Formation and Propagation of Fatigue Cracks in Metals as Affected by Overstressing and Understressing

1. A resume is given of pertinent information regarding the behaviour of metals under repeated stress, special emphasis being placed on understressing, overstressing and strengthening. 2. The evidence of changes taking place in a metal under repeated stress is examined, and the effects observed, together with the information obtained above, are correlated with the dislocation theory of slip. 3. The data available regarding the visual observation of cracks is studied and correlated with the percent of damage during overstressing. Because there is insufficient data, the correlation is inconclusive. Suggestions are made as to the type and amount of data required, and to how it may be obtained. 4. Considering the above information, two experimental procedures are proposed which will add greatly to the practical knowledge of the effects of slip under repeated stress, a study of these effects leading to a much clearer conception of the mechanisms of slip and failure. 5. A study is made of the properties desired in the test material. Armco iron is chosen because, in addition to having all the requisite properties, it has the simplest structure. 6. A study was made of existing fatigue-test machines and none were found to have the desired facilities for testing and examination. Thus a new type machine has been designed and built to satisfy the requirements of the outlined procedure. In addition, it has also been designed so as to effect a considerable saving in the time required for tests.