning, 2009.

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