Cube specimens were tested using the Instron machine under uniaxial compression. The loading was a displacement control mode at 10% strain/min. The cubic specimens were tested in all three possible loading directions. Five tests were performed on these specimens. In all of these tests the overall strain reached 10%. Tests number one and two were performed on the first loading direction of the specimens, are denoted as direction A. Test number three was performed on the second direction of the specimens, direction B.  Tests number four and five were performed on the third direction, direction C, as shown in table 3-1.

Table 3‑1 Test Directions for Isotropic Behavior

Figure 3-56 shows the typical stress strain curve for tests one and two. Both curves were corrected to eliminate the seating error. It can be seen that for the same specimen in the same loading direction the stiffness decreases by 50% when preloaded to a 10% strain. The initial modulus is the slope of the straight line and is shown in the equation where the symbol “x” refers to the strain and the symbol “y” refers to the stress in kPa. The initial modulus line is longer in the case of test number two. That means the yield point strain is higher in the case of re loading. From the same figure it can be seen that both curves reached the same strength at 5%strain. The strength at 1%strain is decreased by 50% after reloading.

Figure 3‑1 Direction A Pre and Post Yield Behavior

Figure 3-57 shows the typical stress strain curves for tests on two perpendicular directions. Test two is for direction “A” after reloading. Test number three is the first loading in direction “B”. A modulus of 3522kPa is reached for direction “B” while the modulus of the same specimen in direction “A” is equal to 2162kPa. The stress strain curve is different for both directions.

Figure 3-58 shows the typical stress strain curves for direction “C”. Test number five is done after the specimen is pre loaded in test number four.  The initial modulus is reduced from a value of 3135kPa to a value of 1288kPa. The shape of the curve is different for the two tests. The initial linear portion is much longer in the preloaded test results. The value of the strength at 10% strain is a slightly increased. The strength at 1% and 5% strain is decreased when preloaded.

Figure 3‑2 Direction A Post Yield and Direction B Pre Yield Behavior

Figure 3‑3 Direction B Pre and Post Yield Stress Strain Behavior

Figure 3-59 shows the results for the preloaded test in directions “A” and “C”. It is obvious that the two stress strain curves are not the same. One of the directions has an initial modulus of 2162kPa while the other has an initial modulus value of 1288kPa. The strength at 10% strain is the same for both directions. The strength at 1% and 5% strain is different for both directions.

Figure 3‑4 Post Yield Behavior for Two Perpendicular Loading Directions