trace evidence: fiber · fibers on clothing/body of one person transfer to clothing/body of second...
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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.
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• 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?
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• Fibers can originate from many sources:
– Carpet
– Clothing
– Linen
– Furniture
– Insulation
– Rope/ligature
– Tape
Where do Fibers Come From?
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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
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How are Fibers Collected?
• Special vacuums
• Sticky tape
• Forceps
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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? . . .
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Fiber Uniqueness
• Type of fiber
• Fiber color
• Number of fibers found
• Where the fiber was found
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Fiber Uniqueness Cont
• Textile the fiber originated from
• Multiple Fiber Transfers
• Type of Crime Committed
• Time between crime and discovery of fibers
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Two Types of Fibers
• Natural
– derived from plants or animal hair
• Synthetic
– manmade
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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)
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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)
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Natural Fibers
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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)
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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.
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Fiber Identification
• https://www.youtube.com/watch?v=cn6K1
m7yH0I
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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.
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Fiber Comparisons
• Microscopic Comparisons
• Color
• Diameter
• Surface markings
• Delustering agents
• Cross-sectional shape
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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
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• Polarized Light Microscopy – determines birefringence
(difference between two refractive indices) using polarized light
Fiber Comparisons (continued)
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• Fourier Transform Infrared Spectroscopy (FTIR) –
analysis of a fiber’s chemical composition based on its ability
to absorb light at different wavelengths
Fiber Comparisons (continued)
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• UV-Visible Microspectrophotometry – distinguishes
slight/subtle color differences based on absorption of light at
different wavelengths
Fiber Comparisons (continued)
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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
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• Fiber – a fine, slender piece of thread or filament
• Yarn – a twisted aggregate of fibers
• Textiles – woven fibers; fabrics
Types of Fibers
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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
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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
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Greater number of fiber types
More unique the material
Greater significance
Significance of Fiber Evidence (continued)
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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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