The Study on Shell-Breaking Characteristics and Crack Formation Patterns of Walnuts (Juglans Sigillata D.) Under Static Loading

Abstract

Obtaining highly intact walnut kernels after shell-breaking is a key step in the deep processing of walnuts. Juglans sigillata D., characterized by tightly bonded internal septa and irregular shell geometry, poses significant challenges for achieving kernel integrity. In this study, a three-dimensional walnut model was developed using 120 walnut samples. Four types of compression heads—flat, concave groove-shaped, inverted conical, and protruding stud-shaped—were employed to perform static loading tests at a constant rate of 10 mm/min from three directions: the head, belly, and umbilical regions. The shell-breaking characteristics and crack formation patterns were systematically investigated. Results showed that the shell-breaking process involves an initial elastic deformation followed by plastic failure. The initial shell-breaking force ranged from 172.3 N to 739.7 N, significantly influenced by both compression head type and loading direction. The highest probability of obtaining intact kernels (over 70%) was achieved by applying a belly-directional load using the concave groove-shaped compression head. Initial crack formation patterns were categorized into six types across three main models, with longitudinal cracks perpendicular to the suture being most favorable for intact kernel acquisition. A finite element-based predictive model for crack initiation demonstrated an accuracy of 85.3%, showing high consistency with observed stress concentration zones and actual crack propagation paths. This study elucidates the mechanical behavior and crack formation patterns during walnut shell-breaking and highlights the relationship between crack formation and kernel integrity, offering valuable insights for the development of efficient walnut shelling equipment.