Moisture-Dependent Mechanical Behavior, Elastic-to-Plastic Transitions, and Structural Failure Mechanics of African Eggplant (Solanum aethiopicum) Under Quasi-Static Compressive Loading

Abstract

This study provides an exhaustive, multi-axial biomechanical and rheological characterization of the African Eggplant (Solanum aethiopicum), focusing on its moisture-dependent deformation mechanics, elastic-to-plastic transitions, and structural failure thresholds under compressive loading. Using an Instron Compressive Testing Machine (AMETEK series EZ 250) at a controlled quasi-static travel speed of 22mm/min, experiments were conducted on the green (S. aethiopicum, Aculeatum group) and white (S. aethiopicum, Kumba group) varieties. Thermo-gravimetric analysis (TGA) established a baseline fresh physiological moisture content averaging 93.550%. Controlled dehydration protocols modulated internal moisture levels (M) from 45% to 95% (wet basis) to explore how internal cell turgor pressure acts as a pressurized hydraulic matrix. The empirical findings validate that moisture levels fundamentally govern structural transitions. High moisture levels (M > 85%) preserve a hyper-turgid cellular framework, creating a stiff, crisp, yet highly brittle response. This state features a high Modulus of Stiffness (M_T = 48.69MPa} at the dry intercept) and causes a rapid fracture immediately following the elastic limit. Conversely, lower moisture levels (M < 70%) induce a profound transition toward viscoelastic and highly plastic deformation, where cell walls wrinkle, buckle, and fold, requiring significantly higher deformation strains before absolute rupture. Multi-axial testing revealed an inherent mechanical anisotropy: transverse loading demonstrates significantly higher resistance, stiffness, and yield force compared to longitudinal loading, showing that the vegetable behaves as an anisotropic structural body with lateral cellular reinforcement. High-confidence empirical regression models mapped parameters such as Transverse Rupture Force (F_T = -10.601M + 535.62, R² = 0.9947) and Transverse Yield Force (Y_T = -3.1416M + 244.9, R² = 0.9982). Plotting Rupture Force against Moisture Content identified a Critical Moisture Threshold range between 75% and 82%. Within this window, the tissue optimizes the balance between structural rigidity and viscoelastic damping, maximizing resistance to mechanical impacts during harvest and transport. To validate these empirical observations, an in-silico Finite Element Analysis (FEA) chapter was integrated to simulate non-linear internal stress distributions and localized cell collapse. Furthermore, a complete multi-way Analysis of Variance (ANOVA) was developed to analyze the interactive effects of cultivar variety and loading orientation. Finally, precise industrial storage room engineering designs were elaborated to preserve fruit moisture within the optimal resilience window. These integrated insights provide a robust quantitative foundation for food logistics infrastructure and postharvest processing machinery design, aiming to mitigate agricultural losses across Sub-Saharan supply chains.