Impact Testing and Ductile-Brittle Transition

Polymeric materials are sometimes subjected to rapid stress loading or impact loads. A number of test methods have been proposed for assessing a materials ability to withstand these loads. Two of the most common methods are the so-called Izod and Charpy (pendulum) impact tests.1 The Izod test is the more popular method for plastic materials whereas Charpy is very common for metals. The Izod2 pendulum impact toughness test is described in the standard ASTM D256. This method requires a minimum of five and preferably ten or more individual readings to get a good average for the impact resistance of a material. The total impact energy depends on both the size of the test specimen and the notch shape and length. A standard specimen is usually used to allow comparison between different materials.

The pendulum impact test involves the measurement of the energy required to break a test specimen that is clamped at the ends and then struck in the center by a pendulum weight. The energy required to break the specimen is obtained from the loss in energy of the pendulum. This energy is simply the difference in potential energy of the hammer before and after the impact such as,

Efrac = m g (hS - hE)

where m is the mass of the hammer, h is its height, and g is the gravitational acceleration (9.81m/s2). The values are reported in terms of absorbed energy per unit of thickness at the notch (such as J/m or ft·lb/in). Alternatively, the results may be reported as absorbed energy per unit cross-sectional area at the notch (J/m2 or ft·lb/in2). The dimensions of an ASTM D256 standard specimen are 63.5 × 12.7 × 3.2 mm (2.5 × 0.5 × 0.125 in).

Pendulum Impact Tester         

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The energy absorbed in fracture has two components. These are the work of plastic deformation due to the formation of a plastic zone around the notch tip and the work required to create the fracture surfaces which is equal to the cohesive energy that has to be overcome to seperate the atoms and molecules on both sides of the crack path. The energy of a ductile fracture is much larger than the energy of a brittle fracture because ductile materials undergo strong plastic deformation before and during fracture which absorbs much more impact energy than the breaking of physical and chemical bonds (fracture surface energy).

All materials undergo a transition from ductile behavior at higher temperatures to brittle behavior at lower temperatures. At higher temperatures the impact energy is comparatively large since the fracture is ductile. But as the temperature is lowered, the impact energy sharply decreases over a narrow temperature intervall as the fracture becomes more brittle. The brittle-ductile transition can also be observed from the fracture surfaces; a ductile fracture sample has fibrous or dull surfaces whereas a brittle sample has granular and shiny fracture surfaces.

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The impact resistance (toughness) of a polymer depends on both intrinsic and extrinsic factors. Important intrinsic factors are molecular structure, molecular weight (distribution), cohesive energy and morphology (crystallinity and crystall structure) to name only a few factors. Important extrinsic factors are temperature, impact speed, shape and weight of the striker, specimen geometry, and notch size and shape.

A high molecular weight and narrow molecular weight distribution generally improves impact resistance, whereas increased crystallinity and voids lower impact resistance.

Notes
  1. Other more sophisticated tests include measurement of the area under the stress–strain curve in a high-speed (rapid) tensile or impact stress test.

  2. The test is named after the English engineer Edwin Gilbert Izod (1876–1946)

  • Summary

    Impact Testing

    The pendulum impact test is used to assess a materials ability to withstand impact loads.

  • The standard test methods for determining Izod pendulum impact toughness of plastic materials is the standard ASTM D256.

  • All materials undergo a transition from ductile behavior at higher temperatures to brittle behavior at lower temperatures.

  • Polymers that yield are usually tougher than polymers that undergo brittle fracture because yielding absorbs more energy than bond rupture.

  • A high molecular weight and narrow molecular weight distribution generally improves impact resistance, whereas increased crystallinity lowers impact resistance.

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