Effects of excipient characteristics and compact geometry on localized strain propagation in powder compacts: Optimized digital image correlation approach

Abstract

Although localized strain critically affects the mechanical integrity of pharmaceutical tablets, conventional macroscopic tests cannot capture spatial strain distributions. This study established an optimized digital image correlation (DIC) framework to quantify localized strain fields.

Highlights

DIC parameters were optimized for reliable strain mapping in pharmaceutical tablets.
Optimized DIC (85,700 lx; 80-px subset; 3-px spacing) yielded full-field strain maps.
Excipient type and grade showed distinct strain patterns at similar tensile strength.
3D fractography linked compact structure to localized failure surface formation.
DIC and Heckel metrics distinguished deformation-mechanism groups.

Illumination intensity (60,000–120,000 lx), subset size (20–120 pixels), and subset spacing (1–6 pixels) were optimized using image sharpness, contrast, and a correlation metric based on the zero–mean normalized sum of squared differences, yielding optimal conditions of 85,700 lx, an 80–pixel subset and 3–pixel spacing. Tablets prepared from three microcrystalline cellulose (MCC) grades, two lactose grades, one co–processed lactose–cellulose grade, and two mannitol grades were evaluated at compaction forces of 3–15 kN.

Although tensile strength increased with compaction force, DIC revealed excipient–dependent strain localization at comparable tensile strength, indicating that macroscopic strength alone cannot describe deformation behavior. Three–dimensional fractured–surface analysis revealed grade–dependent fracture–surface morphologies associated with differences in localized strain distribution. The framework discriminated MCC grade–dependent strain propagation and geometry–induced localization in shaped tablets.

Force–strain coupling and out–of–die Heckel analyses further distinguished grades containing a plastically deforming component from predominantly fragmenting grades. These findings demonstrate that DIC complements excipient selection, punch geometry design, and mechanical integrity assessment of tablets.

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Materials

MCC was purchased from FMC BioPolymer (Avicel® PH–102, Avicel® PH–112, and Avicel® PH–200, Philadelphia, PA, USA). Two lactose grades (Tablettose® 80 and GranuLac® 200) and one co–processed lactose–cellulose grade (Cellactose® 80) were purchased from MEGGLE GmbH & Co. KG (Wasserburg am Inn, Germany). Mannitol was purchased from Roquette Frères (Pearlitol® 200SD and Pearlitol® 300DC, Lestrem, France).

Sang Min Lee, Ki Hyun Kim, Min-Soo Kim, Du Hyung Choi, Effects of excipient characteristics and compact geometry on localized strain propagation in powder compacts: Optimized digital image correlation approach, Powder Technology, Volume 485, Part 2, 2027, 123160, ISSN 0032-5910, https://doi.org/10.1016/j.powtec.2026.123160.