L2_txc6 - Lecture 2 16:00 Mechanical Properties Their...

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Unformatted text preview: Lecture 2 16:00 Mechanical Properties Their responses to applied forces and to recovery from those forces Durability (test ?)-ability to retain its physical integrity under condition of mechanical stress for a reasonable period of time Stress-resistance of deformation (a change in length) developed within a fiber being subjected to a pulling or tensile force o Unit: g/den or N/tex Strain-deformation (lengthening or extension) of a fiber caused by application of a pulling or tensile force o Unit: percentage (%) Tenacity-stress expressed as a force per linear density (grams-force per denier or Newtons per tex) o Breaking tenacity- the tenacity at rupture o Features: (test ?) Flax and olefin have high breaking tenacity, wool and acetate have low breaking tenacity High orientation , high degree of polymerization, high crystallinity, high breaking tenacity Elongation o Extension (lengthening) expressed as a percent of its original length Breaking elongation-elongation at rupture Features: Wool (lots of cream) and nylon have high elongation, cotton and flax have low elongation Higher the crystallinity, lower the elongation at break Initial modulus o Starts out linear (straightening of polymer change , arrangement of chain molecule along the fiber axis) till yield point (slope proportional to intra fiber bonding, slippage of chain molecules) o Rupture point=tenacity at break [graph] o First segment of stress-strain curve is a straight line (stress is proportional to strain); the slope of the line provides a measure of the fibers initial modulus aka Youngs modulus o High modulus, hard to elongate ex cotton vs rayon Work of rupture (toughness) o Ability of a fiber to endure a large deformation without rupture o Area under its stress-strain curve o Polyester and nylon are tough fibers Yield point o The point at which the stress-strain curve flattens and permanent deformation takes place Elastic recovery (elasticity) o Ability of a fiber to return to its initial length after being stressed or elongated o Features: Nylon is good, rayon is poor Polymer chains that are spiraled and folded tend to act like springs, cross-linked chains are stable Hydrogen & ionic bonds may reduce elasticity Resilience (work recovery) o Ability of a fiber to spring back after extension; expressed as the ratio of returned work (x) to total work (x+y) required to deform and release the fiber o Features: Can be measured from stress-strain curve and may predict wrinkle properties Crimped fibers are more resilient than smooth Presence of hydrogen bonds lowers resilience...
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This note was uploaded on 09/27/2011 for the course TXC 6 taught by Professor Na during the Spring '11 term at UC Davis.

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L2_txc6 - Lecture 2 16:00 Mechanical Properties Their...

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