Introductionmarysikes.weebly.com/uploads/6/0/5/0/60508559/2_fibe… · · 2016-10-13Fiber...

Forensic Science: Fundamentals & Investigations, Chapter 4 2

Introduction

o Fibers are used in forensic science to create a link between crime and suspect

o When fibers are transferred directly from victim to

suspect or suspect to victim, this is called: direct

transfer.

o Through normal activities

– We shed fibers

– We picked up fibers

o Very small fibers are classified as trace evidence

o Collecting fibers within 24 hours is critical

Fiber Evidence

A fiber is the smallest unit of a textile material that has a length many times

greater than its diameter. A fiber can be spun with other fibers to form a yarn that

can be woven or knitted to form a fabric.

The type and length of fiber used, the type of spinning method, and the type of

fabric construction all affect the transfer of fibers and the significance of fiber

associations. This becomes very important when there is a possibility of fiber

transfer between a suspect and a victim during the commission of a crime.

http://www.fbi.gov/hq/lab/fsc/backissu/july2000/deedric3.htm#Fiber%20Evidence

Matching unique fibers on the clothing of a victim to fibers on a suspect’s clothing

can be very helpful to an investigation, whereas the matching of common fibers

such as white cotton or blue denim fibers would be less helpful.

The discovery of cross transfers and multiple fiber transfers between the suspect's

clothing and the victim's clothing dramatically increases the likelihood that these

two individuals had physical contact.

Many different natural fibers that come from plants and animals are used in the

production of fabric. Natural plant fibers are produced by stems, leaves

seeds and fruits.

http://www.fireflydiapers.com/articles/diaperarticle_naturalfibersabsorb.htm

Cotton fibers are the plant fibers most commonly used

in textile materials Common polymer is cellulose.

Cotton has only one (1) seed.

The animal fiber most frequently used in the

production of textile materials is wool, and the most

common wool fibers originate from sheep.

Natural Fibers

Images: http://www.trashforteaching.org/phpstore/product_images/YarnWS.JPG

http://www.fbi.gov/hq/lab/fsc/backissu/july2000/deedric3.htm#Fiber%20Evidence

http://www.jivepuppi.com/images/fiber_evidence.jpg

More than half of all fibers used in the production of

textile materials are synthetic or man-made.

Categorized as regenerated fibers and polymers.

Nylon, rayon, and polyester are all examples of synthetic

fibers.

Cross-section of a

man-made fiber

Synthetic Fibers

Fibers under a microscope


How Forensic Scientists Use Fibers

Fiber evaluation can show

• Type of fiber

• Number of fibers

• Color

• Possibility of violence

• Location of suspects

• Point of origin

How Fibers are collected by CSI team.

o tweezers

o special vacuums

o sticky tape



Sampling and Testing

o Shedding—common form of fiber transfer

o Microscopes reveal characteristic shapes and markings

o Infrared spectroscopy reveals chemical structures to differentiate similar fibers

o Destructive Testing Methods

• Burning fibers

• Dissolving fibers in various liquids


Sampling and Testing

Compare fibers found on different suspects with those found at the crime scene

10

Fiber Type Burn Rate/Type Smell Smoke Product Left

Wool

Will not burn on it's

own, burns briefly and

Chars, shrinks away

from flame

Smells like burnt hair Little smoke

Black ash that

crumbles when

poked

CottonFlares into bright flame

and burns rapidly

Smells like burnt

paperLittle smoke Fine gray ash

Nylon

Burns out shortly after

removing from flame,

melts and burns away

Smells like celery Gray smoke

Cream colored

bead, bead

darkens if

PolyesterBurns out shortly after

removing from flame

Mild burnt plastic

smellBlack smoke

Melts and beads

into black mass

LinenFlares into bright flame

and burns rapidly

Smells like burnt

paperLittle smoke

Black ash that

crumbles when

poked

SilkDoesn't burn on own

for extended time

Mild smell, burnt or

charred meatLittle smoke

Black ash that

crumbles when

poked

Fiber Identification by Burning Test


Fiber Classification —Natural Fibers

Animal fibers (made of proteins):

o Wool and cashmere from sheep

o Mohair from goats

o Angora from rabbits

o Hair from alpacas, llamas, and camels

o Silk from caterpillar cocoons(longer fiber does not shed easily)

woven wool

textile



Plant fibers (made of the polymer cellulose):

o Absorb water

o Insoluble in water

o Very resistant to damage from harsh chemicals

o Dissolvable only by strong acids

o Becomes brittle over time



Plant fibers:

o Cotton—most common textile plant fiber (picture) one (1) seed

o Coir from coconuts is durable

o Hemp, jute, and flax from stems grow in bundles

o Manila and sisal from leaves deteriorate more quickly



Mineral Fibers:

o Fiberglass—a fibrous form of glass

o Asbestos—a crystalline structure


Fiber Classification —Synthetic Fibers

o 50% of fabrics are artificially produced

o Examples:

• Rayon

• Acetate

• Nylon

• Acrylic

• Polyester


Fiber Classification —Synthetic Cellulose Fibers

Regenerated Fibers (derived from cellulose or modified natural fibers) Add to notes

o Rayon– Most common in this group

– Imitates natural fibers, but stronger

o Celenese®

– Cellulose chemically combined with acetate

– Found in many carpets

o Polyamide nylon – Cellulose combined with three acetate units

– Breathable and lightweight

– Used in performance clothing


Fiber Classification —Synthetic Polymer Fibers

Synthetic Polymer Fibers

o Petroleum base

o Very different from other fibers

o Monomers join to form polymers

o Fibers are spun together into yarns

o No internal structures

o Uniform diameters


Fiber Classification—Synthetic Polymer Fibers

o Polyester• “Polar fleece”

• Wrinkle-resistant

• Not easily broken down by light or concentrated acid

• Added to natural fibers for strength

o Nylon• Easily broken down by light and concentrated acid

• Otherwise similar to polyester

spandex nylon


Fiber Classification—Synthetic Polymer Fibers

o Acrylic

• Inexpensive

• Tends to “ball” easily

• Substitute for artificial wool or fur

o Olefins

• High performance

• Quick drying

• Resistant to wear


Comparison of Natural and Synthetic Fibers

Visual Diagnostics of Some Common Textile Fibers

under Magnification


Yarns, fabrics, and textiles

o Yarns—fibers (of any length, thick or thin, loose or tight) twisted or spun together

o Blending fibers meets different needs (e.g., resistance to wrinkling)

o Fibers are woven into fabrics or textiles

• Threads are arranged side by side (the warp)

• More threads (the weft) are woven back and forth crosswise through the warp


Weave Patterns


. . . . . . . . . . . . . Summary . . . .

o Fibers are a form of class evidence.

o Fibers are a form of trace evidence.

o Fibers are spun into yarns having specific characteristics.

o Yarns are woven, with different patterns, into clothing or textiles.

o Fiber evidence is gathered using different techniques.


. . . . . . . . . . . . . . . . . Summary

o Fibers are analyzed using burn tests, tests for solubility in different solutions, polarized light microscopy, or infrared spectroscopy.

o Fibers are classified as natural or synthetic.

o Natural fiber sources include: • Animal hair

• Plant seeds, fruit, stems, or leaves

• Minerals.

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