Radial stress and post-compaction mechanics in asymmetric uniaxial compression of microcrystalline cellulose and lactose

This study investigates stress transfer and post-compaction behavior of tablets made from microcrystalline cellulose (MCC) and lactose subjected to instrumented asymmetric uniaxial compression. The objective is to compare the mechanical response of these two model excipients by analyzing axial and radial stress redistribution and its relationship with densification and color gradients observed at the tablet radial surface. Compaction experiments were performed using a compaction simulator enabling the simultaneous measurement of pressures applied by the upper punch and transmitted to the lower punch. This asymmetric configuration allows a direct assessment of stress transmission and residual stress development during compression and before ejection. The results reveal distinct behaviors between MCC and lactose in terms of compressibility, stress transmission, and residual stresses. Unusual residual force profiles, characterized by residual stresses exceeding ejection stresses, are observed for MCC-based formulations at high applied pressures. In contrast, such profiles are not observed for lactose-based formulations, despite higher residual stress levels, thereby highlighting material-dependent relaxation mechanisms. The combined analysis of local densities, measured stresses, and color gradients shows that, for MCC, the intensity and distribution of radial coloration cannot be fully accounted for by density variations or radial stress levels alone. This observation highlights the limitations of a purely stress–density-based interpretation and suggests the possible contribution of additional factors related to stress redistribution and intrinsic material behavior. Overall, this work emphasizes the need to explicitly account for stress transmission asymmetry in uniaxial compression and cautions against interpreting radial color gradients as direct indicators of densification during pharmaceutical powder compaction.