Construction and Verification of Spherical Thin Shell Model for Revealing Walnut Shell Crack Initiation and Expansion Mechanism

Abstract

Walnut shell breaking is the first step of deep walnut processing. This study aims to investigate the mechanical properties and fracture state of the Qingxiang walnut shell under unidirectional load and guide the complete separation of the walnut shell and kernel. The spherical thin shell model of the walnut (the fitting error is less than 5%) was established and verified. The process from the initiation to the expansion of walnut cracks was analyzed. The crack expansion rate was estimated in terms of the crack fracture regularity on the shell’s surface. Based on the momentless theory and finite element simulation analysis, we found that the stress on the shell surface in the concentrated force action region was gradient distributed from inside to outside and that the internal forces were equal in all directions in the peripheral force action region. The unidirectional impact shell-breaking experiments confirmed the reliability of our spherical thin shell model and verified our hypothesis of walnut shell fracture along the longitudinal grain. Our results can provide a theoretical basis for the development and structural optimization of shell-breaking machinery.