摘要
湿膨胀系数用于预报谷物堆密度和研究谷粒应力裂纹产生原因等研究。文献中，计算此系数的参照体积和参照长度都取含水量为零状态下的值。这对计算谷粒湿应力会有误差，应当取初始状态下的值。理论上３个互垂方向的线膨胀系数之和应等于体膨胀系数，而且与方向无关，前者和已发表的实验结果基本一致，后者则不很一致。该文基于二相材料的假定，提出了理论湿膨胀系数，它对实验测定工作有指导意义。该文指出水分迁移会给谷粒造成损伤，应用损伤力学理论研究这个问题将是很有意义的。
The hydroscopic expansion coefficient of grains is an important physical parameter for prediction of bulk densities and study of internal stresses of grain kernel during adsorption and desorption. In the definition expressions of volumetric and linear hydroscopic expansion coefficient, reference volume and characteristic linear length are taken as those when moisture is zero in the literatures. It is pointed out in this paper that for calculation of hydro stress in grain kernel, the values of reference volume and characteristic linear length are better taken as those when grains are in initial moisture state. The sum of linear expansion coefficients in 3 perpendicular directions is equal to its volumetric one theoretically. This is confirmed by most published data. The structures of grain endospem, like rice and corn, apear to be isotropic. However published measurement results on linear hydroscopic expansion coefficients show they are anisotropic. What causes whole kernel shrinking anisotropically is still a problem, which needs to be further studied. Suppose that a grain kernel is a twophase system with solid (dry mass) and pure water, and change in the volume of a grain kernel is due to change in the kernel moisture content, a theoretical coefficient of hydroscopic expansion is derived. Since the air phase inside the kernel does not been considered, theoretical coefficient of hydroscopic expansion should be the upper bound for measurement values and this is confirmed by most published data. This property of theoretical coefficient of hydroscopic expansion is useful for experiment determination of the coefficient. Moisture transfer inside grain kernel will cause kernel damage, such as internal voids and micro cracks. Different drying procedures will produce different level of damage. The internal porosity of kernel may be taken as a damage variable which discribes the degree of kernel damage. Damage mechanics will be a useful theory for further investigation of grain stress cracking.
出处
《农业工程学报》
EI
CAS
CSCD
北大核心
1998年第2期203-208,共6页
Transactions of the Chinese Society of Agricultural Engineering
基金
国家自然科学基金
关键词
谷粒
湿膨胀系数
损伤力学
grain kernel, hydroscopic expansion coefficient, theoretical hydroscopic expansion coefficient, damage mechanics
