While it should be stated, at the commencement, that no finality of agreed opinion has
been reached on the causes of fatigue failure and phenomena it is significant to find that the trend of modern thought is to associate the failure of metals-whether under static or repeated loading-with the consequences, direct or indirect, of slip, i.e., the process of plastic deformation by shear on gliding surfaces. Attention was first drawn to this aspect of fatigue by the classical researches of Ewing and Humfrey, whose work indicated a line of attack which has been followed up by other investigators, and has resulted in the discovery of important facts. It is to be expected that these facts are not entirely consistent with the conclusions drawn by the
earlier investigators from their own work in view of the more refincd experimental methods which have since been made available. The pioneer work of Ewing and Humfrey will never be forgotten, however.