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
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.
To make the testing rig you need
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.
a 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.)
b Place the slice in the testing rig with the cut faces at top and bottom. Carefully and gradually tighten both wing-nuts.
c Record the maximum load reached before the sample breaks.
d 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.
e 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?
1 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
2 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 cm2 and m2.
3 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 Salters Horners Advanced Physics©, University of York Science Education Group.
Diagrams are reproduced by permission of the copyright holders, Heinemann.