基于FDEM的核桃挤压-剪切载荷下破碎机理与减损方法

Crushing Mechanism of Walnut under Extrusion-shear Load Based on FDEM

摘要:

破壳取仁是核桃精深加工的重要环节，针对现有核桃破壳方式脱壳效率低、整仁率不高等问题，本文通过有限元耦合离散单元法（FDEM）模拟核桃在挤压剪切载荷下的破碎过程，提出了一种挤压-剪切破壳方法，测定了温185核桃壳厚、壳仁间隙等物理参数，构建了仿真模型并进行了参数标定。通过定性和定量分析了核桃壳和仁在挤压-剪切载荷下破壳角度对壳仁断裂的影响以及破壳角度和压缩量对核桃壳仁破碎影响规律，明晰了核桃挤压-剪切载荷下破碎机理及核仁损伤的原因。结果表明：核桃壳仁破碎机理为核桃壳在拉伸主导下断裂，在接触表面产生贯穿裂纹，有利于核桃快速破壳，核桃仁在剪切主导下断裂，在破壳过程中核桃仁易与壳发生多点接触，产生应力集中导致断裂发生，不利于保持仁的完整性，各破壳角度下随着压缩量增加，核桃壳受到持续加载的力使其向里运动，导致核桃仁损伤逐渐增加。以上机理表明，若在核桃壳发生初次断裂后不继续施加破壳力，给其一个恢复变形时间，然后继续施加间歇加载力，减少壳与仁的多点接触，可使其在小位移下进行多次破壳，有效降低核桃仁损伤。提出了同向对辊破壳方式实现多次小位移破壳，并进行了试验验证，优化结果表明两辊转速分别为33、28 r/min、破壳间隙为33 mm时，破壳率为96.9%，整仁率为84.3%，与传统对向旋转破壳方式相比，破壳率、整仁率分别提高7.7、3.2个百分点，研究结果可为核桃破壳效果的提升提供理论参考。

Abstract:

Shell cracking to extract the kernel is a crucial step in the advanced processing of walnuts. Addressing the low shelling efficiency and inadequate kernel integrity associated with current cracking methods, the finite discrete element method (FDEM) was employed to simulate the fracture process of walnuts under extrusion-shear loads. A extrusion-shear cracking technique was proposed. Initially, physical parameters such as shell thickness and shell-kernel gaps of the Wen 185 walnuts were measured to construct and calibrate the simulation model. Subsequently, both qualitative and quantitative analyses were conducted to examine how the shelling angle under extrusion-shear loads affected the fracture of the shell and kernel. The relationship between shelling angle, compression amount, and the fragmentation of the walnut shell and kernel was also investigated. This clarified the cracking mechanism under extrusion-shear loads and the reasons for kernel damage. The results showed that the cracking mechanism of walnut shells and kernels involved the following: the walnut shell fractured primarily under tensile stress, generating penetrating cracks on the contact surface, which facilitated rapid shell-breaking. In contrast, the walnut kernel fractured predominantly under shear stress. During the shell breaking process, the walnut kernel tended to come into contact with the shell at multiple points, leading to stress concentration, which caused kernel fractures and hindered the preservation of kernel integrity. As the compression increased at different cracking angles, the continuous force exerted on the walnut shell caused inward movement, progressively increasing the damage to the walnut kernel. Based on these mechanisms, it was suggested that after the initial shell fracture, the application of cracking force should cease to allow for deformation recovery time. Subsequently, applying intermittent loading forces can reduce multiple contact points between the shell and kernel, enabling multiple shell fractures with minimal displacement and effectively reducing kernel damage. Finally, a co-directional roller cracking method was proposed to achieve multiple small-displacement shelling, which was validated through experiments. The preliminary optimization results showed that with roller speeds of 33 r/min and 28 r/min and a shelling gap of 33 mm, the shelling rate reached 96.9%, and the kernel integrity rate was 84.3%. Compared with traditional counter-rotating shelling methods, the shelling and kernel integrity rates were improved by 7.7 percentage points and 3.2 percentage points, respectively. The research result can provide a theoretical reference for enhancing walnut shelling efficiency and kernel integrity.