Fibers as Evidence The Importance of Fibers Fibers

Fibers as Evidence

The Importance of Fibers • Fibers are everywhere • Commonly found at crime scenes • Especially incidents involving personal contact, such as … – assaults – sexual offenses – hit and run victim – breaking and entering

Fibers are Class Evidence • Fibers originate from textiles, thread and yarn that are mass produced, therefore… • A fiber cannot be linked to an individual source

Fibers provided Probative Value • Prove something that is relevant to the crime • Create a connection or association between a suspect and a particular person or place

Value of Fiber Evidence Depends on the ability to narrow the fiber's origin to a limited number of sources

Fibers at a Crime Scene • Can occur when two people come in contact • When contact occurs with an item at a crime scene • Methods of Transfer – Primary transfer- when fiber transferred from fabric directly onto victim or suspect – Secondary transfer - fibers already transferred onto victim or suspect are transferred onto another object or person

Fibers are made of … Many filaments twisted or bonded together forming thread or yarn

Examining Fiber Evidence Several instruments are used… • Stereomicroscope • Comparison Microscope • Polarized Light Microscope • Spectroscopy – an instrument that measure the transmission or absorption of various frequencies of visible and infrared light through a specimen

Microscopic Comparison of Fibers Are these fibers the same? cotton rayon

Polarized Light

Fibers Magnified in Polarized Light

Fibers can be classified as • Natural plant fibers may be more ribbon shaped and may contain twists at irregular intervals • Natural fibers from an animal source look like hair and will often have rough external scale patterns and medulla • Synthetic fibers tend to be smooth and uniform and some may have long striation lines on the outer layer

Plant Fibers Cotton - pure cellulose; strong, tough, flexible, moisture absorbent, not shape retentive – Most common natural fiber – Evidential value almost meaningless Other plant fibers – Flax (linen) – Ramie – Jute (burlap) – Hemp (cannabis plant)

Other plant fibers hemp ramie linen jute Flax fibers viewed with polarized light 14

Animal Fibers Most common animal fiber is Wool from sheep • Fine wool found in clothing • Coarse wool found in carpet Woolen fibers magnified • Wavy • Average length 7 -8 in. • Imbricate cuticle

Other Animal Fibers • Mohair and cashmere — goats • Wool — camel hair • Wool — llama hair • Wool — sheep hair • Angora — rabbit hair • Silk —insect fiber • Vicunas • softest fur • endangered since 1964 • National animal of Peru • $3000 / bolt of their fur

Silk • Silkworm moth • Caterpillar spins a cocoon • 1 cocoon yields about 5 meters of silk thread

Synthetic Fibers • More than half of all fibers used in production of textile materials are man-made • Manufactured fibers can originate from 1) Natural materials • Examples: rayon and acetate 2) Synthetic materials • Examples: nylon, polyester and acrylic

Synthetic Fibers from Petrochemicals § Polyester —most widely used man-made fiber § Nylon —most durable of man-made fibers; extremely light weight § Acrylic —provides warmth from a lightweight, soft and resilient fiber § Spandex —extreme elastic properties § Rayon —chemically-altered cellulose; soft, lustrous, versatile

Shape of Synthetic Fibers • The shape of a man-made fiber can determine the value placed on that fiber. • Cross section of a man-made fiber can be manufacturer-specific.

Synthetic Fibers Cross sections of nylon carpet fibers seen with a scanning electron microscope (SEM)

Polymers • Long chain molecules created when simple molecules called monomers are chemically bonded together creating a polymer § 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.

Testing for Identification § Microscopic observation § Burning—observation of how a fiber burns, the odor, color of flame, smoke and the appearance of the residue § Thermal decomposition—gently heating to break down the fiber to the basic monomers § Chemical tests—solubility and decomposition 23

Testing for Identification § 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 finding fibers for collection 24

Dyes § Components that make up dyes can be separated and matched to an unknown. § There are more than 7000 different dye formulations. § Chromatography is used to separate dyes for comparative analysis. § The way a fabric accepts a particular dye may also be used to identify and compare samples. 25

Identification and Comparison of Fibers § Fourier Transform Infrared analysis (FTIR) — based on selective absorption of wavelengths of light § Optical microscopy —uses polarizing light and comparison microscopes § Pyrolysis gas chromatography-mass spectrometry (PGC-MS) —burns a sample under controlled conditions, separates and analyzes each combustion product 26

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. 27

Weave Terminology § Yarn - a continuous strand of fibers or filaments, either twisted or not § Warp - lengthwise yarn § Weft - crosswise yarn § Blend - a fabric made up of two or more different types of fiber. 28

Weave Patterns 29

Plain Weave § The simplest and most common weave pattern § The warp and weft yarns pass under each other alternately § Design resembles a checkerboard 30

Twill Weave § The warp yarn is passed over on to three weft yarns before going under one § Makes a diagonal weave pattern § Design resembles stair steps § Denim is one of the most common examples 31

Satin Weave § The yarn interlacing is not uniform § Creates long floats § Interlacing weave passes over four or more yarns § Satin is the most obvious example 32

Knitted Fabric Knitted fabrics are made by interlocking loops into a specific arrangement. It may be one continuous thread or a combination. Either way, the yarn is formed into successive rows of loops and then drawn through another series of loops to make the fabric. 33

More about Fibers For additional information about fibers and other trace evidence, check out Court TV’s Crime Library at: www. crimelibrary. com/criminal_mind/forensics/trace/1. html 34