One of the great strengths of human language is its ambiguity and with it the ability to suggest, to imply and also to evoke emotions. From this comes beautiful poetry and powerful drama. However the same language when put to scientific use has to be as unambiguous as possible. Because the language of science borrows many words from standard language it is not surprising if sometimes non-specialists get them mixed up. The two words hard and tough are a case in point. In English they might both be taken to mean ‘difficult’ when applied to a question or ‘macho’ when applied to a man. In material science they have two very different and specific meaning but still many people mix them up.
Toughness is a measure of how much energy is needed per unit volume to fracture a material. As such it is equal to the area under the stress strain graph for the material, up to the point of failure. However the actual figure tends to depend on a number of things such as how quickly the load is applied and so special testing techniques have been developed to study different failure conditions. Related to this is the idea of fracture toughness, this is a measure of the resistance to fracture where a crack is already present.
Fracture toughness has been called the single most important property of an engineering material since it determines the difference between an elegant failure and a catastrophic one. To put that in perspective, an elegant failure is when you are crossing a deep gorge by a bridge and suddenly the metalwork creaks and sags. You know it is time to get off the bridge, NOW! The whole structure may well be beyond repair but you and everybody else on the bridge escape with your lives. A catastrophic failure on the other hand is when you are crossing the next bridge and there is a sickening crack. The next thing you know is that you and the two sections of bridge are falling into the gorge below.
Two methods of measuring fracture toughness.
Hardness is a very tricky concept, not least because it means different things to different people. Even in science the word has many meanings, for example the hardness of water (a measure of the quantity of dissolved mineral) and mineral hardness (the ability of a mineral, such as quartz, to scratch another, e.g. gypsum). However to a material scientist hardness usually means the resistance of a substance to permanent (plastic) deformation. This means that hardness can be due to many different factors. The ultimate hard material would be:
Stiff (a high Young’s Modulus), so that it would resist compression when it was in the elastic region of the stress – strain graph.
A high Compressive Strength (the compression equivalent of yield strength), so that the applied force would be largely applied in the elastic region of the stress – strain graph.
A steeper than usual plastic section of the stress strain graph so that when the plastic region is reached it still takes a large residual compressive stress to achieve a permanent deformation.
There are several ways to measure hardness but one of the commonest is Brinell Test.
A small (usually 10 mm) ball made of a hard material (usually hardened steel) is pressed into the material under test with a known force (usually 3000 kilogram force [kgf]) for a set time (usually 15 seconds). The size of the indentation that is left is then measured with a low power traveling microscope (usually).
The need for the use of quite so may ‘usually’s is because of the need to cover a wide range of materials. The data given so far is typically used for steel and iron. For much significantly harder materials a tungsten carbide ball is used. For significantly softer material the force can be reduced to 1500 or even 500 kgf. For materials other than steels the duration of the indentation is extended to at least 30 seconds. The Brinell hardness is then calculated from the following formula:
The quoted result should always contain full details of the test variables, thus:
120 HBS 10/3000/15: A hardness of 120 BHN was measured using a 10 mm diameter hardened steel (HBS [S for steel]) with a force of 3,000 kgf for 15 seconds.
or
1800 HBW 10/3000/30: A hardness of 1800 BHN was measured using a 10 mm diameter tungsten carbide (HBW [W for wolfram, the original name of tungsten]) with a force of 3,000 kgf for 30 seconds.
The reason for the rather peculiar unit of force is that some branches of engineering are still in the habit of using non SI units in places. The hardness scale was introduced before the complete adoption of the système international and so if the SI unit of force were to be used then all the existing hardness numbers would change. 1 kgf is equal to the weight of a 1 kg mass in standard gravity (approximately 9.81 N since g is approximately 9.81 N kg-1).
Sample |
Ball Diameter |
Ball Material |
Force |
Duration |
Indent Diameter |
|
(mm) |
|
(N) |
(s) |
(mm) |
1 |
10 |
Tungsten Carbide |
29430 |
15 |
1.49 |
2 |
10 |
Hardened Steel |
4905 |
30 |
4.17 |
3 |
10 |
Tungsten Carbide |
29430 |
15 |
5.41 |
