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The Mechanical Testing of Polymers and their Composites

The Mechanical Testing of Polymers and their Composites. By Gordon Williams. Introduction. Standard methods are vital in ensuring reliable data True in all fields – Thus ASTM & ISO If a property is to be claimed it must be backed by 1) a method and 2) statistical analysis ( means ,SD)

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The Mechanical Testing of Polymers and their Composites

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  1. The Mechanical Testing of Polymersand their Composites By Gordon Williams

  2. Introduction • Standard methods are vital in ensuring reliable data • True in all fields – Thus ASTM & ISO • If a property is to be claimed it must be backed by 1) a method and 2) statistical analysis ( means ,SD) • For materials science ;materials development must be supported by proper results.

  3. Introduction • e.g. Toughened polymers (PA,PE), • Filled polymers; glass filled ;particle filled • Nano Composites • Recent confusion in “Nano” field is often a result of poor data.

  4. Test Types • A range from direct product testing to basic properties ,e.g. Drop tests to Young’s Modulus. • Intermediate type, Testing special plaques or bowls. • Basic Tests measure material properties independent of size and method.

  5. Test Types • Two Main Types • Deformation behaviour; to find Young’s Modulus ,Yield stress ,Poisson’s Ratio ,Elongation*,Strength*. • Fracture Behaviour; to find Toughness,K1c,Izod*, Charpy*. • All use Tension, Compression or Bending Tests. • * See later; not usually a good idea.

  6. Testing Machines • A long history e.g. 1880 • Originally to test steel, e.g. Harbour Bridge • Machines in Civil Eng. Dept. from 1908 • All work in a similar way • Load applied via a screw ( Instron) or hydraulic rams (Servo Hydraulic) • Load measured by load cells (must calibrate)

  7. Test machine  Instron 4302

  8. Test machine  Instron 1195

  9. Test machine  MTS 810

  10. Testing Machines • Deformations; Cross head ,Gauge length? Extensometers, videos. • Modern machines are speed controlled, i.e. constant or exponential. • Polymers are rate dependent, i.e. are visco-elastic.

  11. Deformation Properties • Measured in; Tension ,compression or bending. • Tension; simplest but; end effects (dog bones) ,gripping (slipping) ,accurate strains difficult. • Compression; no gripping problems • However ,friction . • Necessary for some yield stress tests.

  12. Stress,  y F E y F 2% 0 Strain,  Definition of Deformation

  13. b W  l Illustrations of Deformation Tension: E= /=(F/)(l/bW) =F/bW = /l

  14. Illustrations of Deformation Compression:

  15. F  b W l Illustrations of Deformation Bending: E= /=(F/)(l/bW)(l 2/4W2)

  16. Deformation Properties • Bending • Good for Modulus (magnification) • However; variable strain , inhomogeniety (flexural modulus) • Interpretation of curves. • Young’s Modulus; elastic property ,polymers are visco-elastic ,rate dependent ,unloading and hysterysis .Usually non- linear.

  17. Deformation Behaviour • Yield Stress; onset of permanent deformation, hard to define. 2% offset ,maximum stress. • “Correct” or “definable” ? • Fracture stress/strain ,strength .Variable (surface finish) .A property? • Fracture Mechanics!! See next week.

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