Measurement of stress gradient inside the cubical powder samples of different dimensions

Many industrial powders when pressed; the formed pellet/compact/tablet can and does result in uneven density distribution. This phenomenon has been well documented by many researchers. From this fact, it can be inferred that a stress gradient is developed inside powder mass during compaction. However, the hypothesized stress gradient during initial stages of powder compaction has not been physically measured yet. To evidence the existence and extent of stress gradient in bulk powder sample during hydrostatic compression, stresses inside cubical sample was measured at the center and surface of cubical test specimen using the Cubical Triaxial Tester with hydrostatic triaxial compression test. For this purpose, a miniature pressure sensor was placed at the center and surface of bulk powder samples with two different cubical test specimen with dimensions of 50.8 x 50.8 x 50.8 mm and 31.8 x 31.8 x 31.8 mm, respectively. The result shows that there is a stress gradient set up during hydrostatic compression. For example, pressure measured at the center of large cubical sample with 100 kPa of isotropic stress is 56.4 (± 2.8) kPa whereas it is 68.2 (± 4.3) kPa with small sample under the same isotropic compression condition. The difference of stress at the center of large and small samples is statistically significant which provides a strong evidence of the existence of stress gradient during compaction.