Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Lecture Demonstration Packages Crack growth in inflated balloons (A) Description of the demonstration
Crack growth in inflated balloons Introduction(A) Scientific backgroundIT resources(A) Description of the demonstrationVideo of the demonstration
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(A) Description of the demonstration

Description of how the demonstration is carried out, including identification of critical points to ensure success, any safety issues etc.  Include also any hints about audience participation, use of volunteers etc.

You can start with slides (see at the IT resources: TWC_crack_growth.ppt).

Slide 1. Crack Growth in Balloons – Stresses in a Pressurised Tube. To describe this slide you can use the Scientific Background / Stresses in a Thin-walled Tube. It’s important to note such points as equations for deriving of hoop and axial stresses, internal pressure of tube and thickness of wall.

Slide 2. Crack Growth in Balloons – Griffith Energy Balance. If you like you can use the Scientific Background / Energetics of Crack Advance. Probably it’s better to highlight the equation for the strain energy release rate in order to make theme clear to understand. Don’t forget to mention the critical flaw size for rubber that is used in demonstration.

Experiment. Now you need the volunteer. Firstly show the fast fracture on one of balloons and poke it. Ask your volunteer to help you and hold the balloon in horizontal way.

Then the fracture behaviour may be shown on balloon with sticky tape. So put the tape on balloon with longest side to the horizontal axis. After creating the flaw make volunteer to pay attention for crack propagation, it would be seen under tape.

The last one should be carried out with tape putted on the balloon across, i.e. along vertical axis. Obviously, in this case it would explode faster. Thank your volunteer for help J So now it’s time to explain what did happen with stresses during the experiment.

Slide 3. Crack Growth in Balloons – Inhibition of Strain Energy Balance. Although there are two comparative pictures, it’s worth to note that the tape constrains the relaxation of rubber and reduces the stress-free region. Due to this point the energy release rate also decreases.

Hints

  • It’s really important to stick the tape properly. Be sure there are no gaps, bubbles and gathers.
  • It’s better to poke balloon in centre of tape in order to crack propagation extends at both ends simultaneously.
  • Look to keep volunteer quite far from needle and not too close to balloon. Because he will be badly surprised when it would explode loud. Or you can warn him to be aware.
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