Abstract
There exist two types of fatigue damage caused by the cyclic plastic strain. One is the damage which is accumulated in the form of a surface irregularity such as slip bands or random notches on the specimen surface and is considered to control the crack initiation life of an unnotched specimen. The other is the decrease of fracture ductility which is found to control the fatigue life at the very high cyclic strain region. So the latter type of damage may contribute to the propagation of the crack at the tip of which the high cyclic strain appears.
In this paper, therefore, in order to obtain some microscopic information about fatigue damage, direct observation of fatigue damage on the specimen surface has been made. Furthermore, new techniques for measuring the amount of the surface irregularity and also the local strain distribution of the very small portion of the specimen has been developed using a scanning electron microscope (SEM). The new techniques have the following characteristics: (1) Electron signals due to the back scattered electrons which depend on the gradient of the reflection surface are analized and integrated along a single scanning line with aid of an A-D convertor and a mini-computer to give the profile of that section, and (2) a micro grid of about 1∼2μm spacing is printed on the specimen surface by so-called“the electron beam exposure of the photo-resist”technique by means of the SEM. Using this grid the microscopic strain distribution within a grain or at the tip of crack can be measured.
