TABLE OF CONTENTS In compression testing, a cylinder or a parallelepiped cube (with one side the one parallel to the loading direction longer than the other two) is subjected to compression between two parallel plates. The plates should have a self-alignment system, and they often ride on one or two hemispherical caps, as shown in Figure 3.13(a). If ceramics are being tested, it is also common to use special ceramic (WC, for instance) inserts between the specimen and the hemispherical caps. This eliminates indentation and plastic deformation of platens. Lubrication between the specimen and the plate is also very desirable, to decrease barreling (nonuniform deformation) of the specimen. (Barreling will be discussed shortly.) The use of a thin Teflon coating, molybdenum disulfide, or graphite is recommended. It is also very important to ensure homogeneous loading of the specimen. This is particularly critical for ceramics, which often fail in the elastic range. It is easy to calculate stresses that arise when one of the parallel sides of a specimen is longer than the other. Figure 3.13(b) shows a specimen with a height difference ? h. The right side will experience a stress ? = E( ? h/ h 2) before the left side is loaded. For a typical ceramic, it is a simple matter to calculate the relationship between ? ?, the difference in stress from one side to the other, from ? h. For example,...
