The document describes an experiment to compare the mechanical properties of a chocolate bar at different temperatures. It instructs the reader to: 1. Chill one chocolate bar in the freezer while leaving another at room temperature. 2. Predict how the properties will change with temperature based on the chocolate's composition. 3. Devise a test to measure the properties, like a 3-point bending test using pencils or slotted masses. 4. Observe and explain any differences in strength or brittleness between the cold and warm bars using terms like tough, brittle or strong. Speculate on why not to use a Crunchie bar.

2. When subjected to stress, it fails without significant
deformation. Failure is often accompanied by a snapping
sound.
The amount of energy per
volume that a material can
absorb before rupturing.
Its ability to withstand an
applied stress without
failure.
the extent in which solid
materials can be plastically
deformed without fracture.
A measure of how strongly
a material opposes the flow
of electric current
Which terms would chocolate
bars have significant values in?

3. When subjected to stress, it fails
without significant deformation.
Failure is often accompanied by a
snapping sound.
The amount of energy per volume
that a material can absorb before
rupturing.
Its ability to withstand an applied
stress without failure.
the extent in which solid materials
can be plastically deformed without
fracture.
A measure of how strongly a material
opposes the flow of electric current

4. Introduction
• Mechanical properties of materials depend
on their microstructures.
• ‘Structure determines function’
• Another important factor in determining
the behaviour is the temperature.
• Here you are asked to consider a material,
predict its response to temperature, make
an observation and explain the effect.

5. Comparing warm and cold bars
1. Chill one of the bars long enough for you to be sure that it has
reached the freezer temperature. You may wish to place a
thermometer near the bar to establish its final temperature.
Remember that freezer temperatures can be around – 20 °C; make
sure that the thermometer goes low enough.
2. Whilst the bar is cooling predict how you expect the mechanical
properties to vary with temperature, relating this to the particles of
which the bar is made up.
3. Also devise a way to test the mechanical properties of the bars.
Eating them and using your teeth as the arbiter is not an option
given the low temperature of the chilled bar. The test should be
quick, probably destructive, and should if possible yield quantitative
estimates of the differences.
6. Make your observations and try to explain the results. Use the
terms you learnt in the earlier part of the course as far as possible:
e.g. is the bar tough, brittle, strong, etc?
7. Speculate on why it is recommended that you do not use a
Crunchie bar.

6. Predict how you expect the mechanical properties to vary
with temperature, relating this to the particles of which the
bar is made up.
I think that…

7. My method to determine the mechanical properties of a
chocolate bar is:
My method…

8. Observations and explanations. Use some of the terms
from the start of the lesson
I found that…

9. Why is it not recommended to use a Crunchie bar?
It is not recommended because…

10. Three-point bending
Pencils
Chocolate bar
Slotted masses