2 Fibers Are considered class evidence Have probative value Are common trace evidence at a crime scene Can be characterized based on comparison of both physical and chemical properties Identification ID: 280769
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FibersSlide2
2
Fibers
Are considered class evidence
Have probative value
Are common trace evidence at a crime scene
Can be characterized based on comparison of both physical and chemical properties
Slide3
Identification
and Comparisonof Synthetic fibersHow much value a fiber has as evidence is related to the criminalist’s ability to trace its origin.Slide4
Evidential
Value of fabrics that can be fitted together at torn edges: IndividualEvidence!!Slide5
What most commonly occurs is that a limited number of fibers are presented to the examiner:Slide6
The
possibility of making a PHYSICAL MATCH is non-existant Is NONEXISTANT!!Slide7
The
examiner must use a side by side comparison.A side-to-side comparison of the control and the crime-scene fibers Slide8
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Fabric
Fabric is made of fibers. Fibers are made of twisted filaments
Types of fibers and fabric
A. Natural
—
animal, vegetable or inorganic
B. Artificial
—
synthesized or created from altered natural sourcesSlide9
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Types of Fibers
Synthetic
Rayon
Nylon
Acetate
Acrylic
Spandex
Polyester
Natural
Silk
Cotton
Wool
Mohair
CashmereSlide10
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Classification
Natural fibers are classified according to their origin:
Vegetable or cellulose
cotton, rayon
Animal or protein
wool, silk
Mineral
asbestos, fiberglassSlide11
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Fiber Comparison
Can you tell the difference(s) between the cotton on
the left and the rayon on the right? Slide12
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Synthetic Fibers
(
Made from derivatives of petroleum,
coal and natural gas)
Nylon
—
most durable of man-made fibers; extremely light weight
Polyester
—
most widely used man-made fiber
Acrylic
—
provides warmth from a lightweight, soft and resilient fiber
Spandex
—
extreme elastic propertiesSlide13
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Fabric Production
Fabrics are composed of individual threads or yarns, made of fibers, that are knitted, woven, bonded, crocheted, felted, knotted or laminated. Most are either woven or knitted. The degree of stretch, absorbency, water repellence, softness and durability are all individual qualities of the different fabrics.Slide14
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Polymers
Synthetic fibers are made of polymers which are long chains of repeating chemical units.
The word polymer means many (
poly
), units (
mer
).
The repeating units of a polymer are called monomers.
By varying the chemical structure of the monomers or by varying the way they are joined together, polymers are created that have different properties.
As a result of these differences, forensically they can be distinguished from one another.
Polymerization videoSlide15
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Fiber Cross-Sections
Synthetic fibers are forced out of a nozzle when they are hot, and then they are woven. The holes of the nozzle are not necessarily round; therefore, the fiber may have a unique shape in cross-section.Slide16
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Testing for Identification; Part 1
Microscopic observation
Dye composition---
determine types of pigments in
dye
Thermal decomposition
—
gently heating to break down the fiber to the basic
monomers
Burning
—
observation of how a fiber burns, the odor, color of flame, smoke and the appearance of the residue
Chemical tests
—
solubility and decomposition
Slide17
Process of
microscopic comparison1. Compare color and diameterSlide18
2. A
comparison of striations and pitting marks is madeIf the color and diameter are Consistent…Slide19
The
SHAPES of the fibers are then compared Slide20
Dye
composition Thin layer chromatographySlide21
Or
MicrospectrophotometerCompares the exact wavelengths of visible light that the fiber absorbsSlide22
Chemical
composition Slide23
Determine
(i) Class and (ii) sub-class of fabricSlide24
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Testing for Identification; Part 2
Density
—
mass of object divided by the volume of the object
Refractive Index
—
measuring the bending of light as it passes from air into a solid or liquid
Fluorescence
—
used for comparing fibers as well as spotting fibers for collectionSlide25
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Collection of
Fiber Evidence
Bag clothing items individually in paper bags. Make sure that different items are not placed on the same surface before being bagged.
Make tape lifts of exposed skin areas of bodies and any inanimate objects
Removed fibers should be folded into a small sheet of paper and stored in a paper bag.Slide26
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Fiber Evidence
Fiber evidence in court cases can be used to connect the suspect to the victim or to the crime scene. In the case of Wayne Williams, fibers weighed heavily on the outcome of the case. Williams was convicted in 1982 based on carpet fibers that were found in his home, car and on several murder victims.
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Plain WeaveSlide28
Twill WeaveSlide29
Satin weaveSlide30