Chapter 4 through physical and ... - Ms. Jones...
Transcript of Chapter 4 through physical and ... - Ms. Jones...
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Forensic Science: Fundamentals & Investigations, Chapter 4 1
Chapter 4 A Study of Fibers and TextilesBy the end of this chapter you will be able to:
o Identify and describe common weave patterns of textile samples
o Compare and contrast various types of fibers through physical and chemical analysis
o Describe principle characteristics used to identify common fibers
o Apply forensic science techniques to analyze fibers
All Rights Reserved South-Western / Cengage Learning © 2012, 2009
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Forensic Science: Fundamentals & Investigations, Chapter 4 2
Introduction
o Fibers are used in forensic science to create a link between crime and suspect; “circumstantial evidence”
o Through normal activities– We shed fibers -We pick up fibers
o Very small fibers are classified as trace evidenceo Collecting fibers within 24 hours is criticalo If different fiber types are found on a suspect and also
are in common with the crime scene, the more likely it is that the suspect was actually there.
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Forensic Science: Fundamentals & Investigations, Chapter 4 3
How Forensic Scientists Use Fibers
Fiber evaluation can show • Type of fiber• Color • Possibility of violence• Location of suspects• Point of origin
• see list of common questions asked by FS
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Forensic Science: Fundamentals & Investigations, Chapter 4 4
Sampling and Testing
o Shedding—common form of fiber transfero Microscopes reveal characteristic shapes
and markingso Infrared spectroscopy reveals chemical
structures to differentiate similar fiberso Destructive Testing Methods
• Burning fibers • Dissolving fibers in various liquids
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Forensic Science: Fundamentals & Investigations, Chapter 4 5
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 6
Fiber Classification —Natural Fibers
Animal fibers (made of proteins): o Wool and cashmere from sheepo Mohair from goatso Angora from rabbitso Hair from alpacas, llamas, and
camelso Silk from caterpillar cocoons
(longer fiber does not shed easily)
woven wool
textile
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Forensic Science: Fundamentals & Investigations, Chapter 4 7
Fiber Classification —Natural Fibers
Plant fibers (made of the polymer cellulose): o Absorb watero Insoluble in watero Very resistant to damage from harsh
chemicalso Dissolvable only by strong acidso Becomes brittle over time
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Forensic Science: Fundamentals & Investigations, Chapter 4 8
Fiber Classification —Natural Fibers
Plant fibers: o Cotton—most common textile
plant fiber (picture) o Coir from coconuts is durableo Hemp, jute, and flax from stems grow in
bundleso Manila and sisal from leaves deteriorate more
quickly
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Forensic Science: Fundamentals & Investigations, Chapter 4 9
Fiber Classification —Natural Fibers
Mineral Fibers: o Fiberglass—a fibrous form of glasso Asbestos—a crystalline structure
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Forensic Science: Fundamentals & Investigations, Chapter 4 10
Fiber Classification —Synthetic Fibers
o 50% of fabrics are artificially producedo Examples:
• Rayon • Acetate • Nylon• Acrylic• Polyester
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Forensic Science: Fundamentals & Investigations, Chapter 4 11
Fiber Classification —Synthetic Cellulose Fibers
Regenerated Fibers (derived from cellulose): 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
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Forensic Science: Fundamentals & Investigations, Chapter 4 12
Fiber Classification —Synthetic Polymer Fibers
Synthetic Polymer Fibers o Petroleum baseo Very different from other fiberso Monomers join to form polymers o Fibers are spun together into yarns o No internal structureso Uniform diameters
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Forensic Science: Fundamentals & Investigations, Chapter 4 13
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
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Forensic Science: Fundamentals & Investigations, Chapter 4 14
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
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Forensic Science: Fundamentals & Investigations, Chapter 4 15
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 16
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
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Forensic Science: Fundamentals & Investigations, Chapter 4 17
Weave Patterns
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Forensic Science: Fundamentals & Investigations, Chapter 4 18
. . . . . . . . . . . . . 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.
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Forensic Science: Fundamentals & Investigations, Chapter 4 19
. . . . . . . . . . . . . . . . . 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.