An electromagnetic fatigue-testing machine is shown in Fig. 3.41 in which an alternating force is applied to the specimen by passing an alternating current of frequency \(f\) through the armature. If the mass of the armature is \(20 \mathrm{~kg}\), the stiffness of the spring \(\left(k_{1}\right)\) is \(1.8 \times 10^{6} \mathrm{~N} / \mathrm{m}\), and the stiffness of the steel specimen is \(135 \times 10^{6} \mathrm{~N} / \mathrm{m}\), determine the frequency of the alternating current that induces a stress in the specimen that is twice the amount generated by the magnets.

Transcribed Image Text:

Armature Specimen (stiffness, k) Spring (stiffness, k) 00000 Magnets FIGURE 3.41 Electromagnetic fatigue-testing machine.