THE INTERPRETATION OF ELECTRON MICROSCOPE FRACTOGRAPHS.
NAVAL RESEARCH LAB WASHINGTON DC
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The report was prepared to aid the relative newcomer to electron fractography in the interpretation of fractographs. Accurate interpretation depends on understanding the effects of electron microscope and replica variables on photographic densities in the fractographs. Briefly, some of these effects are as follows Contrast decreases with increasing electron gun potential and lens aperture size. Electron density decreases and hence photographic density in the printed fractograph increases with the amount of replica material penetrated by the electrons. This, in turn, increases with an increase in the local thickness of the replica or with a decrease in the angle between the local replica surface and the incident electron beam. The thickness of the replica depends on the vacuum in the bell jar during formation of the replica, the angle of the local feature with respect to the depositing metal or carbon, and the distance between the feature and the source of the carbon or metal. During shadowing, convex features cast external shadows and tend to grow larger, while concave features contain their shadows and tend to remain the same size but to fill up with shadowing material. Interpretation is aided by stereoscopic viewing and by precision matching of mating fracture surfaces. Replica fidelity is a function of the choice of replication technique, with the direct carbon process generally being superior, and of the shape of both the features under study and immediately adjacent portions of the replicas. Author
- Metallurgy and Metallography