材料特性数据对成形仿真精度的影响(下)——实验方法的完善

仿真模型的预测能力与精度很大程度上取决于材料模型中所使用的实验数据的力学特性。不同的实验数据描述了材料不同的强化与塑性变形特性,也由此决定了仿真模型不同的预测特性。由于冲压件几何形状不同,产生的成形缺陷与成形极限判据也会有所不同,因此,需要针对成形件的受力特点选择最适合的仿真模型实验数据。但是,目前的实验方法不够完善,不能准确获得所需要的实验数据,或者所得的实验数据并不适用于仿真模型,也就是说,实验数据的匮乏制约了仿真模型的预测精度。根据冲压件的应力分布特点,讨论了可应用于仿真模型并能综合反映材料塑性变形特性的实验方法,在此基础上,进一步探讨了常用的简单加载实验方法的现状以及亟待解决与完善的问题,包括如何更好地获得完整的实验数据,进而通过仿真模型能够充分反映出材料在实验应力状态下与塑性变形相关的力学特性。

材料特性数据对成形仿真精度的影响(上)——实验数据及其应用

板料成形的仿真精度主要取决于描述材料塑性变形特征的塑性本构关系。而塑性本构关系的预测能力则是由在不同应力状态下所获得的实验数据所定义。用于描述板材塑性变形特性的实验数据包括初始屈服、后续强化以及各向异性塑性变形参数等。为了应用不同的实验数据来恰当地表征仿真所用的材料模型,首先需要了解仿真成形零件的主要加载应力特性;其次需要合理应用由不同实验方法所获得的实验数据,使所定义的材料塑性本构模型能够充分反映出材料在整个均匀塑性变形过程中所表现出的材料强化与塑性变形特性,而非局部区域或者某一特定点处的实验特性。分析了应用不同实验数据定义仿真材料塑性本构关系对模拟结果所产生的不同影响。提出在应用材料的一些特殊实验结果时,除了需要考虑选择不同的流动准则,还需要考虑所涉及的塑性本构关系是否可以充分揭示出材料所对应的实验特性。

Unified analytical stressstrain curve for quasibrittle geomaterial in uniaxial tension, direct shear and uniaxial compression

Considering strain localization in the form of a narrow band initiated just at peak stress, three analytical expressions for stressstrain curves of quasibrittle geomaterial (such as rock and concrete) in uniaxial tension, direct shear and uniaxial compression were presented, respectively. The three derived stressstrain curves were generalized as a unified formula. Beyond the onset of strain localization, a linear strain-softening constitutive relation for localized band was assigned. The size of the band was controlled by internal or characteristic length according to gradient-dependent plasticity. Elastic strain within the entire specimen was assumed to be uniform and decreased with the increase of plastic strain in localized band. Total strain of the specimen was decomposed into elastic and plastic parts. Plastic strain of the specimen was the average value of plastic strains in localized band over the entire specimen. For different heights, the predicted softening branches of the relative stressstrain curves in uniaxial compression are consistent with the previously experimental results for normal concrete specimens. The present expressions for the post-peak stressdeformation curves in uniaxial tension and direct shear agree with the previously numerical results based on gradient-dependent plasticity.

Study on the biaxial tensile strength of special made concrete based on the Hill theory

This paper mainly studies on the biaxial tensile strength and its calculative formula of a special made concrete. According to the Hill theory and the uniaxial tensile experiment results, the expression of the concrete biaxial tensile strength is deduced, which is then combined with experiment data and checked. The result shows： the biaxial tensile strength of this kind of concrete is about 15%-30% lower than its uniaxial tensile strength, and the calculative value is basically consistent with the experiment value, with less than 11% difference. This kind of biaxial tension weakening effect is very important to the structure designs of concrete road. The author suggests further research.

Study on Microcapsulation of Onion Oil with Complex Coacervation and Spray Drying Method

[Objective] The aim was to study the microcapsulation of onion oil with double place separation-coacevation and spray drying. [Method] The effect of processing parameters which included the different wall material combination,the ratio between the wall material,the core material content and the feed concentration on the efficiency and yield of microencapsulated was evaluated by single factor experiments. [Result] Through orthogonal optimization experiments, the optimum condition was as follows: the ratio of Arabic gum and maltodextrin ration was 1∶1, the core content was 25%, feed composition was 18%, and the microencapsulation efficiency and yield can reach 80.7% and 95.3% respectively. [Conclusion] The microcapsulaiton of onion oil with complex coacervation spray drying method can increase membrane thickness, improve core stability and prolong shelf life of products.