Page 155 - Mechanical Behavior of Materials

156 Chapter 4 Mechanical Testing: Tension Test and Other Basic Tests

Figure 4.27 System for testing brittle materials such as concrete and stone in compression
with lateral pressure. This system was in use at the U.S. Bureau of Reclamation Laboratories,
Denver, CO, in the 1960s. Lateral pressures up to 860 MPa (125 ksi) could be applied by using
kerosene as the hydraulic fluid, which did not contact the specimen, due to the use of a
neoprene sheath. (From [Hilsdorf 73] as adapted from [Chinn 65]; used with permission.)

Where compressive failure does occur, it is generally associated with a shear stress, so the
fracture is inclined relative to the specimen axis. This type of fracture is evident for gray cast iron,
an aluminum alloy, and concrete in Figs. 4.23 and 4.24. Compare the cast iron fracture plane with
that for tension in Fig. 4.13. The tension fracture plane is oriented normal to the applied tension
stress, which is typical of brittle behavior in all materials.
For brittle materials such as concrete and stone, some engineering applications involve
multiaxial compressive stresses, as in foundations for buildings, bridges, and dams. A testing
arrangement that simulates such conditions by employing hydraulic pressure for multiaxial testing is
shown in Fig. 4.27. The axial pressure system provides a compressive force in the vertical direction,
as for a simple compression test. And the lateral pressure system compresses the specimen laterally
on all sides. The compressive strength of concrete or stone in the axial direction is affected by such
lateral pressure; and for large values of lateral pressure, the compressive strength is substantially
higher than in an ordinary compression test. (This behavior is addressed in some detail later in
Sections 7.7 and 7.8, where methods of predicting strength of brittle materials under multiaxial
stress are considered.)