玉米果穗仿生采摘弯曲疲劳断裂机理研究

Fracture mechanism of snapping was a hot research area in quality and efficiency of corn production. It was also the key problem of agricultural machinery needed to be solved urgently. Bionics was the theory basis of this project. The motion of snapping corn by human hand was bionic model. Mechanism of bending fatigue and fracture of bionic snapping of corn ears was researched. The concrete contents included: hand snapping mechanism was revealed based on analysing the component model, kinematic pair, movement rule and load character of human hand snapping. Bending deformation law of bionic snapping corn ears was revealed based on the bionic snapping test bench, law of bending internal force, and bending defromation equation. Factigue fracture was the evolution of bending deformation. Characteristics of fracture morphology, time-order and spacial distribution of cracks, and coupling rate of cooperative extension were analyzed. Mechanism of bending fatigue and fracture of bionic snapping of corn ears was revealed. Moisture content was the boundary condition and influencing factor of bending factigue fracture. The influences of moisture content on initiation, expanding acceleration, breaking strength and fatigue life of cracks were explored. The regulatory mechanism of moisture content on behaviors of bending fatigue and fracture of bionic snapping was established. It will lay theoretical foundations for developing snapping machine of low corn damage.

采摘断裂机理是玉米生产提质增效的研究热点，也是农业机械领域迫切需要解决的关键问题。本项目以仿生学为理论基础，以人类手工掰玉米动作为仿生模本，研究玉米果穗仿生采摘弯曲疲劳断裂机理。具体研究内容包括：通过分析手工采摘构件模型、运动副、运动规律和载荷特性，揭示手工采摘原理；通过搭建玉米仿生采摘试验台，分析弯曲内力规律，构建弯曲变形方程，揭示玉米果穗仿生采摘弯曲变形规律；以疲劳断裂为弯曲变形演化，通过分析裂纹断口形貌特征、多重裂纹萌生时序特征与分布规律，以及多重裂纹协同扩展耦合速率，揭示玉米果穗仿生采摘交变弯曲疲劳断裂机理；以含水率为弯曲疲劳断裂边界条件和影响因素，探索其对裂纹萌生分布、扩展加速度、断裂强度及疲劳寿命的影响，构建含水率对仿生采摘弯曲疲劳断裂行为的调控机制。本项目为开发具有低损性能的玉米果穗仿生采摘机械奠定理论基础。

针对玉米机械化摘穗中存在的功耗高、籽粒掉粒损失大、秸秆残留多等重大技术难题，以及传统技术中拉伸疲劳断裂产生的损失、损伤的重大科学问题，本项目以仿生学为理论基础，以人类手工掰玉米动作为仿生模本，揭示了玉米果穗仿生采摘弯曲疲劳断裂机理，发现了多重裂纹萌生时序特征和空间分布规律，建立了多重裂纹协同扩展耦合机制和弯曲疲劳裂纹寿命拟合表征方式。.研究发现：（1）当籽粒含水率分别为34.8%、30.2%、25.1%和20.3%时，该临界角度分别为50°~55°、50°~55°、45°~50°和45°~50°；穗柄偏转达到临界角度后，玉米穗柄拉断力分别降低了80%、86%、84%和80%，断裂拉伸量增加了72%、70%、93%和84%；穗柄断裂行为突变的诱因是表层纤维的受力由拉应力变为弯曲正应力，导致裂纹的萌生和扩展机理发生变化。（2）果穗碰撞中籽粒的脱落，主要原因是碰撞中籽粒的受力发生改变，果穗碰撞瞬间，籽粒在相邻籽粒和摘穗板共同作用下，所受合力超过籽粒与果穗间粒柄的连接力，导致粒柄断裂，受力平衡被打破，造成籽粒的脱落与飞溅；当籽粒含水率分别为21.8%、16.7%和13.4%时，运用仿生摘穗理论，籽粒损失率平均降低幅度分别达53.4%、48.6%和47.0%。（3）当籽粒含水率为25.1%，果穗偏转角度为60°时，玉米穗柄断裂的主要原因为弯曲变形，其断裂过程可分为三个阶段，在加载的初始阶段，玉米果穗穗柄表面不产生侧向裂纹，拉伸变形量与载荷基本呈正比关系；在裂纹扩展阶段，穗柄表面萌生横向裂纹，随着载荷增加，裂纹扩展，不断发出轻微开裂声；由于穗柄断面内材料的不均匀性，裂纹的扩展产生波动，“载荷-位移”曲线出现多次锯齿状的下降；在非稳定断裂阶段，当载荷达到最大值后，穗柄被拉断，同时发出较大的响声，由于苞叶的影响及穗柄材料各组分弹性系数的差异，穗柄各部分持续弯曲断裂，“载荷-位移”曲线呈现阶跃式下降。.本项目研究成果被中国农业机械学会评价为：提出了第2种玉米摘穗原理，为创新采摘原理、革新采摘技术、开发仿生采摘装备提供理论支撑。