Chapter 8 Fiber Properties and Identification,Fiber Properties and Identification,Mechanical properties Tensile properties Tensile deformation,Fiber Properties and Identification,Tensile deformation: some concepts Elongation,Strain,Fiber Properties and Identification,Tensile deformation Load: Force applied to extend the fiber Unfair to compare force applied to fibers with different sizes.,Stress Engineering stress,Unit: Pa = N/m2 or dyne/cm2 psi = pound/in2,Fiber Properties and Identification,Tensile deformation For textile yarns and fibers, hard to determine cross-section area use linear density: gf/denier or N/tex,Fiber Properties and Identification,Tensile testing of fibers Testing conditions: ASTM standard for textile testing: 70F (21C) and 65% relative humidity (RH) Stress-strain curves,Fiber Properties and Identification,Stress-strain curves Initial modulus The slope of the first section of the stress-strain curve (ratio of the stress to strain) also called Youngs modulus closely follows Hooks Law smaller modulus, easier to elongate under small stresses. For different applications, different requirements: ordinary apparels: high initial modulus womens sheer hosiery: low initial modulus composites: extremely high initial modulus,Fiber Properties and Identification,Stress-strain curves Initial modulus,Fiber Properties and Identification,Stress-strain curves Yield point The point at which the stress-strain curve flattens or changes its slope significantly,Fiber Properties and Identification,Stress-strain curves Yield point The polymers “yield” or molecular chains start to move and pass one another. The deformation is permanent. The slope of the stress-strain curve after yield point relies on the strength of intermolecular forces. Most fibers have yield point but not cotton and flax due to strong intermolecular forces.,Fiber Properties and Identification,Stress-strain curves Hardening point The polymer molecules become more oriented as strain increases and the fiber reaches a deformation limit, where the molecules have the highest orientation and (stress) hardening occurs.,Fiber Properties and Identification,Stress-strain curves Failure point Where the fiber fails. Sometimes not necessary the maximum stress point. Depends on definition.,Fiber Properties and Identification,Stress-strain curves Tenacity Stress at the point of rupture. Stress at the maximum load. Unit: N/tex, gf/denier,Fiber Properties and Identification,Stress-strain curves Strain or elongation at break % of strain at the point of rupture. Strain at the maximum load.,Fiber Properties and Identification,Stress-strain curves Work of rupture Physics: Work = Force Distance = F s If F = f(s) is not a constant When change of distance Ds is very small, F is almost a constant, thus Dw = F D s or when Ds 0, dw = f(s)ds therefore:,Fiber Properties and Identification,Stress-strain curves Work of rupture Area under load-displacement curve. Specific work of rupture Area under stress-strain curve,Fiber Properties and Identification,Elastic recovery percent of strain being recovered when load is released. Occurs at the initial part of the stress-strain curve where deformation is small and almost no relative movement of molecules. Energy absorbed can be recovered completely. Elastic recovery as cross-link as H-bond and ionic bond as strain or stress ,Fiber Properties and Identification,Resilience (Work of recovery) The ratio of energy recovered to energy absorbed May be extensional, flexural, compressional or torsional. Is a function of deformation. as crimp and cross-link as H-bond and ionic bond as strain Resilience wrinkle e.g. PET vs Cotton and Rayon,Fiber Properties and Identification,Flexural rigidity (stiffness) How easy a fiber can be bent Defined as EI where E = Youngs modulus of the fiber I = Moment of inertia of the fiber Mathematically,Fiber Properties and Identification,Flexural rigidity (stiffness) Thus: for a fiber with a round cross-section,Fiber Properties and Identification,Abrasion resistance Very difficult to measure Determined largely by structure of yarn and fabric. Affected by temperature and moisture content e.g. Nylon has the highest and acetate lowest,Fiber Properties and Identification,Flexibility Defined as number of times a fiber can be bent 180 before it fails. Crystallinity , flexibility Fiber diameter , flexibility ,Fiber Properties and Identification,Time related mechanical properties Creep: under a constant load or stress, deformation or strain increases with time.,Fiber Properties and Identification,Force,Elongation,Time,Time,elastic elongation,creep deformation,instant strain recovery,creep recovery,Permanent deformation,Fiber Properties and Identification,Time related mechanical properties Creep A = elastic elongation (initial) B = creep deformation C = instant strain recovery D = creep recovery E = permanent deformation,Fiber Properties and Identification,Time related mechanical properties Stress relaxation A fiber is