Internal Friction and Fatigue in Metals at Large Strain Amplitudes

A barium titanate transducer coupled to an exponential horn can produce large strains in metals. By using a final horn with a necked down section, it is possible to concentrate the strains in a small specimen. Measurements of the impressed voltage and a voltage from a pick up electrode on the titanate make it possible to calibrate the device so that the internal friction, change in elastic constant, and the strain in the sample are all determined. The measured internal friction can be divided into three strain regions. From 10−6 to 10−5 the internal friction is approximately independent of the strain. For strains from 10−5 to from 2 × 10−4 to 2 × 10−3, depending on the material, the internal friction increases with strain in a reversible anelastic manner. Above this range the internal friction increases very rapidly and for strains twice as large as those for the initial rise, the metal fatigues in a very short time. It appears that the anelastic range is caused by the reversible production of unstable Frank-Read loops. The shape of the internal friction curve allows one to evaluate the dislocation distribution with results in accord with etch pit methods. The final phase appears to be due to irreversible production of Frank-Read loops caused by dislocations cutting through other dislocations. This process produces jogs in the dislocations and vacancies in the metal. These vacancies accumulate to produce fatigue cracks in accord with a theory of Mott's. Temperature and time effects result from the lowering of the activation energy by the applied stress. At liquid nitrogen temperature, the fatigue stress increases until the activation energy for diffusion is nearly zero.