Logo from Scott Cunningham.
Logo from Scott Cunningham.
he Brinell Hardness Test was one of the most widely used hardness tests during World War II. For measuring armour plate hardness the test is usually conducted by pressing a tungsten carbide sphere 10mm in diameter into the test surface for 10 seconds with a load of 3,000kg, then measuring the diameter of the resulting depression. The BHN is calculated according to the following formula:
Copyright © 1995 John C. Russ.
Several BHN tests are usually carried out over an area of armour plate. On a typical plate each test would result in a slightly different number. This is due not only to minor variations in quality of the armour plate (even homogenous armour is not absolutely uniform) but also because the test relies on careful measurement of the diameter of the depression. Small errors in this measurement will lead to small variations in BHN values. As a result, BHN is usually quoted as a range of values (e.g. 210 to 245, or 210–245) rather than as a single value.
The BHN of face hardened armour uses a back slash ‘\’ to separate the value of the face hardened surface from the value of the rear face. For example, a BHN of 555\353–382 indicates the surface has a hardness of 555 and the rear face has a hardness of 353 to 382.
The Brinell Hardness Test described above is called “HB 10/3000 WC” and was the type of test used by the Germans in World War II. Other types of hardness tests use different materials for the sphere and/or different loads. Softer materials deform at high BHN which is why tungsten carbide (a very hard material) is used to measure armour plate. Even so, as the BHN goes above 650 the tungsten carbide ball begins to flatten out and the BHN values indicate a greater difference in hardness than there actually is, while above 739 the ball flattens out so badly that it cannot be used.2
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