The evolution of internal structure within compacts composed of select
ed drugs and direct compression excipients, occurring during the unloa
ding phase of compaction, was studied in an instrumented rotary tablet
press. Utilizing three-dimensional viscoelastic analysis applied to s
uccessive, sequential segments of the unloading phase, elastic and vis
cous parameters were obtained that provide quantitative, incremental m
easures of the changing behavior of the compact during this period. It
is apparent from these studies that the principal feature of this pha
se of compaction is the generation and propagation of internal fractur
es as a result of the release of internal stresses through expansion.
The progression of elastic parameters from positive initial values to
negative terminal values during unloading, coupled with the strong rel
ationships they display to corresponding viscous parameters, supports
this conclusion. These results follow well-established Dugdale fractur
e dynamics in which rate-dependent flow, occurring in the immediate su
rroundings of crack tips, accompanies crack formation and propagation.
The extent of viscous flow depends both on the brittle versus plastic
nature of the substance and on the extent of crack growth.