Practical Activity for 14-16
Students observe brittle fracture during compression of a Crunchie bar, and take measurements to calculate its breaking stress. They appreciate that estimates can be useful.
Apparatus and Materials
For each student group
- Crunchie bar
- Saw-toothed knife or modelling saw, sharp (avoid a pointed type)
- Bathroom scales (1,200 N or 120 kg )
- Testing rig (see technical notes)
Health & Safety and Technical Notes
Make sure that students do not eat the Crunchie bars, since eating anything in the laboratory is hazardous.
Take care when using the sharp saw.
- Wooden beams, approx 35 cm x 5 cm x 2 cm, 2
- Lengths of studding, 15 cm long x 1 cm diameter, 2
- Lock nuts to fit studding, 2
- Wing nuts to fit studding, 2
Construct the rig as shown in the diagram.
- Saw a slice from the Crunchie, about 1.5 cm long. Try to keep the cut as ‘square on’ as possible. (Do not cut the slice in advance, as it goes gooey.)
- Place the slice in the testing rig with the cut faces at top and bottom. Carefully and gradually tighten both wing-nuts.
- Record the maximum load reached before the sample breaks.
- Crunchie has a ‘honeycomb’ structure similar to bone and fractures in a similar way under stress. Observe the way in which the Crunchie sample fractures.
- Measure the cross-sectional area of the Crunchie bar. Calculate the stress required to break the sample.
- Compare this breaking stress with the stress on human leg bone in normal use. How do the properties of Crunchie compare with those of bone?
- This activity was originally developed as part of a teaching sequence based around ‘spare part surgery’ (replacement hip joints). The Crunchie is used as a physical model for bone.
- The maximum load will vary considerably between samples. The reasons for this can be discussed with students and include:
- it is difficult to tighten both wing nuts evenly
- the sample breaks more easily if the cut face is not exactly square-on
- a Crunchie bar contains many irregularities and is not manufactured as a uniform material
- Breaking stress = load that breaks the sample/cross-sectional area.
- The stress calculation can be challenging for many students, particularly if you ask them for an answer in N m -2 , rather than N cm -2 .
- Guide them to express the linear dimensions of the Crunchie in metres before calculating the area – this approach is much less prone to error than attempting to convert between cm 2 and m 2 .
- To compare the stress in human bones with that required to break a Crunchie, students will need to estimate the cross-sectional area of their own leg bones. If your school or college biology department has a skeleton, it would be useful to borrow it so that students can measure the relevant bone thickness. As the area is neither circular nor rectangular, and varies along the length of a bone, students will need to make a sensible estimate rather than an exact calculation.
This experiment comes from University of York Science Education Group:
Diagrams are reproduced by permission of the copyright holders, Heinemann.
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