Page 161 - Mechanical Behavior of Materials

162 Chapter 4 Mechanical Testing: Tension Test and Other Basic Tests

2500 300

σ , Ultimate Strength, MPa 1500 Brinell Vickers 200 σ , ksi u
2000

1000
100
u
500
0 0
0 100 200 300 400 500 600 700
HB or HV, Hardness, kg/mm 2

Figure 4.33 Approximate relationship between ultimate tensile strength and Brinell and
Vickers hardness of carbon and alloy steels. (Data from [Boyer 85] p. 1.61.)

Another hardness test that is somewhat similar to the Vickers test is the Knoop test. It differs
in that the pyramidal indenter has a diamond-shaped base and in the use of the projected area to
calculate hardness.

4.7.3 Rockwell Hardness Test

In the Rockwell test, a diamond point or a steel ball is employed as the indenter. The diamond point,
◦
called a Brale indenter, is a cone with an included angle of 120 and a slightly rounded end. Balls
of sizes ranging between 1.6 mm and 12.7 mm are also used. Various combinations of indenter and
force are applied in the regular Rockwell test to accommodate a wide range of materials, as listed
in Table 4.8. In addition, there is a superficial Rockwell hardness test that uses smaller forces and
causes smaller indentations.
Rockwell tests differ from other hardness tests in that the depth of the indentation is measured,
rather than the size. A small initial force called the minor load is first applied to establish a reference
position for the depth measurement and to penetrate through any surface scale or foreign particles.
A minor load of 10 kg is used for the regular test. The major load is then applied, and the additional
penetration due to the major load is measured. This is illustrated by the difference between h 2 and
h 1 in Fig. 4.34.
Each Rockwell hardness scale has a maximum useful value around 100. An increase of one unit
of regular Rockwell hardness represents a decrease in penetration of 0.002 mm. Hence, the hardness
number is
h
HRX = M − (4.31)
0.002
where h = h 2 − h 1 is in millimeters and M is the upper limit of the scale. For regular Rockwell
hardness, M = 100 for all scales using the diamond point (A, C, and D scales), and M = 130 for all