Test and analysis of the friction characteristics of packaged grain with various stacking modes

Packaged grain stacking serves as the primary storage method for finished grain globally. However, stack inclination and collapse remain persistent safety hazards, threatening both economic sustainability and food security. Current industrial practices lack a systematic understanding of the interlayer friction mechanics governing stack stability, particularly under variable stacking geometries and long-term storage conditions. This study addresses this critical gap by quantitatively investigating the deformation-coupled friction behavior of packaged grain layers, aiming to establish a predictive framework for collapse risk assessment and provide actionable insights into the optimization of storage configurations. The urgency of this work is underscored by escalating global grain storage demands and the need to minimize postharvest losses estimated at 8% to 10% annually because of improper stacking practices.

A dedicated friction performance testing platform was developed to evaluate the interlayer friction dynamics in packaged grain stacks. The grain bag friction testing device was independently developed and mainly consisted of three systems: horizontal tension, vertical load, and measurement. The packaged grain was subjected to horizontal tension provided by a horizontal tensioning system, with a quantifiable vertical load applied to its upper part, and the horizontal tension and horizontal displacement of the packaged amount under different load conditions were obtained by the measurement system. The values obtained by the horizontal tension and horizontal displacement sensors were recorded, and the relationship curve between horizontal tension and horizontal displacement was plotted. Controlled experiments were conducted to analyze key variables, including stacking patterns (i.e., laminated seamless, vertical–horizontal crisscross, and multisided contact arrangements), grain layer deformation, and interlayer interactions. Quantitative metrics, such as equivalent friction coefficients, were measured under incremental vertical loads to characterize friction evolution.

The results showed that the deformation of packaged grain is a significant factor affecting the interlayer friction characteristics. The interlayer contact friction initially increases with the increase in deformation, subsequently reaches the maximum value, and finally tends to be stable. The friction characteristics of packaged grain are affected by the interlayer contact friction performance of the packaging bag, the bite effect between the grain particles on the surface of the packaging bag, and the mutual embedding effect between the bags. Because of the slight outward expansion of the side of the multilayer stacked packaged grain during stacking, the extrusion effect forms on the side of the adjacent packaged grain, and the equivalent friction coefficient of the interlayer contact is larger than that of the single-sided contact. Under the same vertical load, the equivalent friction coefficient of the vertical and horizontal crisscross arrangements (i.e., the lower transverse juxtaposition) of the packaged grain is increased by 41% to 47% compared with the laminated seamless arrangement, and the equivalent friction coefficient of the four-sided contact extrusion arrangement is increased by 38% to 57% compared with the double-sided contact extrusion arrangement.

This study establishes a deformation-coupled friction model for packaged grain, revealing that optimized stacking geometries significantly enhance interlayer friction and collapse resistance. The quantified friction increments under crisscross and multisided contact arrangements provide actionable guidelines for designing stable grain stacks. These findings advance the mechanistic understanding of stack collapse and provide a scientific framework for improving grain storage safety standards.