Impact Properties of Polymeric Solids

1
Impact Properties of Polymeric Solids

• Impact resistance is a measure of a materials
  ability to withstand a sudden applied load
  without failure.
• Impact resistance test results are difficult to
  interpret, given its dependence on the frequency
  of the applied load, the sample geometry and its
  sensitivity to sample imperfections.
• Designing for impact resistance is therefore a
  difficult task, requiring the engineer to
  understand the strengths and limitations of
  various test methods, the different types of
  polymer material failure (brittle, ductile, etc)
  and the mechanisms by which these materials
  dissipate
• inputted energy.

2
High-Speed Static Tensile Testing

• True stress-vs-true strain behavior of polymers
  (taking necking into account) measured at high
  strain rate (typically 100 s-1).
• Note differences in yielding behaviour
• Delrin is an acetal resin
• Lexan a polycarbonate
• Teflon TFE is the
• tetrafluoroethylene
• homopolymer
• Teflon FEP is a copolymer
• of tetrafluoroethylene and
• hexafluoropropylene

3
Instrumented Falling Dart Impact Testing

• Brittle Failure
• Ductile Failure

4
Energy Dissipation - Molecular Relaxation

• Storage modulus E1 and loss modulus E2 as a
  function of temperature at 138 Hz for
  poly(ethylene terephthalate) specimens of
  differing degrees of crystallinity 5 34 and
  50.

5
Energy Dissipation - Shear Yielding

• Shear yielding is a mode of material failure
  wherein the plane of failure is 45 relative to
  the applied stress.
• Where energy dissipation is concerned, yielding
  refers to ductile behaviour wherein the material
  deforms under the applied load.
• Shown right are
• instrumented dart
• impact testing data
• for a material at two
• temperatures.
• Note the differences
• in the extent of
• ductile behaviour.

6
Energy Dissipation - Crazing

• Glassy sections of a polymeric material can
  respond to tension by generating crazes normal to
  the direction of the applied force (without
  lateral contraction).
• Crazes are not cracks, but regions of low polymer
  density made of polymer microfibrils with a void
  fraction comprised of the fluid environment
  (air).
• Unlike cracks, crazes can bear a substantial load
• Shown here is crazing in a polycarbonate dogbone
  and an electron micrograph of a craze tip.

Crazing involves the creation of new surfaces,
and therefore is an important mode of impact
energy dissipation.
7
Energy Dissipation - Crazing

• Craze-fibril diameters range from 0.6 nm to 30 nm
  for those generated far below Tg.
• Strain hardening (increased resistance to
  drawing) is a key factor in stabilizing the
  microstructure, as polymer chain alignment
  increases with increased deformation.
• Shown here is a cyclic stress-
• strain curve for crazing
• bisphenol-A polycarbonate
• Note that the elastic modulus of
• the craze is low (25 of the bulk
• resin) due to its low volume fraction
• of polymer, but large elongation
• aligns microfibrils, thereby raising
• the material strength to approach
• its high value.

8
Failure Modes and Stress Whitening

• Tensile rupture of polyethylene
• (a) brittle fracture at low temperatures (T lt 95
  K)
• (b) necking and rupture (T 100-240 K)
• (c) non-necking ductile fracture of slowly cooled
  low molecular
• weight material (T90-300 K)
• (d) necking and drawing (T gt 0 C).

9
Rubber Toughening

• Brittle plastics such as polystyrene
• can be toughened through
• careful blending to create
• dispersed elastomeric domains.
• Interfacial adhesion is required
• to transmit forces to the
• rubbery phase, thereby
• generating the desired impact
• resistance.
• Shown above is an electron micrograph of crazed
  high-impact polystyrene (HIPS), which is a blend
  of polystyrene, polybutadiene and
  poly(styrene-graft-butadiene).
• Recall that polymer solubility is very limited,
  creating phase-separated blends in most
  commercial applications.
• Large-scale crazing of the material initiates at
  the stress concentrations established by the
  rubbery phase, thereby improving impact
  resistance.

10
Rubber Toughening - Crazing

• Typical uniaxial tensile stress-strain behavior
  of polystyrene (PS), medium-impact PS (MIPS),
  high-impact PS (HIPS), and poly(acrylonitrile-co-s
  tyrene-graft-butadiene) ABS.

11
Fibre Reinforced Polymer Composites

• The engineering definition of a composite is
  restricted to materials formed by alignment of
  strong, stiff fibres in a polymer matrix.
• Advanced composites employ continuous fibres
• Generic composites use chopped fibre
• In advanced composites, the polymer serves as a
  binder
• that aligns fibres in a manner that allows them
  to bear much of the
• applied load.
• This differs from chopped fibre reinforced
  plastics, where mechanical properties are
  dictated by both matrix and reinforcing fibre.

12
Reinforcing Fibres

• E-Glass Cloth has been available since the 1940s
  and is still the most widely used and the most
  economical composite reinforcement. It is made
  from strands of continuous glass filaments plied
  and twisted into yarn. It is chrome finished
  (Volan A) and is suitable for use with all
  polyester, vinyl-ester, and epoxy resins.
• S-2 Glass Cloth was developed by Owens Corning
  for military missile applications. Compared to
  E-Glass, S-2 Glass has much greater tensile
  strength, flexural strength, flexural modulus,
  and compressive strength. S-2 Glass laminates
  also exhibit improved impact resistance,
  toughness, a high-service temperature, and
  reduced weight. Often used for high-performance
  surf and sail boards.
• Carbon fiber, also referred to as graphite fiber,
  is one of the strongest and stiffest
  reinforcements available. When properly
  engineered, carbon fiber advanced composites can
  achieve the strength and stiffness of metal parts
  at significant weight savings. In addition to
  the high strength-to-weight and
  stiffness-to-weight ratios, carbon fibers are
  thermally and electrically conductive, have low
  thermal expansion coefficients, and have
  excellent fatigue resistance.
• Kevlar 49, a high modulus fabric, is an aramid
  fibre designed for plastic reinforcements. It
  displays excellent stability over a wide range of
  temperatures for prolonged periods. Even at a
  temperature as low as -320F (-196C) Kevlar
  shows essentially no brittleness or strength
  loss. Excellent dimensional stability and fatigue
  resistance. It, also, has resistance to chemicals
  and moisture.

13
Reinforcing Fibres
14
Reinforcing Fibres
15
Coupling Agents

• Where interfacial adhesion is inadequate,
  coupling agents and/or compatabilizers are used
  to generate cross-domain bonds.
• Examples include aminosilanes in rubber-silica
  blends and vinyl silanes in unsaturated
  polyester-glass (fibreglass) formulations.
• Coupling agents function by bonding with both
  phases, thereby generating the adhesion needed
  for stress to be communicated between polymer and
  reinforcing agent.