澳洲坚果破壳工艺参数优化及压缩特性的有限元分析

为了提高澳洲坚果破壳整仁率，该文首先以加载速度、加载方向、果壳含水率为试验因素进行正交试验，以破壳后澳洲坚果果仁的整仁率为评价指标，优化出适宜澳洲坚果破壳的最佳工艺参数为：加载速率45 mm/min、沿水平向加载、果壳含水率6%～9%。在此破壳工艺下进行试验，得出澳洲坚果的整仁率最高可达93%。澳洲坚果果壳种脐向、宽度向、水平向的平均破壳力分别为1 018、2 274、1 173 N；弹性模量分别为32.24、68.63、39.65 MPa。说明澳洲坚果是各向异性的，宽度向的抗压能力最强。运用有限元方法对澳洲坚果的3个加载方向进行应力和应变分析，得出较佳的施力方向为水平向，与试验结果一致。故在设计破壳机时，施加于澳洲坚果的破壳力应不小于2 500 N，并应尽量使澳洲坚果沿水平向受力。研究结果为澳洲坚果破壳机的研制提供了理论基础。

Optimization of technical parameters of breaking Macadamia nut shell and finite element analysis of compression characteristics

Macadamia nuts have been successfully cultivated as crops in Australia and the USA and were introduced to China for experimental planting since 1980s. It is rich in fat and protein. The current production is over 700000 tons annually in China, but processing technology for macadamia nuts is undeveloped, especially breaking shell. So it has important significance to optimize technical parameters of breaking macadamia nut shell. Orthogonal design was carried out for optimizing technical parameters of breaking macadamia nut shell. The loading rate, the loading direction and the moisture content of macadamia nut shell were selected as factors and the integrated kernel rate of macadamia nuts was selected as evaluation index in this experiment. Macadamia nuts with different moisture content were selected as test samples, the moisture content of which was obtained by hot air drying at 55℃. Experiment of breaking macadamia nut was carried out in electronic tensile testing machine. The results indicated that the moisture content of macadamia nut shell and the loading direction had more significant effect on the integrated kernel rate than the loading rate. The most optimal combination of technics parameters was that loading rate, loading direction and moisture content of macadamia nut shell were 45 mm/min, horizontal, and 6%-9%, respectively. In this case, the highest integrated kernel rate of macadamia nut was 93%. The compression test was carried out in the macadamia nut shell moisture content of 6%-9% and the loading rate of 45 mm/min. Average shelled forces were 1018, 2274 and 1173 N in hilum, width and horizontal, respectively. The elastic moduli of macadamia nut shell calculated by the Hertz contact stress theory were 32.24, 68.63 and 39.65 MPa in hilum, width and horizontal, respectively. The results indicated that macadamia nut was anisotropic. Compression capability was the strongest in width and the weakest in horizontal. The shape of macadamia nut was close to sphere. The width diameter was near to average diameter of macadamia nut. The thickness was 4.03-4.36 mm at top of macadamia nut and 2.22-2.48 mm at the middle. That showed that the shell thickness of macadamia nut was nonuniform. The shell's material was similar to wood, so poisson's ratio was near to 0.3. Material properties and geometric model of macadamia nut were imported into ANSYS workbench. The distribution of stress and strain of breaking shell of macadamia nut was obtained by finite element analysis of macadamia nut. According to the finite element analysis diagram of macadamia nuts in the 3 loading directions, it could be known that cracks were easy to emerge and expand in horizontal. It was also horizontal which was the most appropriate for breaking macadamia nut shell. The simulation results were consistent with the results of experiment. The biggest shelled force was 2016 N in horizontal, and therefore the shelled force should not be less than 2500 N for ensuring that all nuts would be cracked. The loaded force should be in horizontal when macadamia nut shell was crushed. It provides the design basis for the macadamia nut shell-breaking machine.