Transcript Fibers

Fibers
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Bertino
Introduction
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Fibers are used in forensic science to create a link
between crime and suspect
Through normal activities
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We shed fibers
We picked up fibers
Very small fibers are classified as trace evidence
Collecting fibers within 24 hours is critical
Forensic Science: Fundamentals & Investigations, Chapter 4
How Forensic Scientists
Use Fibers
Fiber evaluation can show
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Type of fiber
Color
Possibility of violence
Location of suspects
Point of origin
Forensic Science: Fundamentals & Investigations, Chapter 4
Sampling and Testing
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Shedding—common form of fiber transfer
Microscopes reveal characteristic shapes
and markings
Infrared spectroscopy reveals chemical
structures to differentiate similar fibers
Destructive Testing Methods
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Burning fibers
Dissolving fibers in various liquids
Forensic Science: Fundamentals & Investigations, Chapter 4
Sampling and Testing
Compare fibers found on different suspects with those
found at the crime scene
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Forensic Science: Fundamentals & Investigations, Chapter 4
Fiber Classification
—Natural Fibers
Animal fibers (made of proteins):
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Wool and cashmere from sheep
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Mohair from goats
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Angora from rabbits
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Hair from alpacas, llamas, and
camels
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Silk from caterpillar cocoons
(longer fiber does not shed easily)
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Forensic Science: Fundamentals & Investigations, Chapter 4
woven
wool
textile
Fiber Classification
—Natural Fibers
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
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Forensic Science: Fundamentals & Investigations, Chapter 4
Fiber Classification
—Natural Fibers
Plant fibers:
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Cotton—most common textile
plant fiber (picture)
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Coir from coconuts is durable
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Hemp, jute, and flax from stems grow in
bundles
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Manila and sisal from leaves deteriorate
more quickly
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Forensic Science: Fundamentals & Investigations, Chapter 4
Fiber Classification
—Natural Fibers
Mineral Fibers:
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Fiberglass—a fibrous form of glass
Asbestos—a crystalline structure
Forensic Science: Fundamentals & Investigations, Chapter 4
Fiber Classification
—Synthetic Fibers
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50% of fabrics are artificially produced
Examples:
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Rayon
Acetate
Nylon
Acrylic
Polyester
Forensic Science: Fundamentals & Investigations, Chapter 4
Fiber Classification
—Synthetic Cellulose Fibers
Regenerated Fibers (derived from cellulose):
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Rayon
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Celenese®
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Cellulose chemically combined with acetate
Found in many carpets
Polyamide nylon
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Most common in this group
Imitates natural fibers, but stronger
Cellulose combined with three acetate units
Breathable and lightweight
Used in performance clothing
Forensic Science: Fundamentals & Investigations, Chapter 4
Fiber Classification
—Synthetic Polymer Fibers
Synthetic Polymer Fibers
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Petroleum base
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Very different from other fibers
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Monomers join to form polymers
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Fibers are spun together into yarns
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No internal structures
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Uniform diameters
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Forensic Science: Fundamentals & Investigations, Chapter 4
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Forensic Science: Fundamentals & Investigations, Chapter 4
Fiber Classification
—Synthetic Polymer Fibers
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Polyester
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“Polar fleece”
spandex nylon
Wrinkle-resistant
Not easily broken down by light or concentrated acid
Added to natural fibers for strength
Nylon
• Easily broken down by light and concentrated acid
• Otherwise similar to polyester
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Forensic Science: Fundamentals & Investigations, Chapter 4
Fiber Classification
—Synthetic Polymer Fibers
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Acrylic
• Inexpensive
• Tends to “ball” easily
• Substitute for artificial wool or fur
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Olefins
• High performance
• Quick drying
• Resistant to wear
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Forensic Science: Fundamentals & Investigations, Chapter 4
Comparison of Natural and
Synthetic Fibers
Visual Diagnostics of Some Common Textile Fibers
under Magnification
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Forensic Science: Fundamentals & Investigations, Chapter 4
Yarns, fabrics, and textiles
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Yarns—fibers (of any length, thick or thin,
loose or tight) twisted or spun together
Blending fibers meets different needs
(e.g., resistance to wrinkling)
Fibers are woven into fabrics or textiles
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Threads are arranged side by side (the warp)
More threads (the weft) are woven back and forth
crosswise through the warp
Forensic Science: Fundamentals & Investigations, Chapter 4
Weave Patterns
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Forensic Science: Fundamentals & Investigations, Chapter 4
Fibers as Evidence
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The quality of the fiber evidence
depends on the ability of the criminalist
to identify the origin of the fiber or at
least be able to narrow the possibilities
to a limited number of sources.
Forensic Science: Fundamentals & Investigations, Chapter 4
As Evidence
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Other morphological features that could
be important in comparing fibers are:
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Lengthwise striations on the surface of the
fiber
The presence of delustering particles that
reduce shine
The cross-sectional shape of the fiber
Forensic Science: Fundamentals & Investigations, Chapter 4
Cross Section
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Forensic Science: Fundamentals & Investigations, Chapter 4
Methods for Comparision
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Infrared spectrophotometry is a rapid
and reliable method for identifying the
generic class of fibers.
The molecules that compose a
manufactured fiber selectively absorb
infrared light to form a characteristic
pattern.
Forensic Science: Fundamentals & Investigations, Chapter 4
IR Spec
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Fiber found on
a postcard
compared to
FBI database
Forensic Science: Fundamentals & Investigations, Chapter 4
Under the Scope
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A most useful physical property of
fibers is that many synthetic fibers
exhibit double refraction or
birefringence when viewed under a
polorizing microscope. Depending on
the class of fiber, each polarized plane
of light will have a characteristic index
of refraction.
Forensic Science: Fundamentals & Investigations, Chapter 4
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Forensic Science: Fundamentals & Investigations, Chapter 4
Collection and Preservation
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Need to identify what might be a carrier of
fiber evidence
Forensic Science: Fundamentals & Investigations, Chapter 4