By using a barium titanate driver attached to a tapered brass horn, containing a specimen of metal, it is possible to stress the specimen to fatigue failure. The internal friction and plastic strain of the specimen can be measured from the ratio of the driving voltage to the pickup voltage from the titanate and from the resonant frequency. It is found that there are two amplitude ranges for which the internal friction and plastic strain vary with stress. The final phase for metals results in a very rapid rise in internal friction and plastic strain, and ends in fatigue failure. The effect of a static stress is to increase the stiffness of metals for small alternating stress amplitudes and to lower the value of the alternating stress required to produce fatigue failure. Germanium is not subject to fatigue failure in the manner of a metal but suffers brittle fracture. A theory based on the action of Frank-Read dislocation loops is shown to agree with the measured results. Fatigue stresses can be increased by metallurgical treatments which reduce the lengths of the Frank-Read sources.