BME83 hw7

BME83 hw7 - BME HW#5 Nigel Chou 15.6) Stress relaxation vs....

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Nigel Chou 15.6) Stress relaxation vs. Viscoelastic creep tests a) In stress relaxation , a specimen is initially strained rapidly in tension to a predetermined and relatively low strain level. After that, the stress necessary to maintain the tension is measured as a function of time. For Viscoelastic creep tests , a stress (usually tensile) is applied instantaneously and is maintained at a constant stress level while strain is measured as a function of time. Both types of tests are carried out at a constant temperature. b) Stress relaxation: relaxation modulus ( E r ( t )) measured using o r t t E ε σ ) ( ) ( = where ) ( t is the measured stress at a certain time t and o is the applied strain level. Viscoelastic creep tests: creep modulus ( E c ( t )) measured using ) ( ) ( t t E o c = where o is the constant applied stress and ) ( t is the strain at time t . 15.11) Factors that influence the tensile modulus of a semicrystalline polymer c) Molecular weight: higher MW → higher tensile modulus . With higher molecular weights, ther is increased chain entanglement, making it harder to pull the chains apart, thus a higher tensile modulus. d) Degree of crystallinity: higher crystallinity → higher tensile modulus . Crystalline regions are closely packed in an ordered and parallel arrangement, having extensive secondary bonding between adjacent chain segments, making the chains harder to pull apart, thus the greater the extent of crystalline regions, the greater is the overall tensile modulus. e) Deformation by drawing: greater extension → higher tensile modulus in direction of drawing (but lower in other directions) . During drawing the molecular chains slip past one another and become highly oriented, increasing the secondary bonding between chains, thus they are harder to pull past each other, thus higher tensile strength. f) Annealing of an undeformed material: after annealing → higher tensile modulus. Heating leads to increase in percent crystallinity, crystal size and perfection, as well as modifications to the spherulite structure. Thus the tensile modulus increases for the same reason as in (b). g) Annealing of a drawn material: undoes the effects of drawing . If held at temperature of drawing, the molecular chains relax and assume random conformations characteristic of the predefomed state, thus the polymer will have the mechanical properties of a material before drawing. 1
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This note was uploaded on 04/09/2008 for the course BME 83 taught by Professor Reichert during the Spring '07 term at Duke.

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BME83 hw7 - BME HW#5 Nigel Chou 15.6) Stress relaxation vs....

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