| 3. Testing, Testing... |
Hardness
You don't want a sweet to be too hard, else you could break your teeth, warranting an emergency visit to the dentist. It happened to me several years ago. I wasn't eating a sweet at the time; I was teaching a Chemistry lesson when a large chunk of a molar fell away. Maybe I was grinding my teeth with frustration...
A hard material is not easily scratched or indented. While we have no absolute measure of hardness, we can work out relative hardness according to whether they are scratched by, or can scratch another material. One of the earliest scales was Moh's hardness scale, ranging from 1 (Talc, a very soft rock) to 10 (diamond, an extremely hard mineral). The scale does not go up in even steps.
In the food industry the Brinnell Hardness Test is widely used. One way of carrying this out is shown in the picture:
The next picture shows how the ball bearing is driven into the sample.
The Brinnell Hardness Number is given by:
BHN = mass of applied load in kg ÷ surface area of indentation indentation in mm2
Notice that the units are not SI. You will often come across that in engineering. Engineers don't give a damn about the purity of the physics. They just want numbers that work!
The equation looks ghastly because the area of an indentation has a rather heavy looking formula:
D is the diameter of the ball bearing, and d is the diameter of the indentation. Both are in millimetres.
A simpler relationship can also be used:
D is the diameter of the ball, while h is the depth of the indentation. Again the measurements are in millimetres.
Stiffness
You have met force extension graphs before and the concept of stiffness as the ratio of force (N) over extension (m), i.e.
F = k/x
We can plot force-extension graphs for various materials.
To make it a fair test, we need to have samples of the same area and length. The graph above shows the stretching of materials, but we can do compression tests on soft materials like cake. You want to have a certain amount of "bounce" in a cake. A dry or stale cake is not pleasant to eat.
Much of the testing is done by specialist and highly expensive machinery.
Key words in Material Science
| Key Word | Definition | Example |
| Stiff | Does not easily change shape when force is applied | Seaside rock |
| Elastic | Returns to original shape when force is removed. | Jelly |
| Plastic | Remains deformed when force is removed | Chewed chewing gum |
| Ductile | Can be readily pulled out into a thinner shape. | Soft jelly |
| Malleable | Can be deformed under compression | Fudge |
| Strong | Requires a large force to break it | Shell of a nut |
| Brittle | Easily cracks | Biscuits |
| Tough | Needs a large force to deform it | Kevlar |
Note that all ductile materials are malleable. However, not all malleable materials are ductile.
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